(497c) Rheology as An in-Situ Technique for Studying the Effect of Fatty Acid Crystallization

Skin-care creams are complex systems whose behavior under flow directly affects their tactile properties or their sensory perception. An industrial skin cream formulation is a complex multi-component system consisting of multiple surfactants and additives in water. To study such a complex system, we have designed an experimental model formulation with two synthetic surfactants and a fatty acid in water, which can be readily formulated in a reproducible manner for laboratory investigations. The fatty acid chosen (palmitic) exists as crystals at room temperature. As crystal nucleation and growth are known to strongly depend on thermal and shear history, a well controlled thermal and shear history is required for reproducible formulations. In particular, mixing in simple shear is challenging due to wall slip accompanying the high viscosities during crystallization. To avoid slip and to enhance mixing, we employ a starch pasting cell (SPC) consisting of a rotating impeller with blades in a temperature controlled cylindrical cup. The SPC tool is shown to be rheologically accurate under the conditions of operation by calibration with both Newtonian and viscoelastic rheology standards. The formulations are made in the starch pasting cell by dissolving and mixing components under shear at 70°C, followed by controlled cooling under shear to room temperature. Further evolution of the microstructure is probed at room temperature small amplitude oscillatory shearing (SAOS). The SPC cell provides a very reproducible method of both formulating and simultaneously monitoring the structure build. SPC rheometry combined with complementary measurements of sample morphology by optical, X-ray and Small Angle Neutron Scattering (SANS) show that a high storage modulus (G'~ 10⁵Pa) is obtained due to the formation of a solid fatty-acid crystal network. The formation of a solid fatty acid crystal network is demonstrated by complementary DCS and polarized light microscopy measurements, which show the presence of birefringent lamellar phases at higher temperature, followed by the formation of fatty acid crystals at 55°C. The crystals further grow and aggregate forming a network as the temperature is reduced to 25°C. A minimum critical wt% of fatty acid is required to achieve percolation and build high modulus structures (of the order of 5-7 wt%). The application of strong shearing during formulation leads to low modulus samples. The dynamics of morphology and rheology development are demonstrated by the breadth of techniques applied.