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Islam MN, Liu Y, Herr AE. Electromigration of Charged Analytes Through Immiscible Fluids in Multiphasic Electrophoresis. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2024:2024.05.29.596534. [PMID: 38853831 PMCID: PMC11160796 DOI: 10.1101/2024.05.29.596534] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2024]
Abstract
Multiphasic buffer systems have been of greatest interest in electrophoresis and liquid-liquid electrotransfer; this study extends that foundation by exploring the interplay of the geometric and viscous properties of an interleaving oil layer on the electrotransfer of a charged analyte from an aqueous solution into a hydrogel. We utilized finite element analysis to examine two complementary configurations: one being electrotransfer of a charged analyte (protein) in an aqueous phase into a surrounding hydrogel layer and another being electrotransfer of the protein from that originating aqueous phase - through an interleaving oil layer of predetermined viscosity and thickness - and into a surrounding hydrogel layer. Results indicate that the presence of an oil layer leads to increased skew of the injected peak. To explain this difference in injection dispersion, we utilize Probstein's framework and compare the Péclet (Pe) number with the ratio between length scales characteristic to the axial and radial dispersion, respectively. The formulation assigns electrotransfer conditions into six different dispersion regimes. We show that the presence or absence of an interleaving oil layer moves the observed peak dispersion into distinct electrotransfer regimes; the presence of an oil layer augments the electrophoretic mobility mismatch between the different phases, resulting in a five-fold increase in Pe and a six-fold increase in the ratio between the axial to radial dispersion characteristic lengths. We further show that oil viscosity significantly influences resultant injection dispersion. A decrease in oil-layer viscosity from 0.08 Pa·s to 0.02 Pa·s results in a >100% decrease in injection dispersion. Our theoretical predictions were experimentally validated by comparing the electrotransfer regimes of three different mineral oil samples. We show that lowering the oil viscosity to 0.0039 Pa·s results in an injection regime similar to that of the absence of an oil layer. Additionally, we measure the migration distance and show that average electromigration velocity over the transit duration is inversely proportional to the viscosity of an interleaving oil layer. Understanding of the impact of electrotransfer of charged species across multiple immiscible fluid layers on peak dispersion informs the design of multiphasic electrophoresis systems.
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Affiliation(s)
- Md Nazibul Islam
- Department of Bioengineering, University of California, Berkeley, CA 94720, USA
| | - Yang Liu
- Department of Bioengineering, University of California, Berkeley, CA 94720, USA
- School of Chemical, Materials and Biomedical Engineering, University of Georgia, Athens, GA 30602, USA
| | - Amy E Herr
- Department of Bioengineering, University of California, Berkeley, CA 94720, USA
- Chan Zuckerberg Biohub, San Francisco, CA 94158
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On the mechanisms of ion adsorption to aqueous interfaces: air-water vs. oil-water. Proc Natl Acad Sci U S A 2022; 119:e2210857119. [PMID: 36215494 PMCID: PMC9586313 DOI: 10.1073/pnas.2210857119] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The adsorption of ions to water-hydrophobe interfaces influences a wide range of phenomena, including chemical reaction rates, ion transport across biological membranes, and electrochemical and many catalytic processes; hence, developing a detailed understanding of the behavior of ions at water-hydrophobe interfaces is of central interest. Here, we characterize the adsorption of the chaotropic thiocyanate anion (SCN-) to two prototypical liquid hydrophobic surfaces, water-toluene and water-decane, by surface-sensitive nonlinear spectroscopy and compare the results against our previous studies of SCN- adsorption to the air-water interface. For these systems, we observe no spectral shift in the charge transfer to solvent spectrum of SCN-, and the Gibb's free energies of adsorption for these three different interfaces all agree within error. We employed molecular dynamics simulations to develop a molecular-level understanding of the adsorption mechanism and found that the adsorption for SCN- to both water-toluene and water-decane interfaces is driven by an increase in entropy, with very little enthalpic contribution. This is a qualitatively different mechanism than reported for SCN- adsorption to the air-water and graphene-water interfaces, wherein a favorable enthalpy change was the main driving force, against an unfavorable entropy change.
