Intermolecular Interactions and the Structure of Fatty Acid−Soap Crystals

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Abstract

Single crystals of NaHP2 (sodium hydrogen dipalmitate) have been prepared from mixtures of NaP (sodium palmitate) and HP (palmitic acid) in ethanol. The phase compound crystallizes in the P21/a space group, with a = 9.906 Å, b = 7.163 Å, c = 45.580 Å, β = 92.78°, and 4 molecules per unit cell. The arrangement of the headgroups is unique among known soap and fatty acid structures by accommodating both hydrogen bonding and electrostatic interactions. Carboxylate and acid-like pairs couple perpendicular to the bilayer to accommodate a “short” hydrogen bond, and the sodium is shared among laterally adjacent carboxylate anions to create a pseudo-six-member ring, which adds to the crystal stability. Molecular dynamic (MD) simulations establish a set of interaction parameters to describe the structure and energetics of acid−carboxylate bonds, which accounts for the solid-state miscibility of HP and NaP. Infrared data, collected by ATR-FTIR on very pure, powdered NaH2P3, Na2H3P5, and NaHP2 acid−soap standards reveal peculiar changes in the carbonyl and hydroxyl spectral regions. These spectra were analyzed on the basis of crystal structure and MD simulation data.

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