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. 2026 Feb 23;11(9):15103–15114. doi: 10.1021/acsomega.5c12204

Tailoring the Stability of W/O/W Liquid Foundations: A Comparative Study of Polyglycerol-Based W/O Emulsifiers

Chunfang Zhu †, Lanlan Zeng ‡, Zhenming Xie †,*, Gaodan Luo †,*, Lai-Hon Chung †, Xilong Tang ‡, Yancheng Tang ‡
PMCID: PMC12980205  PMID: 41835609

Abstract

Water-in-oil-in-water (W/O/W) multiple emulsions are thermodynamically unstable systems characterized by high interfacial free energy and complex thermodynamic driving force promoting internal droplet coalescence or Laplace pressure-driven water migration. In color cosmetics such as liquid foundations, this intrinsic instability is further exacerbated by the incorporation of inorganic pigments (e.g., titanium dioxide (TiO2) and iron oxides), which perturb the delicate interfacial equilibrium and trigger premature phase separation. Utilizing polyglyceryl-10 stearate (PG10S) as a fixed oil-in-water (O/W) emulsifier, this study systematically investigated the stability of these challenging systems by assessing four polyglycerol-based water-in-oil (W/O) emulsifiers with distinct molecular architectures: polyglyceryl-3 polydimethylsiloxyethyl dimethicone (KF-6106), polyglyceryl-6 polyricinoleate (PR-15), polyglyceryl-3 polyricinoleate (PG3PR), and polyglyceryl-2 dipolyhydroxystearate (PGPH). Through a combination of confocal laser scanning microscopy (CLSM), rheometry, differential scanning calorimetry (DSC), surface/interfacial tension analysis, and accelerated stability testing, the structure–property relationships governing the integrity of the multiple-layered structure under both intrinsic and pigment-induced stresses were elucidated. The results demonstrate that the silicone-modified emulsifier (KF-6106) exhibited superior stabilizing efficacy, achieving a high thixotropic recovery of 91.16%. This performance is attributed to its ability to form a resilient, “self-healing” interfacial film with high segmental mobility, which effectively suppresses the spontaneous merging of internal droplets and maintains a robust barrier against the destabilizing effects of pigment particles. These findings establish a mechanistic framework for optimizing emulsifier selection and provide theoretical guidance for the rational design of stable, high-performance W/O/W multiple emulsions in complex, particle-filled systems.

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1. Introduction

Water-in-oil-in-water (W/O/W) emulsions are highly attractive for cosmetics, food, and pharmaceutical applications due to their capacity to coencapsulate hydrophilic and hydrophobic active ingredients while delivering distinctive sensory properties. − In color cosmetics, particularly liquid foundations, the W/O/W architecture is poised to combine the water resistance of W/O systems with the lightweight skin feel of O/W systems, enabling the development of high-performance, long-wear formulations. However, practical implementation necessitates the robust dispersion of pigments and the attenuation of inherent thermodynamic instabilities (notably Ostwald ripening and droplet coalescence), which currently constrain commercial viability. Consequently, achieving long-term stability without compromising sensorial performance remains a central challenge for the adoption of W/O/W emulsions in foundation formulations.

Emulsifier selection is a critical determinant of W/O/W emulsion stability. Emulsifiers adsorb at oil–water interfaces to form interfacial films that provide a mechanical barrier against droplet coalescence. The stability of W/O/W systems is predicated on the integrity of two distinct interfacesthe primary W/O interface and the secondary O/W interfacerequiring synergistic integration of lipophilic and hydrophilic emulsifiers. Although the influence of emulsifier molecular structure on W/O/W stability has been extensively documented in simplified systems, − systematic investigations addressing complex pigmented systemssuch as foundations containing significant loads of titanium dioxide (TiO2) and iron oxidesremain limited.

Recent literature has significantly advanced mechanistic understanding of W/O/W systems. For instance, studies have demonstrated prolonged stability in pigment-free models for the delivery of active ingredients, − while microfluidic techniques have been employed to clarify the role of specific stabilizers, such as polyglycerol polyricinoleate, in food-grade multiple emulsions. Extensive work on particlefree systemsranging from small-molecule emulsifiers to protein-based emulsifiersattributes stability variations to interfacial layer thickness, molecular packing, and bulk viscosity. − While these contributions are valuable, they primarily concern particle-free models or disparate application contexts, and their direct applicability to pigment-loaded cosmetic systems is constrained. Solid pigment particles introduce additional complexity: depending on particle size, surface treatment, and interfacial affinity, pigments may either reinforce interfaces via Pickering-type stabilization or induce destabilization by puncturing, thinning, or displacing adsorbed emulsifier layers. , Analogous particle-interface effects have been reported in other complex fluids. , Because pigment chemistry and surface treatment profoundly influence particle–interface behavior, this study maintains a fixed pigment identity and loading to isolate the specific effects of W/O emulsifier architecture on W/O/W stability.

