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. 2025 Aug 25;10(35):40248–40257. doi: 10.1021/acsomega.5c05381

Rapid Preparation of Liquid Photonic Crystals

Qiang Zhao 1, Xiaoyu Bai 1, Yun Zhang 1, Tongtong Cao 1, Yuhua Cao 1,*
PMCID: PMC12423804  PMID: 40949181

Abstract

This study utilized a colloidal particle self-assembly method to rapidly prepare liquid photonic crystals (LPCs) stably dispersed in water. The submicrospheres with a diameter of 300–500 nm were synthesized by soap-free emulsion copolymerization of styrene (St) and 2-hydroxyethyl methacrylate (HEMA) and characterized by transmission electron microscopy and X-ray photoelectron microscopy. With the content of monomer HEMA increasing, the size and the hydration shell thickness of P­(St-HEMA) (PSH) submicrospheres also increased. The assembly mechanism of liquid crystals was explored by varying the volume fraction, ionic strength, solvent, and temperature. The results indicated that the ordered structure began to form at a low PSH volume fraction of 5% in water. As the ionic strength increased, the reflection wavelength was gradually redshifted, and the reflection intensity decreased. In addition, the addition of polar solvents and elevated temperature could reduce the strength of hydrogen bonding to destroy the ordered structure. It revealed that PSH particles could form long-range ordered structures in water through hydrogen bonding attraction and long-range electrostatic forces, which displayed bright colors. The LPC W/O emulsions and creams with high color saturation were prepared by simple emulsification, and the high-speed shear did not destroy the arrangement of liquid crystals, which laid the foundation for their further direct application in cosmetics.

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1. Introduction

Colloidal photonic crystals have a bionically artificial opal structure, composed of monodispersed colloidal particles with diameters around 100–500 nm. These particles form a periodically arranged three-dimensional structure under gravitational or capillary forces, belonging to three-dimensional photonic crystals. − Due to the unique optical properties, the colloidal photonic crystals have been widely used in outdoor displays, , coatings, colorimetric sensors, and solar energy utilization. , During the preparation of colloidal photonic crystals, the researchers have identified a metastable intermediate state wherein the ordered structure of colloidal particles is encapsulated by the liquid and coexists with the colloidal solution, known as liquid colloidal crystals (LCCs). LCCs consist of a crystalline phase, endowing them with unique properties, such as fluidity, reversible assembly and disassembly, and metastability. These special properties enable LCCs to respond more sensitively to external stimuli and garner significant attention by the researchers.

A traditional method for the synthesis of liquid colloidal PCs was static settling for a long time by gravity and the spontaneous precipitation of polystyrene (PS) particles from their aqueous solution. Yang et al. dispersed SiO2 colloidal particles in the mixed solvents with different boiling points and then selectively evaporated the low-boiling-point solvent at a high temperature to concentrate the colloidal particles. Thereby, LCCs were formed in a supersaturated solution. The method not only avoids the aggregation of colloidal particles caused by the external forces but also promotes the effective dispersion of colloidal particles in high-boiling-point solvents, facilitates the concentration of dilute colloidal solutions, and achieves efficient preparation of LCCs. This approach allows for the preparation of LCCs in solvents with higher viscosity. Wang et al. employed liquid–liquid extraction at room temperature to selectively remove the solvent from the colloidal solution. The colloidal solution can be effectively concentrated without the agglomeration of colloidal particles and finally spontaneously precipitates to form liquid colloid, to further prepare LCCs. A key factor for extraction-based preparation is that the solvent in the colloidal solution must be sparingly soluble in the extractant, while the colloidal particles should not disperse in the extractant. This method can be applied to the preparation of LCCs with various colloidal particles. The Shao group − rapidly prepared LCCs through rotary evaporation and addition of suitable surfactants to achieve an ordered and stable arrangement of the particles. Precrystallized LPCs were obtained with good reversibility and high color saturation. However, it took a long time to set the setting. The volatilization leads to the waste of the low-boiling solvent, and a high boiling solvent is required, which limited the application of LCCs. Furthermore, the rotary evaporation method causes particle agglomeration; therefore, it is needed to add the surfactant to stabilize the dispersion. These methods greatly limit the wider application of LCCs.

