Standard Free Energies Of Transfer Research Articles

The basis of the hydrophobic effect

The property of a molecule that most reliably determines the magnitude of the hydrophobic effect that it will experience is the number of hydrogen–carbon bonds it contains not the accessible surface area of its nonpolar portions. This conclusion follows from an examination of the standard free energies of transfer of alkanes, alkenes, alkadienes, and arenes from water to hexadecane. When the standard free energies of transfer for hydrocarbons in these different classes are plotted as a function of the number of hydrogen–carbon bonds they contain, all of the data fall upon the same line. These standard free energies of transfer are also directly proportional to the number of hydrogen–carbon bonds the hydrocarbons contain. When the same standard free energies of transfer are plotted as a function of the accessible surface areas of the hydrocarbons, the data do not fall upon the same line nor are the standard free energies of transfer directly proportional to the accessible surface areas. An examination of the standard free energies of transfer for the different classes of hydrocarbons from the gas phase to water and from the gas phase to hexadecane reinforces the conclusion that the number of hydrogen–carbon bonds in a molecule rather than its accessible surface area is the basis of the hydrophobic effect. Consequently, estimates of the contribution of different functional groups to the hydrophobic effect providing the free energy of folding of a molecule of protein or providing the free energy of dissociation for the association of two proteins or the association of a ligand with a protein should be made by counting the number of hydrogen–carbon bonds excluded from water rather than computing the accessible surface areas excluded from water.

Propylene carbonate as a nonaqueous solvent for capillary electrophoresis: mobility and ionization constant of aliphatic amines.

The two properties of aliphatic amines were investigated in propylene carbonate as solvent that are decisive for capillary electrophoretic migration: the actual mobilities and the pKa* values. Solutes were eight primary, secondary, and tertiary amines. Roughly, the actual ionic mobilities of the ammonium ions are inversely proportional to the solvent viscosity, fairly obeying Walden's rule. The pKa* values of the cation acids, HB+ (the corresponding acids of the amines, B), were related to the conventional pH* scale of the buffers. Determined from the effective mobilities as a function of the pH*, they are increased by approximately 7 units compared to water. This increase was interpreted based on the concept of the standard free energy of transfer of the individual species in the acid-base equilibrium. The corresponding medium effect on the proton, log mgammaH+ (the logarithm of the transfer activity coefficient mgammaH+) is approximately +8. The medium effect on the free base, B, was obtained from solubility data; it is about -1 and smaller. Plausible values for the medium effect on the cation HB+ (-1 to -2) lead to a sum of the increments, which corresponds with the overall effect, expressed by the change in pKa*. Examination of the individual contributions shows that the drastically lower basicity of propylene carbonate compared to water is mainly responsible for the increase in pKa upon transfer of the acid-base equilibrium of aliphatic ammonium/amine from the aqueous to the organic solvent.

The Chemical Meaning of the Standard Free Energy of Transfer: Use of van der Waals' Equation of State To Unravel the Interplay between Free Volume, Volume Entropy, and the Role of Standard States

Recently, in an attempt to refine the chemical interpretation of the free energy of transfer of a solute between two phases, Honig et al. introduced a “volume entropy” term into the canonical particle density-based expression for the free energy of transfer. This term is used when the solute and solvent have different molar volumes and is identical in form to the Flory−Huggins configurational entropy term. The need for any such correction has been denied by Ben-Naim and others on the basis of thermodynamic, conceptual, and empirical arguments. The purpose of the present work, based on the van der Waals model of a binary mixture, is to provide a simple, chemical interpretation of the standard free energy of transfer of an infinitely dilute solute from one nonideal phase to a second, absolutely immiscible nonideal phase. This approach also provides a clear explanation of the effect of the choice of standard states, and especially of concentration scales, on the interpretation of transfer free energies. The principle virtue of the present work lies in the conceptually simple but qualitatively complete description inherent in the van der Waals treatment of a fluid mixture, as it incorporates both attractive and repulsive interactions. It is shown that the free energy of transfer of a solute between two immiscible van der Waals fluids will be nonzero even for an “ideal point solute” (here defined as an infinitely hard, nonattractive but potentially repulsive particle of zero size at, or acting as if it is at, infinite dilution). This result arises from differences in the molar volumes and hard core diameters of the two bulk fluids (solvents) and is true for transfer free energies based on both mole fraction and molar concentration scales, meaning that neither of these scales provides standard free energies of transfer that purely reflect attractive solute/solvent interactions free from volume entropy effects.

