Remarks concerning the relation between rotator phases of bulk n-alkanes and those of Langmuir monolayers of alkyl-chain surfactants on water

Substantial advances have recently been made in characterizing the structures and phase diagrams of Langmuir monolayers of alkyl-chain surfactants on water. These allow detailed comparisons to be made with the bulk phases formed by n-alkanes, in particular, the rotator phases. It is shown that the untilted Langmuir phases denoted CS, S, and LS, correspond to the herringbone-crystal, distorted rotator without long-range herringbone order (R-I) and hexagonal rotator (R-II), respectively. The quantitative behavior of the distortion and area/molecule in the Langmuir monolayers, their derivatives and jumps at the transitions are shown to correspond more closely to the behavior of n-alkane mixtures where the interlayer interaction is weakened than to that of the pure n-alkanes. It is shown that the tilted phases can be associated with the same three categories regarding distortion and herringbone order that characterize the untilted phases, if the distortion is measured perpendicular to the chain-axis. An ''anomalous'' reduction of positional order with decreasing temperature in the hexagonal phase of Langmuir monolayers is shown to correspond to the same phenomena. observed in bulk n-alkane mixtures. While, due to experimental limitations, it has not been possible to observe the long-range herringbone order in the Langmuir monolayers, the quantitative comparisons to bulk systems where the weak herringbone peak has been observed allow us to associate the S-CS transition with the onset of long-range herringbone order.