Soap and detergent molecules have a similar structure consisting of a polar ionic ‘head’ group and a non-polar hydrocarbon ‘tail’. This molecular structure allows these molecules to form ‘bridges’ between polar and non-polar phases. When the thickener concentration gets large enough, the soap molecules self-assemble into micelles; this concentration is known as the critical micelle concentration (CMC). Micelles are discrete particle-like structures as opposed to a three-dimensional fibrous network structure. Nagarajan defined a packing factor to describe the structure of micelles that self-assemble in an aqueous solution.5 In water, a polar solvent, the polar heads of the molecules are hydrophilic (compatible with water), and the nonpolar tails are hydrophobic (incompatible with water). The packing factor was based on the length of the tail and the area of the head, assuming that the hydrophobic tails point towards each other and away from the polar water solvent. Packing factors predict spherical or cylindrical micelles, or even bilayer structures. Lubricating grease is a non-aqueous system, and the self-assembly of soap thickener molecules is not so straightforward because the hydrophobic tails face outward from the micelle to interact with the non-aqueous solvent. The self-assembled soap structure is described as a reverse or inverse micelle as shown in Figure 2.
Figure 2 Stylized picture of an inverse or reverse micelle along with a 3D molecular model from a computer simulation. Although micelles are pictured as spherical structures, there is a continuum of structures based on the concentrations of water, oil, and surfactant (soap). This continuum includes spherical, cylindrical, lamellar, bicontinuous, inverse, and other mixed structures. Because there is a continuum of shapes associated with thickeners in oil, what shape do the thickener particles actually have in greases? How do these shapes affect grease properties? Particle Shape Theory According to Saatchi et al., thickeners form cylindrical (fibrous), plate-like, or spherical structures in oil. How do these structures relate to grease bleed? As in the work by Saatchi et al., a thickener structure can be modeled as n independent spherical particles arranged in a simple cubic lattice and spaced a distance a apart. The particles have a radius of r with an associated hydrodynamic volume that increases the radius to r’ .[ ] The oil held in this hydrodynamic volume moves, within the assumptions of this model, rigidly with the particles. Oil not associated with the hydrodynamic volume is free to flow. The parameters n, a, r, and r’ have not been set yet. The structure is given in Figure 3.
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