Colloids and Surfaces A: Physicochemical and Engineering Aspects 664 (2023) 131136

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Colloids and Surfaces A: Physicochemical andEngineering Aspects
journal homepage: www.elsevier.com/locate/colsurfa

Formation mechanism of surface modified iron oxide nanoparticles using
controlled hydrolysis reaction in supercritical CO2
Yasuhiko Orita *, Keito Kariya , Thossaporn Wijakmatee , Yusuke Shimoyama
Department of Chemical Science and Engineering, Tokyo Institute of Technology, 2-12-1 S1-33, Ookayama, Meguro-ku, Tokyo 152-8550, JapanH I G H L I G H T S

G R A P H I C A L A B S T R A C T• Oleic acid-modified iron oxide NPs is
synthesized in supercritical CO2.
• Smaller amount of water allows theformation of densely and chemicallymodified NPs.
• Larger amount of water leads to theformation of aggregated NPs.
• The formation of iron oleate complexes
is validated by FT-IR analysis.
• Formation mechanism is discussed byexperimental results and predictedsolubility.

A R T I C L E I N F O

A B S T R A C T

Keywords:
Supercritical carbon dioxide
Iron oxide nanoparticles
Surface modification
Hydrolysis reaction

In the synthesis of surface modified nanoparticles (NPs), supercritical CO2 is one of appealing reaction fields torealize simple and green chemistry approach without using an organic solvent. In this work, surface modified
iron oxide NPs was synthesized in supercritical CO2 with various amounts of water between 5.0 and 90.0 mmol
to elucidate the formation mechanism of surface modified NPs with the hydrolysis reaction. The synthesis was
performed at 30.0 ± 0.9 MPa of CO2, 18 h and 100 ◦ C, where iron(III) acetylacetonate, water and oleic acid wereused as starting materials. As a result, the smaller amount of water was revealed to be key factor to synthesize
more densely modified iron oxide NPs without the aggregation and iron oxyhydroxide formation while the larger
amount of water caused the sever aggregation of less modified NPs and the formation of iron oxyhydroxide.
Additionally, the detailed analysis of iron oleate complex and the phase state prediction suggested that thesmaller amount of water allowed more dominated formation of iron oleate and subsequent its hydrolysis reactionin uniform supercritical CO2 phase, resulting in the formation of densely modified NPs without the aggregation.

* Corresponding author.
E-mail address: orita.y.aa@m.titech.ac.jp (Y. Orita).
https://doi.org/10.1016/j.colsurfa.2023.131136
Received 16 December 2022; Received in revised form 27 January 2023; Accepted 10 February 2023
Available online 14 February 2023
0927-7757/© 2023 Elsevier B.V. All rights reserved.

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1. Introductionoxide NPs with the hydrolysis reaction in scCO2, to selectively synthe­
size densely and chemically modified NPs with the size below 20 nm,
which are important for the application. Iron(III) acetylacetonate andoleic acid of fatty acid were used as precursor and surfactant, respec­tively since they are highly stable and commercially available reagents[9], which makes them suitable starting materials.

