Intermolecular interactions and the structure of fatty acid-soap crystals

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 P2(1)/n space group, with a = 9.906 Angstrom, b = 7.163 Angstrom, c = 45.580 Angstrom, beta = 92.78 degrees, 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.