Energy and Entropy of Formation of Crystalline 1-Pentadecanoic Acid from the Ideal Gaseous Monolayer at the Air/Water Interface

The Helmholtz free energies of compression of monolayers of 1-pentadecanoic acid at the air/aqueous HCl solution interface have been estimated from the two-dimensional gas region to the equilibrium spreading pressure at three temperatures. The free energies are used to obtain the energy and the entropy of formation of the crystalline fatty acid from an ideal two-dimensional gas at the aqueous interface. By comparison with published data on the sublimation of solid fatty acids, the energy and entropy of adsorption of the ideal three-dimensional fatty acid vapor to the ideal two-dimensional vapor at the air/aqueous HCl interface are estimated. The sum of the calculated changes of entropy for the loss of one degree of translational freedom and for the loss of the entire configurational entropy of the alkyl chain is close to the estimated standard entropy of adsorption.