Intermolecular Interactions and the Structure of Fatty Acid−Soap Crystals

Single crystals of NaHP 2 (sodium hydrogen dipalmitate) have been prepared from mixtures of NaP (sodium palmitate) and HP (palmitic acid) in ethanol. The phase compound crystallizes in the P 2 1 / 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 NaH 2 P 3, Na 2 H 3 P 5, and NaHP 2 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.