Enthalpy Components Research Articles

Thermodynamic consequences of stapling side-chains on a peptide ligand using a lactam-bridge: A theoretical study on anti-angiogenic peptides targeting VEGF.

Peptides are promising therapeutic agents for various biological targets due to their high efficacy and low toxicity, and the design of peptide ligands with high binding affinity to the target of interest is of utmost importance in peptide-based drug design. Introducing a conformational constraint to a flexible peptide ligand using a side-chain lactam-bridge is a convenient and efficient method to improve its binding affinity to the target. However, in general, such a small structural modification to a flexible ligand made with the intent of lowering the configurational entropic penalty for binding may have unintended consequences in different components of the binding enthalpy and entropy, including the configurational entropy component, which are still not clearly understood. Toward probing this, we examine different components of the binding enthalpy and entropy as well as the underlying structure and dynamics, for a side-chain lactam-bridged peptide inhibitor and its flexible analog forming complexes with vascular endothelial growth factor (VEGF), using all-atom molecular dynamics simulations. It is found that introducing a side-chain lactam-bridge constraint into the flexible peptide analog led to a gain in configurational entropy change but losses in solvation entropy, solute internal energy, and solvation energy changes upon binding, pinpointing the opportunities and challenges in drug design. The present study features an interplay between configurational and solvation entropy changes, as well as the one between binding enthalpy and entropy, in ligand-target binding upon imposing a conformational constraint into a flexible ligand.

The Influence of Imidazolium Ionic Liquids on the Chromatographic Behavior of Some Aromatic Acids under Reverse-Phase High-Performance Liquid Chromatography Conditions

Using reverse-phase high-performance liquid chromatography, some features of the chromatographic retention of polar aromatic acids (gallic, ferulic, benzoic, and salicylic) on hexadecylsilica gel from aqueous-acetonitrile eluents containing imidazolium ionic liquids (ILs) were studied. The following ILs were used: 1-butyl-3-methylimidazolium bromide [C4MIM][Br], 1-hexyl-3-methylimidazolium bromide [C6MIM][Br], 1-decyl-3-methylimidazolium bromide [C10MIM][Br], and 1-methyl-3-octylimidazolium bromide [MC8IM][Br]. The dependences of retention on the structure of imidazolium ILs and aromatic acids were studied. Based on the temperature dependences of the retention factor, the thermodynamic characteristics of the sorption process (standard enthalpy and entropy component) were calculated. The obtained thermodynamic characteristics of sorption were analyzed, and the influence of the structure of the IL and sorbate on their values was shown. The compensation relationships between the enthalpy and the entropy component of the sorption of the studied aromatic acids were analyzed.

Influence of temperature on non-exchange sorption of aromatic amino acids by anion exchanger AV-17-8

The equilibrium in thearomatic amino acid (tryptophan, phenylalanine, tyrosine and histidine) – anion exchanger AV-17-8 (Cl-) systems at different temperatures was investigated In order to identify the influence of both increased and decreased temperatures on the sorption equilibrium the temperature range was 277-343 K was studied. Sorption isotherms were obtained at 277, 293, and 343 K. The thermodynamic description of sorption systems was carried out using an approach based on the idea that the sorption process is stoichiometric. The integral and differential energy characteristics and the compensation effect between the enthalpy and entropy components for the studied sorption systems were calculated. A change in temperature led to a change in the physicochemical properties of the studied sorbates and sorbent. In the temperature range 277-243 K, the solvent content in the sorbent phase remained unchanged. A low solvent content at a temperature of 333 K and an increase in its amount at 343 K were experimentally established. It should be considered that at different temperatures the ionization constants of functional groups and the regions of existence of ionic forms of amino acids change, but the sorption mechanism does not change, since the recharging of amino acids did not occur. It was established that the process is exothermic for tryptophan and phenylalanine. With increasing temperature, the amount of sorbed substance decreased. The absorption energy was low due to the significant compensatory effect of enthalpy and entropy factors, the differential Gibbs energy was practically independent of temperature due to the stabilizing effect of hydrophobic forces in the sorbent phase. For the poorly soluble amino acid tyrosine, when the temperature increased from 277 to 293 K, a slight increase in sorption was observed, and with a further increase, a decrease to trace amounts was revealed. In the temperature range of 277-293 K, the sorption process was characterized by the formation of a supersaturated solution in the sorbent phase and was endothermic. The sorption of hydrophilic histidine increased with increasing temperature, which correlates with the solvent content in the sorbent phase, including low content at 333 K. The process of histidine sorption is endothermic, characterized by a strong dependence of the differential Gibbs energy on temperature and the absence of a compensation effect between the enthalpic and entropic components.

