J. Meyer, R. Scheuermann, H.H. Wenk: Simple Processing of PEG-free Nanoemulsions and Classical Emulsions

Transcription

1 English Edition International Journal for Applied Science Personal Care Detergents Specialties J. Meyer, R. Scheuermann, H.H. Wenk: Combining Convenience and Sustainability: Simple Processing of PEG-free Nanoemulsions and Classical Emulsions

2 J. Meyer, R. Scheuermann, H.H. Wenk* Combining Convenience and Sustainability: Simple Processing of PEG-free Nanoemulsions and Classical Emulsions Keywords: oil-in-water nanoemulsions, impregnating lotions, PEG-free emulsifier system Convenience and Sustainability A trend towards convenient cosmetic products that can easily be applied is reflected in a growing number of cosmetic wet wipes products (e.g. for baby care or make-up removal) and sprayable emulsions. For stability reasons, an exceptional small droplet size is necessary to obtain low viscous emulsion systems with a good stability profile. Classical O/W emulsions are not able to fulfill the stability requirements as their typical droplet size is too big to prevent creaming of the oil droplets. Finely dispersed emulsion droplets, however, do not undergo sedimentation or creaming because these processes are prevented by the Brownian motion of the small droplets. Such emulsions typically have a blue shining appearance and are called nanoemulsions. Different to microemulsions (typically translucent), nanoemulsions are just kinetically stable and the required production procedures are very specific. Basic differences between microemulsions, nanoemulsions and classical emulsions are illustrated in Table 1. The conventional process for manufacturing nanoemulsions is the PIT method (Phase Inversion Temperature method) which utilizes the temperature-dependent hydrophilicity of ethoxylated emulsifiers (1-3). The use of ethoxylated emulsifiers, however, is seen more and more as a disadvantage. As consumers increasingly prefer natural ingredients in cosmetics, the personal care market is extremely interested in more natural emulsions free of Introduction Anew technology allows easy manufacturing of low-viscosity oil-inwater nanoemulsions without using ethoxylated emulsifiers, without applying heating or cooling steps and without using a homogenizer. The technology is based on a phase inversion process upon dilution with water. The resulting nanoemulsions are highly attractive as impregnating lotions for wet wipes. The possibilities and the mode of action of this new technology are highlighted in this article. Moreover, it is shown how the major benefits could be transferred into a novel cold processable PEG-free emulsifier system that allows the simple production of classical oil-in-water emulsions. Microemulsion Nanoemulsion Classical Emulsion Table 1 Types of o/w emulsion systems Appearance / Typical Particle Size Type of Stability Processing Limitations 58 SOFW-Journal

3 ethoxylated ingredients. Moreover, manufacturing of PIT emulsions requires additional energy consumption for heating and cooling steps. An ideal solution to combine convenience and sustainability aspects therefore should work without a homogenizer, without heating/cooling steps and without using ethoxylated emulsifiers. PIC Emulsion Technology The new PIC emulsion technology fulfills all these requirements. The process is based on oil phases consisting of emollients, emulsifiers and cosurfactants that enable the formation of nanoemulsions upon simple dilution with water. So far two commercial products (1, 2) are available on the market. Both are based on 65-70% of an emollient (Diethylhexyl Carbonate), 20-25% of emulsifiers (Polyglyceryl-4 Laurate, Dilauryl Citrate) and 12% of a cosurfactant. Cosurfactants are interfacially active substances but are not surfactants as they do not form micelles on their own. They are also not emulsifiers as they do not stabilize emulsions. However, their ability to integrate into interfacial films is crucial for this new technology. Surprisingly, the well-known preservative Phenoxyethanol proved to be particularly efficient as a cosurfactant in PIC emulsion systems. Of the two commercially available products, one uses a combination of Phenoxyethanol with alkyl parabens (1), the other one is paraben-free (2). Both products show the typical phase behavior of PIC emulsion systems (Fig. 1). Without any water addition both products based on Diethylhexyl Carbonate consist of 88% emollient/emulsifier mix and 12% cosurfactant (e.g. Phenoxyethanol). Upon dilution with water, a translucent microemulsion-like phase is passed at a ratio of oil phase to water of about 50:50. This phase initially appears to be a microemulsion, however, after standing for some days at room temperature, it separates into a real microemulsion phase and an excess water phase. Nevertheless, within this microemulsion-like phase the interfacial tension between oil- and water phase is very low which leads to its Fig. 1 Dilution process in PIC emulsions translucent appearance (very fine droplet size!). When more water is added to the system, nanoemulsions with a very fine particle size form spontaneously (at above 80% water content in the system). The amount of cosurfactant in the total system is crucial for the whole process as it determines whether a microemulsionlike phase is passed in the dilution process. Only when such a phase is passed, nanoemulsions with a very fine particle size can be obtained. When an emollient with lower polarity is used (e.g. Isohexadecane), a lower content of cosurfactant is needed to pass the microemulsion-like phase in the ternary Fig. 2 Particle size and stability of the DE-system phase diagram. When an emollient with higher polarity (e.g. C12-15 Alkyl Benzoate) is used, typically more cosurfactant is needed. Additionally, in this case it is typically necessary to work with more hydrophilic emulsifiers (e.g. Polyglyceryl-6 Laurate instead of Polyglyceryl-4 Laurate). The microemulsion-like phases for Isohexadecane and C12-15 Alkyl Benzoate are schematically shown in Fig. 1. Fig. 2 shows the small particle size of a nanoemulsion obtained by simple dilution of the DE-system1). Moreover, the excellent long-term stability is reflected by the fact that the particle size does not SÖFW-Journal

