Surface Heterogeneity Effects in Liquid-Solid Chromatography with Mixed Eluents

Abstract

Equations describing liquid-solid chronatography (LSC) systems with ideal surface and bulk phases and heterogeneous adsorbents are derived and discussed. It is shown that for a nixed surface phase formed on a heterogeneous solid surface the dependence of In k_s on In x^l ₁ is linear for higher concentrations of the first solvent and its slope is a function of the ratio of molecular sizes of solute and solvents, heterogeneity parameters and equilibrium constants describing phase exchange between solute and solvent molecules. However, for complete coverage of the surface by molecules of the first solvent this linear dependence becomes the Snyder-Soczewiński relationship, in which the slope depends solely on the ratio of molecular sizes of the solute and solvents. These results and an analysis presented herein also demonstrate that a recently proposed interpretation of the Snyder-Soczewiński equation in incorrect.     

Physicochemical characterization of dilute n-alcohol/biodiesel mixtures by inverse gas chromatography

Inverse gas chromatography (IGC) has been used to determine the physicochemical parameters that characterize solution thermodynamic interactions in biodiesel-n-alcohol solute systems. Such data is of value to chemical engineers and separation scientists in optimizing separation processes to separate alcoholic solutes at low concentrations in soybean oil methyl ester mixtures (biodiesel). The derived activity and Henry's Law coefficient data can be used to rationalize the interaction of four members of an n-alcoholic homologous series and the soya-based methyl ester solvent in terms of such esters as "green" renewable solvents. Sorption isotherm data confirm linear behavior in most cases between the solute (alcohol) vapor state concentrations and their uptake into the biodiesel phase. Overall, the experimentally determined activity coefficients agree well with those predicted by solution thermodynamic theories as well as correlative chemical engineering equations.     

Deep eutectic solvent as an operative media on structure-reactivity relationships

Deep eutectic solvents seem to be environmentally friendly solvents, particularly because they are prepared easily and have very low-vapor pressures under ambient conditions. They are suitable candidates as green solvents for reaction media with special properties. To present this behavior, substitution reactions of some para- and meta-substituted anilines with 1-fluoro-2,4-dinitrobenzene have been spectrophotometrically investigated in varying mole fractions of ethaline as a deep eutectic solvent in dimethyl sulfoxide (DMSO). The measured rate coefficients of the reaction demonstrated a noticeable variation with the increasing mole fraction of ethaline in ethaline-DMSO mixtures. The linear free energy relationship (LFER) of second-order rate coefficients based on Hammett's substituent constants demonstrates a reasonably linear straight line with a negative slope in different mole fractions of ethaline-DMSO mixtures. Another LFER investigation based on the polarity parameters of the media showed a good agreement with hydrogen bond donor and acceptor abilities of the solvent. Non-LFER assay according to the preferential solvation model confirmed differences between the microsphere solvation of the solute molecules and the bulk composition of the solvents.     

Statistical distributions in solutions

Statistical correlations are considered for molecules in a liquid perturbed by solute molecules. A new integrodifferential equation is derived for the conditional distribution functions. This describes the changes in the distribution functions in response to small displacements of the solute molecules, which produce the external perturbing field. An explicit solution is derived in the form of a functional series in exp (–u_s)–1, in which u_s is the potential energy in the interaction of the solute molecules with the particles of the medium. The coefficients in the series are dependent on the unperturbed distribution functions for the pure solvent. The solution is used to examine the decay of the perturbations produced by the solute molecules. For the case of strong solvation, the problem reduces to finding the distribution for the particles of a nearly ideal gas in the external field of a solute molecule. The distribution for a hard-spheres model is calculated in the first approximation with respect to the density of the medium. It is found that solute molecules larger than the particles of the medium stabilize the structure of the solvent, whereas ones smaller than the latter tend to break up the structure.I am indebted to F. M. Kuni for advice and assistance in this work.     

