# Surface characterisation of thermoresponsive microgels based on oligo(ethylene glycol): Adsorption kinetics, dilatational rheology and monolayers

Garbine Aguirre, J Quirosa-Mancilla, C Cara-Galdeano, I Navarro-Arrebola, A Rubio-Andres, M A Fernandez-Rodriguez, Laurent Billon, A Martín-Molina, J Maldonado-Valderrama

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## ► To cite this version:

Garbine Aguirre, J Quirosa-Mancilla, C Cara-Galdeano, I Navarro-Arrebola, A Rubio-Andres, et al. Surface characterisation of thermoresponsive microgels based on oligo(ethylene glycol): Adsorption kinetics, dilatational rheology and monolayers. Journal of Colloid and Interface Science, 2026, 705, pp.139522. 10.1016/j.jcis.2025.139522. hal-05459713

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# Surface characterisation of thermoresponsive microgels based on oligo (ethylene glycol): Adsorption kinetics, dilatational rheology and monolayers

G. Aguirre<sup>a</sup>, J. Quirosa-Mancilla<sup>b</sup>, C. Cara-Galdeano<sup>b</sup>, I. Navarro-Arrebola<sup>c</sup>, A. Rubio-Andres<sup>b</sup>, M.A. Fernandez-Rodriguez<sup>b</sup>, L. Billon<sup>a</sup>, A. Martín-Molina<sup>b,d</sup>, J. Maldonado-Valderrama<sup>b,e,*</sup>

## HIGHLIGHTS

* OEG-microgels are biodegradable, pH and temperature responsive.

* Collapsed OEG-microgels diffuse and adsorb faster onto air-water interface.

* Collapsed OEG-microgels monolayers retain a larger bulge into the bulk.

* Swollen OEG-microgels adsorbed layer are more viscoelastic than collapsed ones.

* Swollen microgels are more susceptible to film rupture under large deformations.

## GRAPHICAL ABSTRACT

<figure><img src="image_5.png" /><figcaption>Surface Characterisation Of Thermoresponsive Microgels Based On Oligo(ethylene Glycol)</figcaption></figure>

## ARTICLE INFO

**Keywords:**

Oligo(ethylene glycol)

Microgels

Surface tension

Dilatational rheology

Monolayers

Elasticity

Viscosity

Non linear deformation

Swelling

Temperature

Thermoresponsive

## ABSTRACT

**Hypothesis:** Biocompatible microgels based on oligo(ethylene glycol) (OEG-based microgels) represent a new generation of microgels designed for biomedical applications. OEG-based microgels undergo swelling or collapse in bulk in response to external stimuli such as temperature or pH. Gaining a deeper understanding of the surface properties of OEG-based microgels is essential for the proper development of stimuli-responsive emulsions and foams.

**Methodology:** Combination of colloidal and surface experimental techniques; hydrodynamic diameter, electro-kinetic, analysis of Langmuir and Gibbs monolayers with in-situ microscopy, Atomic Force Microscopy and dilatational rheology in linear and non-linear regime provide new insights into the surface conformation of OEG-based microgels.

**Results:** Swelling and electrokinetic response of OEG-based microgels are initially examined as a function of pH and temperature to stablish the Volume Phase Transition Temperature (VPTT) at various pHs. Langmuir

<footnote><sup>a</sup> 1 Bio-Inspired Materials Group: functionalities & Self-Assembly, Universite de Pau et Pays d' l'Adour, IPREM, 64000 Pau</footnote>

<footnote><sup>b</sup> Department of Applied Physics, University of Granada, 18071 Granada, Spain</footnote>

<footnote><sup>c</sup> CNR- Institute of Condensed Matter Chemistry and Technologies for Energy, Unit of Genova, 16149 Genova, Italy</footnote>

<footnote><sup>d</sup> Carlos I Institute of Theoretical and Computational Physics, 18071 Granada, Spain</footnote>

<footnote><sup>e</sup> Modelling Nature Scientific Research Unit, University of Granada, 18071 Granada. Spain</footnote>

<footnote>* Corresponding author at: Department of Applied Physics, University of Granada, 18071 Granada, Spain.
E-mail address: julia@ugr.es (J. Maldonado-Valderrama).</footnote>
Adsorption
Interfacial conformation

monolayers of OEG-based microgels show a “fried-egg” structure typical of thermoresponsive microgels, below and above the VPTT, exhibiting similar surface coverage but protruding slightly further into the subphase in collapsed state. Gibbs monolayers of OEG-based microgels reveal that collapsed OEG-based microgels diffuse and adsorb faster onto the surface but rearrange similarly to swollen OEG-based microgels. Finally, the slightly greater protrusion of collapsed OEG-based microgels ultimately determines their dilatational behaviour in both linear and non-linear regimes.

**Conclusions:** Surface conformation of OEG-based microgels show very subtle stimuli responsiveness, which in principle, should not compromise their use as emulsion stabilizers. However, they may need to be considered for fine applications of OEG-based microgels as stabilizers in the rational development of stimuli-responsive foams and emulsions.

# 1. Introduction

Microgels are soft spherical particles of the order of 100 to 1000 nm, structured in a three-dimensional network of cross-linked polymer chains. Due to their highly porous structure, microgels can deform to a large extent, as they translocate pores more than ten times smaller than their sizes (in swollen state) importantly altering their internal structure of individual particles which define the compression of microgels at interfaces. Therefore although microgels have defined volume, in the swollen state do not present a clearly defined interface with the surrounding medium, with both the polymeric network and its solvent being part of the system [1,2].

Microgels respond to external stimuli depending on the polymers or chemical groups that compose them. Volumetric Phase Transition (VPT) is defined as volume change between a swollen and a collapsed state in response to these external stimuli. Among microgels, temperature and pH-responsive ones are one of the most studied types owing to its physiological relevance. In particular, poly(N-isopropylacrylamide) (pNIPAM) based microgels are the main reference, having a volume phase transition temperature (VPTT) close to human body temperature, swelling below and collapsing above this temperature in aqueous solution [3].

However, since acrylamide and NIPAM monomers are suspected of being carcinogen and neurotoxin (especially at a temperature above the VPTT) [4], the use of pNIPAM gels in biomedical applications is to some extent challenged [5,6]. Therefore, there is a need to find alternative stimuli-responsive microgels with higher biocompatibility. Suitable alternatives, presenting a VPTT close to human body temperature, are microgels based on poly(N-vinylcaprolactam) (pVCL) and poly(ethylene glycol) (PEG) [4,7,8]. Specially, a new family of biocompatible and multi-responsive microgels based on oligo(ethylene glycol) (OEG) methacrylate with different chain length to tune the transition temperature with ease, has attracted recent attention [9–17]. Copolymers of OEG methacrylate’s with different chain lengths are considered to overpass classic thermo-responsive polymers as they gather high biocompatibility thanks to not having an amide group in its chemical structure avoiding the release of small toxic amide compounds in the case of hydrolysis in acid conditions (see Fig. S1). These novel microgels are also able to encapsulate high contents of magnetic nanoparticles and other active molecules maintaining colloidal stability, thermoresponsiveness, swelling behaviour and film forming ability [12,15]. Hence OEG-based microgels appear as promising candidates for microgel-stabilised emulsions [12,14,17]. However, the interfacial properties of biocompatible microgels with tuneable thermoresponsiveness, how it relates to emulsion stability and the impact of stimuli responsive swelling of microgels in emulsion properties remains not well understood yet being a gap in the use of their full potential [18–26].