The aforementioned methods to prepare LCCs concentrate on the concentration of colloidal particles in the solution through prolonged standing, evaporation, or rotary evaporation to achieve supersaturated solutions. In the present work, LCCs with good monodispersity and high stability in water at a relatively low concentration were rapidly prepared by colloidal particle self-assembly. When the particles were within an appropriate size range, LPCs exhibited strong Bragg diffraction, resulting in vibrant and bright structural colors. The LCCs formed long-range ordered arrangements in water through short-range hydrogen bonding attraction and long-range electrostatic repulsion between the adjacent particles within the crystals. By comparing the crystalline states of the nanospheres at different volume fractions, the assembly mechanism of submicrospheres in the colloid with low volume fraction was investigated. Additionally, the W/O emulsions where the inner phase was LCCs with high color saturation were quickly prepared by simple emulsification. It can be directly applied to cosmetics as a new pigment, which provides an effective approach to the simple preparation of structurally colored photonic crystals.

2. Experimental Section

2.1. Materials

Styrene (St) and potassium persulfate (KPS) were purchased from National Medicines Corporation Ltd., China. 2-Hydroxethyl methacrylate (HEMA) was purchased from J&K Scientific, China. Palmitic acid ethyl acetate was purchased from Linyi Lusen Chemical Technology Co., Ltd., China. KF-6038 was purchased from Shin-etsu Chemical Industry Co., Ltd., Japan. Carbon black (CB) (BC 1408) was obtained from Tianjin Baochi Chemical Technology Co., Ltd., China. Beeswax was obtained from Suzhou GreenLeaf Daily Commodity Co., Ltd., China. All chemical reagents were used without further purification. Ultrapure water (18.25 MΩ cm) came from the laboratory.

2.2. Preparation of Monodispersed PSH Submicrospheres

The highly charged PSH submicrospheres were prepared by soap-free emulsion polymerization. St was filtered through an alkaline alumina column to remove the inhibitor before use. 15 g of pretreated St, 0.4429 g of HEMA, and 100 mL of ultrapure water were added to a 250 mL four-port flask. After flushing N2 for 1 h, the solution was heated to 70 °C, and 5 mL of KPS (0.2 mmol) was added quickly. Then, it was mechanically stirred in a N2 atmosphere at 350 rpm for 24 h, and PSH particle dispersion was obtained. The obtained dispersion was repeatedly centrifuged and washed with water six times. PSH submicrospheres prepared with a molar ratio of HEMA to St of 1:40 were denoted by 1:40. Repeating the above steps and varying the molar amount of HEMA, a series of PSH submicrospheres were prepared and denoted by 1:x.

2.3. Preparation of Liquid Colloidal Crystals

The prepared PSH suspension was centrifuged at 10,000 rpm for 30 min. The supernatant was removed to obtain semiwet PSH submicrospheres, and its solid content was measured by a drying-weighing method. Afterward, the appropriate amounts of water and carbon black were added and ultrasonicated for 1 h to obtain LCCs with a required volume fraction.

2.4. Preparation of Photonic Crystal W/O Emulsions

0.5–5 wt % CB was added to a 30 vol % PSH submicrosphere dispersion and mixed well. 1 g of this mixture as the aqueous phase was dispersed into 3.95 g of palmitic acid ethyl acetate as the oil phase and 0.05 g of KF-6038 as the emulsifier. Then, the mixture was emulsified by a homogenizer (XHF-DY) at 2800 rpm for 30 s to obtain the photonic crystal W/O emulsion.

2.5. Preparation of Photonic Crystal Cream

W/O cream was prepared using 2.65 g of palmitic acid ethyl acetate and 1.3 g of molten beeswax as the oil phase, and the other conditions were the same as the preparation of W/O emulsion (Section ).

