Submitted to: Chemistry and Physics of Lipids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 22, 2006
Publication Date: March 24, 2006
Citation: Schmidt, W.F., Barone, J.R., Francis, B.A., Reeves III, J.B. 2006. Stearic acid solubility and cubic phase volume. Chemistry and Physics of Lipids. 142(1-2):23-32.
Interpretive Summary: Stearic acid is the predominant fatty acid in triacylglycerols of beef fat and coconut oil (present as the ester). The free acid is used routinely in many commercial products in addition to foods. It is used in polymer formulations as an extrusion aid. As the magnesium stearate in tablets, it helps keep the solid ingredients from falling apart in the bottle, and it also enables the tablet to break apart and release the active ingredient when the tablet is swallowed. The free acid can also be burned as a biofuel. Much work has also been done explaining its properties in aqueous systems such as in lipid membranes or in micelles. The interactions of stearic acid in an non-aqueous chemical environment with other non-aqueous chemicals has not been adequately addressed. Solids researchers have proposed that stearic acids exist in a cubic phase, i.e., they self associate in a group of parallel aligned molecules. We demonstrate that the same explanation holds for predicting the solubility of stearic acid in four of five structurally different solvents. The cubic phase can contain half its volume as a structurally different solvent. The fifth solvent (acetonitrile), however, appears to strongly disrupt the cubic array. Results suggest that molecules which disrupt the stearic acid cubic array may also add to or detract from its desired physical properties in commercial products formulations.
Gel, liquid crystal, and cubic phases have been used to explain some properties of lipid molecules. The cubic phase explanation can be applied to explain the stoichiometry of stearic acid (SA) solubility. A saturated solution is the maximum packing within a volume of liquid that can exist without forming a solid phase. SA solubility was investigated in five structurally diverse organic solvents at six temperatures. At 55 degrees C near both the melting point of SA and the boiling point of the solvents, in four of the five solvents, the data demonstrates that the solvent molecules and SA each occupied a comparable amount of space. Molecular mechanics calculations indicate 25 columns of SA pack uniformly and align as aggregates held together by van der Waals forces. In a range of hydrophilic and hydrophobic organic solvents, with the exception of acetonitrile, each replaces individual columns of SA in the cubic phase with solvents. The lower the temperature is from the solvent boiling point and the SA melting point, the more solvent/solvent interactions define intermolecular forces and molecular packing. SA in the cubic phase appears to orient solvent molecules parallel to SA by van der Waals forces. The same explanation does not work with acetonitrile because it appears to strongly disrupt the cubic phase of SA.