4 change upon 3 months storage. The particle size was determined by dynamic light scattering (DLS) (Malvern HPPS 3.1) of nanoemulsions containing 5.7% of the DE-systems (diluted down to 0.5% right before the measurement in order to get reliable results). Such nanoemulsions can easily be used as impregnating lotions for the preparation of cosmetic wet wipes. A formulation example for a facial wipe is given in Table 2. It is based on 5.7% of the DEsystem and therefore contains 0.7% of a combination of Phenoxyethanol and alkylparaben esters. Additionally, it includes the natural occurring amino acid derivative Creatine which plays a vital role in the energy metabolism of skin cells (4). In order to understand the formation process of the nanoemulsions, the phase behavior of the DE-system upon dilution with water was examined visually and by viscosity and conductivity measurments in tap water and in demineralized water. The results are shown in Fig. 3. When using tap water, the conductivity curves show a steep increase once the emulsion has inverted from W/O to O/W. However, it is more interesting to look at the other two curves. The viscosity curve shows two maxima and a pronounced minimum in-between. Similar curves are known from phase inversion processes based on PIT technology using ethoxylated emulsifiers (5). The viscosity minimum is located in the middle of the microemulsion-like phase (50-55% water content). This minimum can be attributed to a bicontinuous system where neither oil nor water is the outer phase of an emulsion (»zero curvature«of the interfacial film on a macroscopic scale) (6). Such bicontinuous systems are known for their flexibility and their ultralow interfacial tension (7). The conductivity data in demineralized water nicely corresponds with its two maxima and one minimum in-between to the viscosity data. The first maximum occurs in the microemulsion-like phase just in the range where the viscosity minimum appears. This observation can be explained by the change from W/O curvature to a bicontinuous structure, leading to a clearly improved conductivity. At higher water concentrations (around Impregnating Lotion for Facial Wipes Phase A: TEGO Wipe DE Diethylhexyl Carbonate; Polyglyceryl-4 Laurate; 5.70% Phenoxyethanol; Methylparaben; Dilauryl Citrate; Butylparaben; Ethylparaben; Propylparaben; Isobutylparaben) Phase B: TEGO Cosmo C 100 Creatine 0.25% Ethanol 0.50% Demineralized Water 93.55% Processing: Charge the vessel with phase A and add phase B with simple stirring (Important: It is possible to charge the vessel with phase B and add phase A with stirring). Table 2 Formulation example 60%), a conductivity minimum appears together with a viscosity maximum. This effect might be caused by the presence of W/O or bicontinuous domains within an external water phase before the phase inversion is fully completed (5,6). The conductivity decrease at concentrations above 60% demineralized water is then rather expected since it simply reflects the dilution effect of the fully inverted O/W emulsion. As the phase inversion from W/O to O/W occurs at a certain water concentration (~ 55%), it is appropriate to talk about a Phase Inversion Concentration and consequently about PIC emulsions. Fig. 3 Phase behavior of the TEGO Wipe DE System (at 25 C) In PIT emulsions, the phase inversion is a transitional process caused by a continuous change of the interfacial film as a result of the change in hydrophilicity of ethoxylated emulsifiers upon temperature change. A temperature reduction can therefore lead to an inversion process from W/O to O/W emulsions (passing a region with»zero-curvature«and ultralow interfacial tension in-between). In PIC emulsions no such temperature dependent effects are required. The transitional phase inversion is mainly triggered by two components: 1) A suitable emulsifier mixture consisting of a primary nonionic emulsifier 60 SÖFW-Journal