To address the interfacial challenges posed by solid pigments, emulsifiers capable of forming robust yet flexible interfacial films are of particular relevance. We focus on polyglycerol-based emulsifiers, which are favored for cosmetic applications due to their tunable hydrophilic–lipophilic balance (HLB), excellent biocompatibility, and ability to form resilient interfacial films. , For example, previous research showed that the substitution of stearic acid with oleic acid in polyglyceryl-4 esters markedly enhanced W/O emulsion stability under centrifugal stress, highlighting the role of fatty-acid unsaturation in modulating interfacial flexibility. Another study reported that increasing the polyglycerol backbone length from tetraglycerol to decaglycerol raised the HLB, lowered interfacial tension, and promoted lamellar liquid-crystalline phases that enhance resistance to coalescence. These findings collectively illustrate how structural variations in polyglycerol-based emulsifierssuch as fatty acid unsaturation and backbone lengthmodulate interfacial properties critical for W/O/W stability. Building on these insights, these principles were evaluated within a representative, pigmented W/O/W foundation model containing functional levels of common cosmetic pigments.

Herein, we report a systematic comparison of four commercially available polyglycerol-based W/O emulsifiers (KF-6106, PR15, PG3PR, PGPH), utilizing a constant outer O/W emulsifier (PG10S) in a pigmented W/O/W foundation formulation. The inorganic pigments, comprising TiO2 (average primary particle size ∼ 300 nm) and iron oxides (yellow, red, and black), were surface-treated with isopropyl titanium triisostearate (ITT) to enhance hydrophobicity and dispersion stability within the oily phase. These pigments function as insoluble solid particulates localized within the intermediate oil layer of the W/O/W structure. By fixing pigment identity and loading, we control a major source of variability, thereby focusing on how distinct polyglycerol architectures influence W/O/W robustness. Using a multifaceted characterization approachcombining microstructural imaging (CLSM), interfacial thermodynamics (surface/interfacial tension and DSC), and macroscopic stability assessments (droplet-size evolution, accelerated aging, and rheological profiling)we aim to correlate emulsifier molecular structure with practical stability metrics. This work establishes an initial structure–property framework for polyglycerol-based W/O emulsifiers in complex, pigmented W/O/W systems, providing evidence-based criteria for emulsifier selection and advancing the rational design of high-performance W/O/W color cosmetics.

2. Experimental Section

2.1. Materials

Dipropylene glycol (DPG LO+) and triethanolamine (Dow TEA) were supplied by Dow Chemical Co. (USA). Cyclopentasiloxane and PEG/PPG-18/18 dimethicone (Dow Corning 5225C) and polydimethylsiloxane (PMX-200) were obtained from Dow Corning (USA). Phenoxyethanol and ethylhexylglycerin (Euxyl PE 9010) was provided by Ashland (USA). The following polyglycerol-based emulsifiers were acquired from their respective suppliers: polyglyceryl-3 polydimethylsiloxyethyl dimethicone (KF-6106) from Shin-Etsu Finetech (Japan); polyglyceryl-6 polyricinoleate (Hexaglyn PR-15) from Nikko Chemicals (Japan); polyglyceryl-3 polyricinoleate (Cithrol PG3PR) and polyglyceryl-3 diisostearate (Cithrol PG3IS) from Croda (UK); polyglyceryl-2 dipolyhydroxystearate (Dehymuls PGPH) and polyglyceryl-10 stearate (PG10S; Emulgade Verde 10 MS) from BASF (Germany). Diethylhexyl carbonate (Tegosoft DEC) was sourced from Evonik (Germany). Acrylates/C10–30 alkyl acrylate crosspolymer (Pemulen TR-2) was provided by Lubrizol (USA). Propylene carbonate and disteardimonium hectorite and isododecane (Bentone gel ISD V) was supplied by Elementis (UK). 3-octylheptamethyltrisiloxane (OHTS) and cyclopentasiloxane (D5) were obtained from Guangzhou Crystalline Silicon New Materials (China). The surface-treated inorganic pigments, including CI 77891 and isopropyl titanium triisostearate and dimethicone and Alumina (Ti-02 ITD), CI 77492 and isopropyl titanium triisostearate and dimethicone (RP-29 ITD), CI 77491 and isopropyl titanium triisostearate and dimethicone (YP-75 ITD), and CI 77499 and isopropyl titanium triisostearate and dimethicone (BP-50 ITD), were supplied by Shanghai Co-Fun Biotech Corp., Ltd. (China). Magnesium sulfate (MgSO4, AR grade, CAS 7487–88–9) was purchased from Tianjin Yongda Chemical Reagents (China). Methylparaben (MP) was obtained from Huaxin Daily Chemicals (China). Purified water was prepared in-house using a laboratory-grade water purification system. All materials were used as received without further purification.

2.2. Preparation of W/O/W Liquid Foundations

The W/O/W foundation liquid was prepared using a two-step emulsification process, adapted from the method reported by Fu et al. with minor modifications. In the first step, the internal aqueous phase (W1) and the oil phase (O) were prepared separately by heating and mixing until homogeneity was achieved. After cooling the W1 phase to 45 °C, PE 9010 was incorporated. The W1 phase was then gradually introduced into the O phase under continuous stirring at 800 rpm for 10 min at room temperature, followed by high-shear homogenization at 10000 rpm for 3 min to form the primary W/O emulsion. In the second step, the external aqueous phase (W2) was prepared by heating and mixing its components until a uniform solution was obtained. TEA was added below 75 °C to neutralize the thickener, and PE 9010 was incorporated upon cooling to 45 °C. After the W2 phase reached room temperature, the preformed W/O emulsion was slowly added to it under mild agitation at 600 rpm. The mixture was stirred for 3 min and subsequently homogenized at 7000 rpm for 3 min to yield the final W/O/W foundation liquid. All process parameters were kept constant to ensure that variations in stability were uniquely attributable to the chemical structure of the W/O emulsifiers.

