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

Cosmetic oil-in-water emulsions with a stearyl phosphate emulsifier are examined by means of static and dynamic ³¹P nuclear magnetic resonance (NMR) techniques to characterize the molecular properties of the emulsifier in situ. The interfacially bound emulsifier can be deteced by high-resolution 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 distearly 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 ³¹P 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 ³¹P 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.