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Benjamin I. Structure, Thermodynamics, and Dynamics of Thiocyanate Ion Adsorption and Transfer across the Water/Toluene Interface. J Phys Chem B 2022; 126:5706-5714. [PMID: 35861680 DOI: 10.1021/acs.jpcb.2c03956] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Molecular dynamics simulations are used to examine in detail the structure, thermodynamics, and dynamics involve in the adsorption and transfer of the thiocyanate ion (SCN-) across the water/toluene interface. Free energy, hydration structure, and several dynamical properties as a function of the ion location along the interface normal are calculated and contrasted with recent experiments. The free energy profile exhibits a local minimum near the interface corresponding to adsorption free energy relative to bulk water of -6 kJ/mol, in reasonable agreement with experiments. The simulations provide insight into the water surface fluctuations that are coupled to the ion transfer, demonstrating formation of water finger-like structures assisting the transfer process.
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Affiliation(s)
- Ilan Benjamin
- Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States
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Wen B, Sun C, Luo Z, Lu X, Wang H, Bai B. A hydrogen bond-modulated soft nanoscale water channel for ion transport through liquid-liquid interfaces. SOFT MATTER 2021; 17:9736-9744. [PMID: 34643637 DOI: 10.1039/d1sm00899d] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
Ion transport through interfaces is of ubiquitous importance in many fields such as electrochemistry, emulsion stabilization, phase transfer catalysis, liquid-liquid extraction and enhanced oil recovery. However, the knowledge of interfacial structures that significantly affect ion transport through liquid-liquid interfaces is still lacking due to the difficulty of observing nanoscale interfaces. We studied here the evolution of interfacial structures during ion transport through the decane-water interface under different ionic concentrations and external forces using molecular dynamics simulations. The roles of hydrogen bonds in ion transport through interfaces are revealed. We identified a soft nanoscale channel during ion transport through liquid-liquid interfaces and the decane phase under specific external force. The stability of the water channel and the ion transport velocity both increase with ionic concentration due to the layered ordering structures of the water near the channel surface. We observed that the stability and connectivity of the water channel in the decane phase are remarkably improved both by the high increase of the number of hydrogen bonds in the water channel with increasing ionic concentration, and by the conformational change in water molecules near the water channel surface. Our discovery of a soft nanoscale water channel by molecular simulations implies that there is a potential stable passage for ion transport through liquid-liquid interfaces.
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Affiliation(s)
- Boyao Wen
- State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China.
| | - Chengzhen Sun
- State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China.
| | - Zhengyuan Luo
- State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China.
| | - Xi Lu
- Petroleum Exploration and Production Research Institute of Sinopec, Beijing, 100083, China
| | - Haibo Wang
- Petroleum Exploration and Production Research Institute of Sinopec, Beijing, 100083, China
| | - Bofeng Bai
- State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China.
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Morita A, Koizumi A, Hirano T. Recent progress in simulating microscopic ion transport mechanisms at liquid-liquid interfaces. J Chem Phys 2021; 154:080901. [PMID: 33639756 DOI: 10.1063/5.0039172] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Transport of ions through liquid-liquid interfaces is of fundamental importance to a wide variety of applications. However, since it is quite challenging for experimentalists to directly and selectively observe molecules at the interfaces, microscopic mechanisms of ion transport have been largely presumed from kinetic information. This Perspective illustrates recent examples that molecular dynamics simulations with proper free energy surfaces clarified mechanistic pictures of ion transport. The key is a proper choice of coordinates and defining/calculating free energy surfaces in multidimensional space. Once the free energy surfaces for realistic systems are available, they naturally provide new insight into the ion transport in unprecedented details, including water finger, transient ion pairing, and electron transfer.