HEXADECYLTRIMETHYLAMMONIUM BROMIDE + TETRADECYLTRIMETHYL-AMMONIUM BROMIDE MIXED MICELLES IN AQUEOUS GLYCOL OLIGOMERS

The conductances of hexadecyltrimethylammonium bromide (HTAB) + tetradecyltrimethylammonium bromide (TTAB) mixtures over the entire mole fraction range of HTAB (αHTAB) were measured in pure water as well as in the presence of various aqueous ethylene glycol oligomers containing 5, 10 and 20 wt% of each additive in their respective binary mixtures at 30°C. From the conductivity data, the critical micellar concentration (cmc), degree of counter ion association (χ) and the standard free energy of transfer of the surfactant hydrocarbon chain from the medium to the micelle (ΔGO HP ) for HTAB and TTAB were computed. From the conductivity data of mixed surfactants systems, apart from cmc and χ, the regular solution theory parameters were also computed in order to explore the non-ideality in the mixed micelle formation in the presence of additives. The micellar parameters of both kind of surfactants and their mixtures show a significant dependence on the amount as well as on the number of repeating units of glycol oligomers. However, the non-ideality of mixed micelle formation remains unaffected in the presence of additives. These results have been explained on the basis of the medium effects of aqueous additive and it has been concluded that there are no significant interactions of glycol oligomers with the micelles of single and mixed surfactants.

Interfacial Free Energies and Surface Areas of Cycloalkanols and α,ω-Alkanediols at the Air-Water Interface

The surface area occupied at the air/water interface for cycloalkanols and α,ω-alkanediols has been estimated from surface tension measurements. From these data, standard free energies of transfer of the alkanols from the bulk of the solution to the interface were obtained. Cycloalkanols constitute a family whose surface excess diminishes when the size of the alkyl ring increases and whose free energies of transfer at zero surface pressure are similar to those of 1-alkanols of similar hydrophobicities. α,ω-Alkanediols occupy considerably larger surface areas than 1-alkanols, a result that can be interpreted in terms of a different geometrical arrangement of the molecules at the interface. In particular, for alkanediols withn> 7, the data suggest an important contribution of bent conformations to the average arrangement of the molecules in the monolayer. The need to adopt bent conformations to allow the interaction of both hydroxyl groups with the aqueous interface results in a less favorable change in free energy when they are transferred from the solution standard state to the interface at constant surface pressure.

Nonaqueous Liquid/Liquid Interfaces: Surface Activity at the Interface betweenn-Paraffins andN-Alkyl Derivatives of 2-Pyrrolidone

While nonaqueous liquids are widely used commercially, the interfacial chemistry of nonaqueous liquid/liquid systems has received scant attention. In this paper interfacial properties of partially miscible two-component systems ofn-hexadecane with 2-pyrrolidone,N-methyl-2-pyrrolidone orN-ethyl-2-pyrrolidone, and ofn-heptane withN-methyl-2-pyrrolidone at 25°C are presented. The surface activity of the long-chainN-alkyl-2-pyrrolidones at these interfaces was also studied. The adsorption of the long-chain pyrrolidones increases with chain length following a modified form of Traube's rule. Partition coefficients for the long-chain pyrrolidones at low concentrations were measured, and standard free energies of transfer between the coexisting bulk phases and between each bulk phase and the interface were calculated. At higher concentrations, the interfacial tensions of the longer chain pyrrolidones go toward zero as the systems become more miscible. The temperature variation of the hexadecane/N-methyl-2-pyrrolidone and the hexadecane/N-ethyl-2-pyrrolidone interfacial tensions were also measured.

