Measurements have been made of the surface tension of solutions of sodium dodecyl sulphate, dodecyltrimethylammonium bromide, decanol, and of n-dodecyl and n-tetradecyl hexaoxyethylene monoethers in pure formamide and in salt solutions. Adsorbed films of the ionic surfactants exert low surface pressures (about 15 mN m-1 near the solubility limit) and exhibit no discontinuity characteristic of micellar aggregation. The monoethers exert surface pressures up to 25 mN m-I and show a critical micelle concentration of 0.039 mol dm-3 (CI2E6) and 0.013 mol dm-3 (ClsEs) with aggregation numbers of about 40 and 64 respectively at 25°C. The lower surface activity and higher solubility of the surfactants in formamide compared with water indicate the lower stability of adsorbed and micellar states in the former solvent. Similarly the aggregation numbers of the micelles of monoethers in formamide are much lower than those in water.

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Synopsis The manufacturing of toiletries is a complex procedure and many process routes are possible for a given formulation. In the past, considerable attention has been given to the design of manufacturing equipment but there has been relatively little appreciation of the influence of physicochemical interactions during processing. These interactions are likely to be particularly important during the manufacture of dispersions such as emulsions and pastes where phase and interfacial behaviour or particle interactions can be affected by the introduction of electrolytes or surface active species. For emulsions consisting of cetyl trimethyl ammonium chloride/long chain alcohol/water, the final rheological behaviour is governed by the formation of a 'frozen' lamellar structure in the continuous phase. The phase changes occurring during processing can significantly affect the degree of structure obtained. With powder dispersions, the rheology is affected by the introduction of surface active species due to deflocculation of the particles. Even in complex product systems such as toothpastes, these changes due to powder/detergent interactions are still apparent.