# From inverse Pickering emulsion to polyhydroxybutyrate gel loaded with chelators for cleaning of copper surfaces: the stabilization effect of hydrophobized halloysite clay nanotubes

Giulia d'Agostino, Patrycja Petrasz, Wu Qing, Han Zhou, Maartje Stols-Witlox, Loïc Bertrand, Edith Joseph, Giuseppe Cavallaro, Giuseppe Lazzara

► To cite this version:

Giulia d'Agostino, Patrycja Petrasz, Wu Qing, Han Zhou, Maartje Stols-Witlox, et al. From inverse Pickering emulsion to polyhydroxybutyrate gel loaded with chelators for cleaning of copper surfaces: the stabilization effect of hydrophobized halloysite clay nanotubes. Journal of Colloid and Interface Science, 2025, 703, pp.139189. 10.1016/j.jcis.2025.139189. hal-05310213

HAL Id: hal-05310213

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# From inverse Pickering emulsion to polyhydroxybutyrate gel loaded with chelators for cleaning of copper surfaces: the stabilization effect of hydrophobized halloysite clay nanotubes

<figure><img src="image_3.png" /></figure>

Giulia D'Agostino<sup>a,b</sup>, Patrycja Petrasz<sup>c</sup>, Wu Qing<sup>c</sup>, Han Zhou<sup>d</sup>, Maartje Stols-Witlox<sup>d</sup>, Loïc Bertrand<sup>e</sup>, Edith Joseph<sup>c,*</sup>, Giuseppe Cavallaro<sup>b,f,*</sup>, Giuseppe Lazzara<sup>b,f</sup>

## GRAPHICAL ABSTRACT

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

**Keywords:**

Pickering emulsions

Halloysite nanotubes

Polyhydroxybutyrate

Gel

Surface cleaning

Copper substrate

## ABSTRACT

Among innovative gel formulations for metal conservation, a sustainable organogel based on bioderived poly-hydroxybutyrate (PHB), ethyl lactate (EL) and chelating agents has been lately explored, bringing promising results. However, since ligands are usually found in the form of aqueous solutions, both the hydrophobicity of PHB and the not ideal mixture of EL in water should be considered.

In this regard, we have developed a PHB-EL organo-gel, containing the chelating water solution in the form of a water-in-oil (W/O) Pickering emulsion stabilized by hydrophobized halloysite clay nanotubes (HNTs). The HNTs hydrophobization has been achieved by selective functionalization of outer surface with a cationic surfactant (hexadecyltrimethylammonium bromide). The stabilization mechanism of hydrophobized nanotubes to

<footnote><sup>a</sup> Dipartimento di Scienze dell'Antichità, Università degli Studi di Roma “La Sapienza”, P.le Aldo Moro 5, 00185, Roma, Italy</footnote>

<footnote><sup>b</sup> Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Ed. 17 “Stanislao Cannizzaro”, Viale delle Scienze, 90128 Palermo, Italy</footnote>

<footnote><sup>c</sup> Haute Ecole Arc Conservation-restauration, HES-SO, Espace de l'Europe 11, 2000, Neuchatel, Switzerland</footnote>

<footnote><sup>d</sup> Conservation and Restoration of Cultural Heritage, University of Amsterdam, BG2 Turfdraagsterpad 15-17, 1012, XT, Amsterdam, the Netherlands</footnote>

<footnote><sup>e</sup> Université Paris-Saclay, ENS Paris-Saclay, CNRS, Photophysique et Photochimie Supramoléculaires et Macromoléculaires, 91190 Gif-sur-Yvette, France</footnote>

<footnote><sup>f</sup> Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali, INSTM, Via G. Giusti, 9, I-50121 Firenze, Italy</footnote>

<footnote>* Corresponding author at: Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Ed. 17 “Stanislao Cannizzaro”, Viale delle Scienze, 90128 Palermo, Italy

E-mail addresses: edith.joseph@he-arc.ch (E. Joseph), giuseppe.cavallaro@unipa.it (G. Cavallaro).</footnote>
the inverse Pickering emulsion gels has been studied by combination of several methods, including morphological analysis, surface characterization and rheology.

Then, the removal capacity of the gel systems toward corrosion layers from a copper substrate has been tested. Both the cleaning efficiency and the release of residues on the treated surfaces have been analyzed. A controlled cleaning has been obtained through the systematic regulation of the gel application time. Therefore, this work proposes an optimization of the previously studied organo-gel, obtaining a system with a more homogeneous structure due to the action of W/O Pickering emulsion.

# 1. Introduction

In the field of Cultural Heritage, metals have always played a central role being used for utensils, sculptures, jewelry, and decorative arts. Their specific chemical and physical properties, and their consequent deterioration, have led to several studies about metal conservation ensuring the protection of this legacy for future generations.