2.6. Characterization

The morphology of submicrospheres was characterized with scanning electron microscopy (SEM) (SU1510, Hitachi, Japan) at an accelerating voltage of 3.0 kV. The microstructure of the submicrospheres was observed using transmission electron microscopy (TEM) (JEM-2100, Japan Electronics, Japan) under 200 kV accelerating voltage. X-ray photoelectron spectroscopy (XPS) was performed on an Axis Supra X-ray photoelectron spectrometer (AXIS Supra, Kratos, England) at a power of 150 W. A WQF-600N (Beijing Rayleigh Analytical Instrument Co., Ltd., China) Fourier transform infrared (FT-IR) spectrometer was employed to obtain the FT-IR spectrum. The reflectance spectrum was measured by using an FLA5000+ optic spectrometer (Flight Technology Co., Ltd., China). A Nano Brook Zeta PALS (Brookhaven) nanoparticle sizer was employed to test the size of the hydration particle. The photographs were taken by an EOS Kiss X4 digital camera (Cannon).

3. Results and Discussion

3.1. The Effect of HEMA Content on PSH

The amount of the monomer HEMA is an important parameter in the soap-free emulsion polymerization to synthesize PSH submicrospheres. The hydrophilic monomer HEMA was introduced into the polymerization system, and the rate of the polymerization reaction was improved with increasing HEMA concentration in the aqueous phase. On the other hand, HEMA tended to act as an emulsifier at the interface between polymer latex particles and the water phase. Under the premise of fixed St and KPS dosage, the concentration of HEMA in the polymerization system was varied to investigate its effect on the size and monodispersity of PSH submicrospheres. As shown in Figure a, the size of PSH submicrospheres initially decreased and then increased with the concentration of HEMA increased. The hydrophobic St and hydrophilic HEMA copolymerized according to the “homogeneous precipitation nucleation” mechanism. , Thus, increasing the HEMA concentration could lead to an increase in the number of particles formed by homogeneous nucleation. As the number of particles increased, the swelling of the particles decreased because of the St amount fixed, so that the particle size of PSH submicrospheres gradually decreased. However, the shell thickness of the particles could increase with HEMA increased, resulting in the larger particle sizes and roughness of the surface (Figure c,d). Additionally, the polydispersity index (PDI) of PSH submicrospheres was less than or close to 0.08, indicating that PSH submicrospheres had good monodispersity. With the increasing HEMA concentration, the synthesized PSH exhibited a higher surface charge and its hydration layer was gradually increased (Table S1), which was related to the hydrogen bonds between HEMA and water molecules.

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Effect of the HEMA amount on the size and PDI of PSH submicrospheres (a) and the SEM images of PSH submicrospheres with monomer ratios of HEMA to St at 0 (b), 1:200 (c), 1:100 (d), and 1:10 (e), with the amount of St and KPS fixed at 15 g of St and 0.2 mmol of KPS.

3.2. Morphology of PSH Submicrospheres

The morphology of PSH submicrospheres was observed using TEM, and the results are shown in Figure and Figure S1. It could be seen that the surface of PS submicrospheres was smooth and the particle size was relatively large (500 nm) (Figure S1a,b). PSH submicrospheres with different monomer ratios exhibited good sphericity and uniform size, which were necessary to assemble PCs with an excellent structural color. As the amount of HEMA increased, the surface of PSH submicrospheres gradually became rougher because of the phase separation between PS and PHEMA (Figure a–c and Figure S1c–h). Therefore, during the polymerization reaction, PHEMA tended to locate on the surface of the submicrospheres, and the polar hydroxyl groups were introduced onto the surface of PSH submicrospheres. Due to the stronger hydrophobicity of PS, it tended to form the core of the submicrospheres in aqueous environments. PS and PSH particles were pressed into thin sheets, and a PHEMA film was obtained by photopolymerization with HEMA as the monomer. After water droplets dripped onto their surface for 10 s, the water contact angle was measured. As shown in Figure S2, the water contact angles of PS, PSH, and PHEMA were 72, 35, and 30°, respectively. The surface water contact angles of PSH and PHEMA were very close and much smaller than that of PS, which indicated that the surface of PSH was mainly occupied with PHEMA. Based on the above analysis, a schematic diagram of PSH submicrospheres was proposed, as shown in Figure d.

