Single-ion Values Research Articles

Standard partial molar volumes of single aqueous ions composing ionic liquids. Measurement and evaluation of temperature dependent values. Parsing and LSER correlation with molecular descriptors based on DFT/COSMO approach

The densities of highly dilute aqueous solutions (m < 0.2 mol·kg−1) of 50 common ionic liquids (ILs) or related salts were accurately measured at seven equidistant temperatures in the range of (288.15 to 318.15) K and ambient pressure. The examined set of ILs/salts composed of 31 ions was judiciously selected to cover wide ranges of ion sizes and chemical structures and allow reliable evaluation of ionic contributions. The data were duly analyzed to infer related thermodynamic properties and information on the behavior of these systems on a molecular level. Appropriate means of mathematical statistics were used to estimate data uncertainties, assign weights to data correlations, and judge the significance of the results obtained. The standard partial molar volumes V¯∞ of the ILs/salts at infinite dilution were evaluated through fitting the concentration dependence of respective apparent molar volumes to Redlich-Meyer equation and their temperature dependence was correlated to quadratic polynomial. The data presented were compared whenever possible with those available in the literature; good agreement was often found, although some data from the literature were also considerably dispersed or in error. Standard partial molar volumes of ILs/salts obeying ionic additivity were split by least-squares optimization into temperature-dependent single-ion values based on the Conway-Millero extrathermodynamic assumption. The ionic additivity of BV parameter was also identified. Its magnitude for anions is typically greater than that for cations, supporting the fact that an anion dominates the volumetric behavior of aqueous ILs. As the BV,anion and the second temperature derivative ∂2V¯anion∞/∂T2 were found to correlate, the two solute’s water structure making/breaking ability indicators appear to speak consistently. Enhanced values of standard partial molar expansion were observed for both strongly kosmotropic and chaotropic anions and a plausible explanation of this phenomenon was suggested. To correlate and analyze the effect of ionic structure on V¯ion,298∞, Linear Solvation Energy Relationship (LSER) methodology with the recently proposed four quantum chemistry computed descriptors based on DFT/COSMO approach was employed providing excellent fit (SD = 2.48 cm3·mol−1, R2 = 0.9996).

On the Suitability and Validity of “Reference Electrolyte” Method for the Determination of Single Ion Thermodynamic Properties in Solutions

Abstract Much advocated and supposed to be best method for the determination of the Gibbs energies of single ions in any solvent or Gibbs energies of transfer of single ions between two solvents (true for other thermodynamic properties as well) is based on thermodynamics of “reference electrolyte” TATB (tetraphenylarsoniumtetraphenylborate). TATB, composed of isoelectrical counter ions (TA+-tetraphenylarsonium ion Ph4As+ and TB – -tetraphenylborate BPh4 – ) with large hydrophobic groups of equal radii, has been assumed to have identical Gibbs energies of solvation in any solvent and hence can be split equally to get i) Gibbs energies of single ions in any solvent and ii) Gibbs energies of transfer of single ions between two solvents. However, all the extrathermodynamic methods including “reference electrolyte” method for the determination of single ion thermodynamic values, based on Born or modified Born equations and ionic additivity principles are defective and invalid from theoretical and practical points of view. All the single ion values in the literature are thus unreliable. Moreover, different theoretical and experimental facts make “Ph4AsBPh4” unsuitable for equal division of the Gibbs energies (in a solvent) or Gibbs energies of transfer (between two solvents) of the reference electrolyte into single ion values as reference standard to give single ion values of other ions. Single ion thermodynamic values in any solvent or between two solvents are illusive and only the electrical components of the single ions can be determined but not without limitations.

Thermodynamic consistencies and anomalies among end-member silicate garnets.

Materials with the garnet crystal structure include silicate minerals of importance both in geology, on account of their use in geothermobarometry, and industrially as abrasives. As a consequence of the former, there is considerable published thermodynamic information concerning them. We here examine this thermodynamic information for end-member silicate garnets (some of which are synthetic since not all occur in nature) for consistencies and anomalies, using thermodynamic relations between thermodynamic properties that we have established over recent years. The principal properties of interest are formula volume, heat capacity, entropy, formation enthalpy (from which the Gibbs energy may be obtained), and isothermal compressibility. A significant observation is that the ambient-temperature heat capacities of the silicate garnets are rather similar, whereas their ambient-temperature entropies are roughly proportional to their formula volumes. Evaluation of their Debye temperatures implies that their vibrational contributions to heat capacity are fully excited at ambient temperatures. The relatively small isothermal compressibilities of these garnets is related to the rigidity of their constituent silicate tetrahedra. We here establish additive single-ion values for each of the thermodynamic properties, which may be applied in estimating corresponding values for related materials.

Thermodynamic consistencies and anomalies among end-member lanthanoid garnets

Lanthanoid (or lanthanide) garnets are ionic solids of technological importance in their use in electronic materials. They are also of interest in respect of their systematic relationships and as geochemical tracers. As a consequence, there is considerable published thermodynamic information for these garnets.Based principally on the computational results of Moretti and Ottonello (1998) [8], we here examine the thermodynamic information for consistencies and anomalies among the ferri-, alumino-, and gallo-garnets using relations between thermodynamic properties that we have established over recent years. The principal properties of interest are formula volume, heat capacity, entropy, and formation enthalpy (from which the Gibbs free energy may be obtained), and isothermal compressibility. We also establish additive single-ion values for trivalent lanthanide cations which may be applied in estimating properties for related materials.Since the results of the work of Moretti and Ottonello are based upon consistent computational analyses, we should expect generally smooth relations. These are, indeed, found for various of the properties (except for the europium garnets), with some uncertainty in the absolute entropies, and anomalies in the formation enthalpies of europium and ytterbium/lutetium garnets. The results of some more recent experimental work are included in our analyses. Values for the (unknown) promethium garnets are estimated by averaging the properties of the neighbouring neodymium and samarium garnets.

Single-Ion Values for Ionic Solids of Both Formation Enthalpies, ΔfH(298)ion, and Gibbs Formation Energies, ΔfG(298)ion

Formation enthalpies, Δ(f)H(298), are essential thermodynamic descriptors of the stability of materials, with many available from the numerous thermodynamic databases. However, there is a need for predictive methods to supplement these databases with missing values for known and even hypothetical materials, and also as an independent check on the not-always reliable published values. In this paper, we present 34 additive single-ion values, Δ(f)H(298)(ion), from the formation enthalpies of 124 ionic solids, including an extensive group of silicates. In addition, we have also developed an additive set of 29 single-ion formation Gibbs energies, Δ(f)G(298)(ion), for a smaller group of 42 materials from within the full set, constrained by the limited availability of the corresponding experimental data. Such single-ion values may be extended among related materials using simple differences from known thermodynamic values, but always with critical consideration of the results. Using the excellent available data for silicates, we propose that the solid-state silicate ion formation enthalpies can be estimated as -Δ(f)H(298)(silicate)/kJ mol(-1)= -252[n(Si) + n(O)] - 27, where n(X) represents the number of species X in the silicate. More speculatively, we estimate the contribution per silicon and oxygen species as -490 and -184 kJ mol(-1), respectively. Similarly, -Δ(f)G(298)(silicate)/kJ mol(-1)= -266[n(Si) + n(O)] - 7, with the contribution per silicon and oxygen species being -140 and -300 kJ mol(-1), respectively. We compare and contrast these results with the extensive collection of "modified lattice energy" (MLE) ion parameters from the M.S. thesis of C. D. Ratkey. Our single-ion formation enthalpies and the MLE parameters may be used in complementary predictions. While lattice energies, U(POT), entropies, S(o)(298), and heat capacities, C(p,298), of ionic solids are reliably estimated as proportional to their formula volumes (using our Volume-Based Thermodynamic, VBT, procedures), this is not the case in general for thermodynamic formation properties, other than within select groups of related materials.

Solvent effects and ion-solvent interactions from the studies of the solubility and dissociation constants of oxalic acid in aquo + 2-propanol mixtures and determination of single-ion Gibbs energies of transfer from water to 2-propanol + H2O mixtures

The solvent effects on the dissociation of the simplest dibasic organic acid were examined from the determination of the first and second dissociation constants ot oxalic acid conductometrically ana pH-metrically in water + 2-propanol mixtures (0-87 wt% of 2-propanol). The solubility values of oxalic acid and potassium hydrogen oxalate were also determined. The dissociation constants and the solubility values were used to calculate the Gibbs energies of transfer ΔG 0 t of oxalic acid (H 2 OX), bioxalate (HOX-), oxalate (OX 2- ) and potassium (K + ) ions from water to aquo-propanolic mixtures. These represent the 'medium effects' or quantitative measures of solute or ion-solvent interactions of the respective species in going from water to 2-propanol + water mixtures. pK 1 , pK 2 and the solubility values of oxalic acid increase continuously with increase in organic co-solvent. AG 0 t (H 2 OX) and AG 0 t (H + ) are negative i.e. favourable but AG 0 t (H 2 OX), ΔG 0 t (OX 2- ) and AG 0 t (K + ) are in general positive and hence unfavourable for the dissociation processes. The increase in basicity facilitated the dissociation processes but decrease in dielectric constant and H-bonding capability enhances the association processes as the organic co-solvent increases. The pK-values can be suitably utilised to prepare buffer solutions in 2-propanol + water mixtures. The solvent effect and solvation phenomena can be better understood from the collection of single-ion values of transfer. These may be utilised to calculate the solubilities of electrolytes in 2-propanol + water mixtures. Attempts have been made to understand the solvation processes in terms of the structure of the aquo-organic mixtures and Walden products.

Determination of the absolute values of Gibbs energy of hydration or solvation ΔG0 abs(M+ or X-)h or s of monovalent (alkali metal and halide) ions and other thermodynamic parameters in aqueous and non-aqueous solvents using a single standard state and analysis of ΔG0 abs(M+ or X-)h or s values based on cluster-ion solvation method

or X - ) h or s and other thermodynamic parameters in aqueous and non-aqueous solvents (methanol, ethanol, n-propanol, isopropanol, n-butanol, ethylene glycol, propylene carbonate (PC), N-methyl formamide (NMF), acetone, tetrahydrofuran (THF), 1,4-dioxan, acetonitrile) were determined directly using modified Born equation and a single standard state [i.e. AG° and ΔH 0 (but not ΔS 0 ) of H 2 gas and other elements to be zero in their elementary standard states at 1 atm. pressure (1 bar) and 298 K]. The ΔH 0 abs (M + or X - ) h or s and ΔG 0 (M + or X - ) h or s values are calculated considering ion-dipole, ion-quadrupole interactions and without considering the interaction terms. Very few data are available in non-aqueous solvents for comparison. Most of the single ion values (particularly for the anions) suffer from limitations associated with the erroneous principle of division of solvation energies ΔG 0 h or s (MX) or ΔΗ 0 h or s (MX) of electrolytes into single ion values. ΔG 0 (M + or X - ) h values determined using cluster-ion solvation data have also been presented. The values have been claimed to be most accurate in recent years. However, the method involves a number of assumptions of doubtful validity and the values cannot be regarded to be equivalent to ΔG 0 (Μ + or X - ) h or determined by other workers using different methods. Coupling the values of ΔG 0 (M + or X - ) h or s and ΔH 0 (M + or X - ) h or s with ΔG 0 (M + or X - ) g and ΔH 0 (M + or X - ) g values in the gaseous state, ΔG 0 (M + or X - ) Water or org. solvent' ΔH 0 (Μ + or X - ) water or org. solvent and ΔS 0 (M + or X - ) water or org. solvent [values in water and in organic solvents] are determined.

Studies on Hydrophobic Interaction from the Conductometric Studies of Medicinal Compounds (as Salts)

Abstract The Λ0 values and association constants of some salts of medicinal interest in aqueous solution have been determined. The division of Λ0 of salts into single ion values λ0 + and λ0 − have been made using λ0 Na+ and λ0 Cl− values from the literature. The Λ0, λ0 + and λ0 − values have been found to be fairly high in aqueous solution and comparatively lesser than those in acetonitrile. Most of the drugs have been found to be dissociated as ions though association of some of the salts has been observed. The calculations of r ± of ions in water show considerable decrease in the radii values of ions in water from which inference regarding the extent of hydrophobic interactions can be made.

Determination of Basicities of Organic Solvents and Absolute Electrode Potentials (E 0-Values) of Monovalent Ions in Organic Solvents based on Absolute Values of Gibbs Energies of Solvation of Single Ions

Abstract Single ion Gibbs energies of monovalent ions in fourteen solvents (water, methanol, NMF, PC, ethanol, n-propanol, iso-propanol, n-butanol, t-butanol, ethylene glycol, acetone, THF, 1,4-dioxan, acetonitrile) with or without (ion–dipole, ion–induced dipole, ion–quadrupole) interactions were determined. The basicity of the solvents was determined from the absolute values of ΔG 0 solvation(H+) or ΔG 0 solution(H+) based on single standard state utilising the modified Born equation suggested by Lahiri. The method involves no arbitrary assumptions. A single scale for the absolute electrode potentials (E 0 values) has been determined based on calculated Gibbs energies of solvation or solution of single ions. Appropriation of Gibbs energies of solvation of electrolytes into single ion values is not possible. Gibbs energies of formation of electrolytes in solvents must be taken into consideration. Stabilisation energies of electrolytes (MX) in solvents akin to lattice energies in solids were determined.

Real Dimensions of the Drug Ions from Conductometric Studies of Drugs (Present as Salts) in Acetonitrile

AbstractThe Λ0and association constants of salts of some widely used drugs (pharmacological agents) in acetonitrile have been determined. The division of Λ0into single ion values λ0+and λ0−has been made using λ0Na⁺and λ0Cl⁻values from the literature. The λ0+and λ0−values have been found to be considerable in acetonitrile. The results suggest that many of the drugs can be present both as dissociated ions as well as associated molecules which are capable of absorption, dissociation and conduction (of ionic forms) in the lipid layers at very low concentrations. The phenomenon of unsolvation has been utilized to calculate the real dimensions of the drug ions (cations or anions) which are difficult to calculate from bond distances due to their large size and coiled configuration in aqueous solutions.

Ion–Solvent Interactions and Determination of Single-Ion Gibbs Energies of Transfer from the Studies of the Solubility and Dissociation Constants of Oxalic Acid in Aquo + Methanolic Mixtures

Abstract The solvent effects on the dissociation of the simplest dibasic organic acid were examined from the determination of the first and second dissociation constants of oxalic acid conductometrically and pH-metrically in water + methanol mixtures (0–87 wt% of methanol). The solubility values of oxalic acid and potassium hydrogen oxalate were also determined in water + methanol mixtures. The dissociation constant and the solubility values were used to calculate the Gibbs energies of transfer ΔG 0 t of oxalic acid (H2OX), bioxalate (HOX−), oxalate (OX2−) and potassium (K+) ions from water to aquo-methanolic mixtures. These represent the quantitative measures of solute or ion–solvent interactions of the respective species in going from water to methanol + water mixtures. pK 1, pK 2 and the solubility values of oxalic acid increase continuously with increase in organic co-solvent. ΔG 0 t (H2OX) and ΔG 0 t (H+) are negative i.e. favourable but ΔG 0 t (H2OX), ΔG 0 t (OX2−) and ΔG 0 t (K+) are in general positive and hence unfavourable for the dissociation processes. The increase in basicity fecilitated the dissociation processes but decrease in dielectric constant and H-bonding capability enhance the association processes as the organic co-solvent increases. Attempts have been made to understand the solvation processes in terms of the structure of the aquo + organic mixtures and Walden products. The pK values can be suitably utilised to prepare buffer solutions in methanol + water mixtures. The solvent effect and solvation phenomena can be better understood from the collection of single-ion values of transfer. These may be utilised to calculate the solubilities of electrolytes in methanol + water mixtures.

The volumetric and thermochemical properties of YCl 3(aq), YbCl 3(aq), DyCl 3(aq), SmCl 3(aq), and GdCl 3(aq) at T=(288.15, 298.15, 313.15, and 328.15) K and p=0.1 MPa

Relative densities and massic heat capacities have been measured for acidified aqueous solutions of YCl 3(aq), YbCl 3(aq), DyCl 3(aq), SmCl 3(aq), and GdCl 3(aq) at T=(288.15, 298.15, 313.15, and 328.15) K and p=0.1 MPa. These measurements have been used to calculate experimental apparent molar volumes and heat capacities which, when used in conjunction with Young’s rule, were used to calculate the apparent molar properties of the aqueous chloride salt solutions. The latter calculations required the use of volumetric and thermochemical data for aqueous solutions of hydrochloric acid that have been previously reported in the literature. The concentration dependences of the apparent molar properties have been modeled using Pitzer ion interaction equations to yield apparent molar volumes and heat capacities at infinite dilution. The temperature and concentration dependences of the apparent molar volumes and heat capacities of each trivalent salt system were modeled using modified Pitzer ion interaction equations. These equations utilized the revised Helgeson, Kirkham, and Flowers equations of state to model the temperature dependences of apparent molar volumes and heat capacities at infinite dilution. Calculated apparent molar volumes and heat capacities at infinite dilution have been used to calculate single ion properties for the investigated trivalent metal cations. These values have been compared to those previously reported in the literature. The differences between single ion values calculated in this study and those values calculated from thermodynamic data for aqueous perchlorate salts are also discussed.

ΔS°t – as a Structural Probe – a Critical Analysis of the Method and Determination of Structure of Aquo-Alcoholic Mixtures

Abstract Attempts were made for a critical examination of the use of ΔS°t as a structural probe to study the ion induced perturbations on aquo-alcoholic solvent mixtures. The division of the thermodynamic properties of TATB (tetraphenyl arsonium tetraphenyl borate) in solution into equal parts is not free from defects. Naturally, the calculation of single ion values based on TATB is also problematic. Dissection of the single ion values like TΔS°t(ion) as suggested by Kundu is full of errors. Analysis shows TΔS°t(ion) to be an insensitive parameter to determine the structural characteristics of aquo-alcoholic mixtures. Structure of aquo-alcoholic mixtures should be determined from the excess thermodynamic properties of mixing, excess dielectric properties of mixing, excess polarizabilities, the correlation parameter “g” etc. Other properties like ultrasonic sound absorption, excess volume and viscosity of mixing were found to be useful. Ion induced perturbations of any solvent system will be dictated by the structure of the particular solvent system. A knowledge of the structure of the aquo-alcoholic mixtures is imperative to develop an idea regarding ion induced perturbations as mirror image relations because of compensation of ΔH° and TΔS° (and of ΔH°t and TΔS°t) will be observed in any solvent system. The thermodynamic measurements provide a summary of all interaction energies over a long time. To comprehend the rational structural details of the solvent systems, accurate thermodynamic measurements alongwith relaxation experiments providing high time and good structural resolutions are necessary.

Determination of Gibbs Energies of Solvation of Monovalent Ions in Water, Methanol and Ethanol and Re-evaluation of the Interaction Energies

Abstract Single ion Gibbs energies of solvation of monovalent ions in water, methanol and ethanol with or without (ion-dipole, ion-quadrupole) interaction-terms have been determined. Widely different single ion values given in the literature usually do not include interaction terms. The method of calculation of interaction terms needs re-evaluation. The usual methods of calculation of ΔG°t(H+) values appear to be defective and ΔG°t(H+) values given in the literature appear to be too high. An alternative approach for the calculation of ΔG°h or s(H+) is suggested.

Solution Thermodynamics of Ba(II)−Cryptand 222 in Various Solvents and Derived Coordination Data in the Solid State

Enthalpies of complexation of barium and cryptand 222 in N,N-dimethylformamide, dimethyl sulfoxide, acetonitrile, and propylene carbonate at 298.15 K determined calorimetrically are reported. The stability constant for this system in acetonitrile at the same temperature was measured by competitive potentiometric titration using silver electrodes. Gibbs energies of complexation in a variety of solvents are analyzed in terms of corresponding data for the transfer of ligand and free and complexed cation. Standard enthalpies of solution of barium perchlorate in N,N-dimethylformamide, dimethyl sulfoxide, acetonitrile, and propylene carbonate and for barium−cryptand 222 perchlorate in water, methanol, N,N-dimethylformamide, and dimethyl sulfoxide measured calorimetrically at 298.15 K are reported. The data show that in these solvents the former electrolyte is more solvated than the latter. Single-ion values of transfer enthalpies for the free and complexed cations are calculated on the basis of the Ph4AsPh4B convention. These data and those previously reported for the transfer enthalpy of cryptand 222 among these solvents are used to explain the variation observed in the complexation enthalpies of Ba2+ and this ligand in the various solvents. Coordination enthalpies for the process in the solid state derived from six different solvents are for the first time reported. The agreement found between the values derived from independent measurements reflect the reliability of complexation and solution data reported in this paper. The relationship between complexation and solvation entropy of barium is discussed taking into account previous thermodynamic data on univalent cations and cryptand 222. It is concluded that the effective shielding effect of the ligand for univalent cations weakens considerably for barium and breaks completely for some of the lanthanide cations as assessed from thermochemical data on these systems.

Transfer Gibbs energies for ClO3–, BrO3–, IO3–, ClO4–and IO4–anions for water–methanol and water–propan-2-ol mixtures: some quantum-chemical aspects of ionic solvation

The transfer Gibbs energies, ΔtrsGo, of KClO3, KBrO3, KIO3, KClO4, KIO4 and the corresponding caesium salts have been obtained through gravimetric measurements of the solubility in aqueous mixtures with methanol (MeOH) and propan-2-ol (PriOH) at 298.2 K. In addition, the solubility of Ph4P(pic)(pic = picrate anion) as well as K(pic) and KBPh4 and the corresponding caesium salts are reported in these mixtures at 298.2 K. Single-ion values of ΔtrsGo have been calculated using the tetraphenylphosphonium tetraphenylborate (TPTB) assumption. The semiempirical quantum-chemical AM1 method was used for calculations of interaction energy of BPh4– and Ph4P+ with a series of five solvent molecules and energy demands associated with a deformation of these ions. Trends observed for ΔtrsGo are discussed in terms of specific ion–solvent interactions and the structural effect of solvent mixtures.

Partitioning of entropy of solvation : single-ion entropy of solvation in aqueous and non-aqueous solvents and the correlation of electrostatic entropy of solvation with electrostriction viscosity B and nmr B? coefficients

Entropies of hydration of alkali-metal halides have been partitioned on the basis of a correspondence plot with the ionic B coefficient obtained by a method recentiy developed. Single-ion values in six non-aqueous solvents have been obtained from the combination of ionic entropies of hydration with the corresponding entropies of transfer from water to the respective solvent. The values thus obtained agree closely with those obtained by recent workers. Entropies of solvation for different concentric layers surrounding ions have been calculated using the two-layer-model equation developed by Abraham and co-workers based on a continuum model and have been correlated with the electrostriction viscosity B and NMR B′ coefficients. The present study shows that the information obtained from the NMR B′ coefficient is confined to the first concentric layer, whereas the viscosity B coefficient covers all the layers of solvation around the ions. Different energies of activation obtained from these two coefficients in different solvents have been discussed as additional proof for this information. The presence of a highly disordered second layer makes the magnitude of the B and B′ coefficients closer in water and ethylene glycol, but the absence of the second layer makes them different in non-aqueous solvents.

Transfer Gibbs Free Energies of Divalent Anions from Water to Organic and Mixed Aqueous-OrganicSolvents

The standard molar Gibbs free energies of transfer of divalent anions, in particular sulfate, from water into methanol, ethanol, ethylene glycol, dioxane, tetrahydrofuran, acetone, N-methylformamide, N,N-dimethylformamide, pyridine, acetonitrile, and dimethylsulfoxide and mixtures of some of them with water or some other solvent have been obtained from the literature. The tetraphenylarsonium tetraphenylborate extrathermodynamic assumption has been used as far as possible in order to obtain the single ion values. The generally unfavorable transfers from water (or protic solvents) into polar aprotic ones are interpreted in terms of the properties of the anions and of the solvents, dominated by the hydrogen-bonding acidity of water.

Transfer gibbs energies for ClO–3, BrO–3, IO–3, ClO–4 and IO–4 anions for water–acetonitrile and water–tert-butyl alcohol mixtures

The transfer Gibbs energies, ΔtrsG, of KClO3, KBrO3, KlO3, KClO4, KlO4 and the corresponding caesium salts have been obtained through the gravimetric measurement of solubility in aqueous mixtures with acetonitrile (AN) and tert-butyl alcohol (Bu tOH). Single-ion values of ΔtrsG have been calculated using the TATB assumption. The trends observed for ΔtrsG are discussed in terms of specific ion–solvent interactions and the structural effect of the solvent mixtures.

Enthalpies and entropies of solution of electrolytes containing 4-hydroxyphenylazobenzoate and chloro-4-hydroxyphenylazobenzoate anions in water and in methanol. Derived single-ion values for the transfer of these anions from water to methanol

Heats of solution of electrolytes containing 4-hydroxyphenylazobenzoate and 2-chloro-4-hydroxyphenylazo-benzoate anions in water and in methanol have been measured calorimetrically at 298.15 K. Enthalpies of transfer of these electrolytes from water to methanol are reported. Combination of these data with transfer free energies values yields the entropies of transfer of the dissociated 1:1 electrolytes in the water–methanol solvent system. Single-ion quantities of transfer (ΔH°t and ΔS°t) from water to methanol for seven anions based on the Ph4AsPh4B convention are reported. Two different combinations of ΔH°t and ΔS°t values lead to the negative ΔG°t values observed for the transfer of these anions from water to methanol. The transfer process is entropically controlled for ortho and chloro-substituted o-(4-hydroxyphenylazobenzoate) anions. For meta and para and chloro-substituted m- and p-(4-hