Integral equation study of a Stockmayer fluid adsorbed in polar disordered matrices

Abstract

Based on replica integral equations in the (reference) hypernetted chain approximation we investigate the structural features and phase properties of a dipolar Stockmayer fluid confined to a disordered dipolar matrix. The integral equations are applied to the homogeneous high-temperature phase where the system is globally isotropic. At low densities we find the influence of dipolar interactions between fluid (f) and matrix (m) particles to be surprisingly similar to the previously investigated effect of attractive isotropic (fm) interactions: the critical temperature of the vapor-liquid transition decreases with increasing (fm) coupling, while the critical density increases. The anisotropic nature of the dipolar (fm) interactions turns out to play a more dominant role at high fluid densities where we observe a pronounced sensitivity in the dielectric constant and a strong degree of local orientational ordering of the fluid particles along the local fields generated by the matrix. Moreover, an instability of the dielectric constant, which is a precursor of ferroelectric ordering occurring both in bulk Stockmayer fluids and in fluids in nonpolar matrices, is observed only for very small dipolar (fm) couplings.

Vapor-liquid transitions of dipolar fluids in disordered porous media: Performance of angle-averaged potentials

“…On the other hand, dipolar ͑fm͒ interactions 10 have been shown to induce a further decrease of the VL critical temperature but an increase of the critical density, which is again similar to observations made for spherical attractive ͑fm͒ interactions. 9,10 Though not exact, a main advantage of the RHNC approach is that the computational effort is relatively small but still yields very reasonable results when compared to Monte Carlo simulation data both for bulk 19 and for confined dipolar fluids. 17 In computer simulations of bulk Stockmayer fluids this approximation has already proven to be a reasonably accurate ͑and technically much less demanding͒ alternative for using full dipolar interactions as long as the ratio m* 2 ϭ 2 /(⑀ 0 3 ) between dipole-dipole and LJ interactions is not too large.…”

“…9,10 The central piece of the approach are the replicasymmetric Ornstein-Zernike ͑RSOZ͒ equations 12,13 relating the total correlation functions h ff (12), h fm (12), and h mm (12) ͑where the indices refer to fluid and matrix particles, respec-tively͒ to the corresponding direct correlation functions c ff(fm,mm) (12). III are based.…”

“…In a previous study 10 we have considered Stockmayer fluids adsorbed to polar matrices consisting of frozen-in DHS ͑model IIa͒, the structure of which is characterized by the parameter m * 2 ϭ m 2 /(k B T m 3 ) defining the dipolar coupling within the state from which the matrix has been quenched. In a previous study 10 we have considered Stockmayer fluids adsorbed to polar matrices consisting of frozen-in DHS ͑model IIa͒, the structure of which is characterized by the parameter m * 2 ϭ m 2 /(k B T m 3 ) defining the dipolar coupling within the state from which the matrix has been quenched.…”

“…Both shifts become more and more pronounced with increasing polarity of the matrix. 10. 10 in the two parts of Fig.…”

“…Moreover, investigation of a variety of model systems 4 -8 suggest that these shifts are very sensitive to both the interactions among fluid ͑f͒ particles and to the nature of the confining ''matrix'' ͑m͒ ͓and the resulting fluid-matrix ͑fm͒ interactions͔. On the other hand, dipolar ͑fm͒ interactions 10 have been shown to induce a further decrease of the VL critical temperature but an increase of the critical density, which is again similar to observations made for spherical attractive ͑fm͒ interactions. The disordered medium has been represented by a quenched configuration of either hard spheres ͑HS͒ or dipolar hard spheres ͑DHS͒.…”