Influence of Surfactants on the Rheology and Stability of Crystallizing Fatty Acid Pastes

Abstract

Complex fluids containing crystallizing fatty acids are important for consumer care products. The key features of these materials are their ability to support their weight under gravity due to the formation of a fatty acid crystal network, and to yield or flow beyond a critical applied strain. In model formulations comprised of two synthetic surfactants and a fatty acid in water, we have shown that the fatty acid crystal network consists of crystal aggregates linked by a non-crystallized mixed fatty acidsurfactant mesophase. We hypothesize that this mixed surfactant-fatty acid mesophase is critical for the macroscopic stability of the formulations. Rheological measurements combined with differential scanning calorimetry (DSC), X-ray scattering, and polarized light microscopy (PLM) measurements show the importance of surfactant loading on the overall stability of the formulations by linking morphology to rheology. Macroscopically homogeneous formulations are realized with 7-10 wt% of fatty acid. Increasing the fatty acid content without adding surfactant leads to inhomogeneous, phase separating formulations. Although both stable and unstable formulations show the presence of a surfactant-fatty acid mixed phase, a critical loading of surfactants is found to be necessary to create macroscopically homogenous formulations. We demonstrate how the rheology, microstructure and the macroscopic stability can be tuned by varying the relative amounts of surfactants and fatty acid.

Coagel Prepared from Aqueous Octyl β‐d‐Galactoside Solution

“…The coagel consists of entangled three-dimensional crystalline networks with enormous surface area covered with water [3,4]. Such structuring of crystalline architectures in a water system or a water/oil dispersion system confers a progressive increase in hardness or modifies rheological properties [5][6][7][8]. Therefore, the coagel matrices can be potentially used for various applications in the food, pharmaceutical, or cosmetic industries.…”

“…The solubility line of surfactants in the temperature-concentration phase diagram is called the ''Krafft boundary'' [1,2]. Below the boundary, the surfactant molecule precipitates as macroscopically opaque solids or the so-called coagel [1][2][3][4][5][6][7][8][9][10][11][12].…”

“…The formation of the coagel phase has been widely studied for both ionic and nonionic surfactants including fatty acid salts, phospholipids, sodium dodecyl sulfate (SDS), monoglycerides (MG), and ascorbic acid derivatives [1][2][3][4][5][6][7][8][9][10][11][12].…”

Water droplets as fillers: the effect of a dispersed aqueous phase on the rheology and microstructure of fat crystal-stabilized water-in-oil emulsions

“…For example, sorbitan monostearate has been observed to inhibit chocolate fat bloom (Lawler and Dimick, 2002) and alter the crystallization of tristearin (Aronhime et al, 1988). The opposite effect has been observed as well with different emulsifiers where the additives act as crystallization "seeds" to accelerate crystallization and/or alter polymorphic behaviour (Lawler and Dimick, 2002;Thareja et al, 2012).…”

A constitutive equation for thixotropic suspensions with yield stress by coarse‐graining a population balance model

“…Thixotropic media are frequently encountered in large‐scale industrial processes including oil pipeline‐transport, red‐mud in minerals processing, and waste processing at superfund sites . In addition, thixotropic agents are prominent in consumer applications, such as beauty creams, and have even been proposed for gelled hydrocarbon fuel applications . Printing inks and even blood can be considered thixotropic .…”