C60(OH)32 fullerenols: calculated temperature-sensitive isomeric interplay

Model quantum chemical calculations are carried out for the relative populations of two low-energy C60(OH)32 isomers at moderate temperatures, consistently using both enthalpy and entropy components of the Gibbs energy, in order to understand the isomeric interplay at relevant synthetic conditions. The Gibbs energy is based on the M06-2X/6-311++G** energetics and M06-2X/3-21G entropy. Owing to the entropy effects, the relative populations of the C60(OH)32 isomers change quite fast so that the equimolarity between both species is reached already at a temperature of 266 K. The calculations indicate a strong sensitivity of the C60(OH)32 isomeric relative populations to temperature changes (a feature useful for yield optimization with a selected isomer). Calculated data on structure, charge distribution, electronic and IR vibrational spectra are presented, too. Possible roles of the inter-isomeric thermodynamic equilibrium and solubility are discussed. The results, the first of its type for polyhydroxylated fullerenes, are encouraging for further such studies with other isomeric fullerenols.

Protein-Protein Binding Free Energy Predictions with the MM/PBSA Approach Complemented with the Gaussian-Based Method for Entropy Estimation.

Here, we present a Gaussian-based method for estimation of protein–protein binding entropy to augment the molecular mechanics Poisson–Boltzmann surface area (MM/PBSA) method for computational prediction of binding free energy (ΔG). The method is termed f5-MM/PBSA/E, where “E” stands for entropy and f5 for five adjustable parameters. The enthalpy components of ΔG (molecular mechanics, polar and non-polar solvation energies) are computed from a single implicit solvent generalized Born (GB) energy minimized structure of a protein–protein complex, while the binding entropy is computed using independently GB energy minimized unbound and bound structures. It should be emphasized that the f5-MM/PBSA/E method does not use snapshots, just energy minimized structures, and is thus very fast and computationally efficient. The method is trained and benchmarked in 5-fold validation test over a data set consisting of 46 protein–protein binding cases with experimentally determined dissociation constant Kd values. This data set has been used for benchmarking in recently published protein–protein binding studies that apply conventional MM/PBSA and MM/PBSA with an enhanced sampling method. The f5-MM/PBSA/E tested on the same data set achieves similar or better performance than these computationally demanding approaches, making it an excellent choice for high throughput protein–protein binding affinity prediction studies.

Calculation of Energy for RNA/RNA and DNA/RNA Duplex Formation by Molecular Dynamics Simulation

The development of approaches for predictive calculation of hybridization properties of various nucleic acid (NA) derivatives is the basis for the rational design of the NA-based constructs. Modern advances in computer modeling methods provide the feasibility of these calculations. We have analyzed the possibility of calculating the energy of DNA/RNA and RNA/RNA duplex formation using representative sets of complexes (65 and 75 complexes, respectively). We used the classical molecular dynamics (MD) method, the MMPBSA or MMGBSA approaches to calculate the enthalpy (ΔH°) component, and the quasi-harmonic approximation (Q-Harm) or the normal mode analysis (NMA) methods to calculate the entropy (ΔS°) contribution to the Gibbs energy (Delta G_{{37}}^{^circ } ) of the NA complex formation. We have found that the MMGBSA method in the analysis of the MD trajectory of only the NA duplex and the empirical linear approximation allow calculation of the enthalpy of formation of the DNA, RNA, and hybrid duplexes of various lengths and GC content with an accuracy of 8.6%. Within each type of complex, the combination of rather efficient MMGBSA and Q-Harm approaches being applied to the trajectory of only the bimolecular complex makes it possible to calculate the Delta G_{{37}}^{^circ } of the duplex formation with an error value of 10%. The high accuracy of predictive calculation for different types of natural complexes (DNA/RNA, DNA/RNA, and RNA/RNA) indicates the possibility of extending the considered approach to analogs and derivatives of nucleic acids, which gives a fundamental opportunity in the future to perform rational design of new types of NA-targeted sequence-specific compounds.