Microgels interpenetrate and flatten at liquid-fluid interfaces, providing unique interfacial features and similarities/differences compared to solid particles, polymeric or conventional surfactants. Microgels appear swollen/collapsed in dispersion depending on chain solvation and entanglement. However, when adsorbed onto fluid interfaces, microgels in general adopt a so called “fried-egg” structure with

outer layers flattening and stretching at the surface while the denser packed centre protrudes into the bulk [27]. Recent experimental studies focus on the analysis of interfacial elasticity of microgels as a key parameter in modulating their packing at the interfaces [23,28]. The behaviour of microgels at the air/water and oil/water interfaces has been widely studied as relates to emulsion stability, especially for the case of pNIPAM-based microgels (see [29] and references cited therein). However, there are discrepancies among studies. Monteaux et al. suggested that the stability of emulsions decreases above the VPTT due to rearrangements of the microgels at the oil/water interface [30]. Similarly, Maestro et al. state that the ability of pNIPAM particles to stabilise emulsions depends on the microstructure of the pNIPAM-based microgels at the interface showing that the vertical size of pNIPAM monolayers at the air/water decreases as the temperature increases above the VPTT. [31] More recently, it has been pointed out that pNIPAM-based microgels shared between two emulsion droplets in flocculated emulsions are responsible for their stimuli-responsiveness [32]. Interestingly, emulsions stabilised by OEG-based microgels, seem to remain stable upon heating above the VPTT [33], in contrast to emulsions stabilised by pNIPAM-based microgels. [30] According to Tatry et al., this is due to microgels remaining adsorbed on the emulsions despite the collapse induced by the temperature [34]. However, a deeper analysis of the interfacial conformation of non-pNIPAM-based microgels still remains to be done to fully understand this intriguing behaviour and allow fine tuning of stimuli-responsive emulsions. For the case of OEG-based microgels, Welsch and Lyon prepared OEG-sidechain microgels in absence of cross-linking agent and demonstrated how they enhanced deformability and particle spreading on solid surfaces in comparison with cross-linked microgels [35]. Other authors studied the effect of OEG chains incorporated into pNIPAM-based microgels at the dodecane-water interface at two different temperatures below and above the VPTT [27]. They observed that OEG chains influence the dynamics of the microgels at the interface, modulate the VPTT and affect how the microgels deform or spread at the interface. Consequently, the authors concluded that the incorporation of OEG allows adsorption to be controlled beyond what pure pNIPAM-based microgels do [27]. A recent work by Tatry et al. focus on interfacial rheology of pNIPAM films at air/water interface features the advantages of applying combination of Pendant Drop Tensiometry and Langmuir Film Balance [28]. Therein, the mechanical behaviour of this model interface is analysed in terms of microgel structure interfacial packing [28]. Following Tatry et al., we report here a systematic characterisation of OEG-based microgels at the air/water interface, using a range of surface characterisation techniques, including compression monolayer, Atomic Force Microscopy, adsorption kinetics and dilatational rheology in linear and non-linear deformation. Results presented here underpin similarities and differences in the current debate on the surface behaviour of microgels. Moreover, all surface properties are studied as a function of temperature induced swelling, with the aim of contributing to the development of stimuli responsive emulsions stabilised by OEG-based microgels.
## 2. Materials & methods

### 2.1. Materials

Di(ethylene glycol) methyl ether methacrylate (MeO₂MA 95 %, Aldrich), oligo(ethylene glycol) methyl ether methacrylate (OEGMA, monomethyl terminated with 8 EG repeat units, number average weight Mn = 475 g mol⁻¹, Aldrich), methacrylic acid (MAA, Aldrich), oligo (ethylene glycol) diacrylate (OEGDA, number average weight Mn = 250 g mol⁻¹, Aldrich), and potassium persulfate (KPS 99 %, ABCR) were used as received. Citric acid (Sigma-Aldrich) and sodium phosphate dibasic (Na₂HPO₄, Sigma-Aldrich) were used to prepare the buffers. Ultrapure water from a Millipore Milli-Q system (0.054 µS) was used for all buffer preparations. Glassware was thoroughly rinsed in a sequence of 10 % Micro90®, tap water, isopropanol, distilled water and ultrapure water.

### 2.2. Microgel synthesis

The synthesis of P(MeO₂MA-OEGMA-MAA)-based microgel was carried out following the procedure described previously by Boularas et al. [36], using in this case a 2 L jacketed glass reactor [15]. According with these authors, the carboxylic acid groups of the MAA comonomers were incorporated in the synthesis to provide a pH-responsiveness to the microgels. Below we briefly outline the essential aspects of the procedure: once 83.90·10⁻³ mol of MeO₂MA, 9.36·10⁻³ mol of OEGMA, 1.92·10⁻³ mol of OEGDA (cross-linker) and 930 g ultrapure water were charged into a 2 L jacketed glass reactor, the reaction mixture was stirred at 150 rpm and purged with nitrogen for 45 min to remove oxygen at room temperature. Then, 5·10⁻³ mol of MAA dissolved in 30 mL of ultrapure water were added to the reactor and the mixture was heated up to 70 °C. After adding the initiator (0.89·10⁻³ mol of KPS dissolved in 40 mL of degassed water), that introduces negative charges into the microgel network, the polymerization reaction was allowed to continue under nitrogen atmosphere while stirring for 6 h. The reaction mixture was subsequently cooled to 25 °C maintaining the stirring. At the end of synthesis, the microgel was purified by 3 centrifugation cycles (20.000 rpm, 20 min). From the cross-linker and monomers used we obtain a cross-linking density of 1.9 %mol. Additionally, we used pVCL microgels as an example of a standard microgel system with a similar low crosslinking density, 1.6 %mol. The synthesis by precipitation polymerization and characterisation is described in a previous work [37].

### 2.3. Microgel dispersions

The total concentration of solid content in the microgel prepared above was measured by drying the sample in the oven at 70 °C overnight obtaining a $sc = 1.6 \pm 0.3 \% w/w$, which is the concentration of the reference solution (16 g L⁻¹). Dilutions were made from this reference solution in biocompatible buffer prepared from citric acid and disodium phosphate adjusted to the required pH.

### 2.4. Experimental methods

#### 2.4.1. Dynamic light scattering (DLS)

Hydrodynamic diameter ($D_H$) and electrophoretic mobility ($\mu$) of microgels at different temperatures and pHs were measured by DLS on a Zetasizer Nano ZS (Malvern). In all measurements the pH was controlled using different buffered media at an ionic strength <2 mM. Buffered media at low ionic strength were used to avoid the medium's ionic strength effect on the swelling ability of microgel particles. In order to investigate the pH-sensitivity, microgel was diluted to 1.6·10⁻² g L⁻¹ particle concentration and measurements were carried out at 25 °C at pHs of 3.1, 4.4, 6.1 and 7.1. To study the thermal behaviour, measurements were carried out at those pHs from 18 to 58 °C, taking

measurements every 2 °C. Similar device and particle concentration was used to measure the electrophoretic mobility by Electrophoretic Light Scattering (ELS). With the aim of ensure the reproducibility of the microgels synthesis, all experiments were repeated at least three times for different samples obtained from different batch and values plotted are mean values with standard deviations.