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TEM images of PSH (a) 1:200, (b) 1:40, and (c) 1:10 and (d) structural diagram of PSH submicrospheres.

The characteristic functional groups of PSH submicrospheres were further characterized by an infrared spectrometer, and the results are shown in Figure a. Among them, the characteristic peaks of the benzene ring at 1492 and 1452 cm–1 and the stretching vibration absorption peaks of −CH at 3026 cm–1 indicated the existence of the benzene ring in polystyrene. The strong peaks at 1739 and 3476 cm–1 were attributed to the stretching vibration absorption peaks of the carbonyl group (CO) and hydroxyl group (O–H) in HEMA. The elemental composition analysis of the PSH submicrospheres on the surface was conducted using XPS. The full-scan spectrum in Figure b showed that the surface elements of the prepared PS and PSH submicrospheres comprised carbon and oxygen. The C 1s spectrum of PSH in Figure c clearly displayed three peaks with binding energies at 284.8, 286.0, and 289.2 eV, corresponding to C–C, C–O, and CO, respectively. Meanwhile, a π–π* satellite with a binding energy of 291.4 eV appeared in the C 1s spectrum of PS in Figure d, which was attributed to benzene. However, a π–π* satellite did not exist in the C 1s spectrum of PSH, which indicated that the surface of PSH was occupied by PHEMA, and PS were difficult to be detected. The O 1s spectrum (Figure d) of PSH showed the peaks at 532.6 and 533.7 eV, attributed to CO and C–O of PHEMA, respectively, and the peak area of C–O was much larger than that of CO because the number of C–O was twice as much as that of CO in the HEMA molecule. Since it was inevitable that the impurities containing oxygen were introduced during soap-free emulsion polymerization of PS, the O 1s spectrum of PS had two peaks corresponding to CO and C–O. However, the oxygen content in PSH was significantly higher than that in PS. Additionally, with the increase in HEMA content, the ratios of the O 1s/C 1s in the PSH with the molar ratios of HEMA to St as 1:40, 1:20, and 1:10 were 0.1, 0.14, and 0.16, respectively. They are higher than the O 1s/C 1s of 0.05 in the PS sample (Table S2). Meanwhile, the experimental data are much higher than the theoretical calculated data. It was another proof that HEMA was located on the surface of PSH submicrospheres. The analysis of FT-IR and XPS showed that PSH submicrospheres were prepared by copolymerization of HEMA and St monomers.

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FT-IR spectra (a) and XPS spectra of PS and PSH submicrospheres: full-scan (b), C 1s spectrum (c), and O 1s spectrum (d).

3.3. Self-Assembly of Liquid Photonic Crystals

The synthesized PSH colloidal submicrospheres with different volume fractions were centrifuged and ultrasonically dispersed in water to achieve LPCs. The preparation process of liquid crystals is shown in Figure . As the volume fraction of PSH submicrospheres exceeded a certain critical value, the colloidal particles began to precipitate − and formed an ordered arrangement. After being diluted with water to a low concentration dispersion, the particles could still form an ordered arrangement. It means that the equilibrium between the electrostatic repulsion and hydrogen bond attraction drives the submicrospheres to form LPCs.

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Preparation process of liquid crystals.