The corresponding formulations are detailed in Table

1. Formulations of the W/O/W Liquid Foundations .

Category Ingredient Content (wt %)
W1 Water 8.19
DPG 2.4
MgSO4 0.12
MP 0.045
PE 9010 0.135
O KF-6106/PR-15/PG3PR/PGPH 1.5
DEC 4.2
PMX-200 4.2
5225C Formulation Aid 1.3
OHTS 0.647
D5 0.522
PG32IS 0.028
ISD V 0.24
Ti-02 ITD 5.85
RP-29 ITD 0.468
YP-75 ITD 0.094
BP-50 ITD 0.059
W2 TR-2 0.15
PG10S 5
TEA 0.12
PE 9010 0.32
Water to 100
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a

Values are expressed as weight percentages (wt %); the total mass of each batch was 100 g.

2.3. Methods

2.3.1. Image Acquisition with CLSM

The microstructure of the W/O/W liquid foundation was visualized by CLSM following a previously reported protocol. The oil and aqueous phases were selectively stained with Nile Red and fluorescein isothiocyanate (FITC), respectively. Stock solutions (0.1 wt %) were prepared by dissolving the dyes in propylene glycol. An aliquot (10 μL) of each staining solution was added to 2 mL of the foundation, and the mixture was stirred at 300 rpm for 15 min at 25 °C in the dark to ensure uniform labeling. A 20-μL drop of the stained sample was placed on a glass-bottom dish, covered with a coverslip, and examined immediately. Imaging was performed using an LSM 800 microscope with Airyscan (Carl Zeiss, Jena, Germany) equipped with a 40× oil-immersion objective. Nile Red was excited at 561 nm and FITC at 488 nm.

2.3.2. Optical Microscopy and Particle Size Analysis

A drop of W/O/W foundation was placed on a glass slide, gently covered with a coverslip to avoid air bubbles, and examined under an optical microscope (Olympus CX31, Tokyo, Japan). Micrographs were captured at 1000× magnification and analyzed with ImageJ software. At least 200 droplets per sample were measured to determine the particle-size distribution. The mean Feret diameter (D) and its standard deviation (SD) were calculated, with SD serving as an index of droplet-size uniformity.

2.3.3. Physical Stability

Aliquots (8–10 mL) of the fresh emulsion were transferred to 15 mL graduated centrifuge tubes and centrifuged at 3000 rpm for 30 min at 25 °C. The volume of the separated phase was recorded immediately, and the creaming index (CI) was calculated as follows:

CI(%)=(V1V2)×100

where V 1 is the volume of the separated phase and V 2 is the initial total volume of the emulsion. The test was performed in triplicate.

2.3.4. Viscosity

After 24 h storage at 25 °C, the viscosity of each sample was determined using a Brookfield DV2TLV viscometer (Brookfield, Middleboro, USA) equipped with an LV-4 spindle at 30 rpm. A 60-s equilibration time was allowed before recording. Each sample was measured in triplicate, and the mean value (±SD) was reported in mPa·s.

2.3.5. Thermal Stability

Samples (20 g each) were sealed in 30 mL glass vials and stored at 45 ± 1 °C for 30 days. Following storage, the samples were equilibrated to 25 °C and examined macroscopically for signs of phase separation or coalescence.

2.3.6. Cold Stability

Equivalent aliquots were stored at −15 ± 1 °C for 30 days, thawed at 25 °C for 4 h, and subsequently inspected for demulsification or precipitation.

2.3.7. Storage Stability

The emulsions (20 g each) were stored in transparent, sealed bottles at 25 ± 2 °C for 30 days while protected from light. Color and phase homogeneity were evaluated visually.

2.3.8. Surface Tension and Interfacial Tension

The surface tension of the freshly prepared W/O emulsion was determined at 25 °C via the pendant-drop method using a Theta Lite optical tensiometer (Biolin Scientific, Gothenburg, Sweden). A ∼10 μL pendant drop was formed from a 200-μL disposable tip (Optifit, Sartorius, Germany) and imaged at 20 fps for 10 s. Drop profiles were fitted to the Young–Laplace equation to yield the average surface tension.

The interfacial tension between the oil phase (DEC/PMX-200, 1:1 w/w, containing the respective W/O emulsifier) and pure water was measured with a DCAT 21 tensiometer (Dataphysics, Filderstadt, Germany; resolution 0.01 mN·m– 1) using the Wilhelmy plate method. A square platinum foil (0.4 cm edge) was employed. The instrument was calibrated against pure water (71.7 mN·m– 1). Before each measurement, the plate was rinsed with ethanol and flame-cleaned; the immersion speed was 0.5 mm·s– 1 with a target accuracy of ± 0.03 mN·m– 1.

2.3.9. DSC Analysis

DSC analysis was performed on a DSC3 calorimeter (Mettler-Toledo, Greifensee, Switzerland) following a modified method of Ma et al. Briefly, 7–10 mg of the sample was hermetically sealed in an aluminum pan and cooled from 25 °C to −50 °C at 5 °C·min– 1 under a nitrogen purge (30 mL·min– 1).

2.3.10. Rheological Properties

All rheological measurements were conducted on an Anton Paar MCR 102e rheometer (Anton Paar, Graz, Austria) equipped with a Peltier temperature-control unit.