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Affiliation(s)
- Akihiro Morita
- Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
| | - Ai Koizumi
- Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
| | - Tomonori Hirano
- Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
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Benjamin I. Molecular Dynamics Studies on the Effect of Surface Roughness and Surface Tension on the Thermodynamics and Dynamics of Hydronium Ion Transfer Across the Liquid/Liquid Interface. J Phys Chem B 2020; 124:8711-8718. [PMID: 32902279 DOI: 10.1021/acs.jpcb.0c06304] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
Molecular dynamics simulations are used to examine the effect of surface roughness and surface tension on the transfer of the classical hydronium ion (H3O+) across the water/1,2-dichloroethane interface. Free energy of transfer, hydration structure, and dynamics as a function of the ion location along the interface normal are calculated with six different values of a control parameter whose variation modifies the surface tension without impacting the bulk properties of the two solvents. Transfer of the classical hydronium ion across the water/1,2-dichloroethan interface involves the cotransfer of three hydration shell water molecules, independent of the surface tension. However, as the interaction between the two liquids weakens, a rise in interfacial tension and decrease in intrinsic water fingering and capillary fluctuations result in fewer water molecules cotransported with the ion in the second shell and a reduction in the length of the finger that the ion is attached to, consistent with the reduced size of the second hydration shell. First shell water residence time and lateral ion diffusion constants vary with the surface tension in a way that is consistent with the abovementioned structural insight.
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Affiliation(s)
- Ilan Benjamin
- Department of Chemistry and Biochemistry, University of California, Santa Cruz, Santa Cruz, California 95064, United States
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7
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Wen B, Sun C, Zheng W, Bai B, Lichtfouse E. Evidence for water ridges at oil-water interfaces: implications for ion transport. SOFT MATTER 2020; 16:826-832. [PMID: 31840723 DOI: 10.1039/c9sm01791g] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Understanding ion transport across interfaces is of fundamental importance in many processes such as liquid-liquid extraction, phase transfer catalysis, enhanced oil recovery and emulsion stabilisation. However, the factors that control ion transport across interfaces are poorly known due to a lack of knowledge of structural changes at interfaces. We studied here the effects of ionic concentration and external force on the transport of ions across the decane-water interface using classical molecular dynamics simulations. The results show that the evolution of interfacial structures during ion transfer across the interface is controlled by hydrogen bonding and ionic interactions at the interface. We also identified a new mode of ion transfer across the interface at low ionic concentrations, involving a 'water ridge', rather that the classical 'water finger'. In the water ridge mode, hydrogen bonds are not broken due to low ion levels, and the water ridge induces gradual interface deformation. Whereas, at high ionic concentrations, hydrogen bonds are broken by the strong ion electrostatic repulsion, thus inducing the formation of a water finger. We also found that the variation of the Gibbs free energy during ion transfer is directly relevant to the ionic concentration. The water ridge at low ionic concentrations, which displaces more water molecules towards the decane phase, induces less free energy variation than the water finger at high ionic concentrations.
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Affiliation(s)
- Boyao Wen
- State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China.
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Kikkawa N, Wang L, Morita A. Computational study of effect of water finger on ion transport through water-oil interface. J Chem Phys 2016; 145:014702. [DOI: 10.1063/1.4954774] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Affiliation(s)
- Nobuaki Kikkawa
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Lingjian Wang
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Akihiro Morita
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
- Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8520, Japan
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Karnes JJ, Benjamin I. Geometric and energetic considerations of surface fluctuations during ion transfer across the water-immiscible organic liquid interface. J Chem Phys 2016; 145:014701. [DOI: 10.1063/1.4954331] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Affiliation(s)
- John J. Karnes
- Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, USA
| | - Ilan Benjamin
- Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, USA
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10
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Temperature effect in the ion transfer kinetics at the micro-interface between two immiscible electrolyte solutions. Electrochim Acta 2015. [DOI: 10.1016/j.electacta.2015.08.110] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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11
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Kikkawa N, Wang L, Morita A. Microscopic Barrier Mechanism of Ion Transport through Liquid–Liquid Interface. J Am Chem Soc 2015; 137:8022-5. [DOI: 10.1021/jacs.5b04375] [Citation(s) in RCA: 46] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Nobuaki Kikkawa
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Lingjian Wang
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Akihiro Morita
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
- Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8520, Japan
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Abstract
The liquid interface is a narrow, highly anisotropic region, characterized by rapidly varying density, polarity, and molecular structure. I review several aspects of interfacial solvation and show how these affect reactivity at liquid/liquid interfaces. I specifically consider ion transfer, electron transfer, and SN2 reactions, showing that solvent effects on these reactions can be understood by examining the unique structure and dynamics of the liquid interface region.