Surface modified metal oxide nanoparticles (NPs) have attracted
major attention during the last few decades due to their appealing
properties [1]. Surface modification reduces the surface energy of NPs
and induces the strong steric repulsion force between NPs, improving
their dispersibility to solvent and allowing the some practical applica­tion such as thin film fabrication [2], patterning on the substrate [3] anddrug delivery system [4]. Especially, the one-pot synthetic system of
surface modified NPs can prevent the aggregation in the reaction fieldand can control their size and shape that significantly affects the phys­
icochemical properties [5,6].
Various methods have been applied to the direct synthesis of surface
modified NPs such as sol-gel [7], hot-injection [8], heat-up [9] andhydrothermal methods [10]. These wet-based methods can control thesize, shape and surface property of the NPs, however, these methods
typically cause the large amount of waste liquor for the synthesis andwashing [5,9,10], where the disposal and regeneration cost of them isknown as the major problem. Whereas the synthesis in supercriticalcarbon dioxide (scCO2) can be one of the appealing candidates to syn­thesize surface modified NPs because scCO2 can be used as not only
synthesis but also washing and drying solvent for the particle production[11,12], which enables the simple fabrication process without using asolvent. Additionally, scCO2 has high solubility of metal organic pre­cursor and high diffusivity, which allows the formation of uniform re­action field [13]. These merits have promoted the synthesis researchusing scCO2 for inorganic materials such as metal oxide [14], metalhydroxide [11] and metal sulfate [15]. However, the products have beenfrequently observed as aggregates after the synthesis in scCO2 [14,16],
which is probably due to the non-polar property of CO2 that has low
compatibility with the hydrophilic surface of metal oxide.
Introduction of organic surfactant to the synthesis in scCO2 could
modify the surface of metal oxide particles, which effectively changes
their surface property from hydrophilic to hydrophobic that iscompatible with CO2 [17,18]. Additionally, the surface modification canalso induce strong steric repulsion force between particles [5,19].
Furthermore, scCO2 has high solubility of organic surfactant that is
typically used for the synthesis of surface modified NPs such as saturated
and unsaturated fatty acids [20,21]. These expectation and potential of
scCO2 have promoted our recent study about the direct synthesis ofsurface modified NPs using scCO2 as a reaction field. As a result, we
successfully synthesized chemically and densely modified iron oxide
NPs using water and surfactant in scCO2 [22]. Additionally, the hydro­
lysis reaction in scCO2 was suggested to be the key role to synthesize
NPs. However, the effect of water amount on the crystal structure,morphology and surface property of obtained NPs should be clarified tocontrol their properties. In this study, surface modified iron oxide NPs
was synthesized in scCO2 with various amounts of water to understandits effect on the characteristic of obtained NPs and to elucidate theformation mechanism with the hydrolysis reaction.
Iron oxide NPs are the excellent biocompatible material approved by
the US Food and Drug Administration for use in humans [23] and haveappealing magnetic properties such as strong magnetic saturation andsuperparamagnetism [24], which allows many biomedical applications[25–27]. Especially in use as contrast agent, their superparamagnetic
property (below about 20 nm) allows more emphasized magnetic reso­
nance imaging [27]. In use as drug carrier, iron oxide NPs allow the long
blood circulation time in human body due to their small size [28] andcan be magnetically guided to the target site due to their strong mag­netic saturation [24]. For these applications, hydrophobically modified
iron oxide NPs are typically used as the starting material [9,29,30], andtheir surface state are transformed into hydrophilic via the ligand ex­change reaction [29] and polymer coating [30]. However, to achieve theeffective hydrophilization and the water transfer of NPs, densely andchemically modified NPs are required to be synthesized [9,29,30].
Herein, we report the formation mechanism of oleic acid modified iron