Flow curves: viscosity was recorded at 25 °C using a cone-plate geometry (CP25-1; 25 mm, 1°). The shear rate was increased logarithmically from 0.1 to 100 s– 1.

Amplitude sweeps: strain was varied from 0.01% to 100% at 10 rad·s– 1 (25 °C, parallel-plate PP25; 25 mm, 1 mm gap) to determine the linear viscoelastic (LVE) region. Storage (G′) and loss (G″) moduli were plotted against strain.

Thixotropic recovery: evaluated at 25 °C (PP25; 1 mm gap) by a three-step sequence within the LVE region: (i) 1 Hz, 0.1% strain, 3 min (initial complex viscosity, η0); (ii) 100 s– 1 constant shear, 3 min (structure breakdown); (iii) 1 Hz, 0.1% strain, 5 min (recovery, final viscosity ηt). The recovery ratio (%) was calculated as (ηt/η0) × 100.

Temperature sweeps: complex viscosity (η*) was measured within the LVE region (PP25; 1 mm gap, 1 Hz, 0.3% strain). Samples were cooled from 25 to 0 °C, heated to 50 °C, and cooled back to 25 °C at 4.17 °C·min– 1 to evaluate stability.

3. Results and Discussion

3.1. CLSM Characterization of W/O/W Foundation Structures

As illustrated in Figure , all four W/O emulsifiers successfully facilitated the formation of foundation formulations with a characteristic W/O/W architecture. However, distinct variations in droplet morphology and internal compartmentalization were observed. Foundations stabilized with emulsifiers A and C exhibited well-defined multiple emulsion droplets; the enlarged views revealed numerous internal water droplets densely encapsulated within the oil globules. In contrast, droplets stabilized by emulsifiers B and D showed irregular contours, fewer internal compartments, and localized pigment aggregation. These morphological defects suggest the formation of less cohesive interfacial films that are susceptible to deformation and rupture under mechanical stress or osmotic pressure.

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CLSM images of W/O/W liquid foundations prepared with four W/O emulsifiers: (A) KF-6106, (B) PR-15, (C) PG3PR, and (D) PGPH. The left column shows the overview at 400× magnification; the right column shows magnified detail of the red-boxed region. (Green: aqueous phases; Red: oil phase; Black: unstained pigment particles.).

The molecular architectures of the four emulsifiers (Figure ) provide a fundamental basis for interpreting these trends. Emulsifier A features a polydimethylsiloxane (PDMS) backbone with pendant polyglyceryl-3 (PG-3) groups. The exceptional chain flexibility and low rotational energy of the Si–O–Si bonds allow emulsifier A to undergo rapid conformational adaptation at the interface. − This enables the molecule to effectively “heal” interfacial defects induced by pigment migration, thereby maintaining a resilient and self-healing film. In contrast, the performance of the polyester-based emulsifiers B and C underscores the critical role of hydrophilic headgroup dimensions. Both emulsifiers possess hydrophobic polyricinoleate chains, where the cis-double bonds and hydroxyl groups introduce bent, irregular conformations. These features increase chain flexibility, facilitate interfacial packing, and lower interfacial tension, thereby promoting W/O emulsification. However, their stabilizing efficiency differs: the shorter PG-3 headgroup in C promotes a more compact molecular arrangement at the W/O interface. Conversely, the longer polyglyceryl-6 (PG-6) headgroup in B generates significant lateral steric repulsion. This repulsion increases the intermolecular distance, resulting in loose packing that fails to effectively hinder the migration of pigments, which can then penetrate the oil phase and trigger coalescence. Regarding emulsifier D, the combination of a minimal polyglyceryl-2 (PG-2) headgroup and relatively rigid hydrophobic tails appears to introduce excessive steric hindrance and limited lateral mobility, leading to a more “brittle” interfacial film. Consequently, the interface is easily ruptured by pigment-induced stress, compromising the structural integrity of the W/O/W compartments. These molecular-level interpretations align well with the CLSM observations, confirming that the synergy between backbone flexibility and headgroup packing dictates the stability of pigmented multiple emulsions.

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Molecular architectures of W/O emulsifiers: (A) KF-6106, (B) PR-15­(a = 6), (C) PG3PR­(a = 3), and (D) PGPH.

3.2. Optical Microscopy and Particle Size Analysis

Bright-field micrographs and the corresponding particle size distributions of W/O/W foundations prepared using the four W/O emulsifiers are shown in Figures and , respectively. Due to the high pigment content, the internal droplet structure could not be resolved by optical microscopy; however, the multiple-emulsion morphology was previously confirmed via CLSM (Figure ). Therefore, Figures and serve to evaluate the evolution of the outer oil globules under thermal and freeze–thaw stresses.

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Optical micrographs of W/O/W liquid foundations prepared with different W/O emulsifiers under various storage conditions: (A) KF-6106, (B) PR-15, (C) PG3PR, and (D) PGPH.

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Droplet size distributions of W/O/W liquid foundations prepared with different W/O emulsifiers under various storage conditions: (A) KF-6106, (B) PR-15, (C) PG3PR, and (D) PGPH.

