Structure and rheology of semisolid o/w creams containing cetyl alcohol/non-ionic surfactant mixed emulsifier and different polymers.

Oil-in-water (o/w) emulsions for cosmetic use, such as lotions and creams, are complex multiple-phase systems, which may contain a number of interacting surfactants, fatty amphiphiles, polymers and other excipients. This study investigates the influence of two synthetic cationic polymers, Polyquaternium-7 and Polyquaternium-11, and the natural anionic polymer, gum of acacia, on the rheology and microstructure of creams prepared with a non-ionic mixed emulsifier (cetyl stearyl alcohol-12EO/cetyl alcohol) using rheology (continuous shear, and viscoelastic creep and oscillation), microscopy and differential scanning calorimetry (DSC). A control cream containing no polymer was also investigated. The semisolid control cream was structured by a swollen lamellar gel network phase formed from the interaction of cetyl alcohol and the POE surfactant, in excess of that required to stabilize oil droplets, with continuous phase water. Endothermic transitions between 25 and 100 degrees C were identified as components of this phase. Incorporation of cationic polymer into the formulation caused significant loss of structure to produce a mobile semisolid containing larger oil droplets. The microscopical and thermal data implied that the cationic polymer caused the swollen lamellar gel network phase to transform into non-swollen crystals of cetyl alcohol. In contrast, incorporation of gum of acacia produced a thicker cream than the control, with smaller droplet sizes and little evidence of the gel network. Microscopical and thermal data implied that although there were also interactions between gum of acacia and both the surfactant and the swollen gel network phase, the semisolid properties were probably because of the ability of the gum of acacia to stabilize and thicken the emulsion in the absence of the swollen lamellar network.