Cell’s Thermochemistry and I²R Formula

All electrochemical power sources of practical use emit heat during discharge. This heat primarily warms the battery itself. The amount of heat, as well as the heat flow, depend on the discharging rate. In case of the lithium-ion battery, high-rate (fast) process can overheat the battery up to unsafe conditions, causing sometimes fire and explosion. Thus, understanding and quantifying the character of heat releasing processes is important for batteries’ usage, maintenance, modeling, and developing.All of the following reasoning relates to the situation when the battery operates under projected use conditions.Direct calorimetric measurements that provide immediate answer need special equipment, such as, for example, the ARC instrument. However, both entities, that is, total heat and heat flow, can be derived relatively easily employing the general ideas of thermochemistry and thermodynamics. The approach is based on comparing the enthalpy of the redox reaction of the cell and the useful energy produced by the cell during discharge at a certain rate. The latter is measured in a typical battery test; the enthalpy ΔH being the sum of Gibbs’ (free) energy ΔG and the entropy component T ΔS (T is absolute temperature, and ΔS is entropy change, ΔH = ΔG + T ΔS) can be easily determined through additional though simple experiment. Electrochemical objects give a unique opportunity of obtaining both enthalpy components directly by measuring the open circuit voltage (OCV) of the cell at various temperatures and state of charge. Indeed, by definition, the cells’ OCV is in fact the free energy expressed in electric units U = -ΔG/zF, where U is the OCV, F is the Faraday constant, and z is the number of electrons involved in the redox reaction. Further, since dG = -SdT + Vdp (V and p are volume and pressure, respectively), the temperature coefficient of the OCV (∂U/∂ T)p = -ΔS/zF. The talk presents the results of examining of three lithium-ion batteries of extensively used chemistries: lithium cobalt oxide-graphite, lithium iron phosphate-graphite, and lithium nickel cobalt aluminum oxide-graphite. For performing the comparison and analyzing the results, uncomplicated but special computational procedures were developed. Particular attention is given to the effect of the entropy of the reactions on batteries’ thermal behavior. As it was mentioned above, the enthalpy is comprised of Gibbs energy and the entropy member. Since there is a solid-state reaction in the batteries, a good assumption is that entropy should not change much, and the entropy term might be neglected. Of course, such an assumption needs verification. Our investigation showed in which cases this hypothesis is correct, and if it is incorrect – to what degree.Total heat and heat flow depend upon the current of discharge I. In battery engineering practice, it is commonly assumed that this dependence can be expressed in terms of the Joule-Lenz law where the voltage is substituted with resistance and current according to Ohm’s equation. However, the electrochemical electrodes are essentially non-Ohmic elements meaning that there is a non-linear connection between voltage and current. In the present work, empirical relationships have been suggested: Total Heat is proportional to I1/2 , and Heat Flow – to I3/2 . The proportionality factor evidently cannot be interpreted as resistance – first of all, because its unit of measure is different. The explanation of the phenomenon is based on the non-Ohmic nature of the battery behavior.In some experimental series, we measured the temperature of the cells during cycling. The experiments revealed impressive matching of the temperature and entropy profiles.

Calculation of Energy for RNA/RNA and DNA/RNA Duplex Formation by Molecular Dynamics Simulation

The development of approaches for predictive calculation of hybridization properties of various nucleic acid (NA) derivatives is the basis for the rational design of the NA-based constructs. Modern advances in computer modeling methods provide the feasibility of these calculations. We have analyzed the possibility of calculating the energy of DNA/RNA and RNA/RNA duplex formation using representative sets of complexes (65 and 75 complexes, respectively). We used the classical molecular dynamics (MD) method, the MMPBSA or MMGBSA approaches to calculate the enthalpy (ΔH°) component, and the quasi-harmonic approximation (Q-Harm) or the normal mode analysis (NMA) methods to calculate the entropy (ΔS°) contribution to the Gibbs energy (ΔG°37) of the NA complex formation. We have found that the MMGBSA method in the analysis of the MD trajectory of only the NA duplex and the empirical linear approximation allow calculation of the enthalpy of formation of the DNA, RNA, and hybrid duplexes of various lengths and GC content with an accuracy of 8.6%. Within each type of complex, the combination of rather efficient MMGBSA and Q-Harm approaches being applied to the trajectory of only the bimolecular complex makes it possible to calculate the ΔG°37 of the duplex formation with an error value of 10%. The high accuracy of predictive calculation for different types of natural complexes (DNA/RNA, DNA/RNA, and RNA/RNA) indicates the possibility of extending the considered approach to analogs and derivatives of nucleic acids, which gives a fundamental opportunity in the future to perform rational design of new types of NA-targeted sequence-specific compounds.

