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Mechanism of oil-in-water emulsification using a water-soluble amphiphilic polymer and lipophilic surfactant
Journal of Colloid and Interface Science 300 (2006) 141–148 www.elsevier.com/locate/jcis
Mechanism of oil-in-water emulsification using a water-soluble amphiphilic polymer and lipophilic surfactant Eri Akiyama a,∗ , Akio Kashimoto a , Hajime Hotta a , Tomohito Kitsuki b a Kao Corporation, Tokyo Research Lab, 2-1-3, Bunka, Sumidaku, Tokyo 131-8501, Japan b Kao Corporation, Wakayama Research Lab, 1334, Minato, Wakayama 640-8580, Japan
Received 11 October 2005; accepted 23 March 2006 Available online 3 April 2006
Abstract A new O/W (oil-in-water) emulsification system was developed using the amphiphilic polymer HHM-HEC (hydrophobically–hydrophilically modified hydroxyethylcellulose) and a lipophilic surfactant. HHM-HEC was used as a thickener and polymeric surfactant, and the addition of small quantities of various types of nonionic lipophilic surfactant (hydrophilic–lipophilic balance <5) decreased the droplet size of several types of oil due to a lowering of the tension at the water/oil interface. The oil droplets were held by the strong network structure of the aqueous HHM-HEC solution, preserving the O/W phase without inversion. These stable O/W emulsions were prepared without the addition of hydrophilic surfactants and thus show improved water repellency. © 2006 Elsevier Inc. All rights reserved. Keywords: HHM-HEC; Water-soluble polymer; Oil-in-water emulsion; Lipophilic surfactant
1. Introduction Water based emulsions (oil-in-water (O/W)-type emulsions) have been widely applied to cosmetics and toiletries. Actually they have a good watery feeling when applied to skin. However, conventional O/W emulsions contain a large amount of hydrophilic surfactant that is used to obtain fine emulsified particle. Therefore, these cause uncomfortable feeling and skin irritation and lower water repellency. In order to improve cosmetic formulations, such as lotions, creams, sunscreens, and base make-up liquid foundations, it is important to obtain fine O/W emulsions with minimum amount of hydrophilic surfactant. Furthermore, the best is to obtain fine O/W emulsion without hydrophilic surfactant. Recently, functional water-soluble amphiphilic polymers became highly developed [1–3]. Some of these polymers have thickening and emulsifying abilities and have been used in O/W-type emulsions [4–6]. They can reduce the required quantities of low-molecular-weight hydrophilic surfactants and sta* Corresponding author. Fax: +81 3 5630 9342.
E-mail address: [email protected] (E. Akiyama). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.03.066
bilize the emulsions. However, the performance of conventional water-soluble polymers is limited. There have been several reports of O/W-type emulsification systems that contained conventional water-soluble amphiphilic polymers and hydrophilic surfactants [7,8], as well as polysaccharides and polar organic compounds [9]. However, the O/W-emulsions composed of water-soluble polymers and various kinds of oil without hydrophilic surfactant have been little investigated. We prepared amphiphilic polymer HHM-HEC (sodium hydroxyethylcellulose hydroxypropyl stearylether hydroxypropyl sulfonate), a good thickening and emulsifying agent [10,11]. It is a strong acidic polyelectrolyte with a high molecular weight. It has a good solubility in water, high electrolyte tolerance [12], and high physical stability. Intermolecular aggregation occurs above 0.2 wt% and three-dimensional networks [13] are formed in water through the aggregation of hydrophobic alkyl chains in HHM-HEC. It forms thixotropic and elastic gel above 0.6 wt% [11]. These properties enable O/W emulsions containing various kinds of oil to be stabilized in the absence of surfactants. However, the diameters of the oil particles in the reported HHM-HEC/water/oil emulsions were much larger than those of conventional O/W emulsions (with values rang-
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ing from 5 to 50 µm and a mean diameter of 23.6 µm.) Such a coarse emulsion does not have smoothness and it becomes disadvantage when applied to cosmetics and toiletries. It was difficult to combine “fine O/W emulsion” and “no hydrophilic surfactant” with conventional method. In this study, we achieved the creation of the fine O/W emulsion without hydrophilic surfactant. We have succeeded in reducing the size of the oil particles by adding of small quantities of lipophilic surfactant (hydrophilic–lipophilic balance (HLB) <5) in HHM-HEC/water/oil emulsions. This system is unique in that lipophilic surfactants promote the formation of an O/W emulsion without phase inversion. In this paper we describe the emulsification mechanisms of this system. 2. Materials and methods 2.1. Materials 2.1.1. Water-soluble polymers The water-soluble polymers used in this work are listed: (1) Hydrophilically–hydrophobically modified hydroxyethylcellulose (HHM-HEC). (2) Alkyl chain-bearing HEC (R-HEC). (3) HEC (HEC QP-100MH, Union Carbide Corporation). The structures of compounds (1)–(3) are shown in Fig. 1. Compounds (1) and (2) were prepared by the method reported in [10]. The compound (3) was a raw material for the compounds (1) and (2), and the number of glucose unit (7300) was the same for all compounds (1)–(3). HHM-HEC has both alkyl chain moieties and sulfonic acid moieties. The molecular weights and the substitution degrees of the alkyl chains and the sulfonic acid moieties per one HEC unit are listed in Table 1. We prepared two types of HHM-HEC (A) and (B), but they have no significant differences in substitution degrees and properties.
