Model of the dense part of the double layer in the absence of specific adsorption

A model is suggested for the dense part of the double layer in the absence of specific adsorption, which is based on the following assumptions: (1) There are two kinds of adsorbed solvent particles on the electrode surface: associates freely oriented in the double layer field and separate chemisorbed water dipoles; 92) the total number of solvent molecules on the surface is determined by the geometric dimensions of associates and chemisorbed dipoles; (3) account is taken of the electrostatic interaction of associates and chemisorbed dipoles in the surface layer; (4) a discreteness coefficient is introduced, accounting for the degree of influence of the field due to all the other dipoles and associates in the surface layer on the dipole or associate being considered; (5) the dipole moment of a chemisorbed water molecule exceeds the value of μH₂O in the bulk of the solution; (6) account is taken of the decrease with increasing temperature of the average number of water molecules in surface associate.The calculations on the basis of this model describe satisfactorily the shape of the differential capacity curves of the dense layer and their temperature dependence for the system Hg-aqueous NaF solution.     

A two-state solvent model for rigid rods adsorption at charged interfaces

A simple two-state solvent model is developed to describe the adsorption of rigid rods at polarizable interfaces. The adsorption isotherm and the equation of state are determined by means of the lattice theory of strictly regular solutions, assuming that the solvent at the interface exists in the form of monomers with two possible orientations, and the adsorbed rods are oriented normal to the electrode surface. In the case of the mercury/aqueous inert electrolyte interface in the presence of small, polar, aliphatic hydrocarbon derivatives, the model predicts that the variation of the isotherm interaction parameter is determined mainly by the dipole-dipole interactions between the permanent dipoles of the adsorbed molecules. This variation becomes more pronounced as the dipolar interactions between adsorbate-solvent molecules become more intense. The Marshall-Conway treatment for the polarization catastrophe and the approach of Levine et al. in incorporating polarizability effects are also taken into consideration and examined critically.     

Thermodynamics of DL-α-aminobutyric acid induced solvation mechanism in aqueous KCl solutions at 288.15–308.15 K

The solubilities of DL-α-aminobutyric acid in KCl solutions of different concentrations are measured at 288.15–308.15 K. Gibbs energies and entropies have been determined for transfer of α-aminobutyric acid form water to aqueous KCl solution at 298.15 K. The cavity, dipole-dipole and other interactions affecting the solubility, as well as stability of the amino acid in solution are also evaluated. Gibbs energy and entropy of transfer due to interactions are computed to create the model of the complex solute-solvent and solventsolvent interactions. Molar volume, densities, dipole moment of solvent and diameter of co-solvent in aqueous potassium chloride are also evaluated.     

On the Theory of the Stability of Dipole Molecule Solution Interlayers in Apolar Solvents: 1. Thermodynamics of Solution Film

Thermodynamics of the film of solution with apolar solvent and polar solute is analyzed. The expression for calculating disjoining pressure in a film via the potential electric energy of the filed generated by the solute dipoles is derived. The effect of the image forces on the orientation of dipole molecules near the interfaces is considered.     

Soft sticky dipole-quadrupole-octupole potential energy function for liquid water: an approximate moment expansion

A new, efficient potential energy function for liquid water is presented here. The new model, which is referred here as the soft sticky dipole-quadrupole-octupole (SSDQO) model, describes a water molecule as a Lennard-Jones sphere with point dipole, quadrupole, and octupole moments. It is a single-point model and resembles the hard-sphere sticky dipole potential model for water by Bratko et al. [J. Chem. Phys. 83, 6367 (1985)] and the soft sticky dipole model by Ichiye and Liu [J. Phys. Chem. 100, 2723 (1996)] except now the sticky potential consists of an approximate moment expansion for the dimer interaction potential, which is much faster than the true moment expansion. The object here is to demonstrate that the SSDQO potential energy function can accurately mimic the potential energy function of a multipoint model using the moments of that model. First, the SSDQO potential energy function using the dipole, quadruple, and octupole moments from SPC/E, TIP3P, or TIP5P is shown to reproduce the dimer potential energy functions of the respective multipoint model. In addition, in Monte Carlo simulations of the pure liquid at room temperature, SSDQO reproduces radial distribution functions of the respective model. However, the Monte Carlo simulations using the SSDQO model are about three times faster than those using the three-point models and the long-range interactions decay faster for SSDQO (1/r(3) and faster) than for multipoint models (1/r). Moreover, the contribution of each moment to the energetics and other properties can be determined. Overall, the simplicity, efficiency, and accuracy of the SSDQO potential energy function make it potentially very useful for studies of aqueous solvation by computer simulations.     

