Chemistry and physics of lipids

Stearic acid solubility and cubic phase volume.

PMID 16616908


Stearic acid (SA) is highly soluble in structurally diverse solvents. SA/solvent packing within a (24.8 A)3 cubic volume explains the stoichiometry of SA solubility at multiple temperatures in multiple solvents. In the absence of solvent, the cubic volume contains 25 molecules at van der Waals distances from each other. At 55 degrees C, SA occupied half the cubic volume in saturated solution of four structurally diverse solvents. Below 4% SA/volume (e.g. in acetonitrile), the head and foot of each SA molecules on average is more than one solvent molecule away from the head and foot of a neighboring SA molecule. At 50% SA/cubic volume, -CH2- groups on SA molecules are separated from neighboring -CH2- groups on SA molecules by a monolayer of solvent molecules. Lowering the temperature from 55 to 25 degrees C, the volume fraction of SA decreased by a factor of 2 (or more) for every 6 degrees C. Lowering temperature increased the relative number of column of solvent molecules in the cubic phase, and correspondingly, the distance between SA molecules within the cubic volume increased. In three of five solvents, molecular mechanics calculations demonstrated the van der Waals stabilization that occurs from SA/SA affinity in the absence of solvent is similar in magnitude to the van der Waals stabilization from SA/solvent affinity. Methyl-t-butyl ether was less stabilized than hexane, acetone or methanol because the more bulky molecules packed less efficiently within the cubic volume. The most efficient/most stable packing however was still as columns of solvent between columns of SA. The efficiency and stability of SA and solvent packing optimal within the (24.8 A)3 cubic volume. Between 100 and 8% SA, multiple SA molecules present within the cubic volume function as SA aggregates. Both inter- and intra-cubic (phase) volume properties of SA aggregates coexist. Although acetonitrile and SA at the molecular level are both rod shaped, acetonitrile disrupted the packing of SA molecules within the cubic phase. The disrupted packing explains the much lower solubility of SA in acetonitrile than in the other solvents. The same molecular structures (e.g. methanol) can either stabilize or disrupt the packing of aggregated SA molecules, depending upon temperature. The mechanisms of aggregation within cubic volumes could also occur with structurally more complicated lipids. Aggregation and dispersion from such cubic phases could also be present in more complex chemical and/or macromolecular environments.