The effect of the ion exchanger site-counterion complex formation on the selectivities of ISEs

Using the model of ideally associated solution, the effect of ion association of the ion exchanger sites with main and foreign counterions on the selectivity of ISEs based on liquid ion exchangers has been considered. Equations which describe the potentiometric selectivity coefficient as a function of ion association constants in the membrane phase and of standard free energies of transfer of the determined and foreign ions from water to the membrane are obtained for the following main cases: (a) the determined and foreign ions are single-charged; (b) the determined ion is double-charged and the foreign ion is single-charged. It is shown that in the case of single-charged main and foreign ions, the ratio of the ion association constants has a great effect on the potentiometric selectivity of membranes, only if the ion exchanger sites produce less strong associates with the determined counterion as compared with the foreign one. Otherwise, this effect is insignificant. The selectivity for double-charged ions should increase, other things being equal, as the first constant of association of these ions with the ion exchanger sites increases. The effect of producing ion triplets of the type I 2 R (±) on the selectivity of ISEs is also considered. Experimental data are presented which illustrate the effect of the nature of the ion exchanger on the potentiometric selectivity. Some procedures employing the factor of ion association for increasing the potentiometric selectivity of liquid ion exchange membranes are considered.

Effect of Large Changes in Temperature and Pressure on the Thermodynamic Properties of Micellization and on the Distribution Constant of a Polar Solute in Micellar Solutions

Density measurements of pentanol (PentOH)−dodecyltrimethylammonium bromide (DTAB)−water mixtures as functions of both alcohol and surfactant (mS) concentrations were carried out at 0.1 MPa from 45 to 75 °C and at 19 MPa from 25 to 130 °C. The standard (infinite dilution) partial molar volumes and expansibilities of DTAB in water and the corresponding properties in the micellar phase were calculated from the experimental data. As far as PentOH in DTAB micellar solutions is concerned, with the exception of the standard partial molar volume (V°R) data at 130 °C and 19 MPa, all the V°R vs mS trends are monotonic curves with mS. The data of V°R as a function of mS were treated by means of an equation previously used for data obtained at ambient conditions. So it was possible to evaluate simultaneously the distribution constant (K) of PentOH between the aqueous and micellar phases and the standard partial molar volume of alcohol in the aqueous (V°R,f) and the micellar (V°R,m) phases. The standard free energy of transfer (ΔG°t) of PentOH from the aqueous to the micellar phases was calculated from K values. By using experimental literature data for the present system at 25 °C, ΔG°t values as functions of temperature and pressure were calculated. The very good agreement between the simulated and experimental values indicates that the model used also holds at higher temperatures. Equations correlating the standard free energy, enthalpy, and entropy of transfer to temperatures up to 130 °C and pressures up to 19 MPa were proposed.

HYDROPHOBIC EFFECTS IN WATER AND WATER/UREA SOLUTIONS A COMPARISON

The partition of several n-alkanols, from methanol to n-nonanol, between n-hexane and water and between n-hexane and water containing 20 % (w/v) urea has been measured at temperatures ranging from 0 °C to 60 °C. The standard free energy of transfer from water to the urea-containing solution decreases with the length of the alkyl chain, being positive for the small alcohols and negative for the higher alkanols. The same tendency is observed upon all the temperature range considered. On the other hand, the standard entropy of transfer from water to the urea-containing solution increases with the length of the alkyl chain of the alkanol. These results are compatible with a simple description of the urea effect in terms of increasing the entropy of dissolution of the hydrophobic alkyl chain in the aqueous solution.

