Nitriles at Silica Interfaces Resemble Supported Lipid Bilayers

Abstract

Nitriles are important solvents not just for bulk reactions but also for interfacial processes such as separations, heterogeneous catalysis, and electrochemistry. Although nitriles have a polar end and a lipophilic end, the cyano group is not hydrophilic enough for these substances to be thought of as prototypical amphiphiles. This picture is now changing, as research is revealing that at a silica surface nitriles can organize into structures that, in many ways, resemble lipid bilayers. This unexpected organization may be a key component of unique interfacial behavior of nitriles that make them the solvents of choice for so many applications. The first hints of this lipid-bilayer-like (LBL) organization of nitriles at silica interfaces came from optical Kerr effect (OKE) experiments on liquid acetonitrile confined in the pores of sol-gel glasses. The orientational dynamics revealed by OKE spectroscopy suggested that the confined liquid is composed of a relatively immobile sublayer of molecules that accept hydrogen bonds from the surface silanol groups and an interdigitated, antiparallel layer that is capable of exchanging into the centers of the pores. This picture of acetonitrile has been borne out by molecular dynamics simulations and vibrational sum-frequency generation (VSFG) experiments. Remarkably, these simulations further indicate that the LBL organization is repeated with increasing disorder at least 20 Å into the liquid from a flat silica surface. Simulations and VSFG and OKE experiments indicate that extending the alkyl chain to an ethyl group leads to the formation of even more tightly packed LBL organization featuring entangled alkyl tails. When the alkyl portion of the molecule is a bulky t-butyl group, packing constraints prevent well-ordered LBL organization of the liquid. In each case, the surface-induced organization of the liquid is reflected in its interfacial dynamics. Acetonitrile/water mixtures are favored solvent systems for separations technologies such as hydrophilic interaction chromatography. Simulations had suggested that although a monolayer of water partitions to the silica surface in such mixtures, acetonitrile tends to associate with this monolayer. VSFG experiments reveal that, even at high water mole fractions, patches of well-ordered acetonitrile bilayers remain at the silica surface. Due to its ability to donate and accept hydrogen bonds, methanol also partitions to a silica surface in acetonitrile/methanol mixtures and can serve to take the place of acetonitrile in the sublayer closest to the surface. These studies reveal that liquid nitriles can exhibit an unexpected wealth of new organizational and dynamic behaviors at silica surfaces, and presumably at the surfaces of other chemically important materials as well. This behavior cannot be predicted from the bulk organization of these liquids. Our new understanding of the interfacial behavior of these liquids will have important implications for optimizing a wide range of chemical processes in nitrile solvents.

Interfacing with silica boosts the catalysis of copper

“…In contrast, SiO 2 without reducible metal cations usually serves as ‘inert’ support, or plays as shell material to fabricate yolk-shell and core-shell metal nanocatalysts to prevent the sintering of metal components 18 – 24 . Reports on the promotional effects of SiO 2 on heterogeneous catalysis are rare 25 , 26 .…”

Assessment of Simple Models for Molecular Simulation of Ethylene Carbonate and Propylene Carbonate as Solvents for Electrolyte Solutions

“…For example, glycerol carbonate has been studied recently by neutron diffraction and modeling [ 37 ]. Nitriles have received extended study [ 1 , 51 – 53 ], as has acetonitrile [ 54 – 62 ]. Of course, it is the non-aqueous conditions that are of interest here.…”

Analysis of the behaviour of confined molecules using 2 H T 1 nuclear magnetic relaxation dispersion

“…only the tumbling reorientation about the axis perpendicular to the C 3 symmetry axis is perturbed by the surface; the order parameter S remains unchanged. This increases the anisotropy ratio τ ⊥,S /τ ,S close to the Si-surface, as seen in MD simulations [12,13]. The spin relaxation may be dependent on the orientation of the pore with respect to the external magnetic field in the laboratory frame.…”

“…Pure acetonitrile and acetonitrile-water solutions have been subject of a great deal of experimental spectroscopic work as summarised by Griffiths [4], and have also been studied using molecular dynamics (MD) simulations [5] in conjunction with NMR-relaxation measurements [6,7]. More recently, acetonitrile confined in silica pore systems has been studied using NMR spin relaxation measurements [8] and with MD [9][10][11][12][13]. It is well documented that the reorientation of bulk solvent molecules is slowed down at interfaces with solids such as silica glasses or proteins [14].…”