#### 2.4.2. Langmuir-Pockels film balance

Spread monolayers of OEG-based and pVCL microgels were measured by means of a Langmuir-Pockels film balance (of a total area of 238.51 cm²). A Wilhelmy plate pressure measuring system (KSV) with a sensitivity of 0.1 mN m⁻¹ is used to measure the surface pressure ($\pi$) as the total surface area (A) is compressed uniaxially by two parallel barriers. The whole setup is thermostatically controlled and located in a transparent Plexiglas case to avoid air-streams and dust deposition. The absence of surface-active contaminants was verified before every experiment by compressing the pure MilliQ water phase, obtaining values of $\pi < 0.2$ mN m⁻¹ within the whole compression cycle. OEG-based and pVCL microgels suspensions were dispersed in 1:5 isopropanol/water and carefully spread on the subphase by means of a microsyringe (Hamilton®) while observing in-situ the interface via an epi-illuminated optical microscope with a 100× super long working distance objective (Nikon LV150). After an equilibration time of 15 min at the required temperature, the surface pressure-area ($\pi-A$) compression curve was recorded at a constant rate of 10 mm min⁻¹. Samples with different solid mass were spread to cover the whole compression curve (0.896–1.024 mg) and compression curves are expressed as $\pi-A$/mass. Experiments were repeated at least three times and plotted values are mean values with the standard deviations. In-situ images were obtained in another set of isotherms plotting the surface pressure-area per particle ($\pi-A_p$) curve obtained by first compressing the monolayer up to 10 mN m⁻¹ and lowering the pressure in steps down to 0.2 mN m⁻¹, at a constant rate of 10 mm/min⁻¹ between steps. These images were repeated at least three times and plotted values are mean values with the standard deviations. The images were post-processed and analysed by using a custom-made Python script based on the particle tracking code by Crocker and Grier to extract the location of the microgels (ranging from 15,000 to 20,000 in each image) [38].

To further analyse the conformation of microgels at the air/water interface, the monolayer was transferred from the interface to a solid silicon substrate. The silicon substrate was attached to the dipper arm, which was tilted 60° with respect to the interface. Then, the substrate was lowered below the interface position, and the trough was filled with MilliQ water. The monolayer was prepared as described above and compressed up to 5 mN m⁻¹. At that moment, the dipper was lifted at a constant velocity of 10 mm min⁻¹ and the monolayer was deposited on the silicon substrate. This was done both below and above the VPTT, at 25 °C and 45 °C, respectively.

### 2.5. Atomic Force Microscopy (AFM)

Deposited monolayers were imaged by means of atomic force microscopy (AFM, motorized Dimension 3000) in tapping mode (Tap300 Al-G cantilevers, 300 kHz, 40 N m⁻¹, BudgetSensors, Bulgaria). 20 × 20 µm² images were taken with a resolution of 512 pixels². The images were post-processed and analysed by using the software Gwyddion to extract the distribution of heights of each sample with more than 320 microgels per AFM image.

### 2.6. Pendant drop surface film balance

Adsorbed microgel films have been measured in a Pendant Drop Surface Film Balance which has been fully designed and assembled at the University of Granada (UGR) and described in detail elsewhere [39]. The whole system is computer-controlled by software DINATEN® and the images taken are analysed by software CONTACTO® also developed
<figure><img src="image_6.png" /><figcaption>Fig. 1. (a) Hydrodynamic diameter, D<sub>H</sub> and (b) Electrophoretic mobility, µ, of OEG-based microgels as a function of temperature (T) at different pHs; pH 7.1 (yellow squares), pH 6.1 (blue circles), pH 4.4 (green up-triangles), pH 3.1 (purple down-triangles). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

at the UGR (https://core.ugr.es/dinaten/, last access: 07/18/2022).

The pendant drop is produced at the tip of a Teflon capillary which is connected to a microinjector (Hamilton PSD3 Half-Height Syringe Pump). The pendant drop is formed between a diffused light source profile and a video camera CCD (Pixelink®), connected to an optical microscope (Edmund Optics®) which captures drop images. The software DINATEN® detects and analyses the drop profile. It then finds the theoretical solution of the Gauss-Laplace capillarity equation:

$$ \gamma \left( \frac{1}{R_1} + \frac{1}{R_2} \right) = \Delta P = \Delta P_0 - \Delta \rho g z \quad (1) $$

where R<sub>1</sub> and R<sub>2</sub> are the central radii of curvature in the drop, γ is the surface tension of the droplet solution, ΔP<sub>0</sub> is the reference pressure at z = 0 (drop apex), Δρ is the density difference between both fluids, g is the gravity acceleration, and z is the vertical coordinate.

The fitting is optimised by using Axisymmetric Drop Shape Analysis (ADSA) [39,40] and the program provides as outputs the surface tension (γ), the surface area (A) and the volume of the pendant drop. The surface area and volume are controlled and corrected in real time to carry out the experiment at a constant surface area, previously calibrated A<sub>0</sub> = 27 mm<sup>2</sup> during the time required to stabilise of the surface film given by the negligible variation of the surface tension.

The surface dilatational modulus (E) is measured by oscillating the volume of the droplet at a given frequency (ω) and recording the response of surface tension to the applied area deformation. In a general case, E is a complex quantity containing a real part (E'), termed storage modulus, and an imaginary part (E''), loss modulus, which are respectively related to the surface elasticity (ε), and the surface viscosity (η) of the adsorbed layer through the angular phase shift (δ) between the surface tension and area signals, as:

$$ E = E' + iE' = |E|(\cos\delta + i\sin\delta) = \varepsilon + i\eta\cdot\omega \quad (2) $$

The relative values of the interfacial dilatational elasticity and viscosity depend on both, the frequency, and the amplitude of oscillation which are addressed in linear and non-linear deformation, respectively. In the linear regime, the deformation ΔA = (A - A<sub>0</sub>)/A<sub>0</sub> applied to the initial surface area A<sub>0</sub> was fixed to 5 %, while the frequency of the periodic deformation was varied: 0.01, 0.0125, 0.02, 0.033, 0.05, 0.1 and 0.2 Hz. In the non-linear regime, the frequency was maintained at 0.02 Hz while the deformation was varied: ΔA = 5, 10, 20 and 30 %. Five periodic deformations were always applied to the surface layer, followed by 10 s of recording the surface tension at constant A<sub>0</sub> = 27 mm<sup>2</sup>. Computer software DINATEN© records the images of the dilatational experiments in real time which are then processed by computer software CONTACTO©, providing as outputs the dilatational parameters for each

deformation cycle applied.

The whole system is placed on an antivibrational optical table (Kinetec System Inc. Vibrplane) to prevent vibration during the experiments. The pendant drop is kept in a glass cuvette (Hellma®) located in a thermostatically controlled cell which is adjusted with an external temperature control which controls the temperature in real time in the bath. The temperature inside the cuvette is also controlled by a second thermometer located inside the cuvette and close to the pendant drop, hence assuring the stability of the temperature control unit. The surface tension of the clean surface (γ<sub>0</sub>) was measured before every experiment to confirm the absence of surface-active contaminants, yielding values of γ<sub>0</sub> (20 °C) = (72.8 ± 0.2) ·10<sup>-3</sup> N m<sup>-1</sup> and γ<sub>0</sub> (45 °C) = (68.8 ± 0.2) ·10<sup>-3</sup> N m<sup>-1</sup> for air/water and for buffer/water. Values are plotted as a function of the surface pressure (π = γ<sub>0</sub> - γ) which normalises against the surface tension of the empty surface. Adsorption and oscillation experiments were recorded at two different temperatures, below and above the VPTT (25 °C and 45 °C), for different concentrations of microgel in buffer at pH 6.1. Microgel solutions were kept at the required temperature at least during 15 min prior to injection in the pendant drop equipment and measurement. All the measurements were carried out in triplicate, obtaining different standard deviations; plotted values are mean values with the standard deviations as error bars.

## 2.7. Data analysis

All experiments were done at least in triplicate. Mean values and standard deviations are plotted in Figures and displayed in Tables. Student's t-test was used to determine significant differences between mean values in selected data used for explicit comparison. Differences between mean values were considered significant at a level of confidence of 95 % (p < 0.05).

