Liquid Crystals Confined in Nanoporous Silica

Confinement has a nontrivial effect on a condensed matter system due to the finite size effect, the surface effect and the induced disorder. Different techniques such as calorimetric measurement, dielectric spectroscopy, dynamic light scattering, X-ray scattering and quasi-elastic neutron scattering have been widely used to study this important topic. Confined liquid crystals (LCs) have attracted great attention in particular because they are a soft system and their physical properties are more easily affected by the geometrical confinement (entropic) effect and the surface interaction (energetic) effect. In this paper, we study the phase transitions and dynamic behavior of LCs, 5CB and 8CB, confined in SBA-15 and MCM-41 mesoporous silica with different pore diameters ranging from 2.3 nm to 25 nm using static and dynamic light scattering and dielectric spectroscopy techniques. SBA-15 and MCM-41 are composed of uniform 2- dimensional hexagonal arrays of cylindrical pores with a sharp pore size distribution. MCM-41 has non-interconnected pores, while only microporous interconnections are found in SBA-15 pores. Such simple geometry can reduce the confusion introduced by the varying pore size and pore interconnectivity in other matrices, which may alter the properties of interest and complicate the interpretation of the experimental results. Moreover, the pore size can be tuned easily by changing the synthesis conditions. This allows us to systematically study the pore size effect on the confined fluids. The experimental results reveal how the geometric confinement and strong interfacial force affect the phase transition temperature and other physical properties of LCs when the effective dimensionality is changing from 3D to 1D.