New concept of solute distribution around a diffusive crystal-solution interface of a binary Lennard-Jones mixture from the viewpoint of molecular dynamics

Directional crystallization from a binary mixture was performed by pseudo-NpT ensemble molecular dynamics. The initial crystal phase having a face-centered-cubic (fcc) structure grew toward the whole cell according to the temperature gradient in the universal cell. The growing crystal phase was not planar even though the solute molecules grew in two-dimensional coordinates because the solvent molecules disturbed the crystallization of the solute molecules at the diffusive crystal-solution interface. This represented the essential phenomenon of solute distribution during crystallization. Consequently, the growing crystal phase still contained solvent molecules having a liquid structure. The time change of the solute composition in the early phase of crystal growth showed an increase in solute composition as the time step proceeded. The resulting solute composition in this early phase was considered at different temperature gradients in the universal cell and it increased as the temperature of the initial crystal-solution interface increased. A new distribution coefficient model was proposed as a function of the difference between the local solute composition and bulk solute composition in the solution around the crystal-solution interface. The impurity-solvent distribution coefficient could be represented by the new model for faster growth of the lower temperature's initial interface. As regards a better distribution coefficient, there was found to be a very dilute solution phase over the crystal phase. The new variable "distribution rate" instead of the ambiguous variable "growth rate" was considered as a function of temperature gradient in the universal cell.     

Solvatochromic behaviours and structure–spectra relationships of 4-carboxyl-2,6-dinitrophenylazohydroxynaphthalenes

Comprehensive electronic absorption spectra of a new dye series, 4-carboxyl-2,6-dinitrophenylazohydroxynaphthalenes have been investigated in solvents of varying polarities. The solvent dependent UV–vis spectral shifts were analysed using some solvent physical parameters such as refractive index, dielectric function, hydrogen bonding acceptor ability, orientation of polarization and others. The observed spectral shifts were correlated with different solute–solvent interaction mechanisms using simple and multiple linear regression analyses. The results of the curve fitting coefficients enabled us to classify the various interactions of solvents with the dyes and relate the solvatochromic behaviours to the substituent effects on the dye molecules. Charge-transfer complexation occurring between one of the congeners and N,N′-dimethylformamide was extensively studied and discovered to be both concentration- and temperature-dependent.The electronic character and the chemical nature of the solvents as well as the chemical nature of the other substituents, apart from the common hydroxyl group, are important factors for the observed solvatochromic properties of the 4-carboxyl-2, 6-dinitrophenylazohydroxynaphthalenes.     

毛细管气液色谱保留过程的研究

以ＰＥＧ２０Ｍ为代表研究了石英毛细管柱气液色谱保留过程,提出了利用毛细管柱测定分配和吸附常数的公式,并测定了９个温度下的分配和吸附常数。计算了８０℃和１２０℃下４支不同液膜厚度柱上吸附对保留的贡献。结果表明,在薄液膜的柱子上界面吸附对保留具有重要贡献;温度升高可以降低弱极性化合物（如正构烷烃和饱和醚）吸附对保留的贡献,但对其它化合物影响不明显。验证了正构烷烃、２－酮系列和正构伯醇的吸附常数的碳数规律。     

Peculiar properties of water as solute

Water has been investigated for a long time as the most important solvent; the peculiar behavior of water as solute has been studied in binary mixtures with organic solvents, mainly exploring the whole phase diagram. In this Article, we studied the behavior of water in binary mixtures with propylene carbonate in the phase diagram region where water acts as a solute as a function of the water molar fraction X(water). Surface tension measurements, differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR) have been used to investigate the state of water molecules and hydrogen bonds when water is to be considered a solute instead of a solvent, and peculiar and interesting properties were discovered. The interaction of water molecules among themselves and between water and propylene carbonate has been shown to be dependent on the water concentration in the mixtures. All of the measured properties showed a break at X(water) approximately 0.15-0.20 similar to the break due to the critical micellar concentration in surfactant solutions. In particular, from the FTIR spectra, it was possible to deduce that at this concentration water has a transition from pure solute ("multimers" solvated by PC) to cosolvent ("intermediate" and "network" water).     

