The cooling process effect on the bilayer phase state of the CTAC/cetearyl alcohol/water surfactant gel

Gel formation by surfactant molecules is generally controlled by thermodynamics rather than kinetics. Mixtures of long-chain alcohols and surfactants are widely used in various cosmetic and pharmaceutical products. The cooling rate used to produce the products is known to be a key parameter to control the rheological properties and the final texture. However, the physical-chemistry links between the cooling process and the rheological properties are still unknown. Here, we investigated the mechanical properties and microstructure of a surfactant gel formulated with cetyltrimethylammonium chloride (CTAC) and cetearyl alcohol in water by using a multi-technique approach involving rheology, small-angle X-ray scattering (SAXS) and diffusion NMR. The gels, formed by two cooling processes fast and slow cooling, respectively from a high-temperature mixture, have locally a lamellar structure with periodic repeat distances of 31.4 and 28.5 nm for fast and slow cooling, respectively. The cross-polarized microscopy images reveal the presence of multilamellar vesicles (MLVs). These data on the supramolecular aggregates allow rationalizing the mechanical behavior of the two samples. In fact, the elastic, G′, and the viscous, G′′, moduli were detected to be ca. 4 times higher for the fast-cooling (quenched) sample than for the slow-cooling (relaxed) sample.