In colloidal solutions, the crystallization behavior of colloidal units is closely related to their volume fractions in the system. As the volume fraction of colloidal units in the system gradually increases, the colloidal system undergoes a phase transition from amorphous to partially crystalline and finally to fully crystalline. As shown in Figure , the PSH (1:40) colloidal system with a submicrosphere diameter of 346 nm was viewed as an example. When the volume fraction of PSH (φPSH) increased to 5% (Figure b), a weak reflection peak began to appear, which indicated that the dispersion started to transition from a disordered state to an ordered structure at this volume fraction. As the volume fraction continued to increase, the reflection wavelength of the PSH colloidal system gradually blueshifted. The main reason for this was that the charged colloidal particles in the system increased as the volume fraction increased. The long-range electrostatic repulsion formed with the particles increased, and the hydrogen bonding between the particles also increased, resulting in a decrease in particle spacing and the blueshift of the reflection peak. As the ordering of the particle arrangement increased, the reflection intensity increased. ,, When φPSH exceeded 30%, the increase in particle quantity led the colloidal system to transition from the amorphous state to partially crystalline, resulting in a decrease in the average spacing of the colloidal crystal, further blueshifting the reflection peak. When φPSH increased to 40%, beyond its critical volume fraction in a supersaturated state, the average interparticle distance in the colloidal system decreased and the particles began to precipitate and assemble into an ordered arrangement. These ordered colloidal crystals were primarily stabilized by the balance of the long-range electrostatic repulsion, the short-range van der Waals attraction, and the hydrogen bonding between neighboring highly electronegative microspheres. − Additionally, the reflection wavelength and intensity remained unchanged in the process of the mechanical perturbation of PSH liquid photonic crystals. It indicated that the photonic crystal structure was not affected by the mechanical agitation and could exist stably in water, and PSH 1:20 and PSH 1:100 exhibited similar properties. However, the particles with different amounts of HEMA required different volume fractions to initiate the formation of ordered arrangements. For example, the high content of HEMA could form an ordered arrangement (Figure b and Figure S3b) at a lower volume fraction of 5%, while the submicrospheres with a low content of HEMA required a higher volume fraction of 10% (Figure S4c). Figure h,i shows the strongest reflection intensity in the visible-light region at different volume fractions. When the volume fraction of PSH (1:40) was 30%, the reflection intensity was the highest. PSH with diameter of 346 nm was air-dried to form solid photonic crystals (SPCs), in which the reflection wavelengths were measured. It was found that the first-order reflection peak was at 849 nm, which was in the infrared region. The second reflection peak was at 445 nm, which was in the visible-light region with a blue-green color. The bandgaps of LPCs were all greater than those of the corresponding SPCs. This significant difference mainly results from the different lattice parameters between SPCs and LPCs. According to the Bragg diffraction law, the calculated lattice constants of LPCs, interparticle distances (d L) inside the crystalline regions, were larger than the close-packing SPCs (D S), which are shown in Figure S5 and Table S3. It indicated that the LPCs were nonclose-packing. Under the different diffraction orders at fixed volume fraction, the calculated LPC interparticle distances were very close to each other, and all were greater than the particle spacing of the SPCs, which indicated that LPCs were noncompactly filled. The photos of PSH (1:40) LPCs with different volume fractions are shown in Figure S6. When the volume fraction was less than 20%, the color could not be seen owing to the milky liquid, but at more than 20%, the color was shown because of the high reflection strength.

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Reflection spectra (a–g), reflection peak wavelength (h), and strongest intensity in the visible region (i) of PSH LPCs with different volume fractions.

The liquid photonic crystal was frozen by freeze-drying to fix its structure arrangement. The arrangement structure was observed and characterized. As shown in Figure , when φPSH was 1%, the colloidal submicrospheres showed a completely disordered structure. When φPSH increased to 20%, the submicrospheres in the system crystallized and form an orderly arrangement. Finally, when φPSH increased to 30%, the colloidal submicrospheres formed a highly ordered arrangement structure. This was highly in accord with the optical properties presented.

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SEM images of PSH LPCs with different volume fractions of 1% (a), 20% (b), and 30% (c) after freeze-drying treatment.

In a colloidal dispersion, the aggregation and dispersion of charged colloidal nanoparticles and their interactions can be explained by the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. , According to the DLVO theory, the interactions between the nanoparticles in a colloidal system are primarily due to van der Waals attraction and the electrostatic repulsion caused by the electrical double layer of charged surfaces. The self-assembly of PSH particles into a long-range ordered photonic crystal structure may be attributed to the stable dispersion of PSH particles in water. The stability of the PSH particles depends on the thickness of the electric double layer, which is determined by the Debye length of the solvent. The high ionic strength leads to a small Debye length and weakens the electric double layer between the PSH particles. Therefore, the thickness of the PSH electric double layer can be adjusted by varying the ionic strength of the PSH submicrospheres in water. In this study, NaCl was used to increase the ionic strength in water and reduce the thickness of the PSH double layer. The effect of the ion strength on the formation of liquid colloidal crystals was investigated. As shown in Figure a, with the increase in NaCl concentration, the reflection wavelength gradually redshifted and the intensity was decreased gradually. This was contradictory to a common understanding. It was thought that an increase in ion strength shields the electrostatic repulsion, which leads to a blueshift in the reflection wavelength. It revealed that the long-range electrostatic forces as one of the main driving forces decreased, leading to the distance between the particles increasing and the reflection wavelength redshifted. When the NaCl concentration exceeded 1.0 × 10–7 mol/L, the long-range electrostatic forces were completely disrupted, so that the particle arrangement became disordered and the intensity was near to zero. When the ionic strength was increased further, the thickness of the PSH double layer decreased, and the electrostatic repulsion was so small that the coagulation among the particles happened. Hydrogen bonding plays a crucial role in self-assembly of LPCs. By changing solvent polarity or elevating the temperature, the hydrogen bonding can be destroyed; thereby, the ordering arrangement is disturbed. To investigate the effect of hydrogen bonding on the formation of liquid colloidal crystal arrays, a certain amount of propylene glycol was added to the PSH aqueous dispersion, or the dispersion was heated to break hydrogen bonds. As the amount of propylene glycol increased (Figure b), the hydrogen bonding between the particles and water molecules decreased, and the hydration layer became thinner. The surface potentials of the particles were also influenced by the permittivity of the dispersion medium. In Figure b, with an increase in propanediol in water, the permittivity decreased and the surface potentials of the particles also lowered. The distance between the particles decreased, and the reflection wavelength blueshifted under the action of long-range electrostatic force. When the volume percent of polar solvent was 40%, the hydrogen bond force was minimal, and the ordered arrangement was disturbed. In addition, the increase in polar solvent will increase the average effective refractive index (n propylene glycol larger than n water), which will also cause the redshift of the reflection peak wavelength. Eventually, the complete loss of hydrogen bonding resulted in an imbalance of the forces, causing the liquid crystal to become disordered and the reflection peak to disappear (Figure b and Figure S7). The elevated temperature can also break hydrogen bonds, thereby affecting the assembly of the liquid crystal (Figure c). With increasing temperature, the reflection wavelength of liquid crystals showed a trend of a redshift first and then a blueshift. When the temperature rose to 60 °C, the reflected wavelength redshifted and the intensity decreased. It showed that the hydrogen bond attraction between the particles was gradually diminished, so that the order degree of liquid crystals became worse. When the temperature was more than 60 °C, due to the evaporation of water, the volume fraction of the particles increased and the distance between the particles in the dispersion decreased, causing a blueshift in the reflection wavelength. The evaporation of water in the liquid crystal led to close packing of the particles, so the reflection intensity increased and the color became brighter. The temperature of 60 °C was the cutoff temperature at which liquid crystals became disordered, so the intensity was lowest, but the position of the reflection peak was still redshifted. At 70 °C, the particles began to precipitate from the solution and gradually accumulated in order, so the strength increased (Figure c inset). As the temperature enhanced further, the number of accumulated layers was greater, and the degree of order would decrease. So, the intensity of the reflection reduced. The color of the liquid crystal at different temperatures is shown in Figure S8.

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Effect of ion concentration (a), solvent (b), and temperature (c) on LPC assembly.

3.4. LPC W/O Emulsions

Due to the highly precrystalline structure and excellent optical effects of LPCs, they can be directly used as the pigments for the preparation of W/O emulsions. The PSH aqueous dispersion with φ30% was used as the inner water phase, and W/O emulsion was rapidly prepared according to the procedure in Section . Figure a shows that there were 10–100 μm water droplets in the W/O emulsion, and these droplets had the structural color. It confirmed that LPCs formed by self-assembly of PSH in the water system. Because the LPCs were very stable, the high-speed shearing during the emulsification process did not destroy the ordered structure in water droplets. Owing to scattering by the droplet at the tens of micrometers level, the reflection light by the photonic crystals would be dim. An appropriate amount of carbon black (CB) as the dulling material was added to the PSH system to absorb the stray light (Figure b–d). With the increase in the amount of CB, the color of the water droplets gradually became green. The color saturation increased, and it was more and more bright. Additionally, due to the long-range order of the LPCs within the water droplets, the structural colors can change with the observation angle and exhibit a distinct iridescent effect. The photo of the PC W/O emulsion where the φPSH 30% aqueous dispersion contains 3 wt % CB as the inner water phase is shown in Figure S9. The LPC W/O cream using molten beeswax as the oil to solidify the water droplets at room temperature was obtained, as shown in Figure . The blue, green, and fuchsia PC creams were obtained using PSH submicrospheres with the diameters of 307, 346, and 400 nm, respectively. The LPC creams exhibited bright colors against a black background, and they also presented the structural color against a light-colored background, such as on the human arm. It was due to the fact that the doping of CB suppresses the incoherent scattered light and improves the chromaticity of the LPCs. The PC creams could be used directly as cosmetic products. This presents the promising prospects for further application in cosmetics.

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Microscope photos of LPC W/O emulsion where PSH (φPSH 30%) aqueous dispersion added different masses of carbon black (a–d) 0, 1, 3, and 5 wt % as the water phase, palmitic acid ethyl acetate as the oil phase, the mass ratio of water to oil phase at 1:4, and 1 wt % KF-6038 as the emulsifier after emulsifying at 2800 rpm for 0.5 min.

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Photos of blue, green, and fuchsia LPC creams using PSH submicrospheres with the diameters of 307, 346, and 400 nm, respectively (a); these creams against a black background (b) and on a human arm (c). The oil phase contained 33.3 wt % beeswax and 66.7 wt % palmitic acid ethyl acetate, and the water phase contained 3 wt % CB; the other conditions were the same as in Figure .

4. Conclusions

In this paper, the highly charged PSH submicrospheres were prepared by soap-free emulsion polymerization. By adjusting the amount of HEMA, the particle diameter of PSH submicrospheres was controlled, thereby affecting the hydration shell thickness of the particles in water and subsequently influencing the formation of hydrogen bonds in water. The results indicated that PSH could form an ordered arrangement in water at a lower volume fraction through hydrogen bonds and a long-range electrostatic force. With the increase in ionic strength in solution, the thickness of the electric double layer of PSH was reduced so that LCCS changed from an ordered to a disordered state. By changing the solvent or enhancing the temperature, the hydrogen bonds between the particles could be disrupted to destroy the orderly arrangement of the particles. The LPC W/O emulsions with high color saturation were rapidly fabricated by a simple emulsification process. The arrangement of liquid crystals was not destroyed by high-speed shear. The addition of the extinction material CB could reduce light scattering, making the color of the emulsion droplets brighter and showing an obvious iridescent effect. It lays the foundation for its further direct application in cosmetics.

Supplementary Material

ao5c05381_si_001.pdf (788.6KB, pdf)

Acknowledgments

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data will be made available on request.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c05381.

  • Particle diameter, PDI, hydrated particle diameter, hydration layer thickness, and zeta potential of PSH (Table S1); atomic concentrations of C and O and O/C ratio in PS and PSH by XPS characterization (Table S2); calculated lattice constant of the PSH LPCs (Table S3); TEM images of PS and PSH (Figure S1); water contact angles of PS, PSH, and PHEMA (Figure S2); reflection spectrum, reflection peak wavelength, and strongest intensity in the visible region of PSH (1:20) and (1:100) LPCs (Figures S3 and S4); reflection spectrum of PSH (1:40) SPCs (Figure S5); photos of LPCs with different volume fractions of PSH (1:40) (Figure S6); effect of glycerol on PSH LPCs (Figure S7); photographs of PSH aqueous dispersion at different temperatures (Figure S8); photo of the LPC W/O emulsion (Figure S9) (PDF)

The authors declare no competing financial interest.

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Associated Data

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Supplementary Materials

ao5c05381_si_001.pdf (788.6KB, pdf)

Data Availability Statement

Data will be made available on request.

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