skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Comparing Simulated and X-ray-measured Surface Structure: The Case of Ionic Liquids

Abstract

The surface-normal electron density profile of an ionic liquid, [bmim][PF{sub 6}], derived from x-ray reflectivity measurements, is compared with two independent molecular-dynamics simulations. It is shown that a meaningful comparison requires a detailed accounting for both thermal and nonthermal surface roughening effects. The former is due to thermally excited capillary waves, and the latter is due to the molecular zero-point motion and form. These quantities influence very significantly, but differently, the simulated and measured density profiles. Stripping off these effects from both types of profiles yields the intrinsic structure factor of the surface. The simulated intrinsic structure factors are found to deviate considerably from the measured one. The introduction of additional ad hoc surface roughness to the simulated profiles greatly reduces the deviation, however, no physical origin for this effect can be identified. The method employed in this study should prove useful for simulation-experiment comparisons of other liquid surfaces, provided they obey capillary-wave theory, as do almost all liquid surfaces studied to date by x-ray reflectivity.

Authors:
; ; ;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL) National Synchrotron Light Source
Sponsoring Org.:
Doe - Office Of Science
OSTI Identifier:
930581
Report Number(s):
BNL-80790-2008-JA
Journal ID: ISSN 0021-9606; JCPSA6; TRN: US200904%%601
DOE Contract Number:
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 175; Journal Issue: 17
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; LIQUIDS; ORGANIC COMPOUNDS; SURFACE PROPERTIES; MORPHOLOGY; ELECTRON DENSITY; MOLECULAR DYNAMICS METHOD; ROUGHNESS; STRUCTURE FACTORS; national synchrotron light source

