Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)
Article
Structural and Dynamical Properties of Tetra-Alkyl Ammonium Bromide Aqueous Solutions: A Molecular Dynamics Simulation Study Using a Polarizable Force Field Dengpan Dong, Justin B. Hooper, and Dmitry Bedrov J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 22, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Structural and Dynamical Properties of Tetra-alkyl Ammonium Bromide Aqueous Solutions: A Molecular Dynamics Simulation Study Using a Polarizable Force Field Dengpan Dong, Justin B. Hooper, Dmitry Bedrov Department of Materials Science & Engineering, University of Utah, 122 South Central Campus Drive, Room 304, Salt Lake City, Utah 84112, United States
Abstract Understanding the behavior of aqueous solutions containing tetra-alkyl ammonium (TAA) cations is of great significance in a number of applications, including polymer membranes for fuel cells. In this work, a polarizable force field has been used to preform atomistic molecular dynamics (MD) simulations of aqueous solutions containing tetra-methyl ammonium (TMA) or tetra-butyl ammonium (TBA) cations and Br counterions. Extensive MD simulations of TMABr/water and TBA-Br/water systems were conducted as a function of solution composition (ion pair : water molar ratios of 1:10, 1:20, 1:30, 1:63, 1:500) at atmospheric pressure and 298K. Our simulations demonstrate excellent agreement with available experimental data for solution densities and diffusion coefficients of different species as function of solution composition, providing us confidence in analyzed structural and dynamic correlations. Various ion-ion and ion-water spatial distributions and the extent of cation aggregation are discussed in light of changes in the structure of cations hydration shells. The delicate balance between cation ionic core interactions with water and the hydrophobic interactions of alkyl tails leads to nontrivial self-assembly of TAA cations and the formation of an inter-penetrating cationic network at higher concentrations. The ions and water dynamics are strongly coupled with the observed structural correlations and are analyzed in terms of various residence time, diffusion coefficients, and ionic conductivity.
1 ACS Paragon Plus Environment
The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 32
1. Introduction Aqueous solutions of tetra-alkylammonium(TAA) have attracted extensive research interests in the last few decades. A combination of net charge and hydrophobic side chains in a TAA cation, usually paired with a halide counter-ion, makes TAA aqueous solutions an interesting research subject in uncovering effects of hydrophobicity on ionic hydration shell structure, thermodynamics, and dynamics. More importantly, many polymer electrolytes used for membranes in fuel-cell systems often have similar cationic structures as pendant groups in the chain backbone.1 In these systems, the effect of hydrophobic interactions between cations, the details of the hydration shell, and the effect of local structure on cation-anion interactions are nontrivial and, therefore, a detailed analysis of TAA-based aqueous solutions provides further opportunity to comprehend correlations between dynamic and structural properties in more complex systems, such as polyelectrolyte membranes. Since the mobility of charged species directly impacts the ionic conductivity in water-saturated fuel-cell membranes, understanding these correlations allows an important molecular scale insight needed for the dconesign of novel and efficient membrane materials. Atomistic molecular dynamics (MD) simulations based on empirical force fields provide an efficient way to study structural and dynamic properties at atomic/molecular level that are not easily accessible to experimental techniques. 2 Several studies of TAA aqueous solutions using MD simulations have been reported employing variety of force fields.
3,4,5,6,7
The structure of the
hydration shell around hydrophobic solutes has been interpreted using the ‘iceberg’ effect, a concept first proposed by Frank and Evans and which implies that apolar molecules dissolved in water tend to be surrounded by entropically unfavorable water network. 8 Moreover, the resulting hydrophobic interactions between solutes strongly depend on the size of the solute. Smaller
2 ACS Paragon Plus Environment
Page 3 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
solutes, such as tetra-methylammonium (TMA), are expected to cause less perturbation to the surrounding water structure compared to TAA cations with longer tails, e.g. tetrabutylammonium (TBA), that create larger perturbation in water structure leading to stronger hydrophobic attraction between such solutes in dilute solutions. 9 Although previous MD simulations produced a number of consistent predictions for structural properties in TAA aqueous solutions, nevertheless there are several aspects where simulations using different force fields predicted contradictory trends. This is particularly apparent for the cation-cation spatial distribution characterized by radial distribution functions (RDF). For instance, for TMA-Br solutions, ref. [7] reported two distinct peaks in the cation-cation (N-N) RDF in the separation range between 6-10 Å, while in ref. [3] only one peak was reported for the same range of separations. In refs. [3,7], in dilute solutions (
