thermochimica acta ELSEVIER Thermochimica Acta 267 (1995) 159-167 Phase transition of capillary condensed liquids in porous silica: effect of surface hydroxyl groups’ T. Takei*, T. Konishi, M. Fuji, T. Watanabe,,M. Chikazawa Department of Industrial Chemistry, Faculty of Engineering, Tokyo Metropolitan University, l-1. Minami-osawa, Hachioji, Tokyo 192-03, Japan Received 7 December 1994; accepted 6 March 1995 Abstract Phase transition temperatures of capillary condensed organic liquids in porous silica were measured using a 1020/DSC system (Perkin-Elmer Ltd.). The amounts and types of hydroxyl groups on the sample surfaces were precisely controlled to discuss the surface effect on the phase transition temperature. The melting point of the solid phase of benzene in the pores decreased with increasing concentration of the surface hydroxyl groups. On the other hand, in the case of n- hexane, such an effect was not observed. From these results, it is assumed that the liquid structure of capillary condensed benzene in the pores changes with the amounts and types of surface hy- droxyl groups. This consideration is confirmed from IR spectroscopy. Keywords: Phase transition; Capillary condensed liquid; Silica; Surface hydroxyl group 1. Introduction The physical properties and structure of capillary condensed liquid in a small pore are different from those of bulk liquid [ 1,2]. For example, the density [3] and heat capacity [4] of water confined in silica pores change as a function of pore size. It is suggested that the physical properties and structures of capillary condensed liquid in a pore are affected by surface forces of a solid. Thus, the degree of change in the physical properties and structures of capillary condensed liquid in a pore reflects the strength of the interaction between capillary condensed liquid and a solid surface. Porous silica materials are generally used as an adsorbent. It is well known that ad- sorption properties of silica materials remarkably change with their surface structure [5- * Corresponding author. ’ Presented at the 30th Anniversary Conference of the Japan Society of Calorimetry and Thermal Analysis, Osaka, Japan, 31 October-2 November 1994. 0040-603 l/95/$09.50 0 1995 - Elsevier Science B.V. All rights reserved SSD10040-6031 (95) 02475-l