2. Experimental
2.1. Materials
Iron(III) acetylacetonate [Fe(acac)3] (purity over 99 %) and oleic
acid (purity over 65.0 %) were purchased from Wako Pure Chemical
Industries, Ltd. CO2 (purity over 99.9 %) was supplied by Fujii BussanCo., Ltd. Ultra-pure water was prepared using Direct-Q UV3 Water Pu­
rification System supplied by EMD Millipore Corp. and the resistivity
was confirmed to be 18.2 MΩ cm.
2.2. Synthesis
Fig. 1 shows the high-pressure system to synthesize surface modified
NPs in scCO2. Fe(acac)3 of 0.30 mmol, oleic acid of 1.50 mmol and waterof 5.0–90.0 mmol were transferred to the 76 mL volume of a reaction
vessel (TSC-CO2-008; Taiatsu Glass Corp.). The molality of Fe(acac)3,
oleic acid and water corresponded to 0.006, 0.030 and 0.0–1.80 mol
kg− 1, respectively, where the molality was calculated using the volumeof the vessel and CO2 density at the reaction condition. After the vessel
was purged by CO2 under 0.5 MPa for 1 min, liquified CO2 was intro­
duced into the vessel using a HPLC pump (PU-4386; JASCO Co., Ltd.)
until reaching 6.0–7.2 MPa. After reaching the appropriate pressure, thevessel was sunk in the oil bath stirrer, which resulted in the target
temperature of 100 ◦ C and the pressure of 30.0 ± 0.9 MPa. The reaction
was kept for 18 h while the enclosed materials were vigorously stirred.
After the reaction, the vessel was pulled up from the oil bath stirrer andwas depressurized at a rate of about 0.5 MPa min− 1 using a metering
valve (1315G2Y; HOKE Inc.). Finally, the vessel was cooled in a cyclo­
hexane bath at room temperature.
The products were collected by rinsing the vessel successively withhexane/ethanol mixture with the volume ratio of 1:4. To remove theunreacted precursor and surfactant, the products were centrifuged andwashed with the mixture of hexane and ethanol (1:4 by volume). The
final precipitates were dried in a vacuum chamber at 22 ◦ C over 18 h.
2.3. Characterization
The products were analyzed using powder X-ray diffractometer(PXRD) (MiniFlex600-C; Rigaku Corp.) with Cu Kα radiation. The
products were observed using transmission electron microscope (TEM)(H-7650; Hitachi Corp.) operated at 100 kV. High resolution TEM (HR-Fig. 1. Experimental equipment.
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TEM) image and electron diffraction pattern of particle colony wereobtained using a JEOL JEM-2010 F electron microscope operated at
200 kV. For TEM and HR-TEM analysis, the solid products weredispersed in cyclohexane and transferred onto the copper grids withorganic membrane. The mean size, with their standard deviation, of theparticles were determined by visually observing the contrast difference
and estimating the boundary of approximately 200 particles. It wasnoted that attached particles with no space between them was regarded
as single particle while the size of single particle was defined as a
maximum length regardless of the morphologies. The products wereanalyzed using dynamic light scattering method (DLS) (Nano Partica SZ100-Z: HORIBA Ltd.). The products were dispersed in toluene at theconcentration of 0.01 wt % by the sonication and the dispersion was
filtrated through a syringe filter (pore size: 0.2 µm) prior to the analysis.
Thermogravimetric analysis was performed at N2 atmosphere using
thermogravimetric analyzer (TGA) (TGA-50; Shimadzu Corp.), where
the temperature was kept at 100 ◦ C for 20 min and was increased from100◦ to 600◦ C at a ramp rate of 10 ◦ C min− 1. The fourier transform
infrared spectrometer (FT-IR) (FT-IR4100; JASCO Co., Ltd.) was used tocharacterize the surfactant attached to the surface of products and thesurfactant attached to an iron ion.

Table 1Fitted parameters in Eq. (1) with PR equation and in Eq. (4)(a) Temperature dependence of binary interaction parameters in Eq. (1) with PR
equation.

i

(
bij =

CO2(1) + water(2)323－398kij = 0.5821 – 2120/T

SystemTemp. range (K)

k

m (K)

n

CO2(1) + Fe(acac)3(2)CO2(1) + oleic acid(2)313－333
313－323

4.55
7.66

-4940
-7380

4.71
12.0

Fig. 3(a–f) shows the typical TEM images of products synthesized ateach water amount and the size distribution histogram of product syn­thesized at the water amount of 5.0 mmol. Although the large amount ofwater (25–90 mmol) gave the severe aggregation state, the medium
amount of water (10.0 mmol) gave partially aggregated state. Addi­
tionally, the small amount of water (5.0 mmol) gave the significantly
reduced aggregation state of NPs with unimodal size distribution whose
the mean size is 10.2 ± 2.0 nm, as shown in Fig. 3(f). These results
clearly indicate that the smaller amount of water allows the synthesis of
NPs with reduced aggregation state and suggest that the hydrolysis re­action is the key factor to control the size and morphology of surface
modified NPs in scCO2. It was noted that the irregular shaped NPs andthe small aggregates of a few NPs were also observed at the condition
using small amount of water, which may be due the aggregation of verysmall nuclei precipitated in scCO2. Additionally, the DLS analysis ofsame product dispersed in toluene [Fig. 3(g)] showed the mean size of
88.9 ± 6.5 nm that was significantly larger than the size obtained by
TEM, suggesting the formation of secondary clusters in toluene.

