Xian Kong

Molecular Theory for Electrokinetic Transport in pH-Regulated Nanochannels.

Ion transport through nanochannels depends on various external driving forces as well as the structural and hydrodynamic inhomogeneity of the confined fluid inside of the pore. Conventional models of electrokinetic transport neglect the discrete nature of ionic species and electrostatic correlations important at the boundary and often lead to inconsistent predictions of the surface potential and the surface charge density. Here, we demonstrate that the electrokinetic phenomena can be successfully described by the classical density functional theory in conjunction with the Navier-Stokes equation for the fluid flow. The new theoretical procedure predicts ion conductivity in various pH-regulated nanochannels under different driving forces, in excellent agreement with experimental data.

On the hydrophilicity of electrodes for capacitive energy extraction.

The so-called Capmix technique for energy extraction is based on the cyclic expansion of electrical double layers to harvest dissipative energy arising from the salinity difference between freshwater and seawater. Its optimal performance requires a careful selection of the electrical potentials for the charging and discharging processes, which must be matched with the pore characteristics of the electrode materials. While a number of recent studies have examined the effects of the electrode pore size and geometry on the capacitive energy extraction processes, there is little knowledge on how the surface properties of the electrodes affect the thermodynamic efficiency. In this work, we investigate the Capmix processes using the classical density functional theory for a realistic model of electrolyte solutions. The theoretical predictions allow us to identify optimal operation parameters for capacitive energy extraction with porous electrodes of different surface hydrophobicity. In agreement with recent experiments, we find that the thermodynamic efficiency can be much improved by using most hydrophilic electrodes.

Density functional theory study of the capacitance of single file ions in a narrow cylinder

The differential capacitance of a model organic electrolyte in a cylindrical pore that is so narrow that the ions can form only a single file is studied by means of density functional theory (DFT). Kornyshev (2013), has studied this system and found the differential capacitance to have only a double hump shape (the so-called camel shape) whereas other geometries show this behavior only at low ionic concentrations that are typical for aqueous electrolytes. However, his calculation is rather approximate. In this DFT study we find that the double hump shape occurs only at low ionic concentrations. At high concentrations, the capacitance has only a single hump. Kornyshev considers a metallic cylinder and approximately includes the contributions of electrostatic images. Electrostatic images are not easily incorporated into DFT. As a result, images are not considered in this study and the question of whether Kornyshevs result is due to his approximations or images cannot be answered. Simulations to answer this question are planned.

Spreading of a Unilamellar Liposome on Charged Substrates: A Coarse-Grained Molecular Simulation

Supported lipid bilayers (SLBs) are able to accommodate membrane proteins useful for diverse biomimetic applications. Although liposome spreading represents a common procedure for preparation of SLBs, the underlying mechanism is not yet fully understood, particularly from a molecular perspective. The present study examines the effects of the substrate charge on unilamellar liposome spreading on the basis of molecular dynamics simulations for a coarse-grained model of the solvent and lipid molecules. Liposome transformation into a lipid bilayer of different microscopic structures suggests three types of kinetic pathways depending on the substrate charge density, that is, top-receding, parachute, and parachute with wormholes. Each pathway leads to a unique distribution of the lipid molecules and thereby distinctive properties of SLBs. An increase of the substrate charge density results in a magnified asymmetry of the SLBs in terms of the ratio of charged lipids, parallel surface movements, and the distribution of lipid molecules. While the lipid mobility in the proximal layer is strongly correlated with the substrate potential, the dynamics of lipid molecules in the distal monolayer is similar to that of a freestanding lipid bilayer. For liposome spreading on a highly charged surface, wormhole formation promotes lipid exchange between the SLB monolayers thus reduces the asymmetry on the number density of lipid molecules, the lipid order parameter, and the monolayer thickness. The simulation results reveal the important regulatory role of electrostatic interactions on liposome spreading and the properties of SLBs.

