³¹P nuclear magnetic resonance studies on alkyl phosphate emulsifiers in cosmetic oil‐in‐water emulsions

Abstract

Cosmetic oil-in-water emulsions with a stearyl phosphate emulsifier are examined by means of static and dynamic 31 P nuclear magnetic resonance (NMR) techniques to characterize the molecular properties of the emulsifier in situ. The interfacially bound emulsifier can be detected by highresolution NMR spectroscopy, whereas the excess emulsifier exists as a solid lipid phase not detectable by this technique. The emulsions and the emulsifier raw material, consisting of monostearyl phosphate as well as distearyl phosphate, are examined by solid state cross polarization magic angle spinning NMR spectroscopy to prove the existence of solid emulsifier phases in the emulsions. By applying dynamic 31 P NMR methods to the interfacially bound emulsifier, information about the molecular dynamics at the interface is obtained. The results of the T 1 and T 2 relaxation time measurements indicate a restricted motion of the molecules that is dependent on the oil droplet size in the emulsions. This is verified by 31 P NMR pulsed gradient spin echo self-diffusion measurements on emulsions with different droplet sizes. Only about 5 wt% of the total emulsifier used is bound at the interface; the excess forms solid lipid phases. The coverage of the interface seems to be independent of the emulsifier concentration. Only the monoester of the emulsifier raw material shows interfacial activity. Its mobility indicates the two-dimensional environment of the molecules on the surface of the oil droplets.Emulsifiers are amphiphilic molecules that, owing to their hydrophilic and lipophilic molecular segments, can be solvated at the interface of immiscible liquids, thus reducing the interfacial tension. The development of emulsion-based cosmetic products requires efficient emulsifier systems that ensure the stability of the dispersion even after three years of storage. Although the composition of the formulation ingredients is determined mainly from a cosmetic viewpoint, the emulsifier must also stabilize the emulsion independently over a wide range of solution variables such as the pH, characteristics of the oil phase, and the salt concentration (1).In the case of oil-in-water emulsions (o/w emulsions) anionic emulsifiers of the form R-(CH 2 ) n -CH 3 (n = 12-20, R = carboxylate, sulfate, phosphate) are frequently used. Furthermore, the ability of many ionic emulsifiers to form aggregates in the water phase (also denoted as an emulsifier network) is an important stabilization factor for o/w emulsions because of a steric hindrance of droplet coalescence. For the development of new products and formulations, a better comprehension of the molecular properties of the emulsifier and the emulsifier's interfacial interactions is essential.Nuclear magnetic resonance (NMR) spectroscopic methods are frequently applied to complex surfactant systems such as colloids, microemulsions, and emulsions because noninvasive measurements make in situ examinations possible (2-5). NMR spectra of the molecules reflect the chemical environment of the nuclei, which enab...

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