Enthalpies of transfer of pentanol from water to sodium dodecylsulfate-dodecyl-dimethylamine oxide-water mixtures

At a given surfaetant-surfaetant ratio, the enthalpies of transfer AH (W--~ W + S) of pentanol 0.03 m from water to sodium dodecylsulfate (NaDS)-dodeeyldimethylamine oxide-water mixtures as functions of the surfaetants mixture concentration (mr) were determined. For XsaDS = 0.9, AH (W ~ W + S) increases monotonically with mt such as observed for pure surfactants. For XN,DS = 0.12 and 0.3, AH (W---~ W + S) increases with mt up to 0.12 m beyond which it decreases with rat. At XN,DS = 0.6, two monotonic curves can be distinguished in the AH (W---~ W + S) vs. rat trend. Experimental data were fitted through an equation previously reported for additives in pure surfaetants derived by assuming the pseudo-phase transition model for the mieellization and a mass action model for the distribution of the additive between the aqueous and the micellar phases. This method did permit to simultaneously obtain the distribution constant of the alcohol between the aqueous and the micellar phase (and, then, the standard free energy of transfer) and its enthalpy of transfer from the aqueous to the micellar phases. By combining these properties the standard entropies of transfer were calculated. From these results, the excess properties of pentanol in the mixed mieelles were calculated as a function of the mixture composition. The excess enthalpies and entropies are positive and compensate with each other leading to null values for the excess free energies in the whole range of the mixed micelles composition.

Excess adsorption of lysozyme and water at solid-liquid interfaces α

Adsorption isotherms of lysozyme at solid-water interfaces have been studied as a function of protein concentration, ionic strength of the medium, pH and temperature using silica, alumina, carbon, chromium and Sephadex as solid surfaces. Adsorption of lysozyme is affected strongly by change of pH, temperature and ionic strength. In most cases adsorption isotherms attained a state of adsorption saturation. On chromium, lysozyme is either expanded laterally or negatively adsorbed. In some cases, adsorption isotherms were S shaped, showing the existence of some kind of interactions within the adsorbed protein layer. Adsorption of lysozyme on Sephadex at pH 5.0 and 7.5 is negative due to the excess adsorption of water by this material. The standard free energies (Δ G°) of positive and negative adsorption of lysozyme per square meter, signifying the relative affinity of adsorption in the state of monolayer saturation, have been calculated. The magnitude of the standard free energy of transfer (Δ G B°) of one mole of protein from solution to the surface is observed to be 40.3 kJ mol −1 and the value is independent of pH, ionic strength, nature of the surface and temperature.

Solvent effects on initial and transition states in the reaction ofm-fluorophenyltriethyltin with mercuric chloride in methanol and methanol-water mixtures

We have investigated solvent effects on the aromatic electrophilic substitution reaction of m-fluorophenyltriethyltin, 1, with mercuric chloride in order to obtain information about the polarity of the transition state. Second-order rate constants have been determined for the reaction of 1 with mercuric chloride in methanol and methanol–water mixtures at 25·0°C, allowing determination of δΔG‡. Molar standard free energies of transfer (ΔG) for the same solvent mixtures have also been determined for 1 at 25·0°C. Combination of our δΔG‡ and δΔG values with literature data for ΔG of HgCl2 yield values of ΔG(TS), the standard free energy of transfer of the corresponding transition state (TS) in the reaction from methanol to aqueous methanol. It is shown that the reduction in activation energy accompanying replacement of methanol by water is due to initial state destabilization and not to TS stabilization. In fact, the TS is destabilized as methanol is replaced by water. Further analysis permits dissection of the free energies of transfer of the TS into electrostatic and non-electrostatic components. Comparison of these electrostatic components for the 1/HgCl2 system with some model reactions shows that charge development in TS of 1/HgCl2 is approximately 0·5 units of electronic charge.

A thermodynamic study of the ternary system water + glucose + electrolyte at 298.15 K

The standard free energies of transfer of KCl, NaCl, HC1, MgCl 2 and CaCl 2 from water to aqueous solutions of 5, 10, 15, 20, 25 and 30 Wt.% glucose have been determined at 298.15 K from e.m.f. measurements of cells without liquid junctions. It has been shown that the standard free energy of transfer for each of the electrolytes investigated increases linearly with increasing mole fraction of glucose in the mixed media. Free energies of pair interaction have also been calculated: these were found to be positive for NaCl-glucose, HCl-glucose and CaCl 2-glucose pairs, and nearly zero for KCl-glucose and MgCl 2-glucose pairs in water. The experimental results are discussed in terms of solute-solvent and solute-solute interactions in water + glucose + electrolyte systems.