j

(2)

j

bi + bj2

Equation

3.2. Size and morphology

Phase equilibrium calculations were performed to predict the phasestate of synthetic field. Peng-Robison (PR) equation was used for the
calculations of water + CO2 phase equilibria and water solubility in CO2phase [31]. In the calculation, the below mixing rules were employed.
∑∑ ()1/2 ()
1 − kij(1)am =xi xj ai aj
iTemp. range(K)(b) Correlated results by Eq. (4).

2.4. Phase equilibrium calculation∑∑
xi xj bijbm =System)(3)

3.3. Crystal structure
The PXRD, TEM electron diffraction and HR-TEM analysis were
performed to characterize the crystal structure of products. Fig. 4 shows
the PXRD pattern of products synthesized at each water amount. The
products synthesized at the large amount of water (60.0 and 90.0 mmol)
gave the PXRD peaks that were assigned to α-FeOOH (ICSD: 109041)
and α-Fe2O3 (ICSD: 22505). The products synthesized at the medium
amount of water (10.0 and 25.0 mmol) gave only the peaks that wereassigned to α-Fe2O3. The product synthesized at the small amount ofwater (5.0 mmol) gave very broad PXRD pattern. However, the same
product showed the Debye-Scherrer ring for TEM electron diffraction
analysis and the clear lattice plane for HR-TEM analysis, as shown in
Fig. 5, indicating that the obtained NPs have a good crystallinity. Thisdifference may be because of the small particle size and the low volume
portion of iron oxide NPs due to the core-shell structure of iron oxidecore and oleic acid shell [22]. Small particle size (corresponding to small
crystallite size) typically broadens PXRD pattern. Additionally, Fig. 3(a)suggests that obtained NPs is covered with oleic acid, which is detailly
discussed in chapter 3.4, enough to bring the somewhat spacing between
NPs, supporting the formation of core-shell structure [1]. Assuming thecore-shell structure, the volume portion of iron oxide is calculated to be
38.7 % using the core size of iron oxide (10.2 nm) and the carbon chain
length of oleic acid (1.9 nm) [18]. This low volume portion causes the
reduction of actual crystallite volume for PXRD measurement, which
results in the decreasing intensity. Therefore, the small particle size andthe low volume portion of iron oxide NPs possibly result in the broad
PXRD pattern.
Table 2 lists the face spacings calculated by the electron diffraction
pattern [shown in Fig. 5(a)] of obtained NPs, where each face spacing
was assigned to the crystal structure of α-Fe2O3 (ICSD: 22505) andwhere αm and bm are the mixture’s parameters. ai and aj are parametersreflecting attracting forces of pure components i and j. bi and bj are size
parameters of pure component i and j. The phase composition data was
acquired from the literature [32] to decide the binary interaction
parameter kij and its temperature dependence. Additionally, Chrastil’s
equation [33], expressed as follows, was used to calculate Fe(acac)3(III)and oleic acid solubility y to CO2.
lny = klnρr +

m
+n
T

(4)