2.1.2. Oils The oil materials used in this work are listed: (1) Squalane (NIKKOL squalane, NIKKO Chemicals Co., Ltd.). (2) Dimethyl polysiloxane (silicone KF-96A (6cs), Shin-Etsu Chemical Co., Ltd.). (3) Perfluoropolymethylisopropyl ether (FOMBLIN HC/04 Ausimont). 2.1.3. Surfactants The surfactants used in this work are listed: (1) Isostearyl glyceryl ether, GE-IS (HLB 2.5 calculated by the Davies method, HLB 5.0 calculated by the Griffin method, PENETOL GE-IS, Kao Corporation). (2) Dimethicone copolyol (polyoxyethylene methylpolysiloxane copolymer, HLB 2, supplied by Toray Dow Corning Silicone). (3) Polyoxyethylene(20) sorbitan monostearate (HLB 14.9, RHEODOL TW-S120, Kao Corporation). The structures of surfactants (1)–(3) are shown in Fig. 2. 2.2. Preparation of emulsions To prepare the O/W emulsions, HHM-HEC was thoroughly suspended and swollen in water. A mixture of oil and surfactant was then added to the aqueous polymer solution while stirring with a propeller mixer at 400 rpm for 5 min. After this premixing, the emulsions were stirred for 10 min at 25 ◦ C with a laboratory homomixer operating at 5000 rpm. 2.3. Viscosity measurements Viscosity of emulsion was measured using a Toki-Sangyo Model B viscometer (B8L, Rotor No. 4, 6 rpm), which gave measurement errors of <5%.
Table 1 HEC derivatives employed in this work Sample name
Mw
Number of alkyl chain moieties (per monosaccharide unit)
Number of sulfonic acid moieties (per monosaccharide unit)
HHM-HEC (A) HHM-HEC (B) R-HEC HEC
1.8 × 106 1.8 × 106 1.5 × 106 1.5 × 106
0.0033 0.0048 0.0041 0
0.21 0.22 0 0
Fig. 1. Structures of HHM-HEC and HEC.
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Fig. 2. Structures of surfactant.
2.4. Particle size measurements The distributions of droplet sizes in the emulsions were determined using a laser scattering particle size distribution analyzer (HORIBA LA-920). Photomicrographs of the emulsions were obtained using a Nikon OPTIPHOT-2 photomicroscope. Measurements were carried out after appropriate dilution of the sample with water. It was confirmed in advance that the dilution process did not significantly change the particle size. The measurement errors were <0.7%.
(A)
2.5. Interfacial tension measurements The tension of the aqueous polymer solution/oil (including surfactant) interface was measured using a Wilhelmy-type automated surface tension meter (KRÜSS Processor tension meter K122) with a platinum plate. (B)
2.6. Rheological measurements Oscillatory measurements of each aqueous solution of polymer and emulsion were taken with a Modular Compact Rheometer MCR300 (PHYSICA Messtechnik GmbH), which has cone-plate geometry (CP50-1: angle 1.0◦ , diameter 50 mm). All date were collected from a linear viscoelastic region (strain amplitude γ = 1%). The sample was covered by a reservoir filled with water during the measurements to prevent the evaporation of water. 3. Results 3.1. Decrease in particle size induced by lipophilic surfactants The addition of low-HLB surfactants to the HHM-HEC/ water/oil systems caused a large decrease in particle size. Figs. 3A and 3B show optical photomicrograph images of the emulsions with and without surfactants. The distribution of particle size in the emulsions is shown in Figs. 4A and 4B. These emulsions contained 0.5 wt% HHM-HEC (A), 19.5 wt% oil, and 0.5 wt% surfactant. Figs. 5A and 5B show the relationship between the quantity of lipophilic surfactant added and (a) the size of the oil droplets (squalane, dimethyl polysiloxane, and perfluoropolymethylisopropyl ether) and (b) viscosity of the emulsions. These emulsions contain 0.4 wt% HHM-HEC (A) and 20 wt%
Fig. 3. (A) Photomicrographs of HHM-HEC/water/squalane emulsions: (a) without surfactant, (b) with GE-IS (HLB 2.5). (B) Photomicrographs of HHM-HEC/water/dimethyl polysiloxane emulsions: (a) without surfactant, (b) with dimethicone copolyol (HLB 2).
oil. Lipophilic surfactants caused a decrease in size of oil droplets. The smallest oil droplets were obtained with the combination of GE-IS and squalane, or the combination of dimethicone copolyol and dimethyl polysiloxane. High viscosity was also obtained at these combinations. No viscositydecrease was observed and the O/W phase was preserved at high lipophilic surfactant concentrations. Therefore, precise ratios of oil to surfactant are unnecessary in this emulsification system. However, high-HLB hydrophilic surfactants caused a decrease in viscosity and segregation of the emulsion. The viscosity of the HHM-HEC/water/squalane/polyoxyethylene sorbitan monostearate (HLB 14.5) emulsion was 600 mPa s and HHM-HEC/water/dimethyl polysiloxane/polyoxyethylene sorbitan monostearate emulsion was 580 mPa s. (0.4 wt% HHMHEC (A), 19.5 wt% oil, 0.5 wt% polyoxyethylene sorbitan monostearate). Creaming was immediately observed for the both emulsions. 3.2. Interfacial tension Figs. 6A and 6B show the interfacial tension at the aqueous HHM-HEC (A) or HEC solution/oil phase as a function
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Table 2 Effect of type of polymer on state of emulsion (0.5 wt% HEC derivatives, 19.5 wt% squalane, 0.5 wt% GE-IS) Sample name
State of emulsion
Viscosity (mPa s)
Particle size (µm)
HHM-HEC (A) HHM-HEC (B) R-HEC HEC
O/W O/W Creaming Creaming
22,000 23,000 – –
4.0 3.4 5.8 4.4
Table 3 Effect of concentration of HHM-HEC on state of emulsion (19.5 wt% squalane, 0.5 wt% GE-IS) (A)
(B) Fig. 4. (A) Droplet diameter distribution by volume for HHM-HEC/water/ squalane emulsions. (B) Droplet diameter distribution by volume for HHMHEC/water/dimethyl polysiloxane emulsions.
of lipophilic surfactant concentration. In the absence of surfactant the interfacial tension was large. When lipophilic surfactant was added to the oil phase, a significant decrease in the interfacial tension (by approximately a factor of ten) was observed regardless of the type of polymer.
