Structure and rheology of cetomacrogol creams: the influence of alcohol chain length and homologue composition.

Liquid paraffin-in-water emulsions prepared with cetomacrogol 1000 and alcohols cetostearyl (A), cetyl (B), steryl (C) and myristyl (D) were examined by microscopical, particle size analytical and rheological (continuous shear, small strain creep, and oscillation) techniques at 25 degrees as they aged over 30 days. The particle sizes of the emulsions were similar and did not increase significantly with age. Thus the rheological stabilities were not correlated with particle size distributors, but rather with viscoelastic networks formed in the continuous phases when the non-ionic mixed emulsifiers interacted with water. The rheological properties of emulsions B and D differed from those of emulsion C. Emulsion A, of mixed homologue composition, showed some properties similar to each of the pure alcohol emulsions. Emulsions B and D were semi-solid immediately after preparation whereas emulsion C was so mobile initially that small strain data were not derived. On ageing, the consistencies of B and D changed slightly initially, and then remained essentially constant. In contrast, the consistency of emulsion C increased on ageing, especially over the first few days when there was a change from mobile liquid to semisolid. Emulsion A was a semisolid initially but like emulsion C increased in consistency especially over the first 24 h. Continuous shear data indicated that this emulsion was the most resistant to structure breakdown. Microscopical examination supported the view that the networks formed in emulsion A were the most extensive and that stearyl alcohol networks in C formed comparatively slowly. Although the cetomacrogol/pure alcohol networks were diffuse and sometimes crystallized, they did not rapidly disintegrate on storage as did the ionic surfactant/pure alcohol networks examined previously.