where k, m and n are fitting parameters and ρr is the relative density ofCO2. The solubility data was acquired from some literatures to fit theparameters k, m and n for Fe(acac)3(III) [13] and oleic acid [34].
3. Results and discussion
3.1. Phase state
Fitted values, summarized in Table 1, were used to calculate thesolubility of starting materials in CO2 phase at the temperature of 100 ◦ C(Fig. 2), where each plot in Fig. 2 denotes the initial concentrations of Fe(acac)3, oleic acid and water in the synthetic field. The solubilities of Fe(acac)3 and oleic acid at 30.0 MPa showed significant higher values than
the initial concentrations of these materials, indicating that Fe(acac)3and oleic acid were completely dissolved into scCO2 at the synthesis
condition (100 ◦ C, 30.0 MPa). Whereas the water solubility in CO2 at
30.0 MPa showed 1.20 mol kg− 1, suggesting that the phase separation
occurs more than approximately 1.20 mol kg− 1 of the initial water
concentration.
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Fig. 2. The solubility (line) of (a) Fe(acac)3, (b) oleic acid and (c) water in CO2 phase at 100 ℃ and the initial concentration (circle) of these materials.

Fig. 3. (a-e) The typical TEM images of products synthesized at the water amount of (a) 5.0, (b) 10.0, (c) 25.0, (d) 60.0 and (e) 90.0 mmol. (f-g) (f) Size distribution
histogram of the product synthesized at the water amount of 5.0 mmol and (g) its DLS result in toluene.

γ-Fe2O3 (ICSD: 79196). In short, the large amount of water(60.0–90.0 mmol) gave α-FeOOH and α-Fe2O3, the medium amount ofwater (10.0–25.0 mmol) gave α-Fe2O3, and the small amount of water(5.0 mmol) gave α-Fe2O3 and γ-Fe2O3. Herein, as discussed in Chapter
3.1, water phase is suggested to be separated from scCO2 phase when its
initial concentration is more than around 1.20 mol kg− 1 (corresponding
to the water amount of 60.0 mmol for the synthesis). Additionally, thehydrolysis reaction of iron precursor in acidic water frequently forms
α-FeOOH [35]. Therefore, the reduced water amount plausibly allows
the uniform reaction field without the separation of water phase,resulting in the formation of iron oxide without α-FeOOH of iron
oxyhydroxide.

from 100◦ to 600◦ C were relatively low at the large amount of water
between 25.0 and 90.0 mmol, and increased with decrease in the wateramount from 25.0 to 5.0 mmol. These results suggest that the smaller
amount of water leads to the formation of more densely modified NPs.
Fig. 7(a–e) shows FT-IR spectra of products synthesized at each wateramount. The characteristic bands at 2850 and 2900 cm− 1 were assigned
to asymmetric and symmetric stretching modes of -CH2- in the alkyl
chains of oleic acid [36]. The band of free carboxyl group (-COOH)(normally detected at about 1700 cm− 1) in oleic acid was not confirmed
in all conditions. The bands at approximately 1550 and 1400 cm− 1 wereassigned to the asymmetric and symmetric stretching modes ofcarboxylate group (-COO− ) in oleic acid [36]. These results evidentiallyshow that oleic acid do not physical absorb to the surface but chemicallyattach to the surface of iron oxide [21]. Additionally, focusing on the–relative band intensities of -CH2- (2850 and 2900 cm− 1) and -COO
− 1(1530 and 1400 cm ), the smaller amount of water (5.0 mmol) showedsignificantly stronger intensities than those from the larger amount ofwater. Since the band intensity qualitatively reflects the amounts ofmolecules attached to the surface, this result further supports that thesmaller amount of water leads to the formation of more densely

3.4. Surface state
The TG and FT-IR analysis were performed to characterize the oleic
acid attached to the surface of NPs. Fig. 6 shows the TG spectra of
products synthesized at each water amount. The weight loss showed
increasing trend from 300◦ to 450◦ C that corresponded to the thermal
decomposition temperature of oleic acid [21]. The total weight losses
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be collected after the washing and drying, which means no particle
formation without water addition. Therefore, the detected characteristic
bands of -COO– in Fig. 8(a) strongly support the formation of iron oleatecomplexes in scCO2 without water addition. Addtionally, the key
carboxylate region shows the bands at 1576, 1527 and 1431 cm− 1, asshown in Fig. 8(b). The defference between the band at 1431 cm− 1 andother bands corresponded to the bridging (Δ = 145 cm− 1) and bidentate(Δ = 96 cm− 1) coordination modes of -COO–, respectively [20,37,38].
These results suggeste that scCO2 medium with oleic acid allows theformation of iron oleate complexes with these coordination modes via
the ligand exchange reaction between Fe(acac)3 and oleic acid.
Table 2
Face spacings and assigned structures of NPs synthesized at the water amount of
5.0 mmol.