HHM-HEC concentration (wt%)
State of emulsion
Viscosity (mPa s)
Particle size (µm)
0.1 0.2 0.4 0.5 0.8 1.0
Creaming O/W O/W O/W O/W W/O
–
– 6.5 4.1 4.0 4.1 –
620 10,700 10,800 29,800 –
HEC derivatives, 19.5 wt% squalane, 0.5 wt% GE-IS) are shown in Table 2. Small oil droplets (4–6 µm) were obtained for all emulsions, but creaming was observed for R-HEC and HEC emulsions. (Furthermore, stable emulsions with fine particles could not be obtained at any concentration (0.2–1.0 wt%) for the HEC.) Both alkyl chain and sulfonic acid moieties has an effect on state of emulsion containing lipophilic surfactant. As shown in Table 3, fine O/W emulsions were successfully obtained for concentrations of HHM-HEC (A) between 0.4 and 0.8 wt%. These emulsions also contained 19.5 wt% squalane and 0.5 wt% GE-IS. Creaming occurred for low HHM-HEC concentrations (0.1 wt%) because the aqueous phase was too dilute to hold the oil particles, and phase inversion occurred for high HHM-HEC concentrations (1.0 wt%) because the aqueous phase became too viscous to contain the oil particles by stirring.
3.3. Rheology 4. Discussion Oscillatory measurements of the emulsion were carried out to elucidate the relationship among aqueous solution of HHMHEC, the emulsion of the HHM-HEC/water/oil system and the emulsion of the HHM-HEC/water/oil/lipophilic surfactant. The concentration of HHM-HEC in water was constant. (HHMHEC (B) is 0.625 wt% in water.) The results are shown in Figs. 7A and 7B. The aqueous solution of HHM-HEC was predominantly elastic. With regard to Figs. 7Aa and 7Ba, the shape of rheological spectra of the emulsion containing 20 wt% oil were similar to that of the aqueous solution but those of the emulsion containing lipophilic surfactant were a little changed (Figs. 7Ab and 7Bb) 3.4. Effect of polymer type and concentration on state of emulsion The effects of different types of HEC derivatives on the state of O/W emulsions containing a lipophilic surfactant (19.5 wt%
Fine O/W emulsions were formed by lipophilic surfactant and amphiphilic polymer HHM-HEC. Various kinds of oil droplets were minimized by GE-IS or dimethicone copolyol. This phenomenon is characteristic and two important mechanisms in the formation of the emulsification system could be considered: a decrease in interfacial tension caused by lipophilic surfactant and the formation of a three-dimensional network structure by the HHM-HEC polymer in the continuous phase, which holds the emulsified oil particles. 4.1. The role of lipophilic surfactants Figs. 6A and 6B show the lowering of interfacial tension by adding lipophilic surfactants to the oil phase. The size of the oil particles in HHM-HEC/water/oil emulsions decreased with the addition of lipophilic surfactants (Figs. 5Aa and 5Ba). It indicates that lipophilic surfactants are able to decrease in-
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(A)
(B) Fig. 5. (A) Effect of GE-IS concentration on (a) size of oil particles, (b) viscosity. (B) Effect of dimethicone copolyol concentration on (a) size of oil particles, (b) viscosity.
(A)
(B)
Fig. 6. (A) Interfacial tensions at the aqueous solution of polymer/squalane as a function of GE-IS concentration. (B) Interfacial tensions at the aqueous solution of polymer/dimethyl polysiloxane as a function of demethicone copolyol concentration.
terfacial tension as effectively as hydrophilic surfactants even in O/W emulsions and the size of the oil particles decreases with the lowering of interfacial tension. There was no significant difference in the values of interfacial tension between the HEC and HHM-HEC polymers, indicating that the alkyl chains in HHM-HEC do not behave as surfactants [5,6]. Slight adsorption of alkyl chains at oil/water interface can be possible, however it does not have much effect on lowering interfacial tension. The decrease of interfacial tension is simply caused by the assembly of lipophilic surfactant molecules at the interface.
Furthermore, it is preferable that the surfactants are highly soluble in oil phase. When surfactants dissolve well in the oil phase, the assembly of surfactants can be effective (because mobility of surfactants is high) and fine emulsified particles can be obtained. GE-IS dissolves well in squalane, while it does not dissolve in dimethyl polysiloxane or perfluoropolymethylisopropyl ether. Likewise, dimethicone copolyol dissolves only in dimethyl polysiloxane. As a result, the smallest oil droplets and high viscosity of emulsion were obtained with the combination of GE-IS and squalane, or the combination of dimethicone copolyol and dimethyl polysiloxane (Fig. 5A and 5B).
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(A)
(B) Fig. 7. (A) The storage modulus, G (closed), and loss modulus, G (open), of (a) aqueous solution of HHM-HEC and HHM-HEC/water/squalane emulsion; (b) aqueous solution of HHM-HEC and HHM-HEC/water/squalane/GE-IS emulsion at 25 ◦ C. (B) The storage modulus, G (closed), and loss modulus, G (open), of (a) aqueous solution of HHM-HEC and HHM-HEC/water/dimethyl polysiloxane emulsion; (b) aqueous solution of HHM-HEC and HHM-HEC/water/dimethyl polysiloxane/dimethicone copolyol emulsion at 25 ◦ C.
The system described here takes advantage of the poor solubility of lipophilic surfactants in water. In general, the thickening ability of self-assembly polymer is canceled by the addition of large quantities of hydrophilic surfactant. The hydrophobic domains, which are cross-linking point of threedimensional gel network, dissociate due to adhesion of the hydrophilic surfactant to the hydrophobic moiety [14–16]. The decrease in viscosity of the HHM-HEC/water/oil emulsion on addition of hydrophilic surfactants (polyoxyethylene(20) sorbitan monostearate) is thought to be caused by the same effect. Low-molecular-weight hydrophilic surfactants have a high mobility in water and could therefore adhere to the hydrophobic domains in the HHM-HEC network. Conversely, lipophilic surfactants are poorly soluble in water and the hydrophobic domains of the HHM-HEC network are preserved. As a result, the viscosity of the emulsion does not decrease.