Citation Formats

Sloutskin ,E., Lynden-Bell, R., Balasubramanian, S., and Deutsch, M. Comparing Simulated and X-ray-measured Surface Structure: The Case of Ionic Liquids. United States: N. p., 2006. Web. doi:10.1063/1.2361289.
Sloutskin ,E., Lynden-Bell, R., Balasubramanian, S., & Deutsch, M. Comparing Simulated and X-ray-measured Surface Structure: The Case of Ionic Liquids. United States. doi:10.1063/1.2361289.
Sloutskin ,E., Lynden-Bell, R., Balasubramanian, S., and Deutsch, M. Sun . "Comparing Simulated and X-ray-measured Surface Structure: The Case of Ionic Liquids". United States. doi:10.1063/1.2361289.
@article{osti_930581,
title = {Comparing Simulated and X-ray-measured Surface Structure: The Case of Ionic Liquids},
author = {Sloutskin ,E. and Lynden-Bell, R. and Balasubramanian, S. and Deutsch, M.},
abstractNote = {The surface-normal electron density profile of an ionic liquid, [bmim][PF{sub 6}], derived from x-ray reflectivity measurements, is compared with two independent molecular-dynamics simulations. It is shown that a meaningful comparison requires a detailed accounting for both thermal and nonthermal surface roughening effects. The former is due to thermally excited capillary waves, and the latter is due to the molecular zero-point motion and form. These quantities influence very significantly, but differently, the simulated and measured density profiles. Stripping off these effects from both types of profiles yields the intrinsic structure factor of the surface. The simulated intrinsic structure factors are found to deviate considerably from the measured one. The introduction of additional ad hoc surface roughness to the simulated profiles greatly reduces the deviation, however, no physical origin for this effect can be identified. The method employed in this study should prove useful for simulation-experiment comparisons of other liquid surfaces, provided they obey capillary-wave theory, as do almost all liquid surfaces studied to date by x-ray reflectivity.},
doi = {10.1063/1.2361289},
journal = {Journal of Chemical Physics},
number = 17,
volume = 175,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • The properties of the liquid-vapor interface for three simple liquids (water, carbon tetrachloride, and methanol) have been measured using x-ray reflectivity. The measured surface roughness is interpreted using a model that combines the effects of thermally induced capillary waves and the dimensions of the constituent molecules.
  • No abstract prepared.
  • X-ray scattering experiments at room temperature were performed for the ionic liquids n-butyl-trimethylammonium bis(trifluoromethanesulfonyl)imide, [N{sub 1114}][NTf{sub 2}], and methyl-tributylammonium bis(trifluoromethanesulfonyl)imide, [N{sub 1444}][NTf{sub 2}]. The peak in the diffraction data characteristic of charge ordering in [N{sub 1444}][NTf{sub 2}] is shifted to longer distances in comparison to [N{sub 1114}][NTf{sub 2}], but the peak characteristic of short-range correlations is shifted in [N{sub 1444}][NTf{sub 2}] to shorter distances. Molecular dynamics (MD) simulations were performed for these ionic liquids using force fields available from the literature, although with new sets of partial charges for [N{sub 1114}]{sup +} and [N{sub 1444}]{sup +} proposed in this work.more » The shifting of charge and adjacency peaks to opposite directions in these ionic liquids was found in the static structure factor, S(k), calculated by MD simulations. Despite differences in cation sizes, the MD simulations unravel that anions are allowed as close to [N{sub 1444}]{sup +} as to [N{sub 1114}]{sup +} because anions are located in between the angle formed by the butyl chains. The more asymmetric molecular structure of the [N{sub 1114}]{sup +} cation implies differences in partial structure factors calculated for atoms belonging to polar or non-polar parts of [N{sub 1114}][NTf{sub 2}], whereas polar and non-polar structure factors are essentially the same in [N{sub 1444}][NTf{sub 2}]. Results of this work shed light on controversies in the literature on the liquid structure of tetraalkylammonium based ionic liquids.« less
  • In this article we determine the temperature-dependent structure of the tetradecyltrihexylphosphonium bis(trifluoromethylsulfonyl)amide ionic liquid using a combination of X-ray scattering and molecular dynamics simulations. As in many other room-temperature ionic liquids three characteristic intermolecular peaks can be detected in the structure function S(q). A prepeak or first sharp diffraction peak is observed at about q = 0.42 {angstrom}{sup -1}. Long range anion-anion correlations are the most important contributors to this peak. In all systems we have studied to date, this prepeak is a signature of solvation asymmetry. The peak in S(q) near q = 0.75 {angstrom}{sup -1} is the signaturemore » of ionic alternation and arises from the charge ordered separation of ions of the same charge. The most intense diffraction peak near q = 1.37 {angstrom}{sup -1} arises from short-range separation between ions of opposite charge combined with a significant contribution from cationic carbon-carbon interactions, indicating that cationic hydrophobic tails have significant contacts.« less
  • A combination of X-ray scattering experiments and molecular dynamics simulations were conducted to investigate the structure of ionic liquids (ILs) which chemically bind CO 2. The structure functions were measured and computed for four different ILs consisting of two different phosphonium cations, triethyloctylphosphonium ([P 2228] +) and trihexyltetradecylphosphonium ([P 66614] +), paired with two different aprotic heterocyclic anions which chemically react with CO 2, 2-cyanopyrrolide, and 1,2,4-triazolide. Simulations were able to reproduce the experimental structure functions, and by deconstructing the simulated structure functions, further information on the liquid structure was obtained. All structure functions of the ILs studied had threemore » primary features which have been seen before in other ILs: a prepeak near 0.3–0.4 Å–1 corresponding to polar/nonpolar domain alternation, a charge alternation peak near 0.8 Å–1, and a peak near 1.5 Å–1 due to interactions of adjacent molecules. The liquid structure functions were only mildly sensitive to the specific anion and whether or not they were reacted with CO 2. Upon reacting with CO 2, small changes were observed in the structure functions of the [P 2228] + ILs, whereas virtually no change was observed upon reacting with CO 2 in the corresponding [P 66614] + ILs. When the [P 2228] + cation was replaced with the [P 66614] + cation, there was a significant increase in the intensities of the prepeak and adjacency interaction peak. While many of the liquid structure functions are similar, the actual liquid structures differ as demonstrated by computed spatial distribution functions.« less