Fig. 4. The PXRD pattern of products synthesized at the water amount of (a)5.0, (b) 10.0, (c) 25.0, (d) 60.0 and (e) 90.0 mmol.

No

Face spacinga (nm)

1
2
3
4
5

0.242
0.224
0.208
0.168
0.149

Assigned structureb

α-Fe2O3
α-Fe2O3

γ-Fe2O3
γ-Fe2O3
γ-Fe2O3
γ-Fe2O3aFace spacings were calculated by TEM electron diffraction pattern. b The facespacings were assigned to the crystal structure of α-Fe2O3 (ICSD: 22505) and
γ-Fe2O3 (ICSD: 79196).

modified NPs. Fig. 7(f) shows the enlarged FT-IR spectra of iron oxide
NPs synthesized at the water amount of 5.0 mmol. The separation ofasymmetric and symmetric stretching modes for -COO− , Δ, was used todetailly analyze the chemical bonding state of oleic acid. Unidentate,bridging and bidentate ligands are expected for Δ >200 cm− 1,
200 cm− 1> Δ >140 cm− 1 and 110 cm− 1> Δ, respectively [37]. For the
product, two characteristic bands were confirmed at 1519 and
1430 cm− 1, and its difference was 89 cm− 1, suggesting that oleic acid ischemically bonded to the surface as a bidentate mode [20,38].
3.5. Proposed mechanism
To investigate the formation mechanism of surface modified NPs
behind the effect of water amount in scCO2, the synthesis was performed
at 100 ◦ C and 30.0 MPa without water addition. Fig. 8 shows the FT-IR
spectra of wet solid product before aftertreatment (washing and drying).
The characteristic band at 1700 cm− 1 was assigned to -COOH of oleic
acid, in addition, the bands at approximately 1550 and 1400 cm− 1 wereassigned to the asymmetric and symmetric stretching modes of -COO– in
oleic acid, respectively [36]. Herein, the thermolysis rate of Fe(acac)3 is
known as very small, and the calculated conversion at the reaction
condition (100 ◦ C, 18 h) was negligible based on the report from G.
Beech and R. Lintonbon [39]. Additionally, the solid material could not

Fig. 6. TG spectra of products synthesized at the water amount of (a) 5.0, (b)10.0, (c) 25.0, (d) 60.0 and (e) 90.0 mmol.

Fig. 5. (a) The electron diffraction pattern and (b) the high resolution TEM image of NPs synthesized at the water amount of 5.0 mmol.
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Fig. 9. Formation mechanism of surface modified NPs behind the effect ofwater amount in scCO2.

confirmed in our previous research with the same reaction condition(100 ◦ C, 30.0 MPa) [22] and iron oleate formation is validated by the
analysis in previous paragraph, which supports that the hydrolysis of Fe(acac)3 competes the iron oleate formation. When the large or medium
amount of water exist in scCO2, the hydrolysis of Fe(acac)3 should befaster than the iron oleate formation, resulting in the nucleation of lessmodified NPs. Such iron oxide NPs typically has hydrophilic surface thatis low compatible to scCO2 with nonpolar property and causes the low
steric hindrance between NPs, which easily leads to the severe aggre­
gation of less modified NPs (Fig. 9). Since the aggregated NPs are noteasily disaggregated and additionally modified, the final product reflects
the less modified surface, resulting in the relatively small weight loss
and band intensities assigned to oleic acid for TG and FT-IR analysis,
respectively. Additionally, the large amount of water above the solubi­
lity causes the phase separation of water, resulting in the formation ofiron oxyhydroxide (Fig. 9). In contrast to them, the small amount ofwater allows the uniform reaction field and the relatively faster forma­tion of iron oleate than the hydrolysis reaction of Fe(acac)3. As a result,iron oxide NPs is synthesized through the hydrolysis reaction of ironoleate complexes (Fig. 9). As widely known in hydrothermal method [5,6,18], the hydrolysis reaction of iron oleate leads to the nucleation ofchemically and densely modified iron oxide NPs. Such NPs induces thestrong steric hindrance between NPs and has the hydrophobic surface
that is compatible with scCO2, which inhibits the aggregation. There­
fore, it can be concluded that the smaller amount of water allows more
dominated formation of iron oleate and subsequent its hydrolysis reac­tion in uniform scCO2 phase, resulting in the formation of densely andchemically modified iron oxide NPs without the aggregation.