醇类溶剂溶剂化显色极性的理论分析

对一系列醇类溶剂分子进行了理论计算,运用多元线性回归分析方法从分子间相互作用的角度对四种溶剂化显色极性参数(E~T^N,π^*,Py和SPP)进行了理论分析。结果表明,对醇类溶剂而言,参数E~T^N和SPP实质上主要反映的是溶剂的氢键酸性性质;参数π^*中虽然包含了溶剂的极性因素,但同时与溶质-溶剂分子间的电荷转移相互作用有着密切的关系;而参数Py则较好地反映了溶剂的极性性质。     

Detection of Significant Aprotic Solvent Effects on the Conformational Distribution of Methyl 4‐Nitrophenyl Sulfoxide: From Gas‐Phase Rotational to Liquid‐Crystal NMR Spectroscopy

The conformational equilibrium of methyl 4‐nitrophenyl sulfoxide (MNPSO) was experimentally investigated in the gas phase by using microwave spectroscopy and in isotropic and nematic liquid‐crystal solutions, in which the solvents are nonaqueous and aprotic, by using NMR spectroscopy; moreover, it was theoretically studied in vacuo and in solution at different levels of theory. The overall set of results indicates a significant dependence of the solute conformational distribution on the solvent dielectric permittivity constant: when dissolved in low‐polarity media, the most stable conformation of MNPSO proved to be strongly twisted with respect to that in more polar solvents, in which the conformational distribution maximum essentially coincides with that obtained in the gas phase. We discuss a possible explanation of this behavior, which rests on electrostatic solute–solvent interactions and is supported by calculations of the solute electric dipole moment as a function of the torsional angle. This function shows that the least polar conformation of MNPSO is located at a twist angle close to that of the conformational distribution maximum found in less‐polar solvents. This fact, associated with a relatively flat torsional potential, can justify the stabilization of the twisted conformation by the less‐polar solvents.     

A New Equation for Solute Retention in Liquid Chromatography

In consideration of the adsorption of solvent, diluent and solute molecules on the surface of a stationaryphase, a new equation for solute retention in liquid chromatography is presented. This equation includesthree parameters: the displacement equilibrium constant (Ksd) between the solvent and diluent molecules onthe surface of the stationary phase, the total number(N) of the solvent and diluent molecules released fromthe stationary phase after one solute molecule being adsorbed, and the parameter (I) related to the thermody-namic equilibrium constant for the solute adsorption on the stationary phase. Over the whole concentrationrange of the solvent in the mobile phase, the experimental retention data can be well described by this equa-tion, parameters K~, N and I can be obtained by the regression analysis of the experimental retention data,and consequently the number of the solvent and the diluent molecules displaced by one solute molecule fromthe stationary phase can also be derived at different solvent concentrations in the mobile phase,     

蔗糖、葡萄糖对水溶液中丙酮、乙醇和乙腈活度的影响（298.1K）

用液上气相色谱法测定了298.1K下蔗糖和葡萄糖水溶液中丙酮、乙醇和乙腈的活度系数。对实验规律和3个非水组分活度数（lnγ）随糖浓度（m）的变化，从糖分子平伏羟基（e-OH)水化和溶剂混溶序及溶质与糖分子间相互作用的角度作了初步解释。     

Orientational effects of hydrogen bonding in liquid-crystalline solutions containing Schiff bases

Intermolecular hydrogen bonding in binary mixtures containing nematogenic Schiff bases as solvents and proton-donating non-mesomorphic solutes has been considered. Reasons for the anomalous concentration dependences of solute order parameters are discussed. A solution structure model of acetic acid in nematic solvents is proposed; constants of complex-dimer equilibrium and coefficients of the orientational correlation of the non-mesogenic solute are calculated on the basis of this model. Hydrogen bonded complex structure using ¹³C NMR has been studied and stability constants in isotropic solutions in chloroform have been calculated. The influence of the solvent orientational ordering on the complex stability is discussed. Data on the solvation isotopic effects in the solutions investigated, which confirm the adequacy of the model are given.     

内压与Henry常数

建立了一个气体溶解度的新模型,它实际上是Pierotti理论的修正,按照这个模型,稀溶液中的溶质被视为虚拟的完全气体,Henry常数则是1mol完全气体的压力与一个Boltzmann因子的乘积,这个因子取决于溶质分子周围溶剂的内压。对若干气体在有机溶剂和聚合物中溶解度数据检验结果表明,这个模型能满意地用来描述Henry常数随温度的变化规律。计算得到的稀溶液形成的偏摩尔热力学函数也与实验值吻合。     

Thermodynamic functions of solvation of 1,4-dioxane in various solvents at 298.15 K