溶剂分子性质与界面内层微分电容变化特性

依照前文[1]设立的偶极取向分布模型，利用模拟的C1（σ）假想曲线阐析溶剂分子性质对电极／溶液界面内层微分电容的影响趋向，理想的C1（σ）拟合曲线表现出单峰或双峰的两种基本式样，而溶剂分子的极化，各态偶极取向能的差别以及偶极间的相互作用均将导致C1（σ）曲线明显形变．据此，可从分子的性质预测各类电极／溶液界面体系C1（σ）曲线变化特性.     

黄嘌呤及其互变异构体的密度泛函理论研究

嘌呤碱及其衍生物在生物系统中起重要作用。对人具有兴奋和利尿作用的茶碱和咖啡碱就是黄嘌呤的甲基衍生物。黄嘌呤存在多种互变异构体,从理论计算的角度研究这些互变异构体的几何结构、电子结构及相对稳定性,进一步研究溶剂对其结构和性质的影响是有意义旧。密度泛函理论方法既考虑了电子相关,又较其它CI（组态相互作用）或MPn（n级微扰）方法节省机时。其计算结果较好,被广泛应用于研究各种化合物。本文采用密度泛函B3LYP／6—311G方法对14种黄嘌呤可能的互变异构体（见图1）,分别在气相和水相中进行几何构型全自由度优化和能量计算,讨论了异构体的相对稳定性,水的溶剂化作用对异构体的能量、几何构型、电荷分布和偶极矩的影响,探讨了溶剂极性对异构体的能量和偶极矩的影响。     

How hydrophobic buckminsterfullerene affects surrounding water structure

The hydrophobic hydration of fullerenes in water is of significant interest as the most common Buckminsterfullerene (C60) is a mesoscale sphere; C60 also has potential in pharmaceutical and nanomaterial applications. We use an all-atom molecular dynamics simulation lasting hundreds of nanoseconds to determine the behavior of a single molecule of C60 in a periodic box of water, and compare this to methane. A C60 molecule does not induce drying at the surface; however, unlike a hard sphere methane, a hard sphere C60 solute does. This is due to a larger number of attractive Lennard-Jones interactions between the carbon atom centers in C60 and the surrounding waters. In these simulations, water is not uniformly arranged but rather adopts a range of orientations in the first hydration shell despite the spherical symmetry of both solutes. There is a clear effect of solute size on the orientation of the first hydration shell waters. There is a large increase in hydrogen-bonding contacts between waters in the C60 first hydration shell. There is also a disruption of hydrogen bonds between waters in the first and second hydration shells. Water molecules in the first hydration shell preferentially create triangular structures that minimize the net water dipole near the surface near both the methane and C60 surface, reducing the total energy of the system. Additionally, in the first and second hydration shells, the water dipoles are ordered to a distance of 8 A from the solute surface. We conclude that, with a diameter of approximately 1 nm, C60 behaves as a large hydrophobic solute.     

Rigidity of segments of poly (ethyl methacrylate) in CCl14 solutions

Collegial dispersion forces originate from a coordination of the temporary electronic dipoles of a series of molecules. To insure this coordination the nuclei on a row have to replace their individual vibrations by coupled ones. Such row is then rigid. For polymers collegial forces can already be temporarily active in given parts of a solution but are regularly annihilated by intrusion of the solvent. However the regular passage through the rigid state modifies the distribution of the dipole moments during the interval that solvent or ligand molecules are in contact with the segment. In a rigid domain the correlation parameter of Kirkwood g is zero. The existence of such rigidity in solution is demonstrated by the low experimental value of g_mA for the dipoles of the complexes between poly(ethyl methacrylate) and phenols. Due to the large number of segments that separate the complexes the classical explanation of stiffness of the chain due to individual interactions between neighbours does not hold.     