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Affiliation(s)
- Ilan Benjamin
- Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064;
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13
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Uppapalli S, Zhao H. The influence of particle size and residual charge on electrostatic interactions between charged colloidal particles at an oil-water interface. SOFT MATTER 2014; 10:4555-4560. [PMID: 24817608 DOI: 10.1039/c4sm00527a] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
Electrostatic repulsive interaction forces between charged spherical colloidal particles at an oil-water interface are numerically studied by solving the standard three-dimensional Poisson-Nernst-Planck model. We directly compute the electrostatic force on a finite-size spherical particle and our results are applicable to all inter-particle distances without distinguishing short ranges and long ranges. The model successfully captures the scaling relationship of the force and the separation distance (d) between two charged particles at both short ranges (exponential dependence) and long ranges (∼d(-4)). The model also bridges these two ranges and provides quantitative information in the middle range. In addition, by assuming that there is a small residual electric charge at the particle-oil interface, the standard model is capable of quantitatively predicting the repulsive particle-particle interaction force over a large range of the separation distance between two particles. The favorable agreement between experiments and theoretical predictions also leads one to conclude that the standard model adequately describes the particle-particle interactions trapped at the oil-water interface.
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Affiliation(s)
- Sebastian Uppapalli
- Department of Mechanical Engineering, University of Nevada, Las Vegas, NV 89154, USA.
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14
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Kopelevich DI. One-dimensional potential of mean force underestimates activation barrier for transport across flexible lipid membranes. J Chem Phys 2014; 139:134906. [PMID: 24116584 DOI: 10.1063/1.4823500] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
Abstract
Transport of a fullerene-like nanoparticle across a lipid bilayer is investigated by coarse-grained molecular dynamics (MD) simulations. Potentials of mean force (PMF) acting on the nanoparticle in a flexible bilayer suspended in water and a bilayer restrained to a flat surface are computed by constrained MD simulations. The rate of the nanoparticle transport into the bilayer interior is predicted using one-dimensional Langevin models based on these PMFs. The predictions are compared with the transport rates obtained from a series of direct (unconstrained) MD simulations of the solute transport into the flexible bilayer. It is observed that the PMF acting on the solute in the flexible membrane underestimates the transport rate by more than an order of magnitude while the PMF acting on the solute in the restrained membrane yields an accurate estimate of the activation energy for transport into the flexible membrane. This paradox is explained by a coexistence of metastable membrane configurations for a range of the solute positions inside and near the flexible membrane. This leads to a significant reduction of the contribution of the transition state to the mean force acting on the solute. Restraining the membrane shape ensures that there is only one stable membrane configuration corresponding to each solute position and thus the transition state is adequately represented in the PMF. This mechanism is quite general and thus this phenomenon is expected to occur in a wide range of interfacial systems. A simple model for the free energy landscape of the coupled solute-membrane system is proposed and validated. This model explicitly accounts for effects of the membrane deformations on the solute transport and yields an accurate prediction of the activation energy for the solute transport.
