Molecular Simulation Study of Vapor-Liquid Critical Properties of a Simple Fluid in Attractive Slit Pores: Crossover from 3D to 2D Sudhir K. Singh, Ashim K. Saha, and Jayant K. Singh* Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India ReceiVed: NoVember 19, 2009; ReVised Manuscript ReceiVed: February 14, 2010 We present the effect of surface attraction on the vapor-liquid equilibria of square well (SW) ﬂuids in slit pores of varying slit width from quasi 3D to 2D regime using molecular simulation methodologies. Four to ﬁve distinct linear regimes are found for shift in the critical temperature with inverse slit width, which is more prominent at higher surface ﬂuid interaction strength. On the other hand, shift in the critical density and the critical pressure does not show any speciﬁc trend. Nevertheless, critical density and pressure show the sign of approaching toward the 3D bulk value with increase in the slit pore width, H, beyond 40 molecular diameters. The crossover from 3D to 2D behavior for attractive pores is observed around 14-16 molecular diameters, which is signiﬁcantly different from the crossover behavior in the hydrophobic slit pore. Critical properties for H e 2 molecular diameters are indifferent to the surface characteristics. Corresponding state plot displays ﬂuctuating positive deviation of spreading pressure for large pores and negative deviation for small pores from the bulk saturation value. Such behavior is more accentuated at stronger surface-ﬂuid interaction strength. We also present vapor-liquid surface tensions of the SW ﬂuid for different attractive planar slit-pores of variable slit-widths. Vapor-liquid surface tension or interfacial width values are insensitive to the surface-ﬂuid interaction strength for slit width, H e 2 molecular diameters. At a given slit width and temperature, vapor-liquid interfacial width is found to decrease with increasing wall-ﬂuid interaction for H > 2. However, interfacial properties approaches to the bulk value with increasing slit width. On the other hand, surface tension at a reduced temperature displays a nonmonotonic behavior with the change in H, which is in good agreement with the nature of the corresponding scaled interfacial width. 1. Introduction It has been observed that conﬁned ﬂuids in micro- to nanometer pore sizes, regardless of geometry, exhibit minimal to signiﬁcant deviations from bulk thermophysical and structural properties. 1–3 These differences have generated great interest, as conﬁned ﬂuids feature conspicuously in both technology and nature. Recent investigations 4–8 suggest that dimensionality of the system largely determines the behavior of materials under conﬁnements. Several studies by theory, molecular simulations, and experiments have been performed to understand various changes in the equilibrium and the dynamical properties of the conﬁned ﬂuid. 9–22 However, some inherent limitations are always associated with different approaches, which limit its applicabil- ity. For example, experimental approach is not feasible to capture the ﬂuid properties in ultra nanopores or if it does it may give only very approximate information on molecular level details from the existing techniques. On the other hand, elegancy of molecular simulation accompanied with modern day comput- ing power and new efﬁcient algorithms have been useful in investigating the properties of ﬂuids conﬁned in nanopores of few molecular diameters, which can be used to bridge the theory and the experimental outcomes. Moreover, molecular simulation methods can provide a microscopic picture of a ﬂuid in the interaction ﬁeld of the conﬁned space and enable one to examine the underlying physics. Detailed knowledge of the phase coexistence properties of conﬁned ﬂuids is of crucial importance for the interpretation of experimental data on ﬂuids in nanopores. This further can be helpful to optimize the various industrial processes. 23–30 Burgess et al., 31 Keizer et al., 32 Machin, 33 and others 34,35 observed that vapor-liquid critical temperature is suppressed under conﬁne- ment. This decrease in the critical temperature increases as pore size decreases. However, due to wide pore size distribution and irregular pore geometry, it has not been possible to establish a quantitative relation between critical point shift and pore size. In recent years with the discovery of well-deﬁned geometry of mesoporous materials, such as MCM-41, 36 MCM-48, and SBA- 15, 37 allowed direct experimental measurements of critical points with some quantitative linear trends. For example, Thommes et al. 38 and Morishige et al. 39 observed experimentally that the shift in the critical temperature has linear dependence on the inverse pore width. Further, Vishnyakov et al. 40 performed Monte Carlo simulations on carbon slit pore and obtain the similar results as seen experimentally in former investigations; however, the simulations were limited to ﬁve molecular diameters. On the other hand, later investigations of Vortler 41 suggest nonlinear dependence of shift in critical temperature as a more generic behavior in nanopores. In another investigation pertaining to square well ﬂuid, Zhang and Wang 42 studied the shift in the critical temperature in a cylindrical pore for various wall-ﬂuid and ﬂuid-ﬂuid interaction strengths, using DFT calculations, and found a nonmonotonic behavior; additionally, Zhang and Wang’s study 42 on critical density in cylindrical pore indicates monotonic behavior with wall-ﬂuid interactions. In a subsequent work, Singh et al. 43 reported similar observation of shift in critical temperature for square well ﬂuids in slit pores; critical temperature ﬁrst increase with the increase in the wall-ﬂuid interaction strength and decrease subsequently on * Corresponding author. E-mail: jayantks@iitk.ac.in. J. Phys. Chem. B 2010, 114, 4283–4292 4283 10.1021/jp9109942  2010 American Chemical Society Published on Web 03/10/2010