Effects of surface and physical confinement on the phase transitions of cyclohexane in porous silica

Abstract

We have measured the specific heat (C~) of cyclohexane at 120& T &300 K when cyclohexane was physically restricted in porous Spherosil (silica) samples of pore radii 4, 7.5, 15, 30, and 62.5 nm. The behaviors of the monoclinic-to-cubic structural transition and of the melting transition of cyclohexane were determined.As expected, both transition temperatures, i.e. , solid-solid and melting, inversely scaled with the pore radius (R~}. It is argued that the surface heterogeneity, the presence of hydroxyl groups, and the radius of curvature (especially for smaller pores) induce considerable disorder in the adsorbed layers of cyclohexane, thus, resulting in the nucleation of crystalline grains of various sizes rather than in a single crystalline plug of cyclohexane in the porous silica samples. The distribution of the grain boundaries of crystalline cyclohexane produces a specific-heat peak for the melting transition, which almost resembles the A, shape for R~3 0 nm. Unlike the monoclinic-to-cubic transition, the k anomaly of the melting transition does not reflect a logarithmic dependence near the transition temperature. It is also argued, from the comparative observed C~b ehavior of the bulk cyclohexane with the physically restricted cyclohexane, that the cyclohexane liquid is more viscous than the bulk when it is confined in pores of 4 and 7.5 nm.

Temperature Evolution of Sodium Nitrite Structure in a Restricted Geometry

“…there is large and important group of restricted geometry objects, namely materials confined within porous media (hereinafter we will call them confined materials -CM). In recent years properties of CM and in particular various types of phase transitions (PT) (superconducting [1,2], superfluid [3,4], melting-freezing [5,6,7,8,9,10,11] and others PTs [12,13,14,15,16,17,18,19,20,21,22] in different CM have been extensively studied by different experimental methods including calorimetry [5,7,20], NMR [9,21], ultrasonic [8,9] and dielectric [12,14,15] measurements, Raman [10,13], X-ray [12,16,17,18] and neutron scattering [10,14,22,23,24,25,26], differential thermal analysis [19] etc. It has been shown that CM can form either a system of isolated particle...…”

Molecular Dynamics of Tert-butyl Chloride Confined to CPG (7.4, 15.6 nm)

“…The temperatures of solid I -solid II transition, that is the transition to plastic phase, observed for TBC confined to CPG with 15.6 nm as well as 7.4 nm pore diameter is also depressed, 5 K and 21 K, respectively. Similarly to the results for cyclohexane [21,23,25], the extent of the depression, is smaller than that of the freezing temperatures, 17 K and 43 K, respectively.…”

“…The first peak at 247 K is assigned to liquid-solid I transition, while the latter to the solid I -solid II (at 214 K) and solid II -solid III (182.4 K) transitions. The characteristic larger peak at 214 K marks the transition to the plastic phase, in which molecules are very mobile and orientationally disordered [21][22][23].…”

Nanostructured Coatings of Inner Surfaces in Microporous Matrixes

“…The melting and freezing within porous glasses have been observed for materials such as water, organic liquids [5,6], metals with low melting point [7][8][9][10][11], helium [12], oxygen and some other simple liquids [13][14][15]. The ferroelectric phase transition within porous glass has been observed for NaNO 2 and K 2 HPO 4 [16,17].…”