Behaviour of a lyotropic-smectic liquid crystal between mica surfaces A study of the fully hydrated egg-lecithin

Abstract

The behaviour of three egg-lecithin (EL)—water (W) mixtures, 60% (w/w):I, 50% (w/w):II and 30% (w/w):III, squeezed out or sucked in between two mica surfaces was investigated using an apparatus designed by J.N. Israelachvili. Mixture I is a lyotropic-smectic liquid crystal (LSLC), in contrast to II and III in which the LSLC is in equilibrium with a separated phase of excess water. The force F applied to the micasheet holders, the “separation” D between the mica surfaces and the average refractive index were measured for 1 < D < 200 nm. A stepwise-thinning process was mainly observed on compressing mixtures I and II from separation 30 < D < 60 nm down to the minimum reproducible separation D min , 9.0 ± 0.7 nm, which corresponded to the flattening of the mica surfaces. The steps Δ D were equal to multiples of λ = 5.5 ± 0.4 nm for sample I and of λ = 5.3 ± 0.3 nm for sample II. The contact area of the flattened-mica sheets increased with the force F at separation D min . Assuming that an effective elastic parameter can be defined for our composite system (mica sheet + glue), we interpret the F —contact-area dependence using the theory of Johnson et al. and deduce values of average normal pressures or stresses of the order of 10 mN m −2 on the flat mica—mica contact. At such pressures the values of λ and the repeat distance of the LSLC formed by EL + W and measured by X-ray diffraction are similar. The refractive index n of I is independent of separation for D min < D < 50 nm but its value increases when D decreases for II and III. It appears that excess water is selectively extruded when F increases and D decreases. We suggest that, at maximum stress and D min = 9 nm, the two mica surfaces are separated by two poorly hydrated EL bi-layers, while the EL polar groups in contact with the mica surface are almost completely dehydrated.