Comparative Thermodynamic Behavior of Physically Restricted Cyclohexane and Cyclohexanone in Porous Silica
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ABSTRACT We undertook comparative differential scanning calorimeter (DSC) measurements on cyclohexane (C6H12) and cyclohexanone (C6H100), physically confined in porous silica of pore radius 4, 7.5, 15, 30, and 62.5 nim, with a view to ascertain how guest fluid-surface host interactions affected the thermodynamic properties of the confined fluids. Our results can be summarized as follows: (a) No distinct signature of freezing or melting transition was observed for the physically confined cyclohexanone, irrespective of whether the bulk was present outside the pores. However, this was not the case for cyclohexane. (b) The solid-to-solid transition temperature of cyclohexane and cyclohexanone inversely scaled with the pore radius of the host porous silica. (c) The cubic-to-orthorhombic transition of cyclohexanone was strongly influenced by whether the bulk fluid was present outside the pores. In the absence of the bulk, the transition temperature was considerably suppressed relative to the bulk transition temperature. However, in the presence of the bulk, the confined and the bulk transitions occurred at the same temperature. INTRODUCTION When materials are physically confined in ultra small pores [radius (Rp) < 100 nm], their physical and thermodynamic properties considerably change relative to their bulk properties. Phase transition temperatures, e.g., melting and freezing, of physically confined materials are affected by their small sizes and their interaction with the surface of the host confining material. Awschalom et al. [1] probed the molecular dynamics of physically restricted liquid oxygen in porous glasses and found that the freezing temperature of the confined oxygen was lower than its bulk value. In fact, the difference in the transition temperature, AT ( AT = TBU k- Tcoimod, where TBui, and Tcn are the transition temperatures of the bulk and confined fluid, respectively), was found to scale inversely with the pore size of the host glass. In agreement with their results, Mu and Malhotra [2] from their calorimetric studies on cyclohexane confined in porous silica reported that not only the melting temperature was reduced but also the monoclinic-to-FCC transition temperature of the cyclohexane was substantially depressed. Even though both transitions showed linear dependence between AT and Rp', they had different slopes. Jackson and McKenna 3 reported that the heat of fusion of non-polar organic compounds was considerably reduced when the liquids were confined in porous glasses. Similar behavior was observed for cyclohexane physically confined in porous silica [2]. Despite the consistent experimental observations for physically restricted fluids, i.e., linear relationships between AT and Rp",, the interpretations and conclusions have been controversial [17]. For example it has been difficult to separate the effects of finite size from those of surface interaction between the confined fluid and confining solid wall. In an effort to further understand the roles of surface structure and finite size, Malhotra et
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