Accounting for non-optimal interactions in molecular recognition: a study of ion-π complexes using a QM/MM model with a dipole-polarisable MM region

For a quantitative understanding of molecular structure, interaction and dynamics, accurate modelling of the energetics of both near-equilibrium and less optimal contacts is important. In this work, we explore the potential energy surfaces of representative ion-π complexes. We examine the performance of a semi-empirical QM/MM approach and the corresponding QM/MMpol model, where inducible point dipoles are additionally employed in the MM region. The predicted potential energy surfaces of cation-benzene complexes are improved by inclusion of explicit MM polarisation of the π-molecule. For cation-formamide complexes, inducible dipoles appreciably improve energetic estimates at geometries forming non-optimal interactions. Energetic component analysis suggests that the implicit MM polarisation of the fixed charge QM/MM model mirrors the behaviour of the QM/MMpol dipole model for the energetics of near-equilibrium conformations. However, for complexes at less optimal orientations, the QM/MM model exhibits higher errors than the QM/MMpol approach, being unable to capture orientation-dependent variations in polarisation energy.     

Electrostatic properties of aqueous salt solution interfaces: a comparison of polarizable and nonpolarizable ion models

The effects of ion force field polarizability on the interfacial electrostatic properties of approximately 1 M aqueous solutions of NaCl, CsCl, and NaI are investigated using molecular dynamics simulations employing both nonpolarizable and Drude-polarizable ion sets. Differences in computed depth-dependent orientational distributions, "permanent" and induced dipole and quadrupole moment profiles, and interfacial potentials are obtained for both ion sets to further elucidate how ion polarizability affects interfacial electrostatic properties among the various salts relative to pure water. We observe that the orientations and induced dipoles of water molecules are more strongly perturbed in the presence of polarizable ions via a stronger ionic double layer effect arising from greater charge separation. Both anions and cations exhibit enhanced induced dipole moments and strong z alignment in the vicinity of the Gibbs dividing surface (GDS) with the magnitude of the anion induced dipoles being nearly an order of magnitude larger than those of the cations and directed into the vapor phase. Depth-dependent profiles for the trace and z z components of the water molecular quadrupole moment tensors reveal 40% larger quadrupole moments in the bulk phase relative to the vapor which mimics a similar observed 40% increase in the average water dipole moment. Across the GDS, the water molecular quadrupole moments increase nonmonotonically (in contrast to the water dipoles) and exhibit a locally reduced contribution just below the surface due to both orientational and polarization effects. Computed interfacial potentials for the nonpolarizable salts yield values 20-60 mV more positive than pure water and increase by an additional 30-100 mV when ion polarizability is included. A rigorous decomposition of the total interfacial potential into ion monopole, water and ion dipole, and water quadrupole components reveals that a very strong, positive ion monopole contribution is offset by negative contributions from all other potential sources. Water quadrupole components modulated by the water density contribute significantly to the observed interfacial potential increments and almost entirely explain observed differences in the interfacial potentials for the two chloride salts. By lumping all remaining nonquadrupole interfacial potential contributions into a single "effective" dipole potential, we observe that the ratio of quadrupole to "effective" dipole contributions range from 2:1 in CsCl to 1:1.5 in NaI, suggesting that both contributions are comparably important in determining the interfacial potential increments. We also find that oscillations in the quadrupole potential in the double layer region are opposite in sign and partially cancel those of the "effective" dipole potential.     

A new model for predicting activity coefficients in aqueous solutions of amino acids and peptides

A new two-parameter model based on the perturbation of a hard-sphere reference has been developed to correlate the activity coefficients of several amino acids and simple peptides in aqueous solutions. The hard-sphere equation of state used as the reference in the model was proposed recently by Ghotbi and Vera. The perturbation terms coupled with the reference hard-sphere equation of state are attributed to the dispersion forces and the dipole–dipole interactions. The Lennard-Jones and Keesom potential functions are used to represent the dispersion and dipole–dipole interactions, respectively. The results of the new model are compared with those obtained by other models. It is shown that the new model can more accurately correlate the activity coefficients of amino acids and peptides in comparison with the other available models in the literature. The model was also used to correlate the solubility of several amino acids in aqueous solutions. The results show that the model can accurately correlate the solubility of the experimental data over a wide range of temperatures with only two adjustable parameters. New values for Gibbs free energy change, Δg, and enthalpy change, Δh, of the solute, i.e., amino acid for transferring one mole of solute from a saturated solution to a hypothetical aqueous solution with an activity of one molal at temperature 298.15 K are also reported.     