Fig. 7. (a-e) FT-IR spectra of products synthesized at the water amount of (a)5.0, (b) 10.0, (c) 25.0, (d) 60.0, (e) 90.0 and (f) 5.0 mmol: (a-e)
1000–3500 cm− 1 and (f) 1250–1650 cm− 1.

4. Conclusions
In this study, surface modified iron oxide NPs was synthesized in
scCO2 with various amounts of water to reveal the formation mechanismof surface modified NPs with the hydrolysis reaction. The synthesis was
performed at 30.0 ± 0.9 MPa of CO2 and 100 ◦ C for 18 h, where iron(III)acetylacetonate of 0.30 mmol (0.006 mol kg− 1), oleic acid of 1.50 mmol
(0.030 mol kg− 1) and water of 0.0–90.0 mmol (0.0–1.80 mol kg− 1)
were used as starting materials. TEM and XRD analysis revealed that thesmaller amount of water led to the formation of iron oxide NPs without
the aggregation as opposed to the aggregated NPs for the larger amountof water. Additionally, TG and FT-IR analysis supported that the smaller
amount of water led to the formation of more densely modified NPs thanthose from the larger amount of water. These results suggest that thesmaller amount of water is crucial to synthesize densely modified NPs,
resulting in the inhibition of the aggregation. Additionally, the detailed
analysis of iron oleate complex and the phase state prediction of syn­thetic field suggested that the smaller amount of water allowed more

Fig. 8. FT-IR spectra of wet solid product obtained without water addition andaftertreatment (washing and drying): (a) 1000–3000 cm− 1 and (b)1250–1650 cm− 1.

The effect of water amount on the synthesis of iron oxide NPs in
scCO2 can be explained by the mechanism depicted in Fig. 9. The hy­
drolysis reaction of Fe(acac)3 and subsequent particle formation are6

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Colloids and Surfaces A: Physicochemical and Engineering Aspects 664 (2023) 131136

dominated formation of iron oleate and subsequent its hydrolysis reac­tion in uniform scCO2 phase, resulting in the formation of densely andchemically modified NPs without the aggregation. These findings areimportant and useful to design surface modified NPs using the hydro­
lysis reaction in scCO2, contributing to the development of green
chemistry approach based on the utilization of water and CO2.

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CRediT authorship contribution statement
Yasuhiko Orita: Conceptualization, Methodology, Investigation,
Resources, Writing – original draft, Visualization, Supervision, Project
administration, Funding acquisition. Keito Kariya: Methodology, Vali­
dation, Investigation, Visualization. Thossaporn Wijakmatee: Meth­odology, Investigation. Yusuke Shimoyama: Resources, Writing –review & editing, Project administration.
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.
Data availability
The data that has been used is confidential.
Acknowledgements
This work was supported by the Japan Society for the Promotion of
Science, Grant-in-Aid for Early-Career Scientists JP21K14454. This
work was supported by Futaba Foundation (The Futaba Research Grant
Program) in Japan. This work was supported by Shorai Foundation for
Science and Technology in Japan. This work was supported by
Tokuyama Science Foundation in Japan.
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