Thermodynamics of the Solvation of β-Cyclodextrin in Water–Dimethylsulfoxide Solvents

The Gibbs energy of transfer is determined via the interphase distribution of a substance between two immiscible phases. β-cyclodextrin (βCD) from water is used in a mixture with dimethylsulfoxide (χDMSO = 0.0–0.6 mole fraction) at 298 K. The effect of the solvent’s composition has on the thermodynamic characteristics of βCD solvation is analyzed. It is found that increasing the content of DMSO in a mixed solvent leaded to negligible desolvation of cyclodextrin, due probably to the compensation effect between the enthalpy and entropy components of the Gibbs energy of βCD transfer.

An Approach to Computing Solvent Reorganization Energy.

A method for accurate calculation of reorganization free energy of an explicit solvent solvating a solute molecule is presented. The method relies on the knowledge of solvation free energy known either from experimental measurement or from accurate calculation. An important part of this approach lies in the calculation of entropy in solute-solvent interaction free energy using the interaction entropy method, combined with the calculation of enthalpy of solvent reorganization based on a finite number of solvent molecules near the solute molecule. This interaction entropy-solvent reorganization or IESR method enables one to compute both enthalpy and entropy components of the solvent reorganization energy from MD simulation of the solvated system. The calculated results are determined by the molecular force field only without any empirical parameter. The current method is applied to computing the reorganization energies of water solvent solvating a wide range of solute molecules including both hydrophilic and hydrophobic ones. The accuracy of the approach is indirectly verified by the excellent agreement of individual enthalpies and entropies of the solvation energies between theory and experiment.

THERMODYNAMICS OF QUERCETIN SOLVATION IN WATER-DIMETHYLSULFOXIDE SOLVENT

The distribution coefficients of quercetin (QCT) in water-dimethylsulfoxide solvents with a content of dimethylsulfoxide from 0.0 to 0.5 mol. fr. were determined by the method of interfacial distribution of the substance between two immiscible phases: aqueous or water-dimethylsulfoxide solution and n-hexane at 298.2 K. The distribution coefficients are less than one, which indicates a better solvation of quercetin in water and a water-dimethylsulfoxide solvent than in hexane. Changes in the distribution coefficients of quercetin are not correlated with a gradual increase in the content of dimethylsulfoxide (DMSO) in the solvent. Using the obtained values of the distribution coefficients, we calculated the changes in the Gibbs energy of re-solvation of quercetin in water-dimethylsulfoxide solvents. The dependence of the Gibbs energy of QCT re-solvation on the solvent composition has an extreme form with a minimum in the range of DMSO concentrations corresponding to 0.3 mol. fr. A comparative analysis of the effect of a water-dimethylsulfoxide solvent on the change in Gibbs energy of re-solvation of quercetin, nicotinamide and nicotinic acid was carried out. In the case of both nicotinamide and nicotinic acid, an extreme change is observed in the Gibbs energy of re-solvation of particles with a maximum in the region with a low content of non-aqueous component XDMSO ≈ 0.1 mol. fr. The main contribution to the weakening of the solvation of nicotinamide and nicotinic acid is due to the enthalpy component, and with increasing concentration of dimethylsulfoxide there is an increase in the contribution of entropy to the change in the Gibbs energy transfer. An extreme change in the Gibbs energy transfer of quercetin suggests that the minimum on the dependence ∆trGº(QCT) = F(χDMSO) is also a consequence of a change in the prevailing thermodynamic factor in the solvate state of quercetin.

Cage occupancy and dissociation enthalpy of hydrocarbon hydrates

AbstractAn elaborated statistical mechanical theory on clathrate hydrates is applied to exploration of their phase equilibria and dissociation enthalpies. The experimental dissociation pressures of methane, ethane, acetylene, and propane hydrates are well recovered by the method we have proposed. We estimate water/hydrate and hydrate/guest two‐phase coexisting conditions in the temperature, pressure, and composition space in addition to three‐phase equilibrium conditions. It is shown that the occupancy of guest molecules and the two‐phase boundaries in the phase diagram vary depending sensitively on its size. Enthalpy components arising from the host and guest interactions are separately calculated from the temperature dependence of the corresponding free energy values. This enables to evaluate the dissociation enthalpy at any stable and metastable thermodynamic state taking account of the phase transition in the coexisting phase such as melting of ice, notably that along the three‐phase equilibrium line.