极性—非极性双液体系汽液平衡盐效应参数的测定与计算

测定了苯—甲醇— 1_1型电解质 (LiCl、NaBr、KI)、四氯化碳—甲醇— 1_1型电解质 (LiCl、NaBr、KI)两个体系在恒压 ( 1 0 1 .3kPa)条件下的汽液平衡盐效应参数。理论计算以Pierotti的定标粒子理论为基础 ,硬球作用项采用Masterton -Lee方程计算 ,软球作用项采用胡英等人建议的简化的径向分布函数 ,分子间力在Lennard -Jones位能函数基础上计入极性分子间偶极—偶极、偶极—诱导偶极 ,离子与极性分子间的电荷—偶极以及离子与分子间的电荷—诱导偶极的贡献 ,并根据溶剂性质和溶液结构作出一些合理的假设。在此基础上 ,理论计算与实验结果基本相符。     

Electrophoretic motion of a slightly deformed sphere with a nonuniform zeta potential distribution

Electrophoretic motion is analyzed for a rigid, slightly deformed sphere with a nonuniform zeta potential distribution. Hydrodynamics and electrostatics solutions for the deformed sphere with an arbitrary double-layer thickness are determined by using the domain perturbation method. The surface shape and the zeta potential distribution for the deformed sphere are expressed by using the multipole expansion representation. In terms of monopole, dipole, and quadrupole moments of the surface shape and the zeta potential distribution, explicit expressions are obtained for the translational and rotational electrophoretic mobility tensors. The ensemble average for the mobility of the deformed sphere with a uniform orientation distribution is also derived. The utility of the general mobility expression is demonstrated by studying the electrophoretic motion of axisymmetric and ellipsoidal particles. The translational and rotational mobilities of axisymmetric particles are both affected by the monopole, dipole, and quadrupole moments of the zeta potential. For ellipsoidal particles, however, the dipole moment of the zeta potential does not affect the translational mobility, while the rotational mobility depends only on the dipole moment. The mobility of the deformed sphere with either a thick or a thin double layer is also derived.     

电极/溶液界面单分子吸附层的统计力学处理 I. 非水溶液中汞电极的内层微分电容

本文提出电极/溶液界面溶剂化层偶极取向分布模型, 应用统计力学方法及热力学平衡条件导出普遍化的单层吸附等温方程, 其电解质溶液的溶剂组成可以是纯态的或混合物(多组份)的. 文中分别以甲酰胺、碳酸亚乙酯和甲醇等三种纯溶剂的汞/溶液界面为例, 采用曲线拟合计算内层微分电容随表面电荷变化关系。预计本模型处理对汞/水溶液或汞/(混合溶剂)溶液界面仍可适用。     

Effect of the interfacial chemical environment on in-plane electronic conduction of indium tin oxide: role of surface charge, dipole magnitude, and carrier injection

Reaction of the indium tin oxide (ITO) surface with Br?nsted acids (bases) leads to increases (decreases) in its in-plane conductance as measured by a four-point probe configuration. The conductance varies monotonically with pH, suggesting that the degree of surface protonation or hydroxylation controls the surface charge density, which in turn affects the width of the n-type depletion layer and ultimately the in-plane conductance. Measurements at constant pH with a series of tetraalkylammonium hydroxide species of varying cation size indicate that surface dipoles also affect ITO conductance by modulating the magnitude of the surface polarization. Modulating the double layer with varying aqueous salt solutions also affects ITO conductance, though not to the same degree as strong Br?nsted acids and bases. Solvents of varying dielectric constant and proton donating ability (ethanol, dimethylformamide) decrease ITO conductance relative to H2O. In addition, changing solvent gives rise to thermally derived conductance transients, which result from exothermic solvent mixing. The self-assembly of alkanethiols at the surface increases the conductance of ITO films, most likely through carrier population effects. In all cases examined the combined effects of surface charge, adsorbed dipole layer magnitude, and carrier injection are responsible for altering the ITO conductance. Besides being directly applicable to the control of electronic properties, these results also point to the use of four-point probe resistance measurements in condensed phase sensing applications.     