# 3. Results & discussion

## 3.1. Swelling ability of OEG-based microgels in solution

The pH-dependent thermo-sensitiveness of OEG-based microgels has been previously reported elsewhere [15,36]. It is well known that the thermo-responsiveness in microgels is a result of the competition of hydrophobic/hydrophilic interactions [3,41]. In the case of OEG-based microgels, hydrophilic interactions (H-bonds) are formed between hydrophilic segments of the polymer chains and the water molecules, and the hydrophobic interactions are given between ethyl groups (hydrophobic segments) of the polymeric chains. In the present study, considering that microgel particles are also pH-responsive, it is expected that the pH of the medium will alter ionic interactions, hydrogen
bonding and hydrophobic interactions, and therefore, the thermo-
responsiveness [42]. Fig. 1a displays the variation of the hydrody-
namic diameters of OEG-based microgels measured at different pHs and
plotted as a function of temperature. The obtained trend confirms
qualitatively the ability of microgel particles to collapse by temperature
increase due to the disruption of the existing H-bonds and the
strengthening of the hydrophobic interactions in the microgel structure.
Hence, below the VPTT microgels are swollen whereas above this crit-
ical value particles are shrunken or collapsed.

One of the advantages of these OEG-based microgels lies in their dual
pH and temperature responsiveness. As can be seen in Fig. 1a, microgel
particles present significant pH-dependent behaviour. In fact, a previous
work showed that the VPTT of the microgel increases from 35 °C at pH
4.4 to values above 45 °C at pH 7.1 [15]. This was justified in terms of
the ionization of carboxylic groups of MAA units at basic conditions
triggering the water uptake and thus, microgel swelling. In order to
confirm this feature, the electrophoretic mobility of OEG-based micro-
gels was measured for different pHs and temperature (Fig. 1b). As can be
observed, microgel particles present negative electrophoretic mobility
values at basic pHs attributed to the negatively charged carboxylic
groups, which promotes swelling of the microgel by electrostatic
repulsion. Conversely, at acid pH the electrophoretic mobility decreases
approaching null values, which contributes to the collapse of the
microgel at lower temperatures. The increase in the electrophoretic
mobility at pHs above 6 reflects the protonation of carboxylic groups
close to the pKa, in contrast to a weakening of the electrostatic repulsion
that provokes the decrease in size at lower pHs [15,36].

Fig. 1b also shows how electrophoretic mobilities reach low values when the microgels are highly swollen (below the VPTT). This tendency was already reported for different charged thermo-responsive microgels [43,44] and can be explained in terms of an increase of the friction of the particles when they swell. On the contrary, for temperatures above VPTT, the microgel is collapsed reducing the particle friction during their electrokinetic movement and increasing its electrophoretic mobility [43,44]. Apart from the hydrodynamic effects, the increase of the absolute value of the electrophoretic mobility of the microgels with the temperature can be explained in terms of the electrostatic surface potential. Although the slipping plane is not well defined for soft colloids, the electrostatic potential at the surface provides an approximate idea of the electrokinetic behaviour (i. e. Zeta potential) of the particles. In a previous study, we analysed temperature-sensitive nanogels in the presence and in the absence of salt, calculated from explicit coarse-grained simulations and also form a modified Poisson-Boltzmann cell model [45]. Therein, the surface potential of the microgels was computed by integration of the electric field, which in turn was obtained from Gauss' law and the charge profile. As a consequence, the surface electrostatic potential obtained by both simulations and the theoretical model, increases (in absolute values) with temperature as we have observed with the electrophoretic mobility experiments in Fig. 1.

Finally, Fig. 1a shows how the typical sigmoidal behaviour in size vs.
temperature curves of thermo-responsive microgels is partly lost at pH
7.1. Hence, the surface characterisation is made at pH value (pH 6.1),
being the highest at which this sigmoidal form is clearly observed.

## 3.2. Langmuir monolayers of OEG-based microgels spread at the surface

The behaviour of OEG-based microgels spread at the air-water
interface was first addressed by means of compression curves recorded
at the Langmuir-Pockels film balance and plotted in Fig. S2. Tempera-
ture induced swelling does not have a significant effect on the
compression isotherms which practically overlap overcoming the tem-
perature induced swelling regarding the π-A curve and Gibbs elasticity
(Fig. S2). In swollen and collapsed states, as the surface area is com-
pressed, the particles interact, giving rise to an increase of the surface
pressure which proceeds until the end of the compression curve in one
single slope under moderate dilation-compression disturbances (surface

<figure><img src="image_7.png" /></figure>

Fig. 2. Compression curve of OEG-based microgel spread at the water/air interface obtained on a Langmuir-Pockels balance while examined in-situ via optical microscopy. The surface pressure (II) is plotted versus the Area per particle (A_p) calculated from the number of microgels in each image (ranging from 15,000 to 20,000 in each image). The blue points correspond to 25 °C and the red points to 45 °C. The optical microscope insets represent the monolayer at different Π and the colour of the frame indicates the temperature. The bottom left inset shows the distribution of microgel heights for 5 mN m⁻¹ at the two studied temperatures, extracted from the AFM images in Fig. S3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

pressure below 30 mN m⁻¹) (Fig. S2). The surface elasticity exhibits a
marked maximum as a function of the pressure, which is independent of
the temperature induced swelling. The maximal value (20–25 mN m⁻¹)
is lower than that reported for pNIPAM-based microgels while the
compression interaction regimes (I-III, Fig. S2) seem similar to literature
works [43,46]. Fig. S2 also identifies the different interaction regimes.
At large areas and low surface pressure: non interaction (regime I). As
the monolayer is compressed Gibbs elasticity increases as the microgel
external chains interact forming a 2D gel (regime II). The maximum
Gibbs elasticity corresponds to a flattened structure of microgels which
then deform perpendicularly to the interface compensating for the area
reduction (regime III) and decreasing Gibbs elasticity [46].

Fig. 2 shows the detail of the π-Ap curves, in-situ images and mi-
crographs of OEG-based microgels obtained at compression states below
10 mN m⁻¹ (domains I and II). At large interparticle distance, OEG-
based microgels behave as non-interaction particles as visualised in-
situ in Fig. S3A. The in-situ micrographs allowed measuring the Ap
(for more than 15,000 microgels in each image) and the microgel height
as inset in Fig. 2 from the AFM images in Fig. S3B (for more than 320
microgels in each image), via particle location. The Ap shows that the
diameter of the microgel at the surface at 5 mN m⁻¹, Dinterface = 1.14 ±
0.01 µm, increases significantly compared to the hydrodynamic diam-
eter of OEG-based microgels obtained in dispersion (Fig. 1), DH ≃ 750
nm at pH 7.1, resulting in a stretching ratio of Dinterface/DH ≃ 1.52 at 5
mN m⁻¹. When comparing with the pVCL-microgel of similar size and
crosslinking density reported in Fig. S1c, i.e. 1.6 %mol cross-linking,
Dinterface ≃ 1.8 µm, and DH ≃ 925 nm, the stretching ratio is 1.95.
Furthermore, we can interpolate the data in a previous work [47] to
match the crosslinking density of our OEG-based microgels, 1.6 %mol,
to check the corresponding stretching ratio of standard pNIPAM
microgels, i.e. Dinterface ≃ 1.3 µm, and DH ≃ 620 nm, obtaining a ratio of
2.1, quite in agreement with the pVCL-microgel, and significantly larger
than the OEG-based microgels. Thus, the OEG-based microgels seem
stretch less at the interface than the standard pVCL and pNIPAM-based
microgels. This provides evidence of the surface energy minimization of
the microgels at the air/water interface, where microgels stretch to
<figure><img src="image_8.png" /><figcaption>**Fig. 3.** Adsorption of OEG-based microgels at constant surface area. (a) Surface pressure (π) as a function of time (t), followed by dilatational rheology cycles for swollen microgels, 25 °C and (b) collapsed microgels, 45 °C at different concentrations. (c) Final surface pressure (π) versus concentration (C, g L⁻¹) at 25 °C (blue squares) and 45 °C (red triangles). (d) Schematic diagram of the adsorption process of microgels. (e) Dynamic curves highlighting adsorption regimes I, II and III for [OEG-based microgel] = 0.08 g L⁻¹. (f) Linear fits: regime I-k<sub>diff.</sub>(inset), regime II-k<sub>pen</sub> and regime III-k<sub>arr</sub> at 25 °C (blue squares) and 45 °C (red triangles). Asterisks (*) indicate statistically significant differences between the collapsed and swollen states for the same concentration (p < 0.05, Student's t-test). “ns” denotes no significant difference. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

