Extrathermodynamic Assumptions Research Articles

Study of electrolytic solution process using the scaled-particle theory. Part 3.—Effects of thermal dilution on standard thermodynamic functions

A unified approach consisting of the scaled-particle theory in conjunction with the Born charging equation, proposed earlier in this series, has been used to calculate theoretically the standard free energy and entropy of hydration of various electrolytes in the temperature range between the normal freezing point and the critical temperature of water. The theory appears to hold well for the prediction of the entropy of hydration of electrolytes at least up to 300 °C, the highest temperature for which experimental data are at present available for comparison. The values of the individual ionic hydration entropies at different temperatures calculated according to the present theory and as estimated from experimental results by earlier workers using some extrathermodynamic assumptions have, however, been found to be widely different, particularly at elevated temperatures.

Segment coupling in peptide synthesis—II : A simple predictive equation correlating racemization and primary structure

A predictive equation based on extrathermodynamic assumptions is proposed, that allows the prediction of the degree of epimerization in tripeptide model reactions of condensation of peptide segments as a function of the primary structure. The experimental validation of this equation is presented in the field of the 125 tripeptides containing alanine, leucine, phenylalanine, valine and isoleucine, the prediction requires 13 epimerization measurements on reference reactions. Sixteen statistically chosen predictions are checked experimentally showing a good fit in spite of the drastic basic assumptions needed to set up the equation.

Scaled-particle theory and salting constant of tetrahydrofuran in various aqueous electrolyte solutions at 298.15 K

The vapour pressure change upon addition of electrolytes to dilute aqueous tetrahydrofuran (THF) solutions has been determined at 298.15 K. From these data the salting constants ks(Setchenov constant) have been calculated for THF in aqueous electrolyte solutions with: NaCl, NaNO3, NaClO4, NaBr, CsBr, CsNO3, AgNO3, Ni(NO3)2, NiCl2, CaCl2, BaCl2, Na2SO4, CdSO4, CuSO4, LaCl3, Me4NBr, Et4NBr, Pr4NBr, Bu4NBr, NaPh4B and Ph4AsCl. Using classical extra-thermodynamic assumptions single-ion ks values are calculated. It is shown, using the scaled-particle theory (SPT) that a Lennard-Jones 6–12 potential can describe the interaction of the THF molecule with all the inorganic cations. This may be considered as evidence that collisions occur between the THF molecule and the inorganic cations studied, although the ions should be considered as preferentially hydrated. Comparison of the behaviour of dilute THF and benzene aqueous inorganic electrolyte solutions suggests that under these conditions, THF behaves as a non-polar molecule. In the case of the interaction with organic ions, the THF molecule induces, unlike benzene, an additional repulsion term.

Theoretical model-based equations for the linear free energy relations of the biological activity of ionizable substances. 1. Equilibrium-controlled potency

Because of the ambiguities of how to treat ionization in empirical equations which relate biological activity to partition coefficient by use of a (log P)2 term, a theoretical approach to the problem is proposed. Based on a simplified view of assays of potency following in vitro or continuous infusion administration of drugs, equations have been derived from a combination of mass law, equilibrium, and extrathermodynamic assumptions. In general form the equations which relate potency to partition coefficient (P) and degree of ionization (alpha) are the following. If the neutral form reacts with the receptor, log (1/C) =-log [1 + SIGMAM(DIPci) + sigman[aj/Pb(1-alpha4y]] + X. If the ionic form reacts with the receptor, log (1/C) =-log [1 + (1 - Alphan)/(alphan)[sigmam(diPci) + sigman[aj/Pb(1-alphaj)]]] + X. In this generalized model there are m nonaqueous compartments and n aqueous compartments of different pH. The parameters a, b, c, and d can be interpreted in terms of the model. The shape of the log (1/C) vs. log P curve may be asymptotic, linear, or composed of two portions of unequal slope which meet at an optimum or a bend. With the use of these equations it is possible to examine whether the ion or the neutral form is the active species and whether there is hydrophobic bonding to the receptor and/or an inert compartment. The models may be further extended to include terms other than log P and alpha.

