Fatty acids and acid/soap crystals have a bilayer structure, which is held together by hydrogen bonding between head groups. The present ab initio Car-Parrinello molecular dynamics calculations suggest that proton tunneling or sharing along such hydrogen bonds could be the key structural features in the acid/soap crystal but not in the pure acid. This effect seems to be due to sodium cations, which induce a dramatic shortening of the hydrogen bond and its signature should be visible in the experimental vibrational spectra. (C) 2003 American Institute of Physics.