Molecular dynamics for the charging behavior of nanostructured electric double layer capacitors containing room temperature ionic liquids

The charging kinetics of electric double layers (EDLs) is closely related to the performance of a wide variety of nanostructured devices including supercapacitors, electro-actuators, and electrolyte-gated transistors. While room temperature ionic liquids (RTIL) are often used as the charge carrier in these new applications, the theoretical analyses are mostly based on conventional electrokinetic theories suitable for macroscopic electrochemical phenomena in aqueous solutions. In this work, we study the charging behavior of RTIL-EDLs using a coarse-grained molecular model and constant-potential molecular dynamics (MD) simulations. In stark contrast to the predictions of conventional theories, the MD results show oscillatory variations of ionic distributions and electrochemical properties in response to the separation between electrodes. The rate of EDL charging exhibits non-monotonic behavior revealing strong electrostatic correlations in RTIL under confinement.

A multi-scale molecular dynamics simulation of PMAL facilitated delivery of siRNA

The capability of silencing genes makes small interfering RNA (siRNA) appealing for curing fatal diseases such as cancer and viral infections. In the present work, we chose a novel amphiphilic polymer, PMAL (poly(maleic anhydride-alt-1-decene) substituted with 3-(dimethylamino) propylamine), as the siRNA carrier, and conducted steered molecular dynamics simulations, together with traditional molecular dynamics simulations, to explore how PMAL facilitates the delivery of siRNA. It was shown that the use of PMAL reduced the energy barrier for siRNA to penetrate lipid bilayer membranes, as confirmed by the experimental work. The simulation of the transmembrane process revealed that PMAL can punch a hole in the lipid bilayer and form a channel for siRNA delivery. Monitoring of the structural transition further showed the targeting of siRNA through the attachment of PMAL encapsulating siRNA to a lipid membrane. The delivery of siRNA was facilitated by the hydrophobic interaction between PMAL and the lipid membrane, which favored the dissociation of the siRNA–PMAL complex. The above simulation established a molecular insight of the interaction between siRNA and PMAL and was helpful for the design and applications of new carriers for siRNA delivery.

Multiscale simulation of surfactant–aquaporin complex formation and water permeability

Multiscale simulation has been conducted for the formation of a surfactant–protein complex that uses sodium dodecyl sulfate (SDS), a negatively charged surfactant, and aquaporin Z (AqpZ), a membrane protein that facilitates water transport across lipid membranes. A detailed analysis of the molecular driving forces of the self-assembly at different pH values reveals distinctive contributions of electrostatic and hydrophobic interactions to the complex structure and formation kinetics. The electrostatic interactions become more significant at low pH and are responsible for the formation of larger complexes. A comparison of the protein conformation in the SDS complex with that in the lipid bilayer of palmitoyloleoyl phosphatidyl ethanolamine (POPE) shows that the SDS molecules have only marginal effects on AqpZ conformation including the water channel structure. The simulation indicates that AqpZ preserves its secondary structures after being bound with SDS molecules, while the arrangement of the helical structures leads to a coiled-coil to single helix transition as suggested by experiments. AqpZ may lose water permeability either due to the blockage of the water channel by individual SDS molecules or due to the attachment of micelle-like structures at the hydrophilic ends of the water pore. Reconstitution of the AqpZ complex into a POPE bilayer shows that the membrane protein regains its activity after the complete removal of SDS molecules from the protein pores. The molecular insights gained from multiscale simulation will be helpful for future development of AqpZ-embedded membranes.

A molecular theory for predicting the thermodynamic efficiency of electrokinetic energy conversion in slit nanochannels

The classical density functional theory is incorporated with the Stokes equation to examine the thermodynamic efficiency of pressure-driven electrokinetic energy conversion in slit nanochannels. Different from previous mean-field predictions, but in good agreement with recent experiments, the molecular theory indicates that the thermodynamic efficiency may not be linearly correlated with the channel size or the electrolyte concentration. For a given electrolyte, an optimal slit nanochannel size and ion concentration can be identified to maximize both the electrical current and the thermodynamic efficiency. The optimal conditions are sensitive to a large number of parameters including ion diameters, valences, electrolyte concentration, channel size, and the valence- and size-asymmetry of oppositely charged ionic species. The theoretical results offer fresh insights into pressure-driven current generation processes and are helpful guidelines for the design of apparatus for the electrokinetic energy conversion.