Influence of quenched disorder created by nanosilica network on phase transitions in tetracosane

Calorimetric measurements on composites of a long chain n-alkane, tetracosane, doped with nanosilica aerosil particles decorated with a corona of hydrophobic/hydrophilic nature are described. The weakly perturbing random field created by the addition of the aerosil particles has the general effect of weakening all the transitions to, and between, the three different rotator phases that the pure alkane exhibits. One of highlights of the studies is that the strong first order transitions of the pure alkane, viz., from the isotropic liquid to hexagonal rotator phase (Iso-R2), and tilted monoclinic rotator phase to the crystalline phase (R5-Cr), are accompanied by a much weaker supplementary peak at lower temperatures. In analogy with observations made in aerosil composites of liquid crystalline systems, and additionally with information from preliminary Xray diffraction measurements on the currently studied materials, it is reasoned that the subsidiary peaks are nothing but the transformations between the same phases as the corresponding main peaks due to the bulk of the material, but occurring in the vicinity of the surface of the aerosil particles. The nature of the corona of the aerosil particles also seems to play an important role. For the bulk transitions, the reduction in the transition enthalpy is higher for the composites with the hydrophilic particles, than those with hydrophobic particles; the opposite is true for the surface transitions. The data also suggest that the lowering of the transition temperature of the surface transition with respect to its bulk counterpart is more for the hydrophobic composites than the hydrophilic ones. We provide an explanation for these features based on the surface interactions between the particles and tetracosane molecules, and also the strength of the gel network.