The spherical morphology is thermodynamically favorable for minimizing interfacial energy, as regular droplets reduce collision frequency and coalescence. As shown in Figure A, the foundation stabilized with emulsifier A maintained smooth, spherical contours under all conditions, with pigment particles uniformly dispersed. Even after 14 days at −15 °C and subsequent equilibration to 25 °C, despite an increase in mean diameter (Figure ), the globules exhibited well-defined edges and uniform pigment distribution. This superior stability is directly rooted in the molecular architecture of emulsifier A. The PDMS backbone possesses low bond rotation energy and large Si–O–Si bond angles, imparting high segmental mobility even at lower temperatures. − This flexibility allows the interfacial film to remain elastic rather than brittle during water crystallization, facilitating a “self-healing” response to mechanical stress. Generally, lower temperatures lead to reduced kinetic energy from Brownian motion and weakened electrostatic or steric repulsions, which can promote droplet coalescence driven by van der Waals forces.

In contrast, emulsifier B exhibited visible oil exudation (transparent patches) after only 1 day (Figure B). The structure-performance mismatch here lies in the overextended PG-6 headgroup. The longer hydrophilic chain increases lateral steric repulsion between emulsifier molecules, leading to a lower packing density at the W/O interface. This loose molecular arrangement creates “defects” in the oil film, allowing ITT-modified pigments to aggregate and oil to migrate outward. Consequently, although the droplet size increase was moderate, the interfacial cohesion was insufficient for long-term pigmented system stability.

For emulsifier C, a distinct thermal sensitivity was observed. While storage at 25 °C and −15 °C led to an increase in the mean droplet diameter and a broader size distributioncharacteristic of typical coalescencestorage at 45 °C resulted in a smaller, polydisperse population (Figure ), indicating significant droplet rupture. This trend suggests that at 45 °C, the increased thermal energy shifts the adsorption–desorption equilibrium, promoting the partial detachment of PG3PR molecules from the W/O interface into the bulk oil phase. Concurrently, the hydration of the polyglycerol (PG-3) headgroups may weaken at elevated temperatures, reducing their affinity for the aqueous phase and leading to a lower effective interfacial coverage. This reduction in surfactant density creates transiently unprotected regions on the droplet surface, rendering the oil film susceptible to severe rupture during collisions. Furthermore, micrographs after storage at 25 °C (Figure C) revealed extensive transparent oil domains containing small internal water droplets. This segregation of pigment-rich and oil-rich regions reflects inadequate compatibility between the polyricinoleate chains and the hydrophobically modified pigment surface. Although the pigment-oil system is kinetically trapped during initial emulsification, prolonged storage allows for slow, thermodynamically driven aggregation, compromising the integrity of the W/O layer.

Emulsifier D showed the most significant structural failure. At −15 °C, the droplets exhibited a higher mean diameter, whereas storage at 25 and 45 °C led to smaller mean diameters, with the minimum value observed at 25 °C due to severe rupture. Its instability can be interpreted through its relatively rigid hydrophobic structure (dipolyhydroxystearate). While this structure provides strong anchoring in the oil phase, the “rigid” nature of this interface prevents the rapid redistribution of emulsifier molecules to stabilize newly formed interfacial areas. This leads to a “brittle” film character that is easily compromised by the physical stress of pigment particles. At 45 °C, enhanced collision frequency leads to the coalescence of larger droplets until they rupture; however, at 25 °C, slower collision kinetics still allow large droplets to coalesce and eventually rupture without the rapid reformative ability of more flexible emulsifiers. This explains why the system with emulsifier D reached a smaller particle size at 25 °C than at 45 °C.

In summary, the comparative analysis confirms that the relationship between emulsifier structure and system stability is governed by interfacial packing density and backbone flexibility. The silicone-based flexibility of emulsifier A offers the best protection against coalescence, whereas the steric mismatch in B and the structural rigidity of D lead to premature film failure.

3.3. Centrifugal Stability and Viscosity

The viscosity and centrifugal stability of the W/O primary emulsions and their corresponding W/O/W liquid foundations were compared to assess the impact of emulsifier structure on bulk stability. As shown in Figure , the primary W/O emulsions stabilized by emulsifiers B, C, and D exhibited comparable viscosities (3560, 3680, and 3613 mPa·s, respectively), while the viscosity of formulation A was significantly lower (1260 mPa·s). This lower viscosity of the primary emulsion A likely facilitated more efficient droplet breakup during the secondary emulsification process. After conversion to W/O/W foundation, the viscosities followed the order of C (3480 mPa·s) > D (2880 mPa·s) > A (2147 mPa·s) > B (747 mPa·s). This sequence aligns with the droplet size distributions in Figure , supporting the premise that lower-viscosity liquids are more easily fragmented into finer droplets under high-shear processing, whereas the high viscosities of C and D contribute to a more cohesive bulk structure.

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Viscosity and creaming index of W/O primary emulsions and W/O/W liquid foundations prepared with the four W/O emulsifiers: (A) KF-6106, (B) PR-15, (C) PG3PR, and (D) PGPH.

Notably, foundation A was the only system where the W/O/W foundation exhibited a higher viscosity than its precursor W/O emulsion. This implies that the elastic, PDMS-rich interfacial film formed by emulsifier A effectively withstands the secondary shear stress, preserving the internal W/O droplets and potentially promoting the formation of a more complex network. In contrast, the significant viscosity reduction in foundations B and D suggests a partial loss of the internal phase or droplet coalescence during secondary emulsification. For emulsifier B, the overextended polyglycerol chains may lead to a poorly packed interfacial membrane that is susceptible to leakage of the internal water phase under shear, resulting in a substantial loss of hydrodynamic volume and viscosity. Emulsifier D, characterized by a more rigid but less cohesive interface, likely suffered from droplet deformation and rupture, failing to maintain the high viscosity of its primary state. Emulsifier C exhibited only a minor viscosity decrease, indicating relatively robust droplet stabilization throughout the two-step process.

