Infrared external reflection spectroscopic studies of phase transitions in Langmuir monolayers of heneicosanol

Abstract

We report the results of an infrared external reflection spectroscopic study of phase transitions in Langmuir monolayers of heneicosanol. One of our main goals was to examine monolayers which were pure and at equilibrium. The monolayers we have studied were fully relaxed, judging from the exceptionally good long term stability demonstrated by each state on the isotherms. In addition, our fully relaxed systems displayed good reversibility, though each compression–expansion cycle showed a small hysteresis, which may possibly be a real physical property of these systems. The phase boundaries displayed in these fully relaxed systems differed in number and location from those in the partially relaxed systems usually studied. Our second goal was to advance the utility of infrared external reflection spectroscopy for the study of Langmuir monolayer structure, especially molecular conformation and orientation, by examining the hydrocarbon CH2 symmetric and antisymmetric stretching modes as a function of surface pressure, temperature, etc. In pursuit of this goal we have exploited nonimaging optics to improve the signal to noise in these difficult experiments. Our studies of the relaxed structure of Langmuir monolayers of heneicosanol have uncovered new and interesting phase behavior. In general, we find a clear trend showing that at high surface pressure and low temperature the hydrocarbon chains are rather highly ordered, perhaps all-trans; at low surface pressure and high temperature the hydrocarbon chains become somewhat disordered, containing a higher concentration of gauche defects, though that concentration is still lower than that found in the fluid phase. In all of the cases studied the region of the isotherm immediately prior to the highest density phase can be associated with a distinct ordering transition of the hydrocarbon chain. Regarding the molecular orientation, we have obtained data which are often in disagreement with the current x-ray data, the latter taken on what appear to be partially relaxed monolayers. For example, as the surface pressure increases from 0 to 13 dyn/cm for our 20 °C monolayer the hydrocarbon chains continuously become more ordered, and the molecules retain a collective tilt of perhaps 30°–50°. However, the current view is that along this isotherm there is a continuous tilting transition as the surface pressure increases in the range mentioned. We have rationalized this difference by assuming a kinetic model in which the collective tilting of the molecules can respond to a change in area much more quickly than can a reduction in gauche conformation concentration. Finally, we have found that there are no liquid phases present when the surface pressure is nonzero. In this way fatty alcohol monolayers are quite different from phospholipid monolayers. In fact, even at nearly zero surface pressure and fairly low density the fatty alcohol monolayer still exists as a mesophase.