Theory and computer simulation of solute effects on the surface tension of liquids

Application of molecular dynamics simulation in self-assembled cancer nanomedicine

Self-assembled nanomedicine holds great potential in cancer theragnostic. The structures and dynamics of nanomedicine can be affected by a variety of non-covalent interactions, so it is essential to ensure the self-assembly process at atomic level. Molecular dynamics (MD) simulation is a key technology to link microcosm and macroscale. Along with the rapid development of computational power and simulation methods, scientists could simulate the specific process of intermolecular interactions. Thus, some experimental observations could be explained at microscopic level and the nanomedicine synthesis process would have traces to follow. This review not only outlines the concept, basic principle, and the parameter setting of MD simulation, but also highlights the recent progress in MD simulation for self-assembled cancer nanomedicine. In addition, the physicochemical parameters of self-assembly structure and interaction between various assembled molecules under MD simulation are also discussed. Therefore, this review will help advanced and novice researchers to quickly zoom in on fundamental information and gather some thought-provoking ideas to advance this subfield of self-assembled cancer nanomedicine.

Insights into the mechanism of electrostatic field promoting ozone mass transfer in water: A molecular dynamics perspective

Ozone is the main role of ozone-based advanced oxidation process for organic wastewater treatment, which is usually added in water by aeration. However, the low solubility of ozone in water seriously affects the degradation efficiency. In this article, the external electrostatic field (EF) was proposed to improve the ozone solubility in water. The mass transfer characteristics of ozone in a gas-liquid two-phase system under EF were studied by molecular dynamics (MD) simulation. The microscopic mechanism of ozone mass transfer in water promoted by external EF was revealed by analyzing Gibbs dividing surface (GDS) interface structure, interfacial water molecular orientation, surface tension, liquid phase viscosity, hydrogen bond network and ozone self-diffusion coefficient. Our findings reveal that EF can enhance the thickness of GDS region (from 0.4648 nm to 0.4996 nm when EF is 0.2 V/nm) as well as its ozone content. The dipole moment orientation of water molecules also tends to point in the EF direction due to the influence of EF, making the difference in dipole moment orientation of water molecules in the first and second layers of GDS region gradually disappear. In addition, compared with the absence of EF, the existence of external EF can decrease the surface tension (from 77.6162 mN/m to 73.3480 mN/m when EF is 0.2 V/nm) at the gas-liquid interface and the viscosity of liquid phase (from 0.293 mP_a·s to 0.162 mP_a·s when EF is 0.2 V/nm), break the network of hydrogen bond in liquid phase, and increase the mobility of ozone (self-diffusion coefficient of ozone changes from 1.3232 × 10^-6 cm²·s^-1 to 1.8812 × 10^-6 cm²·s^-1 when EF is 0.2 V/nm). All these properties changes indicate that the presence of external EF enhances the ability of ozone to penetrate the interface of the two-phase system, and then improves the ozone mass transfer efficiency in water.

Contributions of higher-order proximal distribution functions to solvent structure around proteins

The proximal distribution function (pDF) quantifies the probability of finding a solvent molecule in the vicinity of solutes. The approach constitutes a hierarchically organized theory for constructing approximate solvation structures around solutes. Given the assumption of universality of atom cluster-specific solvation, reconstruction of the solvent distribution around arbitrary molecules provides a computationally convenient route to solvation thermodynamics. Previously, such solvent reconstructions usually considered the contribution of the nearest-neighbor distribution only. We extend the pDF reconstruction algorithm to terms including next-nearest-neighbor contribution. As a test, small molecules (alanine and butane) are examined. The analysis is then extended to include the protein myoglobin in the P6 crystal unit cell. Molecular dynamics simulations are performed, and solvent density distributions around the solute molecules are compared with the results from different pDF reconstruction models. It is shown that the next-nearest-neighbor modification significantly improves the reconstruction of the solvent number density distribution in concave regions and between solute molecules. The probability densities are then used to calculate the solute-solvent non-bonded interaction energies including van der Waals and electrostatic, which are found to be in good agreement with the simulated values.