Thermodynamic parameters of transfer and solution of oxalic acid in dimethylsulphoxide-water media

The solubility measurements for oxalic acid in DMSO-H 2O mixtures have been performed at four temperatures (298.15, 303.15, 308.15 and 313.15 K) to calculate the thermodynamic parameters of solution and transfer of oxalic acid from water, as reference solvent, to DMSO-H 2O mixed solvent. The solubility data have been discussed in terms of the solubility parameters. A multiple linear regression method, known as the extended Hansen solubility approach, has been used to estimate the partial and total solubility parameters for oxalic acid. The activity coefficients of oxalic acid in water and mixed solvents have been calculated theoretically and found to be unity. The thermodynamic parameters of transfer of oxalic acid have been found to increase with increasing mole fraction of DMSO. The electrostatic parts of the standard free energies of transfer for oxalic acid have been calculated theoretically. These values increase with increasing DMSO content.

Thermodynamics of amino acid dissociation in mixed solvents. 3: Glycine in aqueous glucose solutions from 5 to 45�C

The first thermodynamic dissociation constants of glycine in 5, 15 mass % glucose+water mixed solvents at five temperatures from 5 to 45°C have been determined from precise emf measrements of a cell without liquid junction using hydrogen and Ag-AgCl electrodes and a new method of polynomial approximation proposed on the basis of Pitzer's electrolytic solution theory in our previous paper. The results obtained from both methods agree within experimental error. The standard free energy of transfer for HCl from water to aqueous mixed solvent have been calculated and the results are discussed.

Electrochemical Behavior of the Cd(II)/Cd(Hg) System in Aqueous Mixtures of Diols and Triols

The electrochemical behavior of the Cd(II)/Cd(Hg) couple (Cd(II) concentration ) in aqueous mixtures of 1,4‐butanediol (BD) and glycerol (GL) containing has been studied. The reversible half‐wave potentials, the diffusion coefficients, and the Walden products for Cd(II) have been determined by polarography, whereas the diffusion coefficient of Cd in mercury, the transfer coefficient for Cd(II) electroreduction, and the apparent standard rate constants have been obtained by cyclic voltammetry. The standard free energies of transfer of 1 mole of Cd(II) ions from water to BD‐water and to GL‐water mixtures are found to be always negative, indicating a higher stability of Cd(II) in organic‐aqueous mixtures than in pure water. The break‐up of these values into electrostatic and chemical contributions shows that all organic‐aqueous mixtures used here are more basic than pure water, and that Cd(II) solvation increases with the mole fraction of BD or GL. The structural properties of the liquid mixtures are analyzed using the Walden products at different BD or GL levels. The changes in the kinetics of the Cd(II)/Cd(Hg) system with solvent composition are discussed in terms of existing models. Results obtained are compared with those previously reported in aqueous mixtures of ethylene glycol and 1,2‐propanediol.

A thermodynamic and solvatochromic investigation of the effect of water on the phase-transfer properties of octan-1-ol