A Fragmenting Protocol with Explicit Hydration for Calculation of Binding Enthalpies of Target-Ligand Complexes at a Quantum Mechanical Level.

Optimization of the enthalpy component of binding thermodynamics of drug candidates is a successful pathway of rational molecular design. However, the large size and missing hydration structure of target-ligand complexes often hinder such optimizations with quantum mechanical (QM) methods. At the same time, QM calculations are often necessitated for proper handling of electronic effects. To overcome the above problems, and help the QM design of new drugs, a protocol is introduced for atomic level determination of hydration structure and extraction of structures of target-ligand complex interfaces. The protocol is a combination of a previously published program MobyWat, an engine for assigning explicit water positions, and Fragmenter, a new tool for optimal fragmentation of protein targets. The protocol fostered a series of fast calculations of ligand binding enthalpies at the semi-empirical QM level. Ligands of diverse chemistry ranging from small aromatic compounds up to a large peptide helix of a molecular weight of 3000 targeting a leukemia protein were selected for systematic investigations. Comparison of various combinations of implicit and explicit water models demonstrated that the presence of accurately predicted explicit water molecules in the complex interface considerably improved the agreement with experimental results. A single scaling factor was derived for conversion of QM reaction heats into binding enthalpy values. The factor links molecular structure with binding thermodynamics via QM calculations. The new protocol and scaling factor will help automated optimization of binding enthalpy in future molecular design projects.

Feasibility study of MgSO4 + zeolite based composite thermochemical energy stores integrated with vacuum flat plate solar thermal collectors for seasonal thermal energy storage

A primary drawback of solar thermal technologies, especially in a domestic setting, is that collection of thermal energy occurs when solar irradiance is abundant and there is generally little requirement for heating. Thermochemical Energy Storage (TCES) offers a means of storing thermal energy interseasonally with very little heat loss. A combination of Solar Thermal Collectors (STC) and TCES systems will allow a variety of different heating applications, such as domestic space and hot water heating as well as low temperature industrial process heat applications to be met in a low carbon way. This paper describes and assesses the feasibility of two novel technologies currently under development at Loughborough University; i) an evacuated flat plate STC and ii) composite TCES materials, coupled together into a system designed to store and supply thermal energy on demand throughout the year. The predicted performance of an evacuated flat plate STC is described. The objective of this paper is to evaluate the economic, energy and carbon saving potential of conceptual STC + TCES systems suitable for domestic use. This research uses experimental results from Differential Scanning Calorimeter tests to evaluate the total enthalpy, dehydration enthalpy and sensible component enthalpy of composite TCES materials. The experimental results along with predicted performance of STC are used within a developed model to assess key metrics of conceptual STC + TCES systems feasibility, including; charging time, payback time, cost/kWh, energy savings and CO2 savings. Preliminary results suggest the combination of these two technologies has significant potential for domestic applications.

Surface Properties of Magnesium-Aluminium Hydrotalcite-Like Compound Characterized by Inverse Gas Chromatography

In this paper, the Mg-Al hydrotalcite-like compound (Mg-Al-HTLC) was synthesized by hydrothermal method at 373K. Structure and morphology of Mg-Al-HTLC was obtained with X-ray diffraction (XRD), scanning electron microscope (SEM) and Fourier transform infra-red spectroscopy(FTIR). A series of polar and non-polar molecules were used for probes, surface properties of Mg-Al-HTLC was studied by inverse gas chromatography (IGC) at 353K, 363K, 373K, 383K respectively. The retention volume was utilized for evaluating the free energy of adsorption (-ΔGSP), the dispersive component of the surface energy(γsD), as well as the enthalpy and entropic component(ΔHSP, -ΔSSP). XRD results reveal that the Mg-Al-HTLC has high crystallinity and perfect layered structure. The results of IGC show that Mg-Al-HTLC would adsorb straight-chain alkanes spontaneously, and the values of γsDwere similar at all temperature. It reveals Mg-Al-HTLC is a material with particular characteristics of both acid and base. This study illustrates that, as a method to evaluate the surface properties of material , IGC method is dependable and significant.