reduce the interfacial tension [48]. Moreover, this could also originate the lower Gibbs elasticity obtained for OEG compared to pNIPAM-based microgels in Fig.S2. Particle location of the in-situ and AFM images evidenced also that heating does not affect the diameter of OEG-based microgels at the interface obtaining similar values of Aₚ at two sampled surface pressures in Fig. 2. However, a slight increase of the height, 132 ± 15 nm at 25 °C (from 374 microgels) and 156 ± 14 nm at 45 °C (from 320 microgels), both at 5 mN m⁻¹, was observed between swollen and collapsed OEG-based microgel. These results agree with recent findings for pNIPAM-based microgels [49], proposing that the region remaining immersed in the water subphase remains thermores- ponsive, deswelling and increasing the thickness of the monolayer above the VPTT [48,50]. Further investigation of the interaction of spread microgels are needed to evaluate the contribution interaction of surface chains to the interfacial free energy and assembly [51] will be performed in future works.

### 3.3. Adsorption of Gibbs OEG-based microgels

Fig. 3 shows the adsorption curves of different OEG-based microgel solutions measured in swollen (25 °C) and collapsed (45 °C) state at pH 6.1 including dynamics (Fig. 3a and b), adsorption isotherms (Fig. 3c). Since changes in the pH and ionic strength strongly influence on the adsorption kinetics, a low ionic strength buffer was used to avoid the introduction of an electrostatic barrier to adsorption [34]. The concen- trations of microgel varies between 0.0016 and 1.6000 g L⁻¹. Fig. 3a shows the evolution of the surface pressure as a function of time (t) by adsorption of swollen microgels. For the more diluted samples ([OEG- based microgel] < 0.016 g L⁻¹), the surface pressure barely changes remaining close to that of pure air/water interface. This suggests that there are not enough microgels in the bulk solution to adsorb at the surface and/or the swollen microgel has limited surface activity possibly

owing to its high hydrophilicity. Then, adsorption transition of microgels proceeds in a very narrow range of concentration (0.016 g L⁻¹ < [OEG-based microgel] < 0.08 g L⁻¹) displaying curves in which the surface pressure does not seem to reach a steady value within the time scale of the experiment (Fig. 3a). Finally, for higher concentrations ([OEG-based microgel] > 0.08 g L⁻¹) the surface pressure increases steeply and reaches a stable value, 20–25 mN m⁻¹, corresponding to a saturated surface layer. Fig. 3b shows the analogue curves obtained at 45 °C, for collapsed microgels where the general trend is completely similar to that found for swollen microgels.

This rapid and sharp adsorption profile has been previously reported for pNIPAM-based microgels. [34] In fact, these kinetics appear quite systematically for different microgel particles in literature. [33] The particularity of adsorption of microgels, compared to proteins, polymers or surfactants, relies in two facts. On one hand, the kinetics changes from non-surface active to surface active in a very narrow concentration range. This is one of the main peculiarities of microgel adsorption compared to other surface active agents. The final interfacial pressure attained after adsorption of microgels appears practically independent of the concentration (while only the kinetics is affected). This behaviour is unusual for particles since the equilibrium value is usually a decreasing value of the particle's concentration. It was reported for pNIPAM-based microgels [34,46] and herein, extended to OEG- microgels. Adsorption of microgels is linked to the presence of local- ized moieties so that surface pressure increases only if enough of these moieties adsorb at the surface revealing also a cooperative behaviour and the existence of attractive interactions at the interface. Hence, all solutions reach a similar surface tension (20–25 mN m⁻¹) above a certain concentration. This value aligns with that obtained at the maximum compression in spread monolayers reported in Fig. S2, and also correlates with the value obtained for OEG- and pNIPAM-based at microgels reported elsewhere. [33,52] The rate of change is faster as the
concentration increases but all samples reach eventually the same final value [34]. This is consistent with the formation of a single surface layer in which the microgels spread as much as possible, maximising the surface coverage upon adsorption, and in agreement with the fried-egg structure reported elsewhere [47]. Even diluted samples could potentially provide a saturated surface layer as long as there are enough microgel particles to cover the surface. Then again, more concentrated solution does not seem to promote adsorption of more particles or increase the surface coverage. The main effect of increasing concentration is that saturation of the surface proceeds faster. Fig. 3c compares the values of surface pressure attained after stabilisation of curves in Fig. 3a and b, plotted as a function of concentration, for collapsed and swollen OEG-based microgels. As expected, adsorption isotherms of OEG-based microgels show a very steep variation of the surface pressure with the concentration, varying steeply from negligible to around 25 mN m⁻¹. Again, the drastic change of surface pressure is related to a maximum stretching of the adsorbed microgel which spreads out at the surface to minimise the high free energy of pure air/water interface saturating the surface as soon as there is enough adsorbed material. Moreover, the values of surface pressure obtained for swollen and collapsed OEG-based microgels are just slightly higher for collapsed OEG-based microgels which could relate to the slightly higher protrusion reported by AFM imaging (Fig. 2). These differences were found to be statistically significant for all concentrations, except for the lowest one. However, it is also very subtle and indicative of a similar surface coverage independent of the swelling state supporting the formation of a monolayer. Fig. 3c does show a slight but significant displacement of the curve recorded for collapsed OEG-based microgels to lower bulk concentration indicative of a slightly higher surface activity.

Fig. 3d shows a schematic diagram illustrating the process of adsorption in three regimes which can be identified in the kinetic curves displayed in Fig. 3e which shows explicit comparison of the adsorption kinetics of swollen and collapsed OEG-based microgels at a concentration of 0.0800 g L⁻¹. The obtained curves highlight a faster kinetics for collapsed OEG-based microgels and the three adsorption regimes which can be studied separately to gain more information on the underlying phenomena. Namely, a first adsorption regime characterised by a negligible surface pressure can be analysed on the basis of diffusion-controlled adsorption (Fig. 3d and f, regime I) [53]. A second regime in which polymers from the microgels adsorb onto the surface, characterised by a steep increase of the surface pressure (Fig. 3d and f, regime II). A final regime in which microgels rearrange and surface pressure stabilises (Fig. 3d and f, regime III). This kinetic and phenomenological model has been applied recently applied to study the behaviour of other microgels and sheds light on the characteristic of the adsorption process [54].

According to the Ward and Tordai equation, in a diffusion-controlled adsorption regime the surface pressure is given by eq. (3):

$$ \pi = 2c_b k_B T \sqrt{\frac{D_s t}{3.14}} = k_{dif} \sqrt{t} \qquad (3) $$

where $c_b$ is the bulk concentration, $k_b$ is the Boltzmann constant, $T$ is the temperature, $t$ is the time and $D_s$ is a diffusion coefficient. Hence, plotting the surface pressure as a function $t^{1/2}$ allows determining a linear region in which the adsorption is diffusion-controlled and can be quantified by value of the slope: $k_{dif}$ as can be seen in Fig. 3f-inset for OEG-based microgels below and above the VPTT.

Then, the evolution of the surface pressure in adsorption regimes II and III can be assessed by application of a first order phenomenological equation, derived originally by Graham and Philips and widely applied in surface adsorption kinetics [55].

$$ \ln \frac{\pi_f - \pi_t}{\pi_f - \pi_0} = -kt \qquad (4) $$

were $\pi_f$ is the final surface pressure, $\pi_0$ is the initial surface pressure, $\pi_t$ is

<caption>**Table 1**</caption>

Values of $k_{diff}$ ($k_{dif}$), $k_{penetration}$ ($k_{pen}$) and $k_{rearrangement}$ ($k_{rea}$) [54] of swollen and collapsed OEG-based microgels. Values are obtained linear regression analysis on Fig. 3e and f. Asterisks (*) indicate statistically significant differences between swollen and collapsed states for each k ($p < 0.05$, Student's t-test). "ns" denotes no significant difference.