The creaming indices, measured after centrifugation (3000 rpm, 30 min), ranged from 6% to 13% for all primary W/O emulsions, ranked as D > C > A > B. Despite its low viscosity, emulsifier A yielded a relatively low creaming index, highlighting the effective steric stabilization provided by the PDMS segments. For the final W/O/W foundation, the creaming order was B > A > D > C, which is inversely related to their final viscosities. According to Stokes’ Law, the higher viscosity of the external continuous phase in systems C and D provides greater hydrodynamic resistance against droplet migration, thereby enhancing stability against accelerated gravitational stress.

In summary, emulsifier B exhibited the poorest performance, with a terminal viscosity significantly lower than its primary emulsion, indicating structural collapse. While emulsifier A demonstrated unique “self-strengthening” behavior (viscosity increase), its absolute viscosity remained lower than those of C and D. Although C and D showed minor viscosity losses compared to their primary states, their higher final bulk viscosities contributed to superior centrifugal stability. Therefore, while A, C, and D demonstrate better overall integrity than B, their long-term stability requires further kinetic study.

3.4. High and Low Temperature Tests

The macroscopic stability of the W/O/W foundations was monitored for up to 30 days across various temperatures to rigorously evaluate the long-term performance of the emulsifiers (Figure ). While the microscopic and particle size analyses at day 14 (Section ) captured the early kinetic destabilization, the 30-day macroscopic observations serve to confirm the long-term consequences of these initial structural flaws.

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Macroscopic appearance of W/O/W liquid foundations prepared with different W/O emulsifiers during 14-day and 30-day stability tests under various temperature conditions: (A) KF-6106, (B) PR-15, (C) PG3PR, and (D) PGPH.

At 25 °C, all foundations exhibited relative stability for the first 14 days. However, upon extending the storage to 30 days, foundation C displayed visible milky-white aqueous phase separation and surface pigment flotation, while foundation D developed noticeable surface halos accompanied by slight milky-white leakage. These terminal failures are consistent with the early stage droplet coalescence and pigment-oil segregation observed in the day-14 micrographs (Figure C,D), confirming that the initial interfacial defects eventually evolved into macroscopic phase separation. This progression suggests a gradual breakdown of the primary W/O interface, leading to the sustained leakage of the internal aqueous phase.

Under accelerated aging conditions at 45 °C, the stability disparities became even more pronounced. Foundation A maintained excellent homogeneity for 30 days, corroborating the high interfacial resilience and “self-healing” mobility of the PDMS segments discussed in Section . In contrast, foundation B exhibited a persistent cream layer (∼1 mm). The macroscopic instability of B aligns with the oil exudation observed under the microscope as early as day 1, confirming that its loose molecular packing cannot provide sufficient long-term thermal protection. Foundations C and D, which showed minor pigment flotation at day 14, underwent significant degradation by day 30: C exhibited a heavily mottled surface, while D experienced severe instability characterized by visible milky-white leakage. The pronounced instability of D at day 30 provides the macro-scale validation of the “brittle” film character and droplet rupture already identified in its day-14 size distribution (Figure ).

At −15 °C, the surface mottling observed in B, C, and D at day 14 intensified significantly by day 30, manifesting as larger, irregular, and more continuous speckled textures. This phenomenon is primarily attributed to the increased oil-phase viscosity and the volumetric expansion of internal ice crystals. Such expansion exerts substantial mechanical stress on the interfacial film, creating localized discontinuities or “micro-cracks” within the W/O/W droplets. The intensified mottling in B, C, and D indicates that their interfacial films are relatively “brittle” and prone to fracturing under mechanical strain. Conversely, foundation A demonstrated higher resistance to freeze–thaw stress with minimal surface alterations, further confirming the flexibility and “self-healing” resilience of the silicone-based interfacial network.

In summary, the 30-day macroscopic results provide a conclusive validation of the destabilization mechanisms identified during the 14-day microscopic study. Foundation A demonstrated significantly higher structural robustness across all tested conditions. The robust performance of the silicone-based emulsifier highlights the importance of interfacial elasticity in maintaining the structural integrity of complex pigmented systems. However, further optimization of the oil-phase composition is warranted to fully suppress surface defects during long-term storage.

3.5. Surface Tension and Interfacial Tension Analysis

Surface tension primarily reflects the cohesive intermolecular forces at a liquid surface, whereas interfacial tension (IFT) arises from the imbalance of molecular interactions at the boundary of two immiscible fluids. While a reduction in IFT lowers the thermodynamic driving force for droplet coalescence, , this correlation is often nonlinear in complex multiphase systems such as W/O/W emulsions.

As illustrated in Figures and , the primary W/O emulsion stabilized by emulsifier A exhibited significantly higher surface and interfacial tension values compared to those stabilized by B, C, or D. This indicates that emulsifier A is less effective at reducing the interfacial free energy. However, as established in the preceding stability assessments, the final W/O/W foundation containing emulsifier A demonstrated superior structural integrity. These contrasting observations confirm that static surface or interfacial tension measurements alone are not definitive predictors of long-term stability in pigmented W/O/W systems.

