Polar ordering of smectic liquid crystals within the interfacial region

The triblock organosiloxane liquid crystalline 4-(pentamethyl-disiloxanyl-n{{-}pentyloxy){-}4}^{{'}}$-cyanodiphenyl, which forms a smectic ${S}A$ phase in the bulk, was investigated at the air-water interface. Imaging null ellipsometry was employed for visualization of morphology and thickness determination. The surface pressure-area $({π}{-}A)$ isotherm exhibits several plateaus, each corresponding to the formation of an interdigitated bilayer. The system is ideally suited for assessment of the influence of the interface on the order and structure of the smectic bulk phase. By further compression all subsequently formed bilayers are of identical thickness and the value matches the layer spacing of the SA phase. Despite these apparent similarities, deviations between bulk and interfacial regions were revealed by optical second-harmonic generation. In contrast to the bulk, the first and second bilayers contain an excess of dipoles as well as molecules with tilt. The transition region between the bulk and interface is composed of at least two bilayers. The findings were further verified by surface potential measurements.