<table><thead><tr><th>OEG-based microgel</th><th>$k_{dif}$ (·10<sup>-3</sup> N m<sup>-1</sup> s<sup>-1/</sup>)<sup>2)</sup></th><th>- $k_{pen}$ (·10<sup>-3</sup> s<sup>-1</sup>)<sup>3)</sup></th><th>- $k_{rea}$ (·10<sup>-2</sup> s<sup>-1</sup>)<sup>4)</sup></th></tr></thead><tbody><tr><td>Swollen</td><td>0.039 ± 0.004*</td><td>0.734 ± 0.016*</td><td>1.34 ± 0.11<sup>ns</sup></td></tr><tr><td> collapsed</td><td>0.102 ± 0.006*</td><td>1.224 ± 0.002*</td><td>1.28 ± 0.08<sup>ns</sup></td></tr></tbody></table>

the time dependant surface and k is a first-order rate constant. Fig. 3f displays the plot of $ln \frac{\pi_f - \pi_t}{\pi_f - \pi_0}$ versus time with obtained values from OEG-based microgels which allows identification of adsorption regimes II and III and calculation of their respective slopes: $k_{pen}$ and $k_{rearr}$ using eq. (4), as shown in Fig. 3f. The obtained rate constants for diffusion, penetration and rearrangement regimes below and above the VPTT are displayed in Table 1.

In regime I, $k_{dif}$ appears slightly higher above the VPTT, indicative of a faster diffusion of collapsed OEG-based microgels. This correlates with a smaller size of collapsed OEG-based microgels in solution (Fig. 1) and higher hydrophobicity which should favours diffusion onto the surface. The thermo-sensitivity of the microgel particles is governed by the formation/disruption of hydrogen bonds so the reduced size of microgel reported in Fig. 1 demonstrates the higher hydrophobicity [51]. In regime II, $k_{pen}$ appears again slightly higher for collapsed OEG-based microgels. This again, implies that adsorption would be favoured for more hydrophobic collapsed structures compared to swollen. Numerical values obtained for $k_{pen}$ agree with those reported by Sun et al. for Chitosan microgels who also find a higher value for smaller/collapsed microgels [54]. In regime III, similar $k_{rea}$ values were obtained for swollen and collapsed OEG-based microgels. This parameter quantifies the deformability of species after adsorption onto the surface. Values reported in Table 1 are comparable to that of proteins (β-lactoglobulin) and higher to that of nanoparticles which are clearly less deformable structures than microgels [56]. The similar $k_{rea}$ obtained for swollen and collapsed OEG-based microgels indicates that structural surface rearrangements proceed similarly, which correlates with the similar surface coverage obtained for spread layers (Fig. 2). Numerical values of $k_{rea}$ are one order of magnitude higher than $k_{pen}$ as also reported by Sun et al. for Chitosan-microgels. [54].

### 3.4. Dilatational rheology of adsorbed layers of OEG-based microgels

Fig. 4 shows the surface dilatational elasticity and viscosity of adsorbed OEG-based microgel layers obtained as affected by temperature induced swelling. These measurements were performed after stabilisation of the surface pressure at constant surface area (Fig. 3a and b). The experimental results are shown as a function of concentration, and the dilatational parameters were measured by applying different oscillation frequencies (0.01, 0.02, 0.10 and 0.20 Hz). Fig. S4 shows these same graphs plotted against the surface pressure as follows from Fig. 3c, which will be discussed alongside with Fig. 4.

Firstly, the dilatational elasticity (Fig. 4a and b) of adsorbed OEG-based microgels shows one single maximum, this is indicative of one single conformational regime at the surface in agreement with the discussion above on Langmuir (Fig. 2) and Gibbs (Fig. 3c) monolayers. Secondly, the height and shape of this maximum is practically independent of the applied frequency according to the standard deviation obtained. Figs. S4a and S4b display the evolution dilatational elasticity with surface pressure, which allows comparison with the Gibbs elasticity (Fig. S2). The similarity between both curves, implies again that the surface coverage of adsorbed layers matches that of a single layer, hence confirming that multilayer formation is not happening upon adsorption of OEG-based microgels [52]. Accordingly, OEG-based microgels adsorb
<figure><img src="image_9.png" /><figcaption>Fig. 4. Dilatational rheology of adsorbed layers of OEG-based microgels as a function of bulk concentration, area deformation: 5 %, oscillation frequencies (Hz): 0.01, (upside-down triangles), 0.02 s. (triangles), 0.1 s (circles), 0.2 s (squares) in all figure. (a) Dilatational elasticity (εd, solid symbols) at 25 °C, (b) at 45 °C and (c) at 0.2 Hz. (d) dilatational viscosity (ηd, open symbols) 25 °C and (d), at 45 °C (e) and (f) at 0.01 Hz. Lines are guide for the eye. Asterisks (*) indicate statistically significant differences between the collapsed and swollen states for the same concentration (p < 0.05, Student's t-test). "ns" denotes no significant difference.</figcaption></figure>

onto the surface forming a monolayer while the rest of microgels remain solubilized in bulk aligning with the similar conformation obtained for spread monolayers and adsorbed films. This is an important difference between adsorption of microgels and proteins, which form multilayers [46,52]. The dilatational elasticity of OEG-based microgels is also significantly lower compared to that of globular proteins [52,57] as well as the shape which generally increases to reach a stable value at high concentrations where multilayers form [52]. In contrast, curves in Fig. 4 reduce the dilatational elasticity at high concentrations, resembling the dilatational behaviour of random coil proteins due to adsorption/desorption of segments also as reported by Pinaud et al. for p-NIPAM-based microgels [46]. Accordingly, OEG-based microgels form a weak surface film composed of a monolayer of entangled segments from the shell which are able to adsorb/desorb at high bulk concentrations to compensate the deformations applied.

Dilatational viscosity (Fig. 4d and e) of adsorbed OEG-based microgels displays also one single maximum located at the same concentration that recorded in the dilatational elasticity (Fig. 4a and b). The height of this maximum appears now only noticeable at lowest frequency (0.01 Hz), where the surface layer has more time to allow for relaxations as the surface adapts to the applied deformation. However, in this frequency range the elastic behaviour dominates over viscous in the monolayer (E' > E") evidencing a solid-like behaviour that will be analysed below in further detail.

Fig. 4c and f allow direct comparison of the dilatational elasticity and viscosities of adsorbed layers of swollen and collapsed OEG-based microgels surface layers. In order to maximise elastic and viscous components, these were plotted at the highest (0.10 Hz) and smallest (0.01 Hz) oscillation frequencies, respectively. Both the dilatational elasticity (Fig. 4c) and viscosity (Fig. 4f) are significantly higher at high concentrations for swollen OEG-based microgels, and the corresponding maxima are considerably wider than those of the collapsed state. This behaviour is related to increased interconnections formed between

swollen microgels, analogue to the higher dilatational response reported for pNIPAM microgels with lower crosslinking. [28] Fig. S4c allows comparison of the maxima at different swelling states appearing slightly displaced to lower surface pressure for swollen microgels. This possibly corroborates the formation of a thinner layer as well as the enhanced anchoring points for swollen microgels compared to collapsed as measured in Fig. 2. Similarly, Figs. 4f and S4f show the maximum dilatational viscosity, appearing also wider for swollen OEG-based microgels can be related to desorption of adsorbed segments upon deformation of swollen surface networks. Viscous effects appear minimal in adsorbed OEG-based microgels layers and only detectable at very slow area deformations as a function of concentration (Fig. 4f) and surface pressure (Fig. S4f).