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Surface tension of W/O primary emulsions prepared with the four W/O emulsifiers: (A) KF-6106, (B) PR-15, (C) PG3PR, and (D) PGPH.

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Oil–water interfacial tension of the four W/O emulsifiers: (A) KF-6106, (B) PR-15, (C) PG3PR, and (D) PGPH.

Instead, the stability appears to be predominantly governed by the interfacial rheological properties. While direct interfacial rheological measurements were not performed, the bulk rheological behavior discussed in Section provides compelling indirect evidence of the interfacial film’s mechanical strength. Even at a higher IFT, a cohesive viscoelastic interfacial architecture can serve as a robust physical barrier, effectively suppressing droplet coalescence through the Gibbs–Marangoni effect and steric hindrance.

This is particularly evident for silicone-based emulsifiers like emulsifier A. The flexible PDMS segments possess an exceptionally low rotational energy barrier, conferring high segmental mobility at the interface. − In the context of polymeric surfactants, such conformational flexibility enables the adsorbed film to exhibit viscoelastic behavior, dissipating or storing energy through molecular rearrangement during interfacial deformation. This molecular elasticity allows the film to undergo rapid deformation and recovery without rupturinga hypothesis strongly supported by the near-complete thixotropic recovery (91.16%) observed in our bulk rheological tests. In contrast, the lower IFT of emulsifiers B, C, and D suggests higher surface activity; however, their hydrocarbon-based chains may form more “brittle” or less resilient films that lack the necessary kinetic mobility to withstand secondary emulsification shear. This highlights that for silicone-stabilized multiple emulsions, the kinetic stability provided by interfacial resilience outweighs the thermodynamic requirement for ultralow interfacial tension.

3.6. DSC Analysis

Low-temperature DSC thermography is a robust technique for probing the structural integrity and phase behavior of W/O/W emulsions. In an ideal multiple emulsion, the oil phase acts as a distinct barrier, leading to the segregated crystallization of internal and external aqueous phases, typically characterized by two separate exothermic peaks. , However, as shown in Figure (where crystallization maxima (T c) and peak intensities are explicitly labeled), all four W/O/W liquid foundations consistently exhibited only a single crystallization peak. This is likely due to the high volume fraction of the external aqueous phase (∼70%). Upon freezing, the volumetric expansion of the water phase exerts significant mechanical stress that disrupts the thin interfacial oil film. This structural failure causes the internal droplets to coalesce with the external phase during the cooling ramp, resulting in a single, merged exothermic event. Although the dual-peak structure is lost during the freezing process, the specific position and morphology of this merged peak provide critical insights into the interfacial confinement capability of each emulsifier.

9.

9

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DSC thermograms of W/O/W liquid foundations prepared with different W/O emulsifiers: (A) KF-6106, (B) PR-15, (C) PG3PR, and (D) PGPH.

The T c values for foundations stabilized with emulsifiers A, C, and D were recorded between −21 and −22 °C, whereas foundation B crystallized at a significantly higher temperature (−18.75 °C). This elevated T c in B correlates with its lower bulk viscosity, which facilitates greater water mobility and promotes earlier, heterogeneous ice nucleation. While polyricinoleate is inherently flexible, the extended PG-6 chains in emulsifier B likely form a loosely packed interfacial membrane that fails to effectively hinder water transport.

Significant variations in peak intensity and width were also observed. Foundation D displayed a sharp and intense crystallization signal, attributed to the rapid, cooperative freezing of water molecules poorly confined by its relatively rigid interfacial film. In contrast, the exothermic peaks for A and C were notably broader and of lower intensity. This profile indicates that the elastic, PDMS-rich layer in A and the sterically hindered polyricinoleate configuration in C provide superior interfacial confinement. These architectures impede the translational mobility of water molecules and disrupt the formation of a continuous ice lattice, thereby distributing the crystallization process over a broader temperature range.

3.7. Rheological Analysis

Figure a presents the viscosity profiles of the W/O/W liquid foundations. All foundations exhibited pronounced shear-thinning behavior characteristic of pseudoplastic fluids. The viscosity followed the order C > D > A > B, consistent with Figure . From a structural perspective, this hierarchy is governed by the emulsifier’s ability to promote interdroplet interactions. The higher viscosities of C and D stem from their bulky polyricinoleate chains and rigid orientations, which facilitate the formation of an extensive droplet network. In contrast, the lower viscosity of B reflects the failure of its overextended PG-6 chains to maintain a cohesive bulk structure. From an application perspective, the observed shear-thinning behavior across all foundations is highly relevant to consumer usage. The high viscosity at low shear rates (0.1–1 s–1) is essential for maintaining pigment suspension and overall structural stability during storage. As the shear rate increases toward 100 s–1, the viscosity drops by nearly 2 orders of magnitude, representing the initial breakdown of the emulsion network. While the full spreading process on the skin often involves much higher shear rates (103–105 s–1), the pronounced pseudoplasticity observed within our testing range (0.1–100 s–1) provides a reliable indicator of the system’s ability to transition smoothly upon initial skin contact, ensuring an even distribution of pigments.

10.

10

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Rheological profiles of W/O/W liquid foundations prepared with the four W/O emulsifiers: (a) viscosity curves, (b) thixotropic recovery, (c) temperature sweep, and (d) loss-factor (tan δ) plots. The emulsifiers used are (A) KF-6106, (B) PR-15, (C) PG3PR, and (D) PGPH.