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Affiliation(s)
- Dmitry I Kopelevich
- Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, USA
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González-Fuentes MA, Manríquez J, Antaño-López R, Godínez LA. Kinetic and thermodynamic study of the transfer of anionic polyamidoamine dendrimers across two immiscible liquids. Electrochim Acta 2011. [DOI: 10.1016/j.electacta.2011.06.104] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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17
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Ban YM, Tasseff RA, Kopelevich DI. Non-adiabatic dynamics of interfacial systems: a case study of a nanoparticle penetration into a lipid bilayer. MOLECULAR SIMULATION 2011. [DOI: 10.1080/08927022.2011.566610] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
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18
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Ahn YN, Gupta A, Chauhan A, Kopelevich DI. Molecular transport through surfactant-covered oil-water interfaces: role of physical properties of solutes and surfactants in creating energy barriers for transport. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2011; 27:2420-2436. [PMID: 21309583 DOI: 10.1021/la103550v] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Mechanisms of molecular transport across oil-water interfaces covered by nonionic surfactants are investigated using coarse-grained molecular dynamics simulations. Resistance of the surfactant monolayer to the solute transport is shown to be controlled by dense regions in the monolayer. The dense regions are formed on both sides of the dividing surface and the barrier to the solute transport is created by those of them experiencing unfavorable interactions with the solute. Resistance to the transport of a hydrophobic (hydrophilic) solute increases with the excess density of the head (tail) group region of the monolayer, which in turn increases with the length of the surfactant head (tail) group. Barriers for solute transport through surfactant monolayers are also influenced by the solute size. However, the extent of this influence is determined by the monolayer thickness and the solute structure and composition. For example, it is shown that resistance offered by thin monolayers to transport of linear oligomers is relatively insensitive to the solute length. The barrier sensitivity to the length of these solutes increases with the monolayer thickness. In addition to the static barriers, the solute transport is shown to be affected by dynamic barriers due to a nonadiabatic coupling of the monolayer surface with the solute position and configuration. This coupling leads to deviations of the system dynamics from the minimum energy path. The deviations are most significant in the neighborhood of the static energy barrier, which effectively leads to an increase of the barrier for the solute transport.
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Affiliation(s)
- Yong Nam Ahn
- Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, United States
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Martin-Gassin G, Gassin PM, Couston L, Diat O, Benichou E, Brevet PF. Second harmonic generation monitoring of nitric acid extraction by a monoamide at the water–dodecane interface. Phys Chem Chem Phys 2011; 13:19580-6. [DOI: 10.1039/c1cp22179e] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
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20
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Electrochemically driven transfer of carboxyl-terminated PAMAM dendrimers at the water/dichloroethane interface. Electrochem commun 2010. [DOI: 10.1016/j.elecom.2009.11.007] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
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Gupta A, Chauhan A, Kopelevich DI. Molecular transport across fluid interfaces: coupling between solute dynamics and interface fluctuations. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2008; 78:041605. [PMID: 18999437 DOI: 10.1103/physreve.78.041605] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/01/2008] [Indexed: 05/27/2023]
Abstract
We investigate the transport mechanism of a small hydrophobic solute molecule across two types of fluid interfaces, (i) an interface between two immiscible liquids and (ii) a surfactant-covered liquid-liquid interface. These systems are modeled by coarse-grained molecular dynamics simulations. It is demonstrated that the dynamics of the solute molecule near the interface significantly deviates from Markovian Brownian motion. Specifically, the correlation time of the random force acting on the solute strongly depends on the distance between the solute and the interface and increases by two orders of magnitude within a very narrow (less than 1 nm wide) region near the interface. The slow fluctuations of the random force in this narrow region are caused by capillary waves. The region location and width are determined by interface protrusions caused by attraction between the solute and the hydrophobic phase. We use results of molecular dynamics simulations to develop a stochastic model for the coupled solute-interface dynamics and estimate the rate of the solute transport across the interface. The observed phenomenon appears to be a general feature of mass transport across fluid or flexible membranes. The coupling between the solute transport and the interface fluctuations is the strongest in areas corresponding to a large free energy gradient or near a free energy barrier for the solute transport. This suggests a strong influence of the coupled solute-interface dynamics on the rate of mass transfer across interfaces.