5 which is an O/W emulsifier with suitable hydrophilicity (e.g. Polyglyceryl- 4 Laurate) and a secondary emulsifier that shows a somewhat ionic character in an aqueous phase (e.g. Dilauryl Citrate). 2) A suitable cosurfactant consisting of a small polar head group and a bulky hydrophobic moiety with a certain water solubility. As the water content in the system increases, the cosurfactant is extracted from the interface into the water phase. Within this process the interfacial film curvature inverts from W/O to O/W passing a bicontinuous structure with»zero curvature«(fig. 4). PIC emulsions therefore make use of the phase behavior of intelligent combinations of cosmetic raw materials. In this way they enable to produce nanoemulsions without using ethoxylated ingredients, homogenizers or heating or cooling steps. Transfer to Classical Emulsions As PIC emulsion technology allows to manufacture cosmetically suitable nanoemulsions in a very simple process, a very simple question arises: is it possible to design a»pic emulsifier«that results in nanoemulsion formation regardless of the emulsion composition? Looking at the phase diagrams illustrated in Fig. 1 leads to a very quick answer. No, it is definitely not possible to fulfill that as PIC emulsion technology makes use of the phase behavior of a system. In other words: changing the emollient leads to a different phase behavior and the total systems need to be adjusted in order to obtain a system with PIC behavior. However, what is possible is to use the know-how gained on PIC nanoemulsions systems and transfer it to classical emulsions. By doing that, a novel PEG-free liquid emulsifier system for the easy production of classical O/W emulsions has been developed. The LTP-emulsifier (3) (»Low Temperature Processing«) combines the very efficient Fig. 4 Role of the cosurfactant Fig. 5 The LTP-emulsifier Proposed Mechanism W/O»zero curvature«o/w emulsifying system of PIC emulsions (Polyglyceryl-4 Laurate, Dilauryl Citrate)- a pasty product with excellent selfemulsifying properties - with a liquid component with basic emulsification properties (Sorbitan Laurate) (Fig. 5). As this new emulsifier is fully based on nature-derived ingredients it is ECOCERT approved. The LTP-emulsifier can be used for a wide range of formulations (sprays, lotions and creams for e.g. skin care, sun care, baby care) and is suitable for cold processed systems as well as for hot processed creams. Typically it can be used without any co-emulsifiers at concentrations of %. An example of a typical cold processed O/W body lotion is shown in Table 3. Highly Efficient PIC Emulsifying System One particular quality of this new emulsifying system is that it also allows the cold processing of O/W lotions and sprays using simple stirring equipment. A simple counterflow impeller stirrer (Fig. 6) is used at a very low speed (just 900 rpm) for this purpose. When such stirrer is used for the manufacturing of W/O emulsions as a homogenizer, it is typically used at a speed of rpm. As homogenizers for O/W emulsions on a laboratory scale, systems with significantly higher homogenization intensity such as an Ultra Turrax (Ika, Germany) are typically used. Regardless of the low energy input, the LTP-emulsifier allows to obtain stable O/W lotions in such a simple cold process. A formulation example is given in Table 4. LTP-Emulsifier Very efficient and versatile cold processable O/W emulsifier Liquid Emulsifier Basis 62 SÖFW-Journal

