Isotropic to smectic A phase transitions in a porous matrix: a case of multiporous phase coexistence

The one-dimensional smectic ordering of the liquid crystal 10CB incorporated in the pores of a silica aerogel has been investigated via x-ray scattering. Although the smectic order is made short-ranged by the aerogel host and the amplitude of the associated Bragg-like peak grows continuously with decreasing temperature, part of the first-order character of the 10CB's direct isotropic-smectic phase transition is retained in the discontinuous temperature dependence of the smectic correlation length. This behaviour contrasts with that of materials where the smectic phase develops from a locally orientationally ordered nematic and can be interpreted as a nucleation-type process.