In the investigated frequency range, the loss tangent exhibits values always lower than unity and practically independent of the frequency. This evidences a solid-like behaviour of the interfacial layers [58]. Similarly, the complex modulus appears linear in a double log-plot with the frequency matching the rheological behaviour of a 2D critical gel [58]. Under these conditions, the relaxation mechanisms at the interface can be described following a power law equation where the complex modulus is given by the following equation:

$$|E| = k\omega^n \qquad (5)$$

where, n is the slope of the logarithmic plot of |E| vs frequency and k refers to the value of |E*| at the highest frequency.

According to Dicharry et al., this model relevant to the rheology of a 3D-gel near its gelation point (the so-called critical gel) is still valid to interpret 2D-rheology measurements following the above mentioned conditions. The parameter n provides the structuring degree of interface, being lower as structuration increases. The parameter k (value of E* extrapolated at the frequency of 1 Hz) measures the strength of the interfacial gel increasing as the film strengthens. This model has been
<caption>**Table 2**</caption>

Values of *k* and *n* (eq. 5) [54] obtained for swollen and collapsed OEG-based microgels. The value of *n* represents the degree of structuration of interface, being lower as structuration increases. The value of *k* accounts for the strength of the surface, increasing as the film strengthens. Asterisks (*) indicate statistically significant differences between swollen and collapsed states for the same concentration and parameter (*k* or *n*) (*p* < 0.05, Student's t-test). “ns” denotes no significant difference.

<table><thead><tr><th rowspan="2">[OEG-based microgel] (g L<sup>-1</sup>)</th><th colspan="2">k (mNm<sup>-1</sup>s<sup>n</sup>)</th><th colspan="2">n</th></tr><tr><th>Swollen (25 °C)</th><th>Collapsed (45 °C)</th><th>Swollen (25 °C)</th><th>Collapsed (45 °C)</th></tr></thead><tbody><tr><td>1.6</td><td>10.3 ± 0.4*</td><td>4.51 ± 0.01*</td><td>0.084 ± 0.014<sup>ns</sup></td><td>0.093 ± 0.001<sup>ns</sup></td></tr><tr><td>0.16</td><td>12.6 ± 1.3*</td><td>9.08 ± 0.21*</td><td>0.060 ± 0.030<sup>ns</sup></td><td>0.065 ± 0.008<sup>ns</sup></td></tr><tr><td>0.08</td><td>20.3 ± 1.0*</td><td>12.10 ± 1.40*</td><td>0.038 ± 0.016<sup>ns</sup></td><td>0.060 ± 0.040<sup>ns</sup></td></tr><tr><td>0.048</td><td>21.5 ± 0.3*</td><td>14.70 ± 0.40*</td><td>0.045 ± 0.004<sup>ns</sup></td><td>0.054 ± 0.008<sup>ns</sup></td></tr><tr><td>0.032</td><td>25.1 ± 0.7<sup>ns</sup></td><td>23.70 ± 2.30<sup>ns</sup></td><td>0.026 ± 0.009<sup>ns</sup></td><td>0.060 ± 0.030<sup>ns</sup></td></tr><tr><td>0.016</td><td>22.0 ± 4.0<sup>ns</sup></td><td>20.00 ± 9.00<sup>ns</sup></td><td>0.150 ± 0.180<sup>ns</sup></td><td>0.110 ± 0.160<sup>ns</sup></td></tr><tr><td>0.0016</td><td>-</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

applied in literature to different proteins, surfactants and more recently to microgels [54,59]. This rheological approach has shown that emulsion stability increases with the strength of the interfacial gel.

Table 2 displays the values obtained for *k* and *n* by fitting $|E|$ vs frequency to eq. (5). The values of the dilatational elasticity and the fittings are shown in Figs. S5 a and S5b. Figs. S5c and S5d represent the trend obtained for each parameter as a function of the bulk concentration. The obtained trends illustrate a that the value of *k* decreases from a maximum, corresponding to maximum strength and matching the maximum elastic moduli located between 0.010 and 0.100 g L⁻¹ (Fig. 4). In contrast, *n* shows no significant variation throughout the concentration range, indicative of a constant structuration of the interface (Fig. S5d).

Comparison of values of *k* and *n* obtained for adsorbed layers of

swollen and collapsed OEG-based microgels show slight differences (Figs. S5c and S5d). The values of *k* are in general, significantly higher for swollen OEG-based microgels, demonstrating the higher strength of this surface film. Conversely, no statistically significant difference in the *n* values was observed between swollen and collapsed OEG-based microgels, reflecting a similar structuration of the surface film formed. This observation aligns with the already proposed similar surface rearrangements of swollen and collapsed OEG-based microgels, in which the surface structure and conformation is similar, but entanglement and interconnections are somehow favoured in swollen systems. A reason for this could be the lower effective charge (Fig. 1) and softer structure of swollen OEG-based microgels (Fig. 4). [29] Results from Table 2 are in quantitative agreement with those reported by Sun et al. for chitosan based microgels [54] and appear significantly lower than those obtained for globular proteins [57], in which the presence of disulfide bonds importantly strengthens surface films and enhances structuration. According to values reported in Table 2, both swollen and collapsed OEG-based microgels feature a surface network formed by weak interaction forces [54].

Fig. 5 explores the effect of non-linear deformation on the dilatational behaviour of OEG-based microgels in swollen and collapsed state. The oscillations were made at an intermediate frequency value (0.02 Hz), and the deformation varied: 5, 10, 20 30 %, thus accounting for the non-linear deformation above 5 %. At this relatively low frequency, the dilatational viscosity of the adsorbed layer was negligible (Fig. S6). Fig. 5d and e show how the dilatational elasticity measured remains practically unchanged under non-linear deformations for all concentrations except for one, both in swollen and collapsed state. This is an important new results which highlights the remarkable ability of OEG-based microgels to withstand large deformations, despite exhibiting a relatively low dilatational modulus compared with globular proteins [52]. OEG-based microgels expand and extend their polymeric structure upon adsorption onto the surface developing multiple lateral interactions between adsorbed chains to reach an adsorbed state with minimal energy (Fig. S2, regimes II and III). These monolayers can compensate for the area deformation owing to the region remaining immersed in water which may act as a reservoir of polymeric chains that

<figure><img src="image_10.png" /><figcaption>**Fig. 5.** Schematical representation of a monolayer of OEG-based microgels adsorbed at the air-water interface, with microgels below and above the VPTT: (a) without deformation, (b) under linear dilatational expansion, and (c) under non-linear dilatational expansion. (d) Dilatational elasticity (ε<sub>d</sub>) of adsorbed OEG-based microgels (Fig. 3) at different concentrations. Values plotted as a function of surface area deformation: 5, 10, 20, 30 % at oscillation frequency 0.02 Hz, (d) 25 °C and (e) 45 °C. Lines are guide for the eye.</figcaption></figure>
<caption>**Table 3**</caption>

Summary table of similarities and differences between OEG-based and pNIPAM-based microgels.