The primary challenge in metal conservation is the spontaneous process of corrosion to which metals are inherently prone. Corrosion is a thermodynamically favored process that provokes slow and continuous alteration of artifacts properties. The majority of corrosion processes in nature are electrochemical, involving a combination of physical and chemical reactions, by means of interactions with the surrounding environment [1]. Aside from the presence of oxygen and water, acting as oxidant and electrolyte, respectively, several factors play an important role in corrosion such as the nature of the atmosphere and the pollutants present. Moreover, corrosion is a primary cause of metallic material loss, which, after being mobilized and introduced into biogeochemical cycles, contributes to environmental pollution and poses risks to human health. Taking all these aspects into account, conservation and control technologies are essential for ensuring the preservation of metal artifacts together with the environment [1].

The first phase of metal care consists of tackling the corrosion layer present, usually involving the stabilization or the removal of the reactive compounds present. Corrosion removal typically involves both mechanical and chemical methods, including dry or wet abrasion, and complexing agents or electrochemical reactions, respectively [2]. Since the safety of the artifact is a leading priority in conservation procedures, it is extremely important to find a product that is not only suitable and compatible with the surface, but which also has an appropriate application procedure. Nowadays, gels are the most widely spread systems for conservation cleaning procedures, ensuring a controlled release of the active solutions.

From the early 1990s onwards, gels have been applied for the conservation of several substrates, from wall paintings and stone to easel paintings, and deemed to be essential for materials directly sensitive to water [3–5]. Regarding metal based Heritages, corrosion layers usually turn the substrate to be more porous thus more sensitive to solvents [6]. Thus, gel systems could be perfectly suitable since the liquid phase is confined in a retentive matrix and released gradually [4,7]. Accordingly, their use for metal care is increasing in recent years [8].

In the present study, we optimized a bio-based organogel composed by polyhydroxybutyrate (PHB), ethyl lactate (EL), and ethylenediamine-N,N'-disuccinic acid (EDDS) as chelating agent. PHB is a polymer belonging to polyhydroxyalkanoates, produced by bacteria (e.g., *Pseudomonas* spp., *Bacillus* spp., *Alcaligenes* spp., *Azotobacter* spp.) through intracellular carbon aerobic conversions [9]. They are not harmful to humans or environment, also being biodegradable into safe organic waste [10]. PHB can be dispersed in few organic solvents, such as alkyl or cyclic carbonates or ketones [11–13]. Moreover, it has recently been found that some lactones, such as EL, are also capable of dissolving PHB, going up to 20 % of polymer weight and over 100 °C, subsequently forming a gel if cooled at room temperature [14,15]. Ethyl lactate is a monobasic ester, which is non-corrosive, recyclable, and non-oxide-consuming. It is derived from lactic acid, being part of renewable technologies using biomass as primary source [16]. However, authors

report that ethyl lactate forms non-ideal mixtures in water, with its molecules having a strong tendency to self-associate, even in diluted solutions, leading to immiscibility regions for higher amounts of lactate members [17]. EDDS is an aminopolycarboxylic acid, a hexadentate ligand that is gaining interest because of its metal chelating properties and biodegradability, being considered as a good substitute for the widespread toxic ethylenediaminetetraacetic acid (EDTA) [18,19].

Among innovative gel formulations for metals conservation, a sustainable organogel based on PHB and EL and chelating agents has been lately explored, bringing very promising results [20]. However, since ligands are usually found in the form of aqueous solutions, both the hydrophobicity of PHB and the non-ideal mixture of EL in water should be considered. In this regard, we propose a PHB-EL organogel, containing the chelating water solution in the form of a water-in-oil (W/O) Pickering emulsion trapped inside the polymer gel network.

Pickering emulsion systems are stabilized by means of solid particles instead of the most common surfactants, exhibiting high stability and low toxicity [21,22]. Moreover, the use of solid particles as stabilizers avoids the risk of uncontrolled spreading and penetration of surfactants that can lead to significant damage [23]. Among the sustainable solid particles used as stabilizers, clay minerals are very promising. Halloysite clay nanotubes (HNTs) have been successfully employed in the formulation of oil in water emulsions (O/W). Halloysite is an alumina-silicate clay with a typical length of ca. 0.5–2 µm, while the internal and external diameters range between 10 and 50 and 50–200 nm, respectively. Clay nanotubes are suitable fillers for numerous applications, including tissue engineering [24,25], biomedicine [26–30] and packaging [31,32]. Moreover, HNTs can be used as catalytic supports [33,34], fire retardants [35,36], energy storage [37,38] and adsorbents for remediation [39,40].

Halloysite is formed by a hollow-tubular structure with peculiar interface properties since the core, composed of alumina groups, is positively charged while the shell, composed of siloxane groups, is negatively charged. The opposite surfaces charge allows the selective functionalization of halloysite by exploiting electrostatic interactions. It is important to note that halloysite can be easily dispersed in water having an overall negative charge, as demonstrated by its ζ potential value (−35 mV). Due to their hydrophilicity, pristine HNTs form O/W emulsions in agreement with the contact angle (θ < 90°) at the water/oil interface [23,41,42]. Recent review articles report that O/W Pickering emulsions stabilized by HNTs are suitable for several applications, including cosmetics [43] and pharmaceutics [44]. The inversion of Pickering emulsions (from O/W to W/O) can be achieved by using surfactant functionalized halloysite nanotubes as emulsifying agents. Cationic surfactants can be used to obtain HNTs with a hydrophobic external surface suitable for the preparation of W/O Pickering emulsions. Literature reports that alkyltrimethylammonium bromides are efficient to selectively modify the HNTs shell [45]. Due to their positive headgroups, these molecules are absorbed onto the HNTs shell allowing the formation of nanotubes with a hydrophobic external surface and a more hydrophilic core, with an increased interaction capacity toward more hydrophobic solvents (see Fig. 1S in Supplementary Material).