Thermodynamically consistent derivation of chemical potential of a battery solid particle from the regular solution theory applied to LiFePO4

The chemical potential of lithium in LixFePO4 active cathode nanoparticles and the surface free energy between LixFePO4 and electrolyte were determined with the novel thermodynamically consistent application of the regular solution theory. Innovative consideration of crystal anisotropy accounts for the consistent determination of the dependency of the chemical potential on the mechanistically derived enthalpy of mixing and the phase boundary gradient penalty. This enabled the analytic, thermodynamically consistent determination of the phase boundary thickness between LiFePO4 and FePO4, which is in good agreement with experimental observations. The obtained explicit functional dependency of the surface free energy on the lithium concentration enables adequate simulation of the initiation of the phase transition from FePO4 to LiFePO4 at the surface of active cathode particles. To validate the plausibility of the newly developed approaches, lithium intercalation into the LixFePO4 nanoparticles from electrolyte was modeled by solving the Cahn-Hilliard equation in a quasi-two-dimensional domain.

Molecular Structure Inhibiting Synergism in Charged Surfactant Mixtures: An Atomistic Molecular Dynamics Simulation Study

Synergistic and nonsynergistic surfactant-water mixtures of sodium dodecyl sulfate (SDS), lauryl betaine (C12B), and cocoamidopropyl betaine (CAPB) systems are studied using molecular simulation to understand the role of interactions among headgroups, tailgroups, and water on structural and thermodynamic properties at the air-water interface. SDS is an anionic surfactant, while C12B and CAPB are zwitterionic; CAPB differs from C12B by an amide group in the tail. While the lowest surface tensions at high surface concentrations in the SDS-C12B synergistic system could not be reproduced by simulation, estimated partitioning between surface and bulk shows trends consistent with synergism. Structural analysis shows the influence of the SDS headgroup pulling C12B to the surface, resulting in closely packed structures compared to their respective homomolecular-surfactant systems. The SDS-CAPB system, on the other hand, is nonsynergistic when the surfactants are mixed on account of the tilted structure of the CAPB tail. The translational excess entropy due to the tailgroup interactions discriminates between the synergistic and nonsynergistic systems. The implications of such interactions on surfactant effects in complex, multicomponent atmospheric aerosols are discussed.

Effect of methane-sugar interaction on the solubility of methane in an aqueous solution

In this study, the effect of methane-sugar interaction on the solubility of methane in an aqueous solution at ambient pressure was investigated. Various sugars, such as fructose, glucose, sucrose, maltose, and raffinose, were used, and depending on the type and concentration of sugar, the methane solubility increased from 21.72mg/L (in pure water) to 24.86mg/L. Sugars with a low hydrogen-bonding number between the water and sugar molecules exhibited a large enhancement in methane solubility. The solute partitioning model and molecular dynamics simulations were employed to verify the results obtained for the experimental solubility of methane in aqueous sugar solutions.

Local fluctuations in solution mixtures

An extension of the traditional Kirkwood-Buff (KB) theory of solutions is outlined which provides additional fluctuating quantities that can be used to characterize and probe the behavior of solution mixtures. Particle-energy and energy-energy fluctuations for local regions of any multicomponent solution are expressed in terms of experimentally obtainable quantities, thereby supplementing the usual particle-particle fluctuations provided by the established KB inversion approach. The expressions are then used to analyze experimental data for pure water over a range of temperatures and pressures, a variety of pure liquids, and three binary solution mixtures - methanol and water, benzene and methanol, and aqueous sodium chloride. In addition to providing information on local properties of solutions it is argued that the particle-energy and energy-energy fluctuations can also be used to test and refine solute and solvent force fields for use in computer simulation studies.

Recruitment of class I hydrophobins to the air:water interface initiates a multi-step process of functional amyloid formation

Class I fungal hydrophobins form amphipathic monolayers composed of amyloid rodlets. This is a remarkable case of functional amyloid formation in that a hydrophobic:hydrophilic interface is required to trigger the self-assembly of the proteins. The mechanism of rodlet formation and the role of the interface in this process have not been well understood. Here, we have studied the effect of a range of additives, including ionic liquids, alcohols, and detergents, on rodlet formation by two class I hydrophobins, EAS and DewA. Although the conformation of the hydrophobins in these different solutions is not altered, we observe that the rate of rodlet formation is slowed as the surface tension of the solution is decreased, regardless of the nature of the additive. These results suggest that interface properties are of critical importance for the recruitment, alignment, and structural rearrangement of the amphipathic hydrophobin monomers. This work gives insight into the forces that drive macromolecular assembly of this unique family of proteins and allows us to propose a three-stage model for the interface-driven formation of rodlets.