Transfer activity coefficient of the proton in water-pyridine mixtures. A comparison of some extrathermodynamic assumptions

Various water-pyridine mixtures have been selected in order to compare several of the most popular extrathermodynamic assumptions involved in the determination of the transfer activity coefficient of the proton, γt(H+). Two techniques have been utilized for this purpose: voltammetry [study of the ferrocene, ferricyanide, or thallium(1) systems] and potentiometry at equilibrium (emf measurements of various galvanic cells, including liquid junctions and hydrogen electrode or silver electrode as a test electrode). The assumptions have been classified into various groups [e.g., γt(Zp+)=γt(Zq+) or γt(X−)=γt(Y+)], and the values of γt(H+) have been experimentally determined in each case. The results vary depending upon the basic assumption (several pH units); less important differences (e.g., 0.5 pH unit) occur within a given group, and this may be assigned to the nature of the reference species chosen. A simple model of solvation has been also examined; the application of the law of mass action to the corresponding equilibrium provides results close to the γt(X−) =γt(Y+)type of assumptions which ultimately leads to most self-consistent results.

Solvation of tetrabutylammonium bromide in water + acetonitrile mixtures at 298.15 K from vapour pressure measurements of dilute solutions

The standard free energies of transfer ΔG°t of tetrabutylammonium bromide (n-Bu4NBr) from water to water + acetonitrile mixtures have been determined at 298.15 K from precise vapour pressure measurements of dilute solutions. The excess free energies of water + acetonitrile mixtures have also been measured at the same temperature. The results obained are compared with previous determinations of the ΔG°t of n-Bu4NBr from water to water + acetone and to water + tetrahydrofuran mixtures. Using extrathermodynamic assumptions the contribution of the tetrabutylammonium ion to ΔG°t is calculated. It is shown that the cavity effect of this ion, as deduced from the scaled-particle theory, represents the major part of the experimental ΔG°t value. The remaining contribution to ΔG°t is discussed in terms of the influence of solvent-solvent interactions on ΔG°t; a correlation is noted between the standard free energy of transfer of the tetrabutylammonium ion and the excess entropy function of the binary system studied.

Enthalpies of transfer involving ionic transition states : the redundance of extrathermodynamic assumptions

It is pointed out that the calculation of transition state enthalpies of transfer for reactions involving ions does not necessitate the estimation of thermodynamic quantities for single ions. The procedure is illustrated by data on the reaction between ethyl acetate and sodium hydroxide in water–dimethyl sulphoxide mixtures.

Ionic solvation in water + co-solvent mixtures. Part 4.—Free energies of transfer of single ions from water into water + t-butyl alcohol mixtures

The method used to calculate the free energy of transfer of the proton, ΔG°t(H+), from water into mixtures of water with various co-solvents has now been applied to water + t-butyl alcohol mixtures: the extra-thermodynamic assumptions involved in these calculations are detailed in Part 1. These values for ΔG°t(H+) are then used to separate ΔG°t for the hydrogen halides to give values for ΔG°t for Cl–, Br– and I–; and the values for ΔG°t(Cl–) are used in turn to separate ΔG°t for salts MCl, where M+= Li+, Na+, K+, Rb+, Cs+ and Ag+. Values for ΔG°t for the hydrogen halides and for the salts are obtained from data available on the appropriate electrode potentials. Values for Kc=[(ButOH,H+)solv]/[H+aq][ButOHsolv] have been determined experimentally using the previously described spectrophotometric method with p-nitroaniline. The variations of ΔG°t with solvent composition for the single ions are compared with the variations for ΔG°t in other water + co-solvent mixtures and they are related to solvent structure in the mixtures: a maximum is found in ΔG°t(halide) and –ΔG°t(M+) at mole fractions of t-butyl alcohol ∼0.1.

Ionic solvations in water + Co-solvent mixtures. Part 3.—Free energies of transfer of single ions from water into water + ethylene glycol mixtures

The free energies of transfer for MX (where M = H+, Li+, Na+, K+ and X = Cl–, Br–, I–, OH–) from water into mixtures of ethylene glycol(E) and water, calculated from the available data on electrode potentials, have been separated into free energies of transfer for the individual ions, ΔG°t(M+) and ΔG°t(X–), using values for ΔG°t(H+) calculated by the method involving the extrathermodynamic assumptions specified in Part 1. To calculate ΔG°t(H+), values for Kc=[EH+solv]/[H+aq][Esolv] have been determined using the spectrophotometric method with p-nitroaniline developed earlier. The values for ΔG°t(M+) and ΔG°t(X–) obtained are compared with those already determined for mixtures of water with alcohols and with acetone.