Among alkyltrimethylammonium bromides, hexadecyltrimethylammonium bromide (C16Br) exhibited the highest capacity for hydrophobization of HNTs outer surface [45–47].

In this work, we propose an optimized PHB-based gel with
ethylenediamine-N, N'-disuccinic acid trisodium salt (EDDS) as ligand to form metal complexes with copper. EDDS was dispersed in EL in the form of W/O Pickering emulsion stabilized by clay nanotubes hydrophobized with hexadecyltrimethylammonium bromide. The stability and rheological behavior of the gel system have been investigated to evaluate its suitability for cleaning efficiency, which has been tested on naturally aged copper coupons.

## 2. Experimental

### 2.1. Materials

Halloysite nanotubes (HNTs), Ethyl-lactate (EL), polyhydroxybutyrate (PHB), hexadecyltrimethylammonium bromide (C16Br), ethylenediamine-N,N'-disuccinic acid (EDDS). Cuprite powder (Cu₂O) and Rhodamine B were purchased from Sigma-Aldrich. All the products were employed without any further treatment. Copper coupons were provided by the Haute Ecole Arc Conservation-restauration, Neuchatel, Switzerland.

### 2.2. Halloysite hydrophobic functionalization

The hydrophoobization of halloysite was reached by using the approach reported elsewhere [45]. Firstly, an aqueous solution of C16Br was prepared by solubilizing 2 g of surfactant in 250 g of water and then stirred until a clear solution was obtained. Subsequently, 4 g of HNTs were added to the solution, which was stirred for 48 h. The obtained dispersion was then centrifuged at 7000 rpm to collect the functionalized clay. The supernatant was washed several times with water until the surface tension of water was measured indicating that free surfactant was completely removed. The residual powder was then dried at 80 °C and stored in a desiccator at 25 °C.

### 2.3. W/O Pickering emulsion preparation

Variable amounts of C16Br-HNTs (mass concentrations of 0.2, 0.5 and 0.7 % wt%) were dispersed in 6 g of EL by sonication for 15 min in a ultrasonic bath and stirring for 30 min. Then, 1 g of EDDS was added to the suspension, and the emulsion was homogenized with Vortex-Genie®2 mixer for 5 min. The EDDS/EL ratio was kept at 1:6 as previously reported for PHB-EL-EDDS formulations [9].

### 2.4. Formulation of an organo-gel based on PHB-EL and W/O Pickering emulsion

The functionalized halloysite (C16Br-HNT) was dispersed in EL by sonication for 15 min and stirring for 30 min. Then poly-3-hydroxybutyrate (PHB) was added to the clay dispersion at 7 % (wt%) and stirred at 110 °C in a close vial until the complete dissolution of the polymer. Subsequently, after the addition of EDDS, the dispersion was poured into a Petri dish and cooled at room temperature to obtain the gel.

### 2.5. Cleaning test on copper coupons

The cleaning efficacy of the obtained Pickering emulsion gel (PHB-EL-EDDS-C16BrHNTs) was tested by its application on copper coupons (4 cm × 4 cm), interposing Japanese paper to minimize eventual residues of gel. The organo-gel was assessed on triplicate coupons using different application time (0, 5, 15, 20, 25, 30, 35, 40, and 60 min). Lately, a rinse with ethanol 70 % v/v was performed to avoid any unwanted residues on the surface.

## 3. Methods

### 3.1. Surface tension

Measurements of C16Br-HNTs washing water after centrifugation were conducted using a tensiometer equipped with a Wilhelmy plate (KSV Sigma 70).

### 3.2. Water contact angle

Water contact angle measurements were performed by an instrument provided with a high-resolution CCD camera (OCA 20, Data Physics Instruments), while data were analyzed by SCA 20 software (Data Physics Instruments). The sessile drop method was used by the deposition of a droplet (10.0 ± 0.5 µL) on the samples surface. The contact angle was measured immediately after the deposition of the droplet on the solid surface. All the reported values are a result of three-times repeated measurements.

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** WCA (a) and ζ-potential (b) of HNTs and C16Br-HNTs; FTIR-ATR spectra of C16Br (green), HNTs (blue) and C16Br-HNTs (violet) (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
### 3.3. ζ-potential and Dynamic Light Scattering (DLS)

ζ-potential and Dynamic Light Scattering (DLS) measurements were performed to investigate both HNTs and C16Br-HNTs dispersions by a Zetasizer Nano-ZS (Malvern Instruments) under isothermal conditions (T = 25 °C ± 0.1). Both experiments were conducted on aqueous dispersions with the concentration of 10⁻³ wt%. DLS measurements were performed at a scattering angle of 173° and a wavelength of 632.8 nm.

