Structure of Liquid Propionitrile at Interfaces. 2. Experiment

Ding, Feng; Zhong, Qin; Manfred, Katherine; He, Xiaoxiao; Bender, John S.; Brindza, Michael R.; Walker, Robert A.; Fourkas, John T.

doi:10.1021/jp211061p

Cited by 10 publications

(33 citation statements)

References 59 publications

(125 reference statements)

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“…OKE data for propionitrile confined in sol−gel glasses decay biexponentially at room temperature, with one time constant matching the 2.98 ps collective orientational correlation time of the bulk liquid and the other being 13.3 ps (Figure 5B). 40 The amplitudes of these decays indicate that the thickness of the dynamically inhibited surface layer is ∼4 Å (Figure 5B, inset), in good agreement with simulation. It is possible that the 13.3 ps collective orientational decay time corresponds to the second sublayer at the surface and that there is an additional exponential corresponding to the first sublayer that is too slow to observe in these experiments.…”

Section: ■ Organization and Dynamics Of Acetonitrile At Silica Interf...supporting

confidence: 78%

“…39 Fitting shows that the d + peak arises from two oppositely phased Lorentzians with a small frequency shift between them. 40 As in the case of the r + mode of acetonitrile at the SL interface, these two Lorentzian features presumably arise from the hydrogen-bonded molecules in the first sublayer and the non-hydrogen-bonded molecules in the second sublayer. Overall, the VSFG data agree well with simulation results.…”

Section: ■ Organization and Dynamics Of Acetonitrile At Silica Interf...mentioning

confidence: 90%

“…Ding et al complemented these studies with VSFG and OKE experiments on propionitrile at amorphous quartz interfaces . The VSFG spectrum of propionitrile at the SL interface is remarkably simple, being dominated by the methylene symmetric stretch ( d + ) peak (Figure A).…”

Section: Nitriles With Larger Alkyl Groupsmentioning

confidence: 97%

“…39 Ding et al complemented these studies with VSFG and OKE experiments on propionitrile at amorphous quartz interfaces. 40 The VSFG spectrum of propionitrile at the SL interface is remarkably simple, being dominated by the methylene symmetric stretch (d + ) peak (Figure 5A). The absence of a strong r + peak indicates that the distribution of methyl group orientations is broad, in general agreement with simulation.…”

Section: ■ Organization and Dynamics Of Acetonitrile At Silica Interf...mentioning

confidence: 99%

“…38 ■ NITRILES WITH LARGER ALKYL GROUPS Given that acetonitrile forms repeating LBL structures at silica interfaces, what can be expected to happen when the hydrocarbon tail is longer or more complex? To address this question, simulations 39 and experiments 40 were performed on liquid propionitrile (ethyl cyanide) at the interface with amorphous quartz. The bent propionitrile might foster substantially different interfacial organization than the linear acetonitrile.…”

Section: ■ Organization and Dynamics Of Acetonitrile At Silica Interf...mentioning

confidence: 99%

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Nitriles at Silica Interfaces Resemble Supported Lipid Bilayers

Berne

Fourkas²,

Walker

et al. 2016

Acc. Chem. Res.

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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.

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Section: ■ Organization and Dynamics Of Acetonitrile At Silica Interf...supporting

confidence: 78%

Section: ■ Organization and Dynamics Of Acetonitrile At Silica Interf...mentioning

confidence: 90%

Section: Nitriles With Larger Alkyl Groupsmentioning

confidence: 97%

Section: ■ Organization and Dynamics Of Acetonitrile At Silica Interf...mentioning

confidence: 99%

Section: ■ Organization and Dynamics Of Acetonitrile At Silica Interf...mentioning

confidence: 99%

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Nitriles at Silica Interfaces Resemble Supported Lipid Bilayers

Berne

Fourkas²,

Walker

et al. 2016

Acc. Chem. Res.

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Orientational Time Correlation Functions for Vibrational Sum-Frequency Generation. 3. Methanol

Liu

Fourkas

2015

J. Phys. Chem. C

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Molecular dynamics simulations have been used to study the orientational dynamics of methanol at the liquid/vapor and liquid/silica interfaces. Orientational time-correlation functions for the symmetric and asymmetric methyl stretches of methanol have been calculated to assess the role of reorientation in the vibrational sum-frequency generation (VSFG) spectroscopy of these modes at the two interfaces. We find that internal methyl rotation plays a significant role in suppressing the intensity of the asymmetric methyl stretches at the liquid/vapor interface. The broad orientational distribution of the methyl groups and the properties of the Raman spectra of the asymmetric stretches are also major contributors to the low intensity of these modes in VSFG spectra at this interface. We find that after an initial rapid, inertial decay, there is little coupling among the orientational degrees of freedom of the methyl group, which suggests that time-dependent VSFG spectroscopy may be used to probe interfacial orientational dynamics in this and other hydrogen-bonded liquids.

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Structure of Liquid Propionitrile at Interfaces. 1. Molecular Dynamics Simulations

Liu

Weeks

et al. 2012

J. Phys. Chem. C

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The organization of liquid propionitrile at a hydrophilic silica surface and at the liquid–vapor interface has been studied by molecular dynamics simulations. Analysis has been performed on particle densities and the orientations of different bond vectors. At the silica/liquid interface, propionitrile adopts a lipid-bilayer-like structure in which the carbon–nitrogen bond vector has opposite orientations in two sublayers. To explore the influence of the alkyl group on the interfacial structure, we have also studied the methylene–methyl vector orientations and locations of the methyl groups for molecules in the different sublayers. This analysis reveals that liquid propionitrile forms a compact, entangled bilayer at a silica surface, in contrast to the interdigitated bilayer that has been observed previously for acetonitrile.

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Structure of Liquid Propionitrile at Interfaces. 2. Experiment

Cited by 10 publications

References 59 publications

Nitriles at Silica Interfaces Resemble Supported Lipid Bilayers

Nitriles at Silica Interfaces Resemble Supported Lipid Bilayers

Orientational Time Correlation Functions for Vibrational Sum-Frequency Generation. 3. Methanol

Structure of Liquid Propionitrile at Interfaces. 1. Molecular Dynamics Simulations

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