Cosmetic emulsion preparations

    The solubilization of comparably small amounts of oil components in rinse and shampoo formulations demonstrates the basic emulsification properties which alkyl polyglycosides should be expected to show as nonionic surfactants. However, a proper understanding of phase behavior in multicomponent systems is necessary in order to evaluate alkyl polyglycosides as powerful emulsifiers in combination with suitable hydrophobic coemulsifiers.In general, the interfacial activity of alkyl polyglycosides is determined by the carbon chain length and, to a lesser extent, by the degree of polymerization (DP). Interfacial activity increases with the alkyl chain length and is at its highest near or above the CMC with a value below 1 mN/m. At the water/mineral oil interface, C12-14 APG shows lower surface tension than C12-14 alkyl sulfate lnterfacial tensions of n-decane, isopropyl myristate and 2-octyl dodecanol have been measured for pure alkyl monoglucosides (C8,C10,C12) and their dependence on the solubility of alkyl polyglycosides in the oil phase has been described. Medium-chain alkyl polyglycosides may be used as emulsifiers for o/w emulsions in combination with hydrophobic co emulsifiers.

   Alkyl polyglycosides differ from ethoxylated nonionic surfactants in that they do not undergo temperature-induced phase conversion from oil-in-water (O/W) to oil-in-water (W/O) emulsions.Instead, the hydrophilic/lipophilic properties can be balanced by mixing with a hydrophobic emulsifier such as glycerin mono-oleate (GMO) or dehydrated sorbitol mono-laurate (SML).In fact, the phase behavior and interfacial tension of the alkyl polyglycoside emulsifier system are very similar to those of the conventional fatty alcohol ethoxylates system if the mixing ratio of hydrophilic/lipophilic emulsifier in the non-ethoxylated system is used instead of temperature as the key phase behavior parameter.

   The system for dodecane, water, Lauryl Glucoside and Sorbitan Laurate as a hydrophobic coemulsifier forms microemulsions at a certain mixing ratio of C12-14 APG to SML of 4:6 to 6:4 (Figure 1). Higher SML contents lead to w/o emulsions whereas higher alkyl polyglycoside contents produce o/w emulsions. Variation of the total emulsifier concentration results in a so-called “Kahlweit fish” in the phase diagram, the body containing three-phase microemulsions and the tail single-phase microemulsions, as observed with ethoxylated emulsifiers as a function of temperature.The high emulsifying capacity of the C12-14 APG/SML mixture as compared with a fatty alcohol ethoxylate system is reflected in the fact that even 10 % of the emulsifier mixture is sufficient to form a single-phase microemulsion.

   The similarity of phase inversion patterns of the two surfactant types is not only limited to the phase behavior, but also can be found in the interface tension of the emulsifying system.The hydrophilic – lipophilic properties of the emulsifier mixture reached equilibrium when the Ratio of C12-14 APG/SML was 4:6, and the interfacial tension was the lowest. Notably, a very low minimum interfacial tension (approx. 10^-3 mN/m) was observed using the C12-14 APG/SML mixture.

   Among alkyl glycosides containing microemulsions, the reason for the high interfacial activity is that hydrophilic alkyl glycosides with larger glucoside-head groups and hydrophobic co-emulsifiers with smaller groups are mixed at the oil-water interface in an ideal ratio. Hydration (and the effective size of the hydration head) is less dependent on temperature than is the case with ethoxylated nonionic surfactants. Thus, parallel interfacial tension is observed only for the slightly temperature-dependent phase behavior of the non-ethoxylated emulsifier mixture.

This provides interesting applications because, unlike fatty alcohol ethoxylates, alkyl glycosides can form temperature-stable microemulsions. By varying the surfactant content, the type of surfactant used, and the oil/water ratio, microemulsions can be produced with specific properties, such as transparency, viscosity, modification effects, and foaming properties. Co-emulsifier in the mixed system of alkyl ether sulphate and non-ion, the expanded microemulsion area is observed, and can be used to formulate concentrate or fine particle oil-water emulsions.

An evaluation has been made of pseudoternary phase triangles of multicomponent systems containing alkyl polyglycoside/SLES and SML with a hydrocarbon (Dioctyl Cyclohexane) and alkyl polyglycoside/SLES and GMO with polar oils (Dicaprylyl Ether/Octyl Dodecanol),They demonstrate the variability and extent of areas for o/w, w/o or microemulsions for hexagonal phases and for lamellar phases in dependence upon the chemical structure and mixing ratio of the components. If these phase triangles are superimposed on congruent performance triangles indicating for example foaming behavior and viscosity properties of the corresponding mixtures, they provide a valuable aid for the formulator in finding specific and well-designed microemulsion formulations for e. g. facial cleansers or refatting foam baths. As an example, a suitable microemulsion formulation for refatting foam baths can be derived from the phase triangle.