Cross-Sectional Area Increase at Phase Transition on Compression: An Unexpected Phenomenon Observed in an Amide Monolayer

• Published in: Journal of physical chemistry. C 2010-09, Vol.114 (37), p.15695-15702
• Main Authors: Brezesinski, G, Stefaniu, C, Nandy, D, Dutta Banik, S, Nandi, N, Vollhardt, D
• Format: Article
• Language: English
• Subjects: C: Surfaces, Interfaces, Catalysis
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp104358z

At low temperatures (T ≤ 10 °C), the surface pressure−area (π−A) isotherms of some amphiphilic amides reveal a striking second critical point indicating the existence of a second phase transition between two condensed phases. Studies of 3-hydroxy-N-tridecyl propanoic acid amide (HTPA) monolayers have shown that this phase transition between two condensed phases is accompanied by an abrupt change of important 2D lattice parameters. Thorough grazing incidence X-ray diffraction (GIXD) studies have revealed a new phenomenon. The cross-sectional area (A 0) of the alkyl chain of HTPA jumps from a smaller value in the phase at the lower surface pressure to a larger value in the phase existing at higher pressure. The opposite behavior should be expected and is usually noted in the lattice structures of Langmuir monolayers. The phase diagram of the HTPA monolayers is constructed on the basis of the equilibrium π−A isotherms. The fact that two condensed monolayer phases exist and the phase observed above the fluid/condensed transition at higher temperature has a structure similar to that observed at high pressure and low temperatures, as observed by the GIXD experiments, can be thermodynamically explained by the generalized equation of state for Langmuir monolayers for the existence of two condensed phases. The IRRA spectra did not show a substantial change in the band positions below and above the phase transition between the two condensed states, but differences in the dichroic ratio indicate changes in the hydrogen bonding system. The computational studies, using the ab initio level of theory for the headgroup and the molecular mechanical theory for the alkyl chain part, provide a reasonable explanation for the unexpected, novel finding that the cross-sectional area of the alkyl chains jumps to larger values at the phase transition on increasing pressure. It is shown that shortening of the hydrogen bond separation at higher pressure in the headgroup region obviously drives the increase in separation between the alkyl chains, in reasonable agreement with the GIXD results.