6 Light O/W Lotion Cold Processing Phase A: TEGO Care LTP Sorbitan laurate, Polyglyceryl-4 Laurate, 1.50% Dilauryl Citrate TEGOSOFT CI Cetearyl Isononanoate 10.00% TEGOSOFT DEC Diethylhexyl Carbonate 3.50% TEGOSOFT OP Ethylhexyl Palmitate 1.10% TEGO Carbomer 140 Carbomer 0.15% TEGO Carbomer 141 Carbomer 0.15% Xanthan Gum 0.10% Phase B: TEGO Cosmo C 100 Creatine 0.50% Glycerin 3.00% Water 80.00% Phase C: Sodium Hydroxide (10 % in water) q. s. Phase Z: Parfum, Preservative q.s. Processing: 1. Mix ingredients of phase A. 2. Combine phase A and B without stirring. 3. Homogenize. 4. Add phases C and Z and stir well. Viscosity = 18 Pas (Brookfield RVT Spincle 4/5 rpm) (25 C) Table 3 Formulation example Cold processed PEG-free O/W lotion (without homogenization step) Phase A: TEGO Care LTP Sorbitan laurate, Polyglyceryl-4 Laurate, 2.0% Dilauryl Citrate TEGOSOFT OP Ethylhexyl Palmitate 9.60% Mineral Oil (30 mpas) 9.00% TEGO Carbomer 140 Carbomer 0.10% TEGO Carbomer 141 Carbomer 0.10% Phase B: Water, demineralized 75.00% Glycerin 3.00% Phase C: Sodium Hydroxide (10 % in water) q. s. Phase Z: Parfum, Preservative q.s. Processing: 1. Add phase B to phase A without stirring. 2. Stir for 5 minutes (MIG stirrer 900 rpm). 3. Add phases C and Z. 4. Stir again for 5 minutes (MIG stirrer 900 rpm). Table 4 Formulation example Fig. 6 MIG stirrer (counterflow impeller stirrer) Fig. 7 shows the differences in droplet size (micrographs), visual appearance (photographs) and viscosity. Visually both emulsions have a homogeneous appearance and a bright white color (no difference is observed). The micrograph pictures reveal the expected difference in particle size that is, however, not reflected in any weakness in the stability profile of the emulsions. From a viscosity point of view, stronger homogenization leads to lower emulsion viscosity as the Carbomer network in the aqueous phase of the emulsions is shear-sensitive. In general it can be said that the LTPemulsifier allows the simple production of O/W emulsions in a more sustainable production process as energy savings are obtained as no heating or cooling steps are needed and optionally - no homogenizer has to be used. Conclusions Understanding the interfacial behavior of cosmetic raw materials can lead to innovative solutions for cosmetic products. Due to the interfacial properties of Phenoxyethanol it was possible to develop a new technology for the easy production of cosmetic nanoemulsions. This PIC emulsion technology results in nanoemulsions upon simple dilution with water without using ethoxylated ingredients, heating or cooling steps or a homogenizer. Moreover, it was possible to transfer the benefits of the efficient emulsifier system from these PIC emulsion systems to SÖFW-Journal

7 classical emulsions. In this way, it was possible to develop a flexible, easy-touse and cost efficient PEG-free emulsifier for the simple production of classical O/W emulsions. This new emulsifier even allows the production of typical O/W lotions by just using simple stirring equipment. Both developments show that is very well possible to satisfy the market trend towards more convenient products and at the same time pushing for more sustainable cosmetic ingredients and production processes. SIMPLE STIRRING HOMOGENIZED Footnotes: (1) TEGO Wipe DE (INCI: Diethylhexyl Carbonate, Polyglyceryl-4 Laurate, Phenoxyethanol, Methylparaben, Dilauryl Citrate, Ethylparaben, Butylparaben, Isobutylparaben) (2) TEGO Wipe DE PF (INCI: Diethylhexyl Carbonate, Polyglyceryl-4 Laurate, Phenoxyethanol, Dilauryl Citrate) 0.2% Carbomer 140/141 η = 21 Pas* (Brookfield Sp. 4/5 rpm) Fig. 7 Emulsions prepared with and without homogenizer 0.2% Carbomer 140/141 η = 14 Pas* (Brookfield Sp. 4/5 rpm) (3) TEGO Care LTP (INCI: Sorbitan Laurate, Polyglyceryl-4 Laurate, Dilauryl Citrate) References (1) K. Shinoda, H. Saito, J. Colloid Interface Sci., 30 (1969), 258 (2) T. Förster, F. Schambill, W. von Rybinski; J. Disp. Sci. Technol., 13 (1992), 183 (3) T. Engels, T. Förster, W. von Rybinski, Colloids and Surfaces A, 99 (1995), 141 (4) U. Wollenweber, P. Lersch, SÖFW Journal, 130 (3) (2004), 11 (5) J. Allouche, E. Tyrode, V. Sadtler, L. Chopin, J. L. Salager, Langmuir, 20 (2004), 2134 (6) J. Meyer, G. Polak, R. Scheuermann; Cosmetics & Toiletries, 122 (1) (2007), 61 (7) R. Strey, Current Opinion in Colloid and Interfacial Science, 1 (1996), 402 * Author s address: Jürgen Meyer Ralph Scheuermann Hans Henning Wenk Evonik Goldschmidt Personal Care Essen Germany juergen.meyer_dr@evonik.com 64 SÖFW-Journal