Published: November 30, 2011 r2011 American Chemical Society 506 dx.doi.org/10.1021/la203994v | Langmuir 2012, 28, 506–516 ARTICLE pubs.acs.org/Langmuir Diffusion within Ultrathin, Dense Nanoporous Silica Films Thomas C. McDermott, Taslima Akter, J. M. Don MacElroy,* Damian A. Mooney, Michael T. P. McCann, and Denis P. Dowling UCD School of Chemical and Bioprocess Engineering, the SEC Strategic Research Cluster and the Centre for Synthesis and Chemical Biology, the Conway Institute, University College Dublin, Belfield, Dublin 4, Ireland 1. INTRODUCTION A major field of ongoing study in gas mixture separation technology involves the development of novel composite cera- mic membranes suitable for use across a broad range of process conditions. A common theme in these studies is the use of support membranes composed of a mesoporous/microporous substrate materials (e.g., α-alumina/γ-alumina, Vycor) with a minimum pore size of approximately 5 nm, in some cases supplemented with a further metal or silicon oxide microporous layer with pore dimensions of 13 nm, upon which, as illustrated in Figure 1, is deposited a submicrometer thick permselective layer of silica containing pores in the angstrom range (see for example refs 14). The latter layer is the selective coating which itself is typically 30100 nm thick. In order to more fully understand the transport mechanisms within these composite materials and, in particular, how one may develop optimum conditions for both maximum permselectivity and maximum permeation rate, exploratory simulation studies are reported in this work in which equilibrium molecular dynamics simulations are conducted to evaluate the diffusive transport characteristics of simple gases (He, N 2 , and CO 2 ) within model dense silica media over a range of temperatures. These studies are guided by prior work reported in ref 5 and, most importantly, by two observations which may be drawn from recent work on permeation within composite thin silica membranes: 1,3 (1) For a range of simple gases (He, H 2 , CO, CO 2 and CH 4 ) the permeances reported in refs 1 and 3 within composite films of silica prepared via chemical vapor deposition of alkoxysilanes decreased exponentially with increasing thickness of the depos- ited film. This is contrary to Fickian predictions in which the permeance is inversely proportional to the membrane thickness. (2) All gases indicated a finite, nonzero permeance for the thickest SiO 2 films deposited. In these studies and in other work reported earlier by Gavalas and co-workers 69 the primary goal has been to develop the fabrication protocols for composite membranes for use in the separation of H 2 from high temperature process gas streams, and it has been demonstrated that high permselectivities with mod- erately high permeances for H 2 can be achieved in such systems. However, the results reported in refs 1 and 3 clearly suggest that if optimally designed membranes are to be fabricated for gas mixture separation for more general applications (and particu- larly for “difficult” mixtures such as CO 2 ,O 2 , and N 2 in high temperature postcombustion or oxy-fuel carbon capture tech- nologies) then ultrathin permselective coatings with thicknesses less than 2550 nm will need to be produced. This will require an understanding of the underlying mechanism for the observed exponential dependence of the permeance on membrane Received: October 12, 2011 Revised: November 23, 2011 ABSTRACT: In this work the origin of permselectivity in dense silica films which possess a pore structure with pore sizes commensurate with the molecular size of the diffusing gas species is investigated. Much of the recently reported work in this field has involved the development of composite membrane films, and while it is generally assumed that the transport process of the gas species within the selective layer of these films is activated in nature, there are anomalies with this simplified picture. In this paper a new model is developed which, for the first time, explains the permselective behavior of the thin selective coatings ubiquitous to membrane separation processes. The model involves the existence of two primary transport domains within the solid film, one of which rapidly conducts the permeating gas (under non-Fickian conditions), while the second domain involves a slow diffusion mode characterized by normal Fickian transport. To validate the model, molecular dynamics simulations are conducted for diffusion of a number of simple gases (He, N 2 , and CO 2 ) within silica glasses over a range of solid densities. The silica media employed in these studies are based on a novel approach developed in this work for the construction of three-dimensionally periodic atomistic structures of silica of arbitrary density in which network bond connectivity is ensured. The results obtained from this work are in qualitative agreement with experimental observations and confirm the existence of dual mode transport which is central to the interpretation of the permselectivity in composite membranes systems.