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Affiliation(s)
- Ashish Gupta
- Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611-6005, USA
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Daikhin LI, Urbakh M. What is the origin of irregular current oscillations in the transfer of ionic surfactants across liquid/liquid interfaces? J Chem Phys 2008; 128:014706. [DOI: 10.1063/1.2812281] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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25
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Partenskii MB, Miloshevsky GV, Jordan PC. The Theoretical Challenge Posed by Low-Voltage Membrane Electroporation, Viewed from the Perspective of Continuum and Molecular-Level Models. Isr J Chem 2007. [DOI: 10.1560/ijc.47.3-4.385] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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26
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Laforge FO, Sun P, Mirkin MV. Shuttling Mechanism of Ion Transfer at the Interface between Two Immiscible Liquids. J Am Chem Soc 2006; 128:15019-25. [PMID: 17105314 DOI: 10.1021/ja0656090] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The transfers of hydrophilic ions between aqueous and organic phases are ubiquitous in biological and technological systems. These energetically unfavorable processes can be facilitated either by small molecules (ionophores) or by ion-transport proteins. In absence of a facilitating agent, ion-transfer reactions are assumed to be "simple", one-step processes. Our experiments at the nanometer-sized interfaces between water and neat organic solvents showed that the generally accepted one-step mechanism cannot explain important features of transfer processes for a wide class of ions including metal cations, protons, and hydrophilic anions. The proposed new mechanism of ion transfer involves transient interfacial ion paring and shuttling of a hydrophilic ion across the mixed-solvent layer.
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Affiliation(s)
- François O Laforge
- Department of Chemistry and Biochemistry, Queens College-CUNY, Flushing, NY 11367, USA
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27
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Frank S, Schmickler W. Ion transfer across liquid∣liquid interfaces from transition-state theory and stochastic molecular dynamics simulations. J Electroanal Chem (Lausanne) 2006. [DOI: 10.1016/j.jelechem.2006.02.032] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
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28
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Affiliation(s)
- Ilan Benjamin
- Department of Chemistry, University of California, Santa Cruz, California 95064, USA
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29
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Daikhin L, Kornyshev A, Kuznetsov A, Urbakh M. ITIES fluctuations induced by easily transferable ions. Chem Phys 2005. [DOI: 10.1016/j.chemphys.2005.03.029] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
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Markin VS, Volkov AG, Volkova-Gugeshashvili MI. Structure of Nonpolarizable Water/Nitrobenzene Interface: Potential Distribution, Ion Adsorption, and Interfacial Tension. J Phys Chem B 2005; 109:16444-54. [PMID: 16853091 DOI: 10.1021/jp0529220] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Adsorption of hydrophobic and hydrophilic ions at the nonpolarizable interface between two immiscible electrolyte solutions was investigated. The results were analyzed in three different models: (i) Gouy-Chapman model, (ii) ions as hard spheres, and (iii) ion pair formation at the interface. In the Gouy-Chapman model, an analytical expression for the interfacial tension was obtained. It predicts that interfacial tension should be proportional to the square root of the electrolyte concentration, which does not agree with experimental data. Modeling ions as hard spheres only slightly improves the agreement. The third model of interfacial ion pairing as the main origin of adsorption was analyzed using the amphiphilic isotherm (Markin-Volkov isotherm). A good agreement between ion-pairing theory and experimental values was achieved. The MV isotherm takes into account the limited number of adsorption sites, final size of molecules, complex formation at the interface, and interaction between adsorbed particles. The analysis revealed repulsion between adsorbed tetraalkylammonium ions at the nitrobenzene/water interface and demonstrated linear dependence between adsorption site area and the size of a molecule.
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Affiliation(s)
- Vladislav S Markin
- Department of Anesthesiology, UT Southwestern Medical Center, Dallas, Texas 75390-9068, USA.
