Synchrotron X-ray investigations into the lamellar gel phase formed in pharmaceutical creams prepared with cetrimide and fatty alcohols

Semisolid liquid paraffin-in-water emulsions (aqueous creams) prepared from cetrimide/fatty alcohol mixed emulsifiers, and ternary systems formed by d ispersing the mixed emulsifier in controlled percentages of water were exam ined as they aged using a combination of low and high angle X-ray diffracti on measurements (Daresbury Laboratory Synchrotron Radiation Source). The re sults were correlated with the rheological properties measured in earlier s tudies. The cationic emulsifying wax showed phenomenal swelling in water. T he reflection that incorporates interlamellar water increased continuously from 74 Angstrom at 28% water to over 500 Angstrom at 93% water. The trend was not influenced by the method of incorporation of the components and swo llen lamellar phase was also identified in the corresponding emulsion. The swelling, which was due to electrostatic repulsion, was suppressed by salt and was reduced when the surfactant counterion was changed from Br- to Cl-. Changes in rheological properties on storage and in the presence of salt w ere correlated with changes in water layer thickness. High angle diffractio n confirmed that the hydrocarbon bilayers were in the hexagonal alpha-cryst alline mode of packing. Ternary systems and creams prepared from pure alcoh ols, although initially semisolid, were rheologically unstable and broke do wn. Low angle X-ray study into the kinetics of structure breakdown showed t hat the swollen lamellar gel phase formed initially swells even further on storage before separating. (C) 2000 Elsevier Science B.V. All rights reserv ed.