Dynamics of plastic and liquid cyclohexane in bulk and in porous glasses studied by NMR methods

Abstract

Cyclohexane was investigated both in bulk and in porous glasses with pore diameters between 4 and 208 nm in the temperature range 136 K _< T_< 300 K. The methods involved were field-eycling NMR relaxometry, field-gradient NMR diffusometry, transverse-relaxation spectroseopy, and differential seanning calorimetry (DSC). The field-cycling data for the bulk material can best be described assuming translational modulation of interrnolecular dipole-dipole coupling. This interpretation is conf'mned by expefiments with different degrees of deuteration, and is in accordance with diffusion coefficients determined with the aid of field-gradient diffusometry. The confinement in pores produces substantial ehanges in the phase behaviour and in molecular dynamics. For pote diameters of 30 nm and above, a non-frozen two monolayers thick film on the surface retains a diffusivity about one order of magnitude lower than in the bulk liquid, but two orders of magnitude larger than in the bulk plastic phase. Experiments indieate an exchange mechanism between this layer and the crystallite inside the pore. In glass with a pote diameter of 4 nm, all applied methods eorroborate DSC results of the virtual absence of a phase transition and reveal a continuously decreasing translational mobility down to temperatures more than 100 K below the bulk liquid/cubic phase transition temperature.

NMR investigations of hexamethyldisilane confined in controlled pore glasses: Pore size distribution and molecular dynamics studies

“…Similar observations have been reported for cyclohexane confined in porous glasses [5] and silica gels [24]. In the liquid region, the T~ values measured with standard and modified inversion-recovery methods are identical within experimental error.…”

“…Thus, the reduced melting point of a confined liquid may be attributed to the tiny size of the crystals, with the large surface-to-volume ratio, that form in the pores [3]. Moreover, the geometrical restrictions and surfaces have a large influence on both the translational and rotational behavior of the confined molecules [4,5]. In this respect, relaxation and diffusion studies give valuable information on pore geometries, and on the important process of mass transport.…”

“…By using organic molecules, forming soft plastically crystalline phases on solidification, as adsorbates, damage to the pore structures caused by ice formation can be avoided. NMR is a well-established method for characterizing porous materials and for studying the phase behavior and dynamics of fluids in confined geometry [4][5][6][7]. NMR self-diffusion measurements on confined substances are demanding due to short spin-spin relaxation times T: that severely attenuate the measured signal even when the stimulated spin-echo technique is being used.…”

NMR studies of the molecular dynamics of tert-butyl chloride confined in the mesoporous molecular sieve MCM-41

“…The dynamic and thermodynamic behaviour of liquids can be strongly affected by the spatial restrictions, by interactions with the pore surfaces, and by disorder introduced by the porous medium. [1][2][3][4][5][6][7][8][9][10][11] The dynamical behaviour of liquids confined to narrow pores has received much attention from theoreticians 12,13 and experimentalists. [5][6][7][8][9] First-order phase transitions have been studied by experiments on the melting and freezing of confined fluids.…”

“…[5][6][7][8][9] First-order phase transitions have been studied by experiments on the melting and freezing of confined fluids. [1][2][3][4][5][6][7][8][9][10][11] The freezing and melting transitions are broadened, and occur at different temperatures from those in bulk. The shift in the transition temperature is inversely proportional to pore size and cannot be accounted for by supercooling, since it occurs for melting as well as for freezing.…”