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Chorny I, Benjamin I. Hydration Shell Exchange Dynamics during Ion Transfer Across the Liquid/Liquid Interface. J Phys Chem B 2005; 109:16455-62. [PMID: 16853092 DOI: 10.1021/jp051836x] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
We examine using molecular dynamics simulations the rate and mechanism of water molecules exchange around the Li(+) and Na(+) ions during ion transfer across the interface between water and nitrobenzene. As the ions are transferred from the water to the organic phase, they keep their first hydration shell and an incomplete second shell. The rate of water exchange between the first shell and the rest of the interfacial water molecule decreases during the transfer, which is consistent with an increase in the barrier along the ion-water potential of mean force. While in bulk water the exchange of water molecules around the Li(+) follows an associative (A) or associative interchange (I(a)) type mechanism, the fraction of exchange events of type A increases at the interface. In contrast, while in bulk water the exchange of water molecules around the six coordinated Na(+) hydrated species mainly follows a dissociative mechanism, the situation at the interface involves an equilibrium interchange between the four- and five-coordinated hydrated ion. Simulation of the reversed process, in which the hydrated Li(+) ion is transferred to the aqueous phase, shows the same general behavior as a function of location from the interface.
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Affiliation(s)
- Ilya Chorny
- Department of Chemistry, University of California, Santa Cruz, California 95064, USA
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Hill AW, Benjamin I. Influence of Surface Tension on Adsorbate Molecular Rotation at Liquid/Liquid Interfaces. J Phys Chem B 2004. [DOI: 10.1021/jp0467806] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Andrew W. Hill
- Department of Chemistry, University of California, Santa Cruz, California 95064
| | - Ilan Benjamin
- Department of Chemistry, University of California, Santa Cruz, California 95064
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Verdes C, Urbakh M, Kornyshev A. Surface tension and ion transfer across the interface of two immiscible electrolytes. Electrochem commun 2004. [DOI: 10.1016/j.elecom.2004.04.022] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
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Cherepanov DA, Feniouk BA, Junge W, Mulkidjanian AY. Low dielectric permittivity of water at the membrane interface: effect on the energy coupling mechanism in biological membranes. Biophys J 2003; 85:1307-16. [PMID: 12885673 PMCID: PMC1303247 DOI: 10.1016/s0006-3495(03)74565-2] [Citation(s) in RCA: 109] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
Protonmotive force (the transmembrane difference in electrochemical potential of protons, ) drives ATP synthesis in bacteria, mitochondria, and chloroplasts. It has remained unsettled whether the entropic (chemical) component of relates to the difference in the proton activity between two bulk water phases (deltapH(B)) or between two membrane surfaces (deltapH(S)). To scrutinize whether deltapH(S) can deviate from deltapH(B), we modeled the behavior of protons at the membrane/water interface. We made use of the surprisingly low dielectric permittivity of interfacial water as determined by O. Teschke, G. Ceotto, and E. F. de Souza (O. Teschke, G. Ceotto, and E. F. de Sousa, 2001, PHYS: Rev. E. 64:011605). Electrostatic calculations revealed a potential barrier in the water phase some 0.5-1 nm away from the membrane surface. The barrier was higher for monovalent anions moving toward the surface (0.2-0.3 eV) than for monovalent cations (0.1-0.15 eV). By solving the Smoluchowski equation for protons spreading away from proton "pumps" at the surface, we found that the barrier could cause an elevation of the proton concentration at the interface. Taking typical values for the density of proton pumps and for their turnover rate, we calculated that a potential barrier of 0.12 eV yielded a steady-state pH(S) of approximately 6.0; the value of pH(S) was independent of pH in the bulk water phase under neutral and alkaline conditions. These results provide a rationale to solve the long-lasting problem of the seemingly insufficient protonmotive force in mesophilic and alkaliphilic bacteria.
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Affiliation(s)
- Dmitry A Cherepanov
- Division of Biophysics, Faculty of Biology/Chemistry, University of Osnabrück, Osnabrück, Germany
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