Microscopic models for quantum mechanical calculations of chemical processes in solutions: LD/AMPAC and SCAAS/AMPAC calculations of solvation energies

Our previously developed approaches for integrating quantum mechanical molecular orbital methods with microscopic solvent models are refined and examined. These approaches consider the nonlinear solute–solvent coupling in a self-consistent way by incorporating the potential from the solvent dipoles in the solute Hamiltonian, while considering the polarization of the solvent by the potential from the solute charges. The solvent models used include the simplified Langevin Dipoles (LD) model and the much more expensive surface constrained All Atom Solvent (SCAAS) model, which is combined with a free energy pertubation (FEP) approach. Both methods are effectively integrated with the quantum mechanical AMPAC package and can be easily combined with other quantum mechanical programs. The advantages of the present approaches and their earlier versions over macroscopic reaction field models and supermolecular approaches are considered. A LD/MNDO study of solvated organic ions demonstrates that this model can yield reliable solvation energies, provided the quantum mechanical charges are scaled to have similar magnitudes to those obtained by high level ab initio methods. The incorporation of a field-dependent hydrophobic term in the LD free energy makes the present approach capable of evaluating the free energy of transfer of polar molecules from non polar solvents to aqueous solutions. The reliability of the LD approach is examined not only by evaluating a rather standard set of solvation energies of organic ions and polar molecules, but also by considering the stringent test case of sterically hindered hydrophobic ions. In this case, we compare the LD/MNDO solvation energies to the more rigorous FEP/SCAAS/MNDO solvation energies. Both methods are found to give similar results even in this challenging test case. The FEP/SCAAS/AMPAC method is incorporated into the current version of the program ENZYMIX. This option allows one to study chemical reactions in enzymes and in solutions using the MNDO and AM1 approximations. A special procedure that uses the EVB method as a reference potential for SCF MO calculations should help in improving the reliability of such studies.     

Real-time dipole orientational imaging as a probe of ligand-protein interactions

Single-molecule orientational imaging using total internal reflection fluorescence microscopy has been employed to investigate the dynamics of a protein-ligand system. Emission patterns from single tetramethylrhodamine (TMR)-biocytin molecules bound to streptavidin show that the TMR dipole adopts a limited number of favored orientations. The angular trajectories of individual dipoles exhibit remarkably similar patterns that are characteristic of single TMR molecules interacting with a relatively homogeneous population of nanoenvironments. Analysis of the polar and azimuthal angle distributions reveals a tendency for the dipole to assume three primary and two secondary orientations. Autocorrelation analysis of the dipole trajectories shows a predominantly bimodal behavior in the reorientation rates with the slow and fast components corresponding to the primary and secondary orientations, respectively. A number of mechanisms by which the observed orientations might be stabilized have been considered, in particular specific interactions between the zwitterionic TMR probe and charged residues on the streptavidin surface. Variations in the reorientation rates have been discussed in terms of local thermal fluctuations in the protein.     

2,6-二巯基嘌呤互变异构体热力学稳定性的密度泛函理论研究

利用密度泛函(DFT)B3LYP/6-311G(d,p)方法,水相计算采用自洽反应场(SCRF)中的Onsager模型,对气相和水相中可能存在的13种2,6-二巯基嘌呤互变异构体进行了全优化,并计算了各异构体的热力学参数、偶极矩及原子净电荷。计算结果表明,不论是气相还是水相,二硫酮DTP(1,3,7)是最稳定的异构体。溶剂化效应使各异构体的稳定性均增强,偶极矩大者其稳定性显著增大。溶剂化吉布斯自由能与异构体在两相中偶极矩之差存在相关性。二硫酮DTP(1,3,7)在水相中与致癌物BPDE进行亲核取代反应时,二硫酮DTP(1,3,7)中的S10原子优先进攻亲电试剂BPDE.     

Non-ideality in Born-free energy of solvation in alcohol-water and dimethylsulfoxide-acetonitrile mixtures: Solvent size ratio and ion size dependence

Recent extension of mean spherical approximation (MSA) for electrolyte solution has been employed to investigate the non-ideality in Born-free energy of solvation of a rigid, mono-positive ion in binary dipolar mixtures of associating (ethanol-water) and non-associating (dimethylsulfoxide-acetonitrile) solvents. In addition to the dipole moments, the solvent size ratio and ion size have been treated in a consistent manner in this extended MSA theory for the first time. The solvent-solvent size ratio is found to play an important role in determining the non-ideality in these binary mixtures. Smaller ions such as Li⁺ and Na⁺ show stronger non-ideality in such mixtures compared to bigger ions (for example, Cs⁺ and Bu₄N⁺). The partial solvent polarization densities around smaller ions in tertiary butanol (TBA)-water mixture is found to be very different from that in other alcohol-water mixtures as well as to that for larger ions in aqueous solutions of TBA. Non-ideality is weaker in mixtures consisting of solvent species possessing nearly equal diameters and dipole moments and is reflected in the mole fraction dependent partial solvent polarization densities.