4.2. The role of the HHM-HEC network structure in the continuous phase In general, low-HLB surfactants are used to make W/O-type emulsions. Indeed, GE-IS (HLB 2.5) and dimethicone copolyol (HLB 2) are known as W/O-type surfactants. However, in our case the emulsions remain O/W-type without the occurrence of phase inversion at high lipophilic surfactant concentrations and thus differ from conventional emulsions that consist of only low molecular weight media. The type of emulsion (O/W or W/O) is not determined by the morphology of the surfactants (the hydrophobic–hydrophilic balance), but by the rheological properties of the polymers in the continuous phase. Ishihata et al. reported that the thixotropy of the continuous phase had an effect on the stability of O/W emulsions containing glyceryl monostearate [9]. The aqueous solution of HHM-HEC shows
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Fig. 8. Schematic drawings of (a) aqueous solution of HHM-HEC, (b) HHM-HEC/water/oil emulsion, (c) HHM-HEC/water/oil emulsion with lipophilic surfactant.
non-Newtonian flow behavior, and has a high yield value (i.e., thixotropic). Oscillatory measurements reveal that the aqueous solution of HHM-HEC is predominantly elastic (the value of G was larger than that of G ) above 0.6 wt% [11]. Thixotropic and elastic gel is formed by the aggregation of alkyl chains in HHM-HEC structure. The gel structure of HHM-HEC in the continuous phase has the ability to hold oil particles, including lipophilic surfactants (Figs. 8a and 8c) [10,11]. Phase inversion was avoided due to the low mobility of the HHMHEC polymers in the continuous phase, which is derived from a high-molecular-weight, semi-rigid backbone [17] and strong intermolecular aggregates of HHM-HEC. Emulsions with highly expanded networks in the continuous phase are not subject to phase inversion. When only oils were emulsified in aqueous solution of HHM-HEC (0.625 wt% in water), the shape of rheological spectrum of the emulsion was similar to that of the aqueous solution of HHM-HEC (Figs. 7Aa and 7Ba). The viscoelastic properties of HHM-HEC in disperse medium is overwhelming and dominates the rheological properties of the emulsion. On the other hand, when oils including lipophilic surfactant were emulsified, the elastic properties became strong (Figs. 7Ab and 7Bb). These rheological properties were based on both an emulsification by surfactant and network structures of HHMHEC. Using a preparative method in which oil and a lipophilic surfactant are added to a HHM-HEC solution, emulsions of the O/W-type are predominantly made. The aggregation of hydrophobic moieties of HHM-HEC in water takes place prior to emulsification, and the C18 alkyl chain domains in HHM-HEC are considered to exist solely as cross-linking point in water, not to assemble at the oil/water interface like low-molecular-weight surfactants [18]. The oil particles are emulsified in the HHMHEC network structure under the condition of low interfacial tension promoted by the lipophilic surfactants. In this system we conclude that several properties of the polymer in aqueous solution are important. These are structure, concentration, elasticity and thixotropy. This is explained by the observations that both HEC and R-HEC were unable to form stable O/W emulsions with lipophilic surfactants (Table 2). Stable emulsions with fine particles could not be obtained at 0.2– 1.0 wt% HEC. An aqueous solution of HEC is a viscous fluid (not elastic) at 1.0 wt% and has no yield value [11]. 0.5 wt% HEC emulsion showed apparent creaming even when it does
not contain lipophilic surfactant [11]. An aqueous solution of R-HEC is thixotropic and elastic, but the solubility of R-HEC in water is low and the expansion of network structure is not sufficient. In fact, the aqueous solution of R-HEC was slightly turbid [11]. The states of network of HEC and R-HEC in continuous phase are not able to hold oil including lipophilic surfactants. Similarly, the semi-dilute aqueous solution (0.1 wt%) of HHM-HEC was also unable to form stable O/W emulsions (Table 3). HHM-HEC was not thixotropic or elastic below 0.2 wt% (in the region of intramacromolecular association of HHMHEC) [11]. On the other hand, between 0.4 and 0.8 wt%, fine O/W emulsions were obtained. This concentration region is corresponding to the region of intermolecular association of HHM-HEC [11]. Therefore, in order to make stable O/W emulsions using water-soluble polymers and lipophilic surfactants, it is necessary to choose a suitable type of polymer in an appropriate quantity, otherwise phase inversion, creaming or segregation easily occurs. 5. Summary Fine O/W emulsions were obtained using a HHM-HEC/ water/oil/lipophilic surfactant system. It is supposed that the three-dimensional networks formed by HHM-HEC play a vital role in this system. The strong network structure of HHM-HEC is able to hold oil particles, including the lipophilic surfactant, without the occurrence of phase inversion. Conventional polymers were unable to form O/W emulsions with lipophilic surfactants because their network structures and viscoelastic properties were not sufficient to hold oil particles in a stable fashion. This system utilizes the advantages of lipophilic surfactants: their ability to lower interfacial tension and their poor solubility in water. Low interfacial tension decreases the diameter of the oil droplets and poor solubility prevents dissociation of the hydrophobic domains of HHM-HEC in water. When this type of emulsion is applied to the skin, it has a comfortable watery feeling and shows improved water repellency because it does not contain hydrophilic surfactants. (Re-emulsification is inhibited.) There are many potential applications of such systems for cosmetics and toiletries. An investigation of the effects of the degree of alkyl chain and sulfonic moiety substitution in HHM-HEC on the properties and stability of the emulsions are now in progress.
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Acknowledgments We are very grateful to Dr. K. Kita, T. Nishioka, and Dr. T. Ihara for their help with synthesis of the polymers. We also thank K. Kawakami, a researcher in the Kao Corporation, for useful advice on the preparation of emulsions. References [1] J.E. Glass, Water-Soluble Polymers: Beauty with Performance, Advances in Chemistry Series, vol. 213, American Chemical Society, Washington, DC, 1986. [2] J.E. Glass, Polymers in Aqueous Media, Advances in Chemistry Series, vol. 223, American Chemical Society, Washington, DC, 1989. [3] R.Y. Lochhead, Cosmet. Toiletr. 107 (1992) 131. [4] H. Tanaka, Fragrance J. 8 (1998) 79. [5] R.Y. Lochhead, C.J. Rulison, Colloids Surf. A 88 (1994) 27. [6] H. Fukui, K. Akiyoshi, J. Sunamoto, Bull. Chem. Soc. Jpn. 69 (1996) 3659.