<table><thead><tr><th></th><th>OEG-based microgels</th><th>pNIPAM-based microgels</th></tr></thead><tbody><tr><td rowspan="3">Colloidal properties</td><td>High biocompatibility<sup>10-12</sup></td><td>Carcinogen and neurotoxin<sup>4</sup></td></tr><tr><td>Chain lengths' dependent transition temperature<sup>16-17*</sup></td><td>Non-dependent transition temperature</td></tr><tr><td>Lower stretchability at the interface (1.63 at 5 mN m<sup>-1</sup>)</td><td>Higher stretchability (1.97 at 5 mN m<sup>-1</sup>)<sup>47</sup></td></tr><tr><td rowspan="2">Surface properties</td><td>Langmuir adsorbed films</td><td>Lower Gibbs elasticity (20-25 mN m<sup>-1</sup>)</td><td>Higher Gibbs elasticity (50-55 mN m<sup>-1</sup>)<sup>46</sup></td></tr><tr><td>Gibbs adsorbed films</td><td>Temperature independent surface coverage with slightly greater thickness of the monolayer above VPTT</td><td>Similar behavior<sup>48,50</sup></td></tr><tr><td></td><td>Dilatational rheology</td><td>Adsorption profile with final surface tension (20-25 mN m<sup>-1</sup>) independent of concentration</td><td>Similar profile<sup>32,52</sup></td></tr><tr><td></td><td></td><td>Slightly higher dilatational response for swollen microgels</td><td>Similar behavior<sup>29</sup></td></tr></tbody></table>

adsorb/desorb enduring the large deformations applied in Fig. 5d and e while maintaining the surface coverage unchanged.

Fig. 5d and e show only one curve each in which the dilatational elasticity is altered in the non-linear regime. Namely, the most diluted samples, 0.0016 g L-1 and 0.0160 g L-1 for swollen and collapsed states, respectively. These concentrations provide a very low surface coverage, evidenced in Figs. S4 and correspond to interaction regime I (Fig. S2). Under these extreme circumstances, a very large deformation could either promote empty spacing (upon expansion) or interaction of very flattened microgels (upon compression). These large alterations in the surface interaction of microgels are only noticeable in non-linear deformations and have an impact in the dilatational response. In fact, the swelling state of the OEG-based microgel is determinant in the effect of large deformations at low surface coverage. The dilatational elasticity of adsorbed swollen OEG-based microgels decreases (0.0016 g L<sup>-1</sup>, Fig. 5d) while that of collapsed microgel increases (0.016 g L<sup>-1</sup>, Fig. 5e). This different response could be due to the slight structural differences evidenced in Fig. 2 between collapsed and swollen systems. The existence of a thicker immersed region, or chain reservoirs, could provide additional support against large deformations, somehow strengthening the structure of collapsed OEG-based microgels at the surface entering interaction regime II (Fig. S2). This is schematized in Fig. 5a, b and c. It is hypothesized that the slightly thicker surface morphology of collapsed OEG-based microgels can spread further on the surface increasing promoting interactions and increasing the elastic response when subjected to non-linear deformations. Conversely, swollen microgels, which spread further at the surface in the very diluted samples, cannot withstand large deformations which lead to the formation of empty spaces promoting interaction regime I (Fig. S2).

## 4. Conclusions

This work shows a detailed colloidal and surface characterisation of OEG-based microgels, a new class of multiresponsive systems that are emerging as promising alternatives to widely studied pNIPAM-based microgels, due to their potentially greater biocompatibility. Emulsions stabilised by OEG-based microgels remain stable even above the VPTT [33]. Conversely, some emulsions stabilised by pNIPAM-based microgels show different response to temperature resulting sometimes in destabilisation, [30] or not [32]. In order to unravel the underlying phenomena, this work provides a systematic analysis of the interfacial structure and dilatational rheology of OEG-based microgels.

In aqueous solution, OEG-based microgels collapse as temperature increases. Their volume phase transition temperature (VPTT) ranges from 35 °C at acidic pH (4.4) to 45 °C at near-neutral pH (7.1). Additionally, electrophoretic mobility measurements indicate that the effective charge of OEG-based microgels increases with pH and temperature.

In-situ analysis of the behaviour of Langmuir monolayers of OEG-based microgels at the air/water interface showed that temperature

does not affect significantly either the compression curves or the surface conformation of these particles, which exhibit a similar 'fried-egg' surface conformation both below and above the VPTT [48]. However, AFM measurements on OEG-based microgels indicate that the swelling state does affect the height of the monolayers at the surface, as collapsed deposited microgels exhibit a slightly thicker conformation than swollen ones, similar to the swelling-dependent height variation reported for pNIPAM microgels at interfaces [46,48]. From stretching ratios at the interface OEG-based microgels seem to stretch less compared to pNIPAM and pVCL microgels of similar size and cross-linking density.

At sufficiently high bulk concentrations, OEG-based microgels adsorb at the air/water interface, forming single monolayer where they tend to spread as much as possible. As a result, even diluted samples can saturate the interface, so that increasing the concentration primarily affects adsorption kinetics rather than surface coverage, in agreement with the behaviour reported for their pNIPAM counterparts [33,34]. Furthermore, the swelling state of these microgels affects their adsorption kinetics. Calculation of kinetic parameters demonstrate that collapsed microgels, being smaller and more hydrophobic, diffuse faster from the bulk to the interface and penetrate into the surface easily, in agreement with observations made for other microgels [53,54]. Surface rearrangement remains similar in both swollen and collapsed states, and this is consistent with the preceding characterisation of spread microgels at the interface and comparable to values reported for other soft particles providing quantification [54,56].

Dilatational interfacial rheology of OEG-based microgels reveal a single maximum, which closely matches Gibbs elasticity for monolayers, confirming the formation of a monolayer with remaining microgels solubilized in the bulk. Alternatively, dilatational viscosity is only significant at the lowest oscillation frequency. Swollen microgels exhibit a higher and broader maximum in dilatational elasticity than collapsed ones, likely due to increased interconnections between their polymeric segments, which enhance their dilatational response, in agreement with previous studies on low-crosslinked pNIPAM microgels [28]. Analysis of dilatational elasticity as a function of frequency suggests that swollen microgels form stronger surface monolayers compared to their collapsed counterparts, as also reported for chitosan microgels [54].

In the non-linear regime, dilatational elasticity remains practically unchanged with increasing deformation for all concentrations except for the most diluted samples. In this interaction regime I, the film dilatational response increases with the applied deformation for collapsed microgels but decreases for swollen. The explanation of this phenomenon is found in the slight conformational differences envisaged at low surface coverage in Fig. 2.

The results presented here contribute to the ongoing discussion about interfacial behaviour of microgels, highlighting both similarities and differences with previous studies, which are summarised in Table 3. Moreover, by analysing surface properties focusing on the effect of temperature, this work aims to support the development of stimulus-responsive emulsions using OEG-based microgels. Future work could
focus on extending this study to explore the effect of microgel charge on their interfacial behaviour and rheological properties [51]. Since OEG-based microgels exhibit dual responsiveness, understanding the effect of varying the pH can provide additional information on surface interactions. Moreover, there is an urgent need to develop models that reproduce the behaviour of microgels at interfaces which will provide physical basis for the 2D conformation of microgels.

## Author contribution

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

## CRediT authorship contribution statement

G. Aguirre: Writing – original draft, Resources, Methodology, Formal analysis, Data curation. J. Quirosa-Mancilla: Writing – original draft, Methodology, Formal analysis, Data curation. C. Cara-Galdeano: Visualization, Methodology, Formal analysis, Data curation. I. Navarro-Arrebola: Data curation. A. Rubio-Andres: Data curation. M.A. Fernandez-Rodriguez: Writing – original draft, Methodology. L. Billon: Resources, Methodology. A. Martín-Molina: Writing – review & editing, Project administration. J. Maldonado-Valderrama: Writing – review & editing, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

This work was funded by Grants C-EXP-187-UGR23 and C-ING-208-UGR23 awarded by Consejería de Universidad, Investigación e Innovación and by ERDF Andalusia Program 2021-2027, PID2023-1493870B-100 and PID2020-116615RA-100 awarded by MICIU/AEI/10.13039/501100011033 and ERDF, a way of making Europe. M. A.F.R. acknowledges funding from an EMERGIA grant with reference EMC21_00008. J.M.V. acknowledges COST-Action INFOTECH-DATA CA24145. Funding for open access charge: Universidad de Granada / CBUA

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2025.139522.

## Data availability

Data will be made available on request.

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