ChemInform Abstract: MEDIUM EFFECTS FOR SINGLE IONS IN DIMETHYL SULFOXIDE, N,N′‐DIMETHYLFORMAMIDE, AND PROPYLENE CARBONATE

AbstractDie Löslichkeitsprodukte von TlCl, TlBr und TlJ in den im Titel genannten Lösungsmitteln wurden bestimmt.

Medium Effects for Single Ions in Dimethyl Sulfoxide, N,N′-Dimethylformamide, and Propylene Carbonate

Abstract The solubility products of thallium halides were measured in dimethyl sulfoxide, N,N′-Dimethylformamide, and propylene carbonate for the purpose of obtaining the values of the medium effect for these electrolytes on transfer from water to these solvents. The medium effects for cations were estimated from the standard potentials of metal/ion couples obtained by the method of the corrected rubidium scale. The medium effects for halide ions were calculated from the medium effects for thallium halides and the thallium ion. A comparison of extrathermodynamic assumptions was made for the values of the medium effect for the silver ion as estimated by various methods ; the method of the corrected rubidium scale was found to give a reasonable result.

Acid—base reactions in sulfolane

The glass electrode was found to follow the Nernst relation in sulfolane when junction potentials are accounted for, and was used to determine the basicity of some amines in sulfolane. Using extrathermodynamic assumptions, the amine basicities appeared different from those in water mostly because of the large values of the solvent activity coefficient of the neutral amines. Values obtained with the glass electrode in acidic media (HCl, HBr, HClO4, HSbCl6) are in agreement with results reported using other methods (H0, conductivity). The pH of 1 M strong acid in sulfolane is estimated to be ≈−12 from potential measurements of the hydrogen electrode in HClO4 solutions.

Acid-base reactions in sulfolane

Summary The glass electrode was found to follow the Nernst relation in sulfolane when junction potentials are accounted for, and was used to determine the basicity of some amines in sulfolane. Using extrathermodynamic assumptions, the amine basicities appeared different from those in water mostly because of the large values of the solvent activity coefficient of the neutral amines. Values obtained with the glass electrode in acidic media (HCl, HBr, HClO4, HSbCl6) are in agreement with results reported using other methods (H0, conductivity). The pH of 1 M strong acid in sulfolane is estimated to be ≈−12 from potential measurements of the hydrogen electrode in HClO4 solutions.

Ionic equilibria in protic and in dipolar aprotic solvents

Abstract

The standard potentials of silver–silver halide electrodes and ion solvation in dimethyl sulphoxide–water mixtures at 25 °C

Standard e.m.f.s of the cells H2|HX|AgX–Ag (X = Cl, Br, or I) in 10, 60, 70, and 80%(w/w) dimethyl sulphoxide–water mixtures are reported. These, together with values for mixtures of other compositions determined earlier, were used to calculate the molar free energies of transfer of the halogen acids from water to the mixed solvents. Single-ion values were derived therefrom by use of the extrathermodynamic assumptions of Feakins et al. and discussed in terms of ion solvation. It is shown that the solvophilicity of the ions is largely determined by their acid–base' interactions with the solvent molecules. The structural features of the solvents are briefly discussed.

A review of electrochemistry in non-aqueous solvents

Abstract

Solvation of ions. XIII. Solvent activity coefficients of ions in protic and dipolar aprotic solvents. A comparison of extrathermodynamic assumptions

Solvation of ions. XIII. Solvent activity coefficients of ions in protic and dipolar aprotic solvents. A comparison of extrathermodynamic assumptions

645. Acid–base properties of hydrogen peroxide–water mixtures

Abstract : The dissociation constants of a number of weak acids in hydrogen peroxide-water mixtures have been redetermined. These acids fall into two categories typified by picric acid and acetic acid, respectively. It is shown that the difference between these groups is not due to the formation of peracids, but may be explained in terms of the solvation properties of the undissociated acids resulting in widely different molar free energies of transfer from water to hydrogen peroxide-water mixtures. The solubility of picric acid and the solubility products of a number of salts have been measured over a range of hydrogen peroxide concentrations, and molar free energies of transfer estimated. If certain extrathermodynamic assumptions are accepted, it is shown that all values of free energy of transfer are small except for those of the H3O(+) and O2H(-) ions, and the undissociated acids in the acetic class. (Author)