[7] P. Taylor, Colloid Polym. Sci. 274 (1996) 1061. [8] R. Pons, P. Taylor, T.F. Tadros, Colloid Polym. Sci. 275 (1997) 769. [9] S. Ishihata, S. Togiya, M. Ohta, T. Kotani, in: Abstracts of 19th Meeting of the International Federation of Societies of Cosmetic Chemists, 1996, p. 5. [10] T. Ihara, T. Nishioka, T. Kitsuki, H. Kamitani, Chem. Lett. 33 (2004) 1094. [11] E. Akiyama, A. Kashimoto, K. Fukuda, H. Hotta, T. Suzuki, T. Kitsuki, J. Colloid Interface Sci. 282 (2005) 448. [12] K. Kawakami, T. Ihara, T. Nishioka, T. Kitsuki, Y. Suzuki, Langmuir 22 (2006) 3337. [13] K. Kuroda, K. Fujimoto, J. Sunamoto, K. Akiyoshi, Langmuir 18 (2002) 3780. [14] B. Jönsson, B. Lindman, B. Kronberg, K. Holmberg, Surfactants and Polymers in Aqueous Solution, Wiley, New York, 1998. [15] B. Magny, I. Iliopoulos, R. Audebert, L. Piculell, B. Lindman, Prog. Colloid Polym. Sci. 89 (1992) 118. [16] R. Tanaka, J. Meadows, P.A. Williams, G.O. Phillips, Macromolecules 25 (1992) 1304. [17] K. Glinel, J. Huguet, G. Muller, Polymer 40 (1999) 7071. [18] H.S. Kang, S.R. Yang, J.D. Kim, S.H. Han, I.S. Chang, Langmuir 17 (2001) 7501.
Mechanism of oil-in-water emulsification using a water-soluble amphiphilic polymer and lipophilic surfactant Eri Akiyama a,∗ , Akio Kashimoto a , Hajime Hotta a , Tomohito Kitsuki b a Kao Corporation, Tokyo Research Lab, 2-1-3, Bunka, Sumidaku, Tokyo 131-8501, Japan b Kao Corporation, Wakayama Research Lab, 1334, Minato, Wakayama 640-8580, Japan
Received 11 October 2005; accepted 23 March 2006 Available online 3 April 2006
Abstract A new O/W (oil-in-water) emulsification system was developed using the amphiphilic polymer HHM-HEC (hydrophobically–hydrophilically modified hydroxyethylcellulose) and a lipophilic surfactant. HHM-HEC was used as a thickener and polymeric surfactant, and the addition of small quantities of various types of nonionic lipophilic surfactant (hydrophilic–lipophilic balance <5) decreased the droplet size of several types of oil due to a lowering of the tension at the water/oil interface. The oil droplets were held by the strong network structure of the aqueous HHM-HEC solution, preserving the O/W phase without inversion. These stable O/W emulsions were prepared without the addition of hydrophilic surfactants and thus show improved water repellency. © 2006 Elsevier Inc. All rights reserved. Keywords: HHM-HEC; Water-soluble polymer; Oil-in-water emulsion; Lipophilic surfactant
1. Introduction Water based emulsions (oil-in-water (O/W)-type emulsions) have been widely applied to cosmetics and toiletries. Actually they have a good watery feeling when applied to skin. However, conventional O/W emulsions contain a large amount of hydrophilic surfactant that is used to obtain fine emulsified particle. Therefore, these cause uncomfortable feeling and skin irritation and lower water repellency. In order to improve cosmetic formulations, such as lotions, creams, sunscreens, and base make-up liquid foundations, it is important to obtain fine O/W emulsions with minimum amount of hydrophilic surfactant. Furthermore, the best is to obtain fine O/W emulsion without hydrophilic surfactant. Recently, functional water-soluble amphiphilic polymers became highly developed [1–3]. Some of these polymers have thickening and emulsifying abilities and have been used in O/W-type emulsions [4–6]. They can reduce the required quantities of low-molecular-weight hydrophilic surfactants and sta* Corresponding author. Fax: +81 3 5630 9342.
E-mail address: [email protected] (E. Akiyama). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.03.066
bilize the emulsions. However, the performance of conventional water-soluble polymers is limited. There have been several reports of O/W-type emulsification systems that contained conventional water-soluble amphiphilic polymers and hydrophilic surfactants [7,8], as well as polysaccharides and polar organic compounds [9]. However, the O/W-emulsions composed of water-soluble polymers and various kinds of oil without hydrophilic surfactant have been little investigated. We prepared amphiphilic polymer HHM-HEC (sodium hydroxyethylcellulose hydroxypropyl stearylether hydroxypropyl sulfonate), a good thickening and emulsifying agent [10,11]. It is a strong acidic polyelectrolyte with a high molecular weight. It has a good solubility in water, high electrolyte tolerance [12], and high physical stability. Intermolecular aggregation occurs above 0.2 wt% and three-dimensional networks [13] are formed in water through the aggregation of hydrophobic alkyl chains in HHM-HEC. It forms thixotropic and elastic gel above 0.6 wt% [11]. These properties enable O/W emulsions containing various kinds of oil to be stabilized in the absence of surfactants. However, the diameters of the oil particles in the reported HHM-HEC/water/oil emulsions were much larger than those of conventional O/W emulsions (with values rang-
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ing from 5 to 50 µm and a mean diameter of 23.6 µm.) Such a coarse emulsion does not have smoothness and it becomes disadvantage when applied to cosmetics and toiletries. It was difficult to combine “fine O/W emulsion” and “no hydrophilic surfactant” with conventional method. In this study, we achieved the creation of the fine O/W emulsion without hydrophilic surfactant. We have succeeded in reducing the size of the oil particles by adding of small quantities of lipophilic surfactant (hydrophilic–lipophilic balance (HLB) <5) in HHM-HEC/water/oil emulsions. This system is unique in that lipophilic surfactants promote the formation of an O/W emulsion without phase inversion. In this paper we describe the emulsification mechanisms of this system. 2. Materials and methods 2.1. Materials 2.1.1. Water-soluble polymers The water-soluble polymers used in this work are listed: (1) Hydrophilically–hydrophobically modified hydroxyethylcellulose (HHM-HEC). (2) Alkyl chain-bearing HEC (R-HEC). (3) HEC (HEC QP-100MH, Union Carbide Corporation). The structures of compounds (1)–(3) are shown in Fig. 1. Compounds (1) and (2) were prepared by the method reported in [10]. The compound (3) was a raw material for the compounds (1) and (2), and the number of glucose unit (7300) was the same for all compounds (1)–(3). HHM-HEC has both alkyl chain moieties and sulfonic acid moieties. The molecular weights and the substitution degrees of the alkyl chains and the sulfonic acid moieties per one HEC unit are listed in Table 1. We prepared two types of HHM-HEC (A) and (B), but they have no significant differences in substitution degrees and properties.
2.1.2. Oils The oil materials used in this work are listed: (1) Squalane (NIKKOL squalane, NIKKO Chemicals Co., Ltd.). (2) Dimethyl polysiloxane (silicone KF-96A (6cs), Shin-Etsu Chemical Co., Ltd.). (3) Perfluoropolymethylisopropyl ether (FOMBLIN HC/04 Ausimont). 2.1.3. Surfactants The surfactants used in this work are listed: (1) Isostearyl glyceryl ether, GE-IS (HLB 2.5 calculated by the Davies method, HLB 5.0 calculated by the Griffin method, PENETOL GE-IS, Kao Corporation). (2) Dimethicone copolyol (polyoxyethylene methylpolysiloxane copolymer, HLB 2, supplied by Toray Dow Corning Silicone). (3) Polyoxyethylene(20) sorbitan monostearate (HLB 14.9, RHEODOL TW-S120, Kao Corporation). The structures of surfactants (1)–(3) are shown in Fig. 2. 2.2. Preparation of emulsions To prepare the O/W emulsions, HHM-HEC was thoroughly suspended and swollen in water. A mixture of oil and surfactant was then added to the aqueous polymer solution while stirring with a propeller mixer at 400 rpm for 5 min. After this premixing, the emulsions were stirred for 10 min at 25 ◦ C with a laboratory homomixer operating at 5000 rpm. 2.3. Viscosity measurements Viscosity of emulsion was measured using a Toki-Sangyo Model B viscometer (B8L, Rotor No. 4, 6 rpm), which gave measurement errors of <5%.
Table 1 HEC derivatives employed in this work Sample name
Mw
Number of alkyl chain moieties (per monosaccharide unit)
Number of sulfonic acid moieties (per monosaccharide unit)
HHM-HEC (A) HHM-HEC (B) R-HEC HEC
1.8 × 106 1.8 × 106 1.5 × 106 1.5 × 106
0.0033 0.0048 0.0041 0
0.21 0.22 0 0
Fig. 1. Structures of HHM-HEC and HEC.
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Fig. 2. Structures of surfactant.
2.4. Particle size measurements The distributions of droplet sizes in the emulsions were determined using a laser scattering particle size distribution analyzer (HORIBA LA-920). Photomicrographs of the emulsions were obtained using a Nikon OPTIPHOT-2 photomicroscope. Measurements were carried out after appropriate dilution of the sample with water. It was confirmed in advance that the dilution process did not significantly change the particle size. The measurement errors were <0.7%.
(A)
2.5. Interfacial tension measurements The tension of the aqueous polymer solution/oil (including surfactant) interface was measured using a Wilhelmy-type automated surface tension meter (KRÜSS Processor tension meter K122) with a platinum plate. (B)
2.6. Rheological measurements Oscillatory measurements of each aqueous solution of polymer and emulsion were taken with a Modular Compact Rheometer MCR300 (PHYSICA Messtechnik GmbH), which has cone-plate geometry (CP50-1: angle 1.0◦ , diameter 50 mm). All date were collected from a linear viscoelastic region (strain amplitude γ = 1%). The sample was covered by a reservoir filled with water during the measurements to prevent the evaporation of water. 3. Results 3.1. Decrease in particle size induced by lipophilic surfactants The addition of low-HLB surfactants to the HHM-HEC/ water/oil systems caused a large decrease in particle size. Figs. 3A and 3B show optical photomicrograph images of the emulsions with and without surfactants. The distribution of particle size in the emulsions is shown in Figs. 4A and 4B. These emulsions contained 0.5 wt% HHM-HEC (A), 19.5 wt% oil, and 0.5 wt% surfactant. Figs. 5A and 5B show the relationship between the quantity of lipophilic surfactant added and (a) the size of the oil droplets (squalane, dimethyl polysiloxane, and perfluoropolymethylisopropyl ether) and (b) viscosity of the emulsions. These emulsions contain 0.4 wt% HHM-HEC (A) and 20 wt%
Fig. 3. (A) Photomicrographs of HHM-HEC/water/squalane emulsions: (a) without surfactant, (b) with GE-IS (HLB 2.5). (B) Photomicrographs of HHM-HEC/water/dimethyl polysiloxane emulsions: (a) without surfactant, (b) with dimethicone copolyol (HLB 2).
oil. Lipophilic surfactants caused a decrease in size of oil droplets. The smallest oil droplets were obtained with the combination of GE-IS and squalane, or the combination of dimethicone copolyol and dimethyl polysiloxane. High viscosity was also obtained at these combinations. No viscositydecrease was observed and the O/W phase was preserved at high lipophilic surfactant concentrations. Therefore, precise ratios of oil to surfactant are unnecessary in this emulsification system. However, high-HLB hydrophilic surfactants caused a decrease in viscosity and segregation of the emulsion. The viscosity of the HHM-HEC/water/squalane/polyoxyethylene sorbitan monostearate (HLB 14.5) emulsion was 600 mPa s and HHM-HEC/water/dimethyl polysiloxane/polyoxyethylene sorbitan monostearate emulsion was 580 mPa s. (0.4 wt% HHMHEC (A), 19.5 wt% oil, 0.5 wt% polyoxyethylene sorbitan monostearate). Creaming was immediately observed for the both emulsions. 3.2. Interfacial tension Figs. 6A and 6B show the interfacial tension at the aqueous HHM-HEC (A) or HEC solution/oil phase as a function
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Table 2 Effect of type of polymer on state of emulsion (0.5 wt% HEC derivatives, 19.5 wt% squalane, 0.5 wt% GE-IS) Sample name
State of emulsion
Viscosity (mPa s)
Particle size (µm)
HHM-HEC (A) HHM-HEC (B) R-HEC HEC
O/W O/W Creaming Creaming
22,000 23,000 – –
4.0 3.4 5.8 4.4
Table 3 Effect of concentration of HHM-HEC on state of emulsion (19.5 wt% squalane, 0.5 wt% GE-IS) (A)
(B) Fig. 4. (A) Droplet diameter distribution by volume for HHM-HEC/water/ squalane emulsions. (B) Droplet diameter distribution by volume for HHMHEC/water/dimethyl polysiloxane emulsions.
of lipophilic surfactant concentration. In the absence of surfactant the interfacial tension was large. When lipophilic surfactant was added to the oil phase, a significant decrease in the interfacial tension (by approximately a factor of ten) was observed regardless of the type of polymer.
HHM-HEC concentration (wt%)
State of emulsion
Viscosity (mPa s)
Particle size (µm)
0.1 0.2 0.4 0.5 0.8 1.0
Creaming O/W O/W O/W O/W W/O
–
– 6.5 4.1 4.0 4.1 –
620 10,700 10,800 29,800 –
HEC derivatives, 19.5 wt% squalane, 0.5 wt% GE-IS) are shown in Table 2. Small oil droplets (4–6 µm) were obtained for all emulsions, but creaming was observed for R-HEC and HEC emulsions. (Furthermore, stable emulsions with fine particles could not be obtained at any concentration (0.2–1.0 wt%) for the HEC.) Both alkyl chain and sulfonic acid moieties has an effect on state of emulsion containing lipophilic surfactant. As shown in Table 3, fine O/W emulsions were successfully obtained for concentrations of HHM-HEC (A) between 0.4 and 0.8 wt%. These emulsions also contained 19.5 wt% squalane and 0.5 wt% GE-IS. Creaming occurred for low HHM-HEC concentrations (0.1 wt%) because the aqueous phase was too dilute to hold the oil particles, and phase inversion occurred for high HHM-HEC concentrations (1.0 wt%) because the aqueous phase became too viscous to contain the oil particles by stirring.
3.3. Rheology 4. Discussion Oscillatory measurements of the emulsion were carried out to elucidate the relationship among aqueous solution of HHMHEC, the emulsion of the HHM-HEC/water/oil system and the emulsion of the HHM-HEC/water/oil/lipophilic surfactant. The concentration of HHM-HEC in water was constant. (HHMHEC (B) is 0.625 wt% in water.) The results are shown in Figs. 7A and 7B. The aqueous solution of HHM-HEC was predominantly elastic. With regard to Figs. 7Aa and 7Ba, the shape of rheological spectra of the emulsion containing 20 wt% oil were similar to that of the aqueous solution but those of the emulsion containing lipophilic surfactant were a little changed (Figs. 7Ab and 7Bb) 3.4. Effect of polymer type and concentration on state of emulsion The effects of different types of HEC derivatives on the state of O/W emulsions containing a lipophilic surfactant (19.5 wt%
Fine O/W emulsions were formed by lipophilic surfactant and amphiphilic polymer HHM-HEC. Various kinds of oil droplets were minimized by GE-IS or dimethicone copolyol. This phenomenon is characteristic and two important mechanisms in the formation of the emulsification system could be considered: a decrease in interfacial tension caused by lipophilic surfactant and the formation of a three-dimensional network structure by the HHM-HEC polymer in the continuous phase, which holds the emulsified oil particles. 4.1. The role of lipophilic surfactants Figs. 6A and 6B show the lowering of interfacial tension by adding lipophilic surfactants to the oil phase. The size of the oil particles in HHM-HEC/water/oil emulsions decreased with the addition of lipophilic surfactants (Figs. 5Aa and 5Ba). It indicates that lipophilic surfactants are able to decrease in-
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(A)
(B) Fig. 5. (A) Effect of GE-IS concentration on (a) size of oil particles, (b) viscosity. (B) Effect of dimethicone copolyol concentration on (a) size of oil particles, (b) viscosity.
(A)
(B)
Fig. 6. (A) Interfacial tensions at the aqueous solution of polymer/squalane as a function of GE-IS concentration. (B) Interfacial tensions at the aqueous solution of polymer/dimethyl polysiloxane as a function of demethicone copolyol concentration.
terfacial tension as effectively as hydrophilic surfactants even in O/W emulsions and the size of the oil particles decreases with the lowering of interfacial tension. There was no significant difference in the values of interfacial tension between the HEC and HHM-HEC polymers, indicating that the alkyl chains in HHM-HEC do not behave as surfactants [5,6]. Slight adsorption of alkyl chains at oil/water interface can be possible, however it does not have much effect on lowering interfacial tension. The decrease of interfacial tension is simply caused by the assembly of lipophilic surfactant molecules at the interface.
Furthermore, it is preferable that the surfactants are highly soluble in oil phase. When surfactants dissolve well in the oil phase, the assembly of surfactants can be effective (because mobility of surfactants is high) and fine emulsified particles can be obtained. GE-IS dissolves well in squalane, while it does not dissolve in dimethyl polysiloxane or perfluoropolymethylisopropyl ether. Likewise, dimethicone copolyol dissolves only in dimethyl polysiloxane. As a result, the smallest oil droplets and high viscosity of emulsion were obtained with the combination of GE-IS and squalane, or the combination of dimethicone copolyol and dimethyl polysiloxane (Fig. 5A and 5B).
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(A)
(B) Fig. 7. (A) The storage modulus, G (closed), and loss modulus, G (open), of (a) aqueous solution of HHM-HEC and HHM-HEC/water/squalane emulsion; (b) aqueous solution of HHM-HEC and HHM-HEC/water/squalane/GE-IS emulsion at 25 ◦ C. (B) The storage modulus, G (closed), and loss modulus, G (open), of (a) aqueous solution of HHM-HEC and HHM-HEC/water/dimethyl polysiloxane emulsion; (b) aqueous solution of HHM-HEC and HHM-HEC/water/dimethyl polysiloxane/dimethicone copolyol emulsion at 25 ◦ C.
The system described here takes advantage of the poor solubility of lipophilic surfactants in water. In general, the thickening ability of self-assembly polymer is canceled by the addition of large quantities of hydrophilic surfactant. The hydrophobic domains, which are cross-linking point of threedimensional gel network, dissociate due to adhesion of the hydrophilic surfactant to the hydrophobic moiety [14–16]. The decrease in viscosity of the HHM-HEC/water/oil emulsion on addition of hydrophilic surfactants (polyoxyethylene(20) sorbitan monostearate) is thought to be caused by the same effect. Low-molecular-weight hydrophilic surfactants have a high mobility in water and could therefore adhere to the hydrophobic domains in the HHM-HEC network. Conversely, lipophilic surfactants are poorly soluble in water and the hydrophobic domains of the HHM-HEC network are preserved. As a result, the viscosity of the emulsion does not decrease.
4.2. The role of the HHM-HEC network structure in the continuous phase In general, low-HLB surfactants are used to make W/O-type emulsions. Indeed, GE-IS (HLB 2.5) and dimethicone copolyol (HLB 2) are known as W/O-type surfactants. However, in our case the emulsions remain O/W-type without the occurrence of phase inversion at high lipophilic surfactant concentrations and thus differ from conventional emulsions that consist of only low molecular weight media. The type of emulsion (O/W or W/O) is not determined by the morphology of the surfactants (the hydrophobic–hydrophilic balance), but by the rheological properties of the polymers in the continuous phase. Ishihata et al. reported that the thixotropy of the continuous phase had an effect on the stability of O/W emulsions containing glyceryl monostearate [9]. The aqueous solution of HHM-HEC shows
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Fig. 8. Schematic drawings of (a) aqueous solution of HHM-HEC, (b) HHM-HEC/water/oil emulsion, (c) HHM-HEC/water/oil emulsion with lipophilic surfactant.
non-Newtonian flow behavior, and has a high yield value (i.e., thixotropic). Oscillatory measurements reveal that the aqueous solution of HHM-HEC is predominantly elastic (the value of G was larger than that of G ) above 0.6 wt% [11]. Thixotropic and elastic gel is formed by the aggregation of alkyl chains in HHM-HEC structure. The gel structure of HHM-HEC in the continuous phase has the ability to hold oil particles, including lipophilic surfactants (Figs. 8a and 8c) [10,11]. Phase inversion was avoided due to the low mobility of the HHMHEC polymers in the continuous phase, which is derived from a high-molecular-weight, semi-rigid backbone [17] and strong intermolecular aggregates of HHM-HEC. Emulsions with highly expanded networks in the continuous phase are not subject to phase inversion. When only oils were emulsified in aqueous solution of HHM-HEC (0.625 wt% in water), the shape of rheological spectrum of the emulsion was similar to that of the aqueous solution of HHM-HEC (Figs. 7Aa and 7Ba). The viscoelastic properties of HHM-HEC in disperse medium is overwhelming and dominates the rheological properties of the emulsion. On the other hand, when oils including lipophilic surfactant were emulsified, the elastic properties became strong (Figs. 7Ab and 7Bb). These rheological properties were based on both an emulsification by surfactant and network structures of HHMHEC. Using a preparative method in which oil and a lipophilic surfactant are added to a HHM-HEC solution, emulsions of the O/W-type are predominantly made. The aggregation of hydrophobic moieties of HHM-HEC in water takes place prior to emulsification, and the C18 alkyl chain domains in HHM-HEC are considered to exist solely as cross-linking point in water, not to assemble at the oil/water interface like low-molecular-weight surfactants [18]. The oil particles are emulsified in the HHMHEC network structure under the condition of low interfacial tension promoted by the lipophilic surfactants. In this system we conclude that several properties of the polymer in aqueous solution are important. These are structure, concentration, elasticity and thixotropy. This is explained by the observations that both HEC and R-HEC were unable to form stable O/W emulsions with lipophilic surfactants (Table 2). Stable emulsions with fine particles could not be obtained at 0.2– 1.0 wt% HEC. An aqueous solution of HEC is a viscous fluid (not elastic) at 1.0 wt% and has no yield value [11]. 0.5 wt% HEC emulsion showed apparent creaming even when it does
not contain lipophilic surfactant [11]. An aqueous solution of R-HEC is thixotropic and elastic, but the solubility of R-HEC in water is low and the expansion of network structure is not sufficient. In fact, the aqueous solution of R-HEC was slightly turbid [11]. The states of network of HEC and R-HEC in continuous phase are not able to hold oil including lipophilic surfactants. Similarly, the semi-dilute aqueous solution (0.1 wt%) of HHM-HEC was also unable to form stable O/W emulsions (Table 3). HHM-HEC was not thixotropic or elastic below 0.2 wt% (in the region of intramacromolecular association of HHMHEC) [11]. On the other hand, between 0.4 and 0.8 wt%, fine O/W emulsions were obtained. This concentration region is corresponding to the region of intermolecular association of HHM-HEC [11]. Therefore, in order to make stable O/W emulsions using water-soluble polymers and lipophilic surfactants, it is necessary to choose a suitable type of polymer in an appropriate quantity, otherwise phase inversion, creaming or segregation easily occurs. 5. Summary Fine O/W emulsions were obtained using a HHM-HEC/ water/oil/lipophilic surfactant system. It is supposed that the three-dimensional networks formed by HHM-HEC play a vital role in this system. The strong network structure of HHM-HEC is able to hold oil particles, including the lipophilic surfactant, without the occurrence of phase inversion. Conventional polymers were unable to form O/W emulsions with lipophilic surfactants because their network structures and viscoelastic properties were not sufficient to hold oil particles in a stable fashion. This system utilizes the advantages of lipophilic surfactants: their ability to lower interfacial tension and their poor solubility in water. Low interfacial tension decreases the diameter of the oil droplets and poor solubility prevents dissociation of the hydrophobic domains of HHM-HEC in water. When this type of emulsion is applied to the skin, it has a comfortable watery feeling and shows improved water repellency because it does not contain hydrophilic surfactants. (Re-emulsification is inhibited.) There are many potential applications of such systems for cosmetics and toiletries. An investigation of the effects of the degree of alkyl chain and sulfonic moiety substitution in HHM-HEC on the properties and stability of the emulsions are now in progress.
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