The relationship between emulsifier architecture and structural recovery was further elucidated via thixotropic tests (Figure b). Foundation A exhibited a near-quantitative recovery ratio of 91.16% (Table ). This superior resilience is directly attributable to the unique siloxane backbone of emulsifier A. Unlike the hydrocarbon chains in B, C, and D, the PDMS segments possess an exceptionally low rotational energy barrier, conferring “molecular elasticity” to the interface. − Under high shear, these flexible chains can temporarily disentangle and align without fully desorbing; upon cessation of shear, their high segmental mobility allows for rapid re-entanglement and restoration of the interfacial film. Conversely, the lower recovery of C (80.32%) and D (75.57%) suggests that their more rigid, sterically hindered structures undergo partial, irreversible damage due to less cohesive interfacial adsorption. While higher bulk viscosity increases flow resistance, it can also lead to networks that, once broken, are slow to reform due to restricted molecular mobility. In contrast, foundation B showed significant fluctuations during recovery, likely because its low viscosity renders the microstructure highly susceptible to minor perturbations. Although B achieved an 87.74% recovery, its low absolute viscosity leaves the system vulnerable, underscoring that a recoverable interfacial film must be coupled with adequate mechanical strength to ensure long-term stability.

2. Thixotropic Recovery Parameters of the W/O/W Liquid Foundations.

  First-stage η* mean/mPa·s Second-stage η mean/mPa·s Third-stage η* mean/mPa·s Thixotropic recovery ratio (%)
W/O/W A 5868.42 ± 49.12 265.62 ± 1.44 5349.69 ± 285.10 91.16%
W/O/W B 1400.28 ± 18.73 107.88 ± 2.05 1228.72 ± 96.12 87.74%
W/O/W C 14712.62 ± 62.03 348.08 ± 7.60 11817.68 ± 757.16 80.32%
W/O/W D 9824.52 ± 107.58 256.05 ± 4.70 7424.06 ± 569.00 75.57%
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The temperature dependence of the complex viscosity (η*) is shown in Figure c. The parameter η*, integrating both viscous and elastic components, is sensitive to microstructural changes such as droplet dispersion and interfacial strength. Foundation B possessed the highest tan δ and significant fluctuations above 35 °C (Figure d), indicating high internal energy dissipation. The overextended PG-6 headgroup in Emulsifier B likely creates a “loose”, highly hydrated interfacial layer that is sensitive to thermal agitation, explaining its inability to suppress internal phase leakage.

During cooling (25 to 0 °C), the η* values of A, C, and D increased, attributed to enhanced van der Waals and hydrogen-bonding interactions that promote a denser surfactant network. , Upon subsequent heating from 0 to 50 °C, foundation A showed a modest decrease in η*, whereas C and D underwent an abrupt drop near 5 °C. This collapse is attributed to the melting of ice crystals in the external phase: crystal growth can perforate the oil film, and the transient imbalance between surfactant reservoirs prevents rapid readsorption. Upon returning to 25 °C, the η* of foundation A was virtually identical to its initial value, while C and D remained below their baselines. Consequently, the extent of irreversible internal damage follows the order C > D > A, confirming that the flexible siloxane architecture of emulsifier A confers a “self-healing” capability, whereas hydrocarbon-based emulsifiers form more static, fragile films that lack the kinetic mobility to repair the interface after extreme thermal stress.

4. Conclusions

In this study, the influence of different W/O emulsifier structures on the stability of pigmented W/O/W foundations was systematically investigated. The results demonstrate that the molecular structure of the primary emulsifier is the decisive factor in maintaining the structural integrity of these complex systems under thermal and freeze–thaw stresses. Foundation A, stabilized by a silicone-based polyether emulsifier, exhibited superior long-term stability (up to 30 days) compared to foundations using traditional hydrocarbon-based polyglycerol fatty acid esters. This exceptional performance is attributed to the high segmental mobility and low bond rotation energy barrier of the PDMS backbone, which facilitates the formation of a resilient, “self-healing” interfacial film capable of buffering mechanical stresses induced by ice crystal growth and thermal agitation.

Furthermore, this study highlights that the stability of pigmented multiple emulsions is governed by a delicate balance between emulsifier structure and pigment-interface interactions. While our findings isolate the critical role of interfacial elasticity, a more granular understanding of the pigments’ physicochemical propertiesincluding particle size, surface hydrophobic treatment, and compatibilityremains a vital area for future research. Deepening the investigation into pigment-interface coupling will be essential for fully suppressing long-term surface defects. Additionally, exploring blended emulsifier systems presents a promising direction; hybrid interfacial layers, combining silicone-based flexibility with hydrocarbon-based pigment compatibility, could offer synergistic protection. Finally, the optimization of process parameters, such as secondary emulsification shear intensity and duration, represents another valuable path for future industrial scale-up. These perspectives provide a theoretical framework for the rational design of high-performance emulsifiers and the development of robust stabilization strategies for multiphase cosmetic and pharmaceutical delivery systems.

Acknowledgments

The authors gratefully acknowledge financial support from the Basic and Applied Basic Research Foundation of Guangdong Province, China (Grant No. 2024A1515012801). For the purpose of language editing only, the authors utilized the AI tools Gemini, ChatGPT, Kimi, and Deepseek. The authors carefully reviewed, refined, and validated all resulting text and bear complete responsibility for the scientific content and integrity of the final manuscript.

The authors declare no competing financial interest.

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