### 3.4. FTIR microscopy

Coupons were examined through a Thermo Nicolet (Thermo Fisher Scientific) iN10 MX imaging microscope fitted with a mercury-cadmium-telluride (MCT) detector cooled with liquid nitrogen. Spectra (4000–650 cm⁻¹, 4 cm⁻¹ spectral resolution, 10 scans) were recorded in attenuated total reflection (ATR) mode using a germanium crystal. All data were processed using the OMNIC-Picta software (Thermo Fisher Scientific).

### 3.5. Thermogravimetric analysis

Thermogravimetry (TGA) measurements were performed by means of a Discovery TGA 550 (TA Instruments) under the nitrogen flows of 60 cm³·min⁻¹ and 40 cm³·min⁻¹ for the sample and the balance, respectively. TGA analyses were conducted on C16Br-HNTs and the relative pure compounds from room temperature to 800 °C with a scanning rate of 20 °C·min⁻¹. A quantitative measurement of guest molecule (C16Br) loading was provided using the rule of mixtures as reported in literature [48]. Details of the calculations are presented in Supporting Information.

### 3.6. Optical microscopy

A NeXcope NE710, TiEsseLab, optical microscope, furnished with a digital camera, was used to study the morphological characteristics of both W/O Pickering emulsion and PHB-based gels. Specifically, the structure of Pickering emulsions was characterized by fluorescence microscopy. To this purpose, 10 µL of Rhodamine B (excitation wavelength of 545 nm; emission wavelength of 566 nm) was mixed with 0.5 mL of emulsion sample. The microscope pictures were collected at excitation wavelength of 545 nm. All the images were further examined using Fiji-ImageJ v.2.16.0 software to determine the size distribution of EDDS droplets in W/O emulsion.

### 3.7. Scanning electron microscope

The surface morphology of the clays was studied by a Jeol JSM-IT700HR InTouchScope™ Scanning Electron Microscope coupled with energy dispersive X-ray spectrometry (EDX) for elemental analysis. High vacuum mode was used, with an acceleration voltage of 10 kV and working distance of 9.5–10.5 mm. To avoid charging under the electron beam, each sample was coated with a layer (≈ 4 nm) of gold in argon, using a Jeol JFC-1200 Fine Coater machine.

### 3.8. Rheological measurements

The viscoelastic properties of the PHB-based gels, both with and without Pickering emulsion, were analyzed by means of a rheometer (Discovery HR-1, TA Instruments), equipped with a parallel plate geometry (40 mm diameter, 1 mm gap). Frequency sweep tests were performed varying angular frequency from 0.01 to 100 rad s⁻¹ and with 1 % constant strain amplitude. Each measurement was conducted at ambient temperature (T = 25 °C).

### 3.9. X-Ray Fluorescence (XRF)

The corrosion layer on coupons and the quantity of copper contained inside the gels after cleaning were evaluated by a NITON® XL3t portable XRF spectrometer. Analyses were performed on 4 cm × 4 cm coupons and 1 × 1 cm wet gel immediately after cleaning metal coupons at different application time, respectively.

### 3.10. Colorimetric analysis

A colorimetric analysis was performed to monitor the colour changing of the coupons after the cleaning process. The parameters were measured using a portable colorimeter (X-Rite Ci6x Series™) and X-Rite Software for data processing. L* (lightness), a* (red-green) and b* (yellow-blue) variables were measured to obtain the total colour differences (ΔE), using the following equation

$$ \Delta E^* = \sqrt{(\Delta L^*)^2 - (\Delta a^*)^2 - (\Delta b^*)^2} \quad (1) $$

### 3.11. Eddy current measurements

Eddy current measurements were performed to quantify the thickness of the corrosion layer present on copper coupon surfaces. Data were acquired with a PHYNIX Surfix®S instrument and a combination probe FN 3.5. The instrument was calibrated using a precision standard 8-µm sheet (PHYNIX Surfix®) placed on a bare copper coupon. For each point of interest, measurements were in triplicate.

### 3.12. Raman spectroscopy

A Renishaw® VirsaTM Raman Analyser was employed for single-point analysis with 4 cm⁻¹ spectral.

resolution, using a 532 nm frequency diode excitation laser and a 50× Nikon objective. Data elaboration was performed through Wire 5.4 software (Renishaw®).

## 4. Results and discussion

### 4.1. HNTs hydrophobization: adsorption of C16Br onto Halloysite shell

HNTs was selectively functionalized with hexadecyltrimethylammonium bromide (C16Br), following the procedure reported by Cavallaro et al. [45].

Both pristine and functionalized halloysite dispersions in water were monitored after 20 min of sonication. Pristine HNTs formed a stable dispersion in water, while the C16Br-HNTs aqueous suspension evidenced sedimentation processes in agreement with the hydrophobization of the halloysite outer surface (Supplementary Material, Fig. 2S).