A New Computational Method for Protein–Ligand Binding Thermodynamics

We develop a new computational method for protein–ligand binding thermodynamics that combines alchemical transformations and the liquid theory. This method requires only the solute configurations during alchemical transformations, and solvent effects are incorporated through the solvation free energy. It is demonstrated that the new method yields the relative binding free energy that agrees well with the one from the conventional explicit solvent‐free energy simulations. The development reported here opens up new possibilities for faster binding free energy computations and easier access to the enthalpy and entropy components.

Enthalpies of Glycine Protonation in Water–Acetone and Water–Ethanol Solvents

Heat effects of the reaction of glycine protonation at 298 K in water–ethanol (µ = 0.1 (NaClO4)) and water–acetone (µ = 0.3 (NaClO4)) solvents of various compositions are determined via calorimetry. The enthalpies of the resolvation of glycinium ions in aqueous solutions of ethanol and acetone are calculated using the literature data. It is established that a minor change in the heat effect of the reaction of glycine protonation in water–ethanol and water–acetone solvents is associated with the mutual compensation of the enthalpies of proton, glycinium-ion, and glycine resolvation in these media. It is shown that the contribution from the enthalpy component to the change in the Gibbs energy of this reaction is insignificant.

EFFECT OF PRESSURE ON EXCESS THERMODYNAMIC CHARACTERISTICS OF WATER + FORMAMIDE MIXTURE

Using the experimental data on the densities at atmospheric pressure and compressibility coefficients, k=(Vo-V)/Vo, of water + FA mixture the changes in the following thermodynamic parameters were calculated under the pressure increase up to 100 MPa within the temperature range from 288.15 to 323.15 K: excess molar Gibbs energy, ΔPo→PGmE, excess molar entropy ,ΔPo→PSmE, and excess molar entropy ΔPo→PHmE. It was established that ΔPo→PGmEvalues were negative over the whole concentration range and minima appeared on ΔPo→PGmE= f(x2) functions at x2≈0.33. The pressure growth up to 100 MPa resulted in ΔPo→PGmE absolute values increase within entire concentration and temperature intervals. The changes in entropy component, -(ΔPo→PTSmE), of ΔPo→PGmEvalues were almost canceled by the enthalpy component changes. Minimal values of ΔPo→PSmE corresponded to x2≈ 0.33, exactly at that composition 2Н2О-FA associate formed. The isobaric temperature lowering caused the structure ordering also at x2≈ 0.33. The pressure growth promoted the increasing in exothermicity of the mixing enthalpies, HmE, of water and formamide. The changes in HmE value under the mixture compression are indicative of the larger exothermal contribution from new H-bonds formation as compared with the endothermic contribution from the decreasing in the total amount of hydrogen bonds. The temperature lowering decreases ΔPo→PHmEvalues as well; maximal isotherms dispersion is observed at concentrations corresponding to maximal content of 2:1 or 1:1 associates of water and FA.

Thermodynamics of Swelling of Polyacrylamide and Poly(methacrylic acid) Lyophilized Xerogels in Water

The structure and thermodynamic functions of water mixing with polyacrylamide and poly(methacrylic acid) xerogels prepared via drying of chemically crosslinked hydrogels equilibrium swollen in water are studied by methods of scanning electron microscopy, low-temperature nitrogen adsorption, differential scanning calorimetry, isothermal interval sorption of water vapor, and microcalorimetry. The liquid-phase drying in air at 70°C and solid-phase lyophilic drying under vacuum are used. Samples dried by different methods have dissimilar structures, which significantly affects the thermodynamics of the interaction of xerogels with water. The Flory–Huggins parameter of interaction of polyacrylamide and poly(methacrylic acid) with water and also its enthalpy and entropy components are independent of the procedure of xerogel preparation. The influence of the latter component manifests itself only in the thermodynamics of the relaxation of the nonequilibrium glassy structure of polymer.

Thermodynamics of the Sorption of Benzimidazoles on Octadecyl Silica Gel from Water–Methanol Eluents

The standard enthalpy and entropy component of transferring benzimidazoles from water-methanol solutions to surfaces of octadecyl silica gel are determined using reversed-phase high-performance liquid chromatography (RP HPLC). The dependences between the enthalpy and polarizability of the molecules of the studied benzimidazoles, the enthalpy and the entropy factor are studied, and the influence of the quantitative composition of the water-methanol solution on the enthalpy are studied.