Effect of headgroup dissociation on the structure of Langmuir monolayers

We present results of grazing incidence X-ray diffraction studies concernin g the effect of pH (and thus, of headgroup dissociation) on Langmuir monola yers of fatty acids, in the absence of any divalent cations in the aqueous subphase. An increase in pH transforms the distorted-hexagonal S phase firs t to the partially disordered Rotator-I phase with less distortion, and the n to the completely disordered Rotator-II phase with an undistorted hexagon al structure. The S-Rotator-I and Rotator-I-Rotator-II transitions are push ed to lower temperatures with increase in pH. The fact that the effects of pH increase are almost identical to the effects of increasing temperature i ndicates the important role of headgroup-headgroup interactions in these mo nolayer phases.