The hydrophobization process of HNTs shell was corroborated both by water contact angle (θ), ζ-potential and DLS measurements. As shown in Fig. 1a, the adsorption of cationic surfactant determined a relevant enhancement of the water contact angle (from 30° to 103°). According to literature [47], the increase of water contact angle is consistent with surface hydrophilization of halloysite because of the presence of the surfactant alkyl chains on the HNTs shell. Contrarily, the adsorption of surfactant onto the inner surface do not influence the surface properties of halloysite because the alkyl chains are confined within the HNTs cavity. Namely, the loading of surfactant inside the HNTs cavity do not affect the hydrophilic character of halloysite, which possess a high density of OH groups on the external interface.

As shown by ζ-potential data (Fig. 1b), C16Br-HNTs exhibited an inversion of surface charge (+ 41.8 ± 0.2 mV) as compared to pristine halloysite (−34.7 ± 0.2 mV). This can be attributed to the neutralization of the negative charges of halloysite external surface and the consequent
<figure><img src="image_6.png" /><figcaption>Fig. 2. SEM micrographs and relative EDX mapping of HNTs (a) and C16Br-HNTs (b).</figcaption></figure>

prevalence of the inner positive charges [49]. DLS experiments highlighted that the adsorption of cationic surfactant on the HNTs shell affect the aqueous mobility of the clay nanotubes. We determined that the selective functionalization of halloysite outer surface determines a reduction of the aqueous diffusion of HNTs in agreement with the hydrodynamic diameters (502 ± 12 nm and 629 ± 18 nm for HNTs and C16Br-HNTs). As reported in literature [45], the adsorption of surfactant on the HNTs outer surface can promote aggregation processes between the nanotubes because of hydrophobic interactions between the hydrocarbon chains.

FTIR-ATR was used to compare the spectra of modified HNTs with their pristine components (C16Br and HNTs) (Supplementary Material, Fig. 3S). Pristine HNTs evidenced the characteristic peaks at 3694 cm⁻¹ and 3621 cm⁻¹, which are attributed to the stretching vibrations of inner-surface OH groups [50]. As concerns C16Br, we observed FTIR

bands at 2920 cm⁻¹ and 2851 cm⁻¹ due to the CH₂ asymmetric and symmetric stretching modes of the surfactant, respectively. Interestingly, these signals were shifted to smaller wavenumbers for C16Br/HNTs as a consequence of the packing of alkyl chains confirming the adsorption of the surfactant onto the halloysite surface (Fig. 1c).

The morphological structure of the clays was investigated by SEM-EDX analyses. The micrographs.

(Fig. 2) showed that the surfactant addition did not alter the HNTs shape, despite a thin coating is visible along the external surfaces, reducing the sharpness of their outlines. This morphological change confirms halloysite surface modification, being consistent with similar findings observed for other clay/surfactant hybrid materials [45,51]. Furthermore, elemental mapping with EDX revealed a homogeneous distribution of bromine (Br) and carbon (C) in the functionalized halloysite. As expected, these elements were not detected in pristine

<figure><img src="image_7.png" /><figcaption>Fig. 3. Fluorescence microscopy of EDDS-EL mixture (a) and EDDS-EL W/O Pickering emulsion (b); EDDS was labelled with Rhodamine B, while EL phase is not stained and appears black.</figcaption></figure>
halloysite confirming the actual C16Br adsorption onto HNTs surface.

Thermogravimetry measurements were then performed to estimate the surfactant loading (2.36 ± 0.1 wt%) in the modified HNTs using the rule of mixtures on the thermogravimetric curves [52] (Supplementary Material, Fig. 4S). As reported elsewhere for alkyltrimethylammonium bromides/HNTs composites, we calculated the surface coverage degree (77%) of halloysite assuming a monolayer adsorption of the surfactant. It should be noted that the coverage degree was estimated by considering the head group area (1.15 nm²) of C16Br [53] and the area (58.1 m² g⁻¹) of halloysite shell [54].

## 4.2. EDDS/Ethyl lactate W/O Pickering emulsion formulation and characterization

### 4.2.1. C16Br-Halloysite nanotubes and their emulsion

The obtained functionalized Halloysite (C16Br-HNTs) played a key role in the water-in-oil Pickering emulsion formulation. Indeed, optical microscopy analysis of the initial EDDS/EL system, in the absence of HNTs, revealed a non-ideal dispersion of EL mixed with EDDS in water solution (Fig. 3a). Oppositely, Fig. 3b shows the optical images of EDDS droplets inside EL medium in the Pickering emulsion stabilized by the functionalized HNTs. It should be noted that EDDS was labelled with

Rhodamine B (excitation wavelength of 545 nm; emission wavelength of 566 nm). The continuous EL phase is not stained and appears black in the optical microscope measurements. No further tests using Rhodamine B as marker were conducted, because the fluorescent dye influenced the morphology of the system.