The octan-1-ol–water partition coefficient has played a central role in quantitative structure–activity relationships and has been applied to drug design, to model biological membranes, and to predict toxicity and transport of pollutants. The room temperature solubility of water in octan-1-ol is determined to be 2.5 mol dm–3, which is equivalent to a mole fraction of 0.29. Despite this very high solubility very little is known about the effect of water on the chemical properties of the octan-1-ol phase. We have used head-space gas chromatography to measure the free energy of transfer of 14 chemically very different solutes, from neat octan-1-ol to octan-1-ol saturated with water. In addition, the infinite dilution activity coefficients of a series of 11 alcohols were measured in both phases. In general, the presence of water in octan-1-ol increases the concentration of polar and hydrogen-bonding solutes and decreases the concentration of non-polar solutes in this phase. For all solutes, the effect of water on the change in the standard free energy of transfer was less than 200 cal mol–1. These small changes were validated by investigating the effect of added water on the solvatochromic properties of the indicators N,N-diethyl-p-nitroaniline, p-nitroanisole, p-nitro-aniline, p-nitrophenol, ET-30, bis[α-(2-pyridyl)benzylidene-3,4-dimethylaniline]bis(cyano)iron(II), Phenol Blue, and Nile Red in octan-1-ol. Regardless of the indicator used the addition of water had almost no effect on the wavelength of maximum adsorption. These results show that the water is almost completely associated with octan-1-ol and scarcely effects the properties of the media.

Free energy and enthalpy changes for the process of transfer from gas and from dilute aqueous solutions of some alkanes and monofunctional saturated organic compounds

Standard free energies, ΔsolvG°(oct), and enthalpies, ΔsolvH°(oct), of solvation in octan-1-ol of some alkanes (heptane, octane), ketones (propan-2-one, butan-2-one, hexan-2-one, heptan-4-one), ethers (dipropyl ether, dibutyl ether, tetrahydropyran), alkanols (butan-1-ol, butan-2-ol, 2-methylpropan-2-ol) and amines (pyrrolidine, N-methylpyrrolidine, and piperidine) with open-chain and cyclic structure have been determined at 298.15 K. The ΔsolvG°(oct) values were obtained from measurements of partial vapour pressures of dilute solutions and the ΔsolvH°(oct) values, by adding the heats of solution, determined by calorimetry, to known values of enthalpy of vaporization.These data are compared with the standard free energies and enthalpies of solvation in water, and the standard thermodynamic functions for the ideal transfer process of the solutes from pure water to pure octan-1-ol, δtrX°(w–oct)(X=G, H) are calculated.For the examined solutes, hydrocarbons and monofunctional saturated organic compounds, the thermodynamic functions of solvation in octanol and in water are closely correlated to the position of the functional group in the molecular skeleton. The values of the enthalpy of transfer from water to octan-1-ol are also related more to the topologic characteristics of the solute molecules than to their size or to the nature of their functional group. In contrast, the size of the molecules, as well as the presence of a functional group, are important with regard to ΔtrG°(w–oct). An increase of 1 cm3 mol–1 in the intrinsic volumes corresponds to an increase in the value of the partition coefficients of ca. 15%. The substitution of a part of the hydrocarbon surface with a polar surface produces a very large increase in the values of the partition coefficients.The values of the enthalpy of transfer from water to octan-1-ol are always positive, in contrast to the standard free energies of transfer. The entropic term operates by always favouring the transfer towards the alcoholic phase.The potential ability of anhydrous octanol to extract organic solutes of various molecular structure from water-saturated octanol and from hexadecane is also evaluated and discussed.

Absolute Standard Free Energies of Transfer of the Hydroxyl Ion from Water to Some Aquo-protic Solvents at 298.15 K

Absolute Standard Free Energies of Transfer of the Hydroxyl Ion from Water to Some Aquo-protic Solvents at 298.15 K

The Influence of Medium on the Stability of Coordination Compounds. III. The System CuSO4-NaClO4-Organic Solvents-H2O

Abstract The concentration stability constants for CuS04 in various mixed solvents, viz ethanol-water, isopropa-nol-water and dioxane-water, having a composition of organic solvent of 0, 5, 10, 15,20 and 25 mass per cent were measured spectrophotometrically at 298 ±0.5 K using NaC104 as supporting electrolyte. The total ionic strength range of the solutions are 0.06-4.0mol dm−3 and pH = 4–5. The thermodynamic stability constants CuS04 were obtained by a polynomial approximation based on Pitzer's theory. Finally, the standard free energies of transfer of the coordination reaction, ΔG°…, from pure water to mixed solvent were obtained and the electrostatic part of ΔG°… was calculated based on Beveridge's theory.