Kirkwood-Buff integrals for ideal solutions

The Kirkwood-Buff (KB) theory of solutions is a rigorous theory of solution mixtures which relates the molecular distributions between the solution components to the thermodynamic properties of the mixture. Ideal solutions represent a useful reference for understanding the properties of real solutions. Here, we derive expressions for the KB integrals, the central components of KB theory, in ideal solutions of any number of components corresponding to the three main concentration scales. The results are illustrated by use of molecular dynamics simulations for two binary solutions mixtures, benzene with toluene, and methanethiol with dimethylsulfide, which closely approach ideal behavior, and a binary mixture of benzene and methanol which is nonideal. Simulations of a quaternary mixture containing benzene, toluene, methanethiol, and dimethylsulfide suggest this system displays ideal behavior and that ideal behavior is not limited to mixtures containing a small number of components.

The effect of urea on the morphology of NaCl crystals: A combined theoretical and simulation study

It has been known for over a century that the presence of cosolvents such as urea and formamide can alter the morphology of NaCl crystals grown from solution. To help understand this effect we have been developing a theoretical approach based on the Kirkwood-Buff (KB) theory of solutions, and have combined this with computer simulations of the interation of urea with different crystal faces of NaCl. In this way one can predict the effect of urea on the thermodynamic stability of different NaCl faces, with atomic level detail provided by the simulations. We observe that urea is preferentially excluded from 100 and 111 crystal faces, but is less excluded from 111 faces which present chloride ions at the surface. The results indicate that the 111 face is stabilized in urea solutions and promotes the formation of octahedral over cubic NaCl crystals. The approach is totally general and can be applied to understand a variety of interfacial properties. Furthermore, we apply KB theory to study several other issues regarding the simulation of crystal growth.

Using surface tension data to predict differences in surface and bulk concentrations of nonelectrolytes in water

Recently, we developed a quantitative interpretation of surface tension increments (STI) of salts, acids, and bases in terms of the solute (or salt ion) partitioning model (SPM). Here, we obtain an analogous SPM-based interpretation of surface tension increments of nonelectrolytes, which yields local-bulk partition coefficients (K(p)) quantifying the accumulation or exclusion of these solutes in the local region near the air-water surface, and the amount of water per unit area of that region (b1σ). Sucrose exhibits the largest positive STI (approximately 1.4 ergs cm(-2) Osm(-1)). Assuming that K(p) = 0 for sucrose (i.e. that it is completely excluded from the surface of water), these STI provide a minimum estimate of b1σ of 0.20 H(2)O/Å(2), or a minimum thickness of the surface region of approximately two layers of water at bulk density. This is the same value as obtained previously from analysis of surface tension and hydrocarbon solubility increments of Na(2)SO(4) and also for the interaction of glycine betaine with anionic carboxylate surface, indicating that this quantity is not a function of the type of solute or surface investigated and therefore that it may represent the molecular thickness of the region. Partition coefficients of other nonelectrolytes investigated range from moderately excluded (e.g urea) to moderately accumulated (e.g. glycerol, ethylene glycol); strongly accumulated surface active solutes (e.g. mono-substituted alcohols) were not included in this analysis. Partition coefficients for many salt ions obtained from STI and hydrocarbon solubility increments fall in a rank order which corresponds to the Hofmeister series for protein folding and protein solubility, indicating a common pattern of accumulation or exclusion of salt ions at the air-water surface and nonpolar surfaces of dissolved hydrocarbons and proteins; no such patterns are observed for nonelectrolytes.

Quantifying accumulation or exclusion of H+, HO-, and Hofmeister salt ions near interfaces

Recently, surface spectroscopies and simulations have begun to characterize the non-uniform distributions of salt ions near macroscopic and molecular surfaces. The thermodynamic consequences of these non-uniform distributions determine the often-large ion-specific effects of Hofmeister salts on a very wide range of processes in water. For uncharged surfaces, where these nonuniform ion distributions are confined to the first few layers of water at the surface, a two-state approximation to the distributions of water and ions, called the salt ion partitioning model (SPM) has both molecular and thermodynamic signiicance. Here, we summarize SPM results quantifying the local accumulation of H+, exclusion of HO-, and range of partitioning behavior of Hofmeister anions and cations near macroscopic and molecular interfaces. These results provide a database to interpret or predict Hofmeister salt effects on aqueous processes in terms of structural information regarding amount and composition of the surface exposed or buried in these processes.