The standard changes in enthalpy during the solvation of 1,4-dioxane in methanol, ethyl acetate, DMF, and acetonitrile were determined from calorimetric data and compared with the literature data for a series of solvents with different polarities. The standard changes in the Gibbs energy during the solvation of 1,4-dioxane in a wide series of solvents were calculated from the activity coefficients reported in the literature. The variation of the solvation functions of low-polar 1,4-dioxane in the series of solvents was found to be consistent with the enthalpy-entropy compensation rule. The results for 1,4-dioxane were compared with those for its open-chain analog and related large cyclic molecules. The electrostatic interactions of the solute with the solvents did not markedly affect the thermodynamic characteristics of ether in media with different polarities, but affected the interaction of the solute with the solvent more significantly. The solvation of the small ring of 1,4-dioxane in aprotic solvents was accompanied by a more significant exothermal effect than in the case of its open-chain analog. The conclusion was drawn that the enthalpies of the formation of hydrogen bonds between 1,4-dioxane and the associated water and chloroform molecules in solution were smaller in magnitude than the bonds of the similar open-chain polyether.     

The statistical thermodynamics of solvation

Statistical mechanics provides the basis for a rigorous theory of molecules and molecular complexes in chemically inert media. It is found that the free energy of solvation may be calculated, provided the distribution function for the solvent molecules around the solute molecules is known. A relation is deduced between the volume change on dissolving, the free energy of solvation, and the isothermal compressibility of the pure solvent. The general equations are applied to a solution of solid spheres in a structureless medium. The theoretical deductions are found to be in qualitative agreement with the thermodynamic solvation parameters for oxygen and argon in inert solvents.     

低密度溶液中溶剂的重组织性质

分析了溶液的微观结构,结果表明,单个溶质粒子影响其周围的溶剂的结构,溶质粒子间的相互作用也将影响溶剂的结构,溶质对溶剂结构的影响称作溶剂的重组织.提出了二阶重组织能及二阶重组织熵等概念,可以描述在两个溶质粒子发生碰撞时对其周围溶剂结构的影响.利用二元系的集团展开理论,给出了溶剂的一阶、二阶重组织能和重组织熵的表达式.统计热力学分析给出了溶剂-溶剂径向分布函数与溶质和溶剂化学势之间的关系,给出了无限稀溶液模型是否成立的宏观判据.提出的理论可用于低密度的二元溶液.     

Modeling solvent effects on electron-spin-resonance hyperfine couplings by frozen-density embedding

In this study, we investigate the performance of the frozen-density embedding scheme within density-functional theory [J. Phys. Chem. 97, 8050 (1993)] to model the solvent effects on the electron-spin-resonance hyperfine coupling constants (hfcc's) of the H2NO molecule. The hfcc's for this molecule depend critically on the out-of-plane bending angle of the NO bond from the molecular plane. Therefore, solvent effects can have an influence on both the electronic structure for a given configuration of solute and solvent molecules and on the probability for different solute (plus solvent) structures compared to the gas phase. For an accurate modeling of dynamic effects in solution, we employ the Car-Parrinello molecular-dynamics (CPMD) approach. A first-principles-based Monte Carlo scheme is used for the gas-phase simulation, in order to avoid problems in the thermal equilibration for this small molecule. Calculations of small H2NO-water clusters show that microsolvation effects of water molecules due to hydrogen bonding can be reproduced by frozen-density embedding calculations. Even simple sum-of-molecular-densities approaches for the frozen density lead to good results. This allows us to include also bulk solvent effects by performing frozen-density calculations with many explicit water molecules for snapshots from the CPMD simulation. The electronic effect of the solvent at a given structure is reproduced by the frozen-density embedding. Dynamic structural effects in solution are found to be similar to the gas phase. But the small differences in the average structures still induce significant changes in the computed shifts due to the strong dependence of the hyperfine coupling constants on the out-of-plane bending angle.     

Solvent effects on 17O nuclear magnetic shielding: N-methylformamide in polar and apolar solutions

The solvent effect on ¹⁷O isotropic shielding was computed by different methods: the polarizable continuum model and a mixed approach, including a few real solvent molecules treated as the solute. The experimental data show that the behavior of protic and aprotic solvents is markedly different: we found that the continuum approach describes well the observed shielding at various dielectric constants for aprotic solvents, while the mixed procedure is needed when hydrogen bonds to the magnetic centre are present.Contribution to the Jacopo Tomasi Honorary Issue