### 4.2.2. Effect of clay concentration on emulsion properties

The droplet size in a Pickering emulsion can be straightforwardly controlled by varying the concentration of solid particles used as emulsifiers [43]. In this regards, C16Br-HNTs were added to the emulsions in different concentrations (namely, 0.2, 0.5, and 0.7 % wt%). A statistical analysis was conducted to determine the distribution of the size (i.e., radius) of the EDDS droplets. Only the analysis related to the 0.2 % C16Br-HNTs Pickering emulsion showed a bimodal distribution, with the main peak at 3.8 µm and a shoulder centred at 7 µm. The other distributions (0.5, 0.7 % C16Br-HNTs) were fitted with a Gaussian model (Supplementary Material, Table 1S). As shown in the graph reported in Fig. 4, distributions are unimodal and droplets radii range between 0.72 and 15 µm. Noteworthy, it can be easily observed how particles radii decrease by increasing C16Br-HNTs: indeed, we registered average radii values of 4.3, 2.5, and 2.3 µm for EDDS droplets, at 0.2, 0.5, and 0.7 % of clay, respectively (Fig. 4). As reported in literature,

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Optical micrographs of W/O Pickering emulsions and radius distribution of EDDS droplets in EL. The dashed lines represent the Gaussian fit (corresponding R² values are reported in Supplementary Material, Table 1S).</figcaption></figure>
<caption>**Table 1**</caption>

ΔE values and corrosion thickness of treated copper coupon as functions of gel application times.

<table><thead><tr><th>ΔE</th><th>Corrosion thickness / µm</th><th>Gel application time / min</th></tr></thead><tbody><tr><td>0</td><td>2.37 ± 0.4</td><td>0</td></tr><tr><td>1.55 ± 0.13</td><td>2.13 ± 0.3</td><td>5</td></tr><tr><td>3.43 ± 0.16</td><td>1.87 ± 0.4</td><td>15</td></tr><tr><td>5.02 ± 0.62</td><td>1.63 ± 0.4</td><td>20</td></tr><tr><td>7.21 ± 0.44</td><td>1.38 ± 0.5</td><td>25</td></tr><tr><td>9.78 ± 0.26</td><td>1.12 ± 0.3</td><td>30</td></tr><tr><td>14.12 ± 0.71</td><td>0.40 ± 0.3</td><td>35</td></tr><tr><td>19.11 ± 1.15</td><td>0.15 ± 0.4</td><td>40</td></tr><tr><td>28.06 ± 0.20</td><td>0.06 ± 0.4</td><td>60</td></tr></tbody></table>

the decrease of the droplet sizes induces an enhancement of the colloidal stability of Pickering emulsions [23,55].

### 4.3. Pickering emulsion in PHB gel: morphology, rheology and stability

The Pickering emulsion in gel and the PHB-based gel containing the EDDS-EL mixture were compared. The PHB-EL-EDDS gel demonstrates clear evidence of phase separation due to the non-ideal mixture of EL in aqueous solution (Fig. 5a). This result aligns with macroscopic observations where the two phases can simply be distinguished by differences in colour, confirming the absence of a stable network (Supplementary

Material, Fig. 5S). In contrast, no phase separation was observed in the Pickering emulsion gel, which appears significantly more homogeneous even at the microscopic level (Fig. 5a). This structural difference is consistent with the improved stabilization provided by the Pickering emulsion, which prevents coalescence and promotes uniformity within the system.

Although both microscopic and macroscopic observations already highlighted clear structural differences between the two gels, rheological frequency-sweep measurements were performed to assess their mechanical behavior and internal stability. Across the entire frequency range, the storage modulus (G') is consistently higher than the loss modulus (G") for both samples, outlining their solid-like behavior. Notably, the Pickering gel shows significantly higher values of both G' and G" as compared to the PHB-EL-EDDS gel, indicating a stronger and more structured internal network (Fig. 5b). Moreover, we calculated the tanδ, by G"/G' ratios, for the whole frequency range between 10⁻¹ and 10² cm⁻¹. The results (Supplementary Material, Fig. 6S) confirmed the solid-like behavior of both gel formulations since tanδ values are lower than 1. It was noticed that tanδ is almost constant within the whole angular frequency range.

Further differences between the two gel systems were observed by TGA, which allowed us to explore their solvent retention capacities. Fig. 5c illustrates the mass vs time curves for ethyl lactate (EDDS), PHB-EL-EDDS, and PHB-EL-EDDS-C16BrHNTs under isothermal conditions (T = 70 °C). Ethyl lactate exhibits a rapid and complete mass loss within

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Optical micrographs (a) and frequency sweep curves (b) of PHB-EL-EDDS and PHB-EL-EDDS-C16BrHNTs gels. Isothermal thermogravimetric curves (c) at 70 °C for EDDS, PHB-EL-EDDS and PHB-EL-EDDS-C16BrHNTs.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 6. Raman spectra of EDDS solution before (violet) and after (blue) being mixed with a cuprite reference powder, and of PHB gels before (green) and after 1 h-treatment (yellow) on copper coupon. The main Raman bands related to EDDS-Cu complexes are marked by dashed lines (a). Pictures of gels application, interposed Japanese paper, and cleaned coupons after 1 h-treatment with PHB-EL-EDDS and PHB-EL-EDDS-C16BrHNTs (b); FTIR-ATR spectra of each compound present in the gel and of the PHB-EL-EDDS-treated coupon (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

the first 2 min, while both gel systems demonstrate slower weight loss kinetics, suggesting the role of the gel network in delaying solvent evaporation. On this basis, we can estimate the solvent evaporation rate by the following eq. (2):

$$ (\mathrm{dm}/\mathrm{dt})_{\mathrm{gel}} / (\mathrm{dm}/\mathrm{dt})_{\mathrm{s}} \approx P_{\mathrm{gel}} / P_{\mathrm{s}} \quad (2) $$

where $(\mathrm{dm}/\mathrm{dt})_{\mathrm{gel}}$ and $(\mathrm{dm}/\mathrm{dt})_{\mathrm{s}}$ are the mass loss rates for gel system and pure solvent, respectively, corresponding to their relative vapor pressure ($P_{\mathrm{gel}}$ and $P_{\mathrm{s}}$, respectively). As concerns the PHB-EL-EDDS gel, the vapor pressure calculated corresponds to $0.159 \pm 0.002$ Pa, while EDDS/EL Pickering emulsion in PHB gel exhibited a significantly lower pressure of $0.045 \pm 0.002$ Pa. It should be noted that the vapor pressure of pure EDDS is $0.235 \pm 0.005$ Pa. Given that the evaporation rate is proportional to the vapor pressure, as described by the Hertz-Knudsen equation, [56] we can assess that in PHB-EL-EDDS gel there is a more rapid phase transition from liquid to vapor, which may be attributed to a lower degree of molecular encapsulation or weaker intermolecular interactions within the gel matrix. In contrast, the EDDS/EL Pickering emulsion in PHB gel likely benefits from the enhanced stabilization at the water-oil interface, which restricts molecular mobility reducing evaporation rate.

## 4.4. Cleaning of corrosion layers from copper coupons

### 4.4.1. Coupons characterizations

Before applying and comparing the gels to the coupons, a comprehensive characterization was conducted to assess the composition of corrosion layer present on the coupons surface. Firstly, elemental analysis was carried out by means of X-Ray Fluorescence (XRF) spectroscopy, which evidenced the presence of Cu ($99.8 \pm 0.2\%$) and some traces ($< 0.01\%$) of Fe, S, and P. Subsequently, Raman spectroscopy was performed in order to ascertain the nature of the corrosion phases. Copper oxide in the form of cuprite ($Cu_2O$) was detected by the most characteristic band at ca. $217 \mathrm{cm}^{-1}$, correlated by signals at 217, 300, and most remarkably, at $635 \mathrm{cm}^{-1}$, which is a marker for amorphous cuprite (Supplementary Material, Fig. 7S) [57]. No other compounds were identified.

### 4.4.2. Evidence of Cuprite complexation by EDDS in the cleaning gel

A preliminary cleaning test was carried out by applying the two gel formulations (PHB-EL-EDDS and PHB-EL-EDDS-C16BrHNTs) on metal coupons for 1 h. The cleaning efficiency of the gel formulations can be attributed to the EDDS affinity for copper from cuprite complexation. The conditional stability constant for Cu-EDDS complexes as a function of pH provide an indication of the complexation efficiency [58]. Since
reaching a value of ≥6 is often considered the threshold for efficient chelation, EDDS can be considered suitable for chelating Cu(II) ions within a pH range of 3 to 12. Given that the gel system employed in this study possesses a pH of approximately 6.6, it is reasonable to conclude that the conditions necessary for complexation are effectively established during the cleaning process.

Raman spectra of both EDDS solution and EDDS mixed with cuprite reference powder were collected to obtain standard signals for Cu-EDDS complexes. The main difference between spectra is that some peaks are absent in the spectrum of pure EDDS but appear clearly in the spectrum of EDDS mixed with cuprite reference powder. Specifically, Raman signals at ca. 573, 758, 1004, 1082, and 1108 cm⁻¹ may indicate Cu-EDDS complexes (Fig. 6a). Peaks in the spectral region 1000–1200 cm⁻¹ are diagnostic of Cu-EDDS complexes and were attributed to bending and torsion vibrations of metal cations bonded to the amino groups of EDDS [59]. This confirms the formation of complexes.

The effectiveness of complexation was assessed directly on the gels before and after treatment of the copper coupon. A clear distinction was observed between the two samples: after treatment of the coupon, the gel exhibits the Raman peaks previously identified as EDDS complexes, which are otherwise absent for the pristine gel (Fig. 6a).

### 4.4.3. Application and comparison between the two gel formulations

Cleaning tests to compare the two gel formulations (PHB-EL-EDDS and PHB-EL-EDDS-C16BrHNTs) were carried out on metal coupons for one hour. A layer of Japanese paper was interposed between the gels and the metal surface to avoid direct contact and minimize chelating agent residues on the substrate. Each gel application was followed by a rinse with ethanol 70 % v/v to avoid any unwanted residual material on the surface. The PHB-EL-EDDS gel produced an irregular cleaning effect, suggesting an unequal distribution of the chelating agent. In contrast, the Pickering emulsion gel achieved a more uniform removal of cuprite, as confining solvents improves cleaning uniformity on delicate surfaces. [60] This behavior was further confirmed by visual observations of the interposed Japanese paper: in both cases, a blue coloration – indicative of complexation of EDDS with copper ions – was observed, but the distribution of the blue colour was notably more irregular in the tissue used with the PHB-EL-EDDS gel (Fig. 6b).

Furthermore, µ-FTIR spectroscopy in Attenuated Total Reflection mode (ATR) analyses was performed on 1-h-treated coupons to ascertain the presence of gel residues. The coupon treated with PHB-EL-EDDS gel shows characteristic absorption bands at ~1567 cm⁻¹ and ~1390 cm⁻¹, attributed to asymmetric and symmetric vCO₂ bands, which are also observed from the EDDS spectrum (Fig. 6c). This result indicates the presence of residual EDDS on the coupon surface, suggesting an uncomplete of the chelating agent under the tested conditions. In contrast, the coupon treated with PHB-EL-EDDS-C16BrHNTs gel shows no detectable EDDS-related signals, with no residual chelating agent on the metal surface.

### 4.4.4. PHB-EL-EDDS-C16BrHNTs gel cleaning efficiency

Based on the comparative assessment of the cleaning formulations, the Pickering emulsion-based gel demonstrated superior performances. Consequently, all subsequent cleaning tests were conducted exclusively with this formulation.

Cleaning tests were performed at different gel application time – 0, 5, 15, 20, 25, 30, 35, 40, and 60 min – to assess the influence of contact time on cleaning efficiency. For each application time, colorimetric measurements were recorded to quantify the effectiveness of the cleaning process. ΔE values increased with longer application times, indicating the progressive removal of the brown cuprite corrosion layer (Table 1). Concurrently, Eddy Current measurement was performed to monitor corrosion thickness and its progressive reduction during the cleaning process, registering a decrease from 2.37 µm on the untreated coupon to just 60 nm after 60 min of gel application (Table 1). Specifically, from 0 to 30 min, both parameters exhibit a linear trend with

time: ΔE increases steadily, indicating a gradual and continuous colour change, while thickness decreases linearly, suggesting uniform thinning of the corrosion layer, due to material removal. After 30-min-gel application, a variation in behavior occurs: ΔE continues to increase with a steeper linear trend while the thickness rapidly approaches zero, indicating that the cuprite layer has been almost completely removed. In addition, XRF analysis, performed on the dry gel after treatment at different application times, revealed a progressive increase in copper percentage going from 0 to 42 wt%. This increasing trend may thus be interpreted as a more quantitative indicator of the complexation process, complementing both colorimetric and Eddy Current data in confirming the effective thinning of cuprite over time (Supplementary Material, Fig. 8S).

Raman spectra were collected on the coupons treated with PHB-EL-EDDS-C16BrHNTs gel at different application time. The characteristic peaks of cuprite — most notably at 92, 144, 217, 300 and 635 cm⁻¹, are clearly observed in the untreated sample and still visible after 40 min of treatment (Supplementary Material, Fig. 9S). This indicates only a partial removal of the corrosion layer after 40 min. In contrast, after 1 h of treatment, the Raman spectrum does not show the cuprite peaks anymore, confirming the effective removal of the corrosion from the surface to non-detectable levels. These findings demonstrate that a minimum gel application time of ca 1 h is required to achieve complete removal of the cuprite corrosion layer.

## 5. Conclusions

In this work, a novel sustainable cleaning system for corroded copper surfaces was developed by incorporating a W/O Pickering emulsion into a PHB-based organo-gel. The emulsion was stabilized by halloysite nanotubes functionalized with hexadecyltrimethylammonium bromide, leading to the formation of inorganic reverse micelles with high affinity to ethyl lactate. This approach enabled the homogeneous co-dispersion of ethyl lactate and EDDS, overcoming the EL tendency to self-associate when in aqueous solutions. The dispersion of the active components within the emulsion droplets contributed to the overall stability and effectiveness of the system. The cleaning efficiency of the studied systems was evaluated by applying and comparing the gel formulations on naturally corroded copper coupons. The formulated organo-gel showed an improved performance compared to the analogous system without the Pickering emulsion, having precise control over the degree of cleaning depending on the application time. These results highlight the potential of Pickering-emulsion-based organo-gels as a promising formulation for the sustainable cleaning of historical metal heritage.

### CRediT authorship contribution statement

**Giulia D'Agostino:** Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Patrycja Petrasz:** Writing – review & editing, Formal analysis, Data curation. **Wu Qing:** Writing – original draft, Formal analysis, Data curation. **Han Zhou:** Formal analysis, Data curation. **Maartje Stols-Witlox:** Investigation, Formal analysis, Data curation. **Loïc Bertrand:** Methodology, Formal analysis, Data curation. **Edith Joseph:** Writing – review & editing, Supervision, Conceptualization. **Giuseppe Cavallaro:** Writing – review & editing, Validation, Supervision, Data curation, Conceptualization. **Giuseppe Lazzara:** Supervision, 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

Authors acknowledge funding provided through the GoGreen project of the European Commission (GA no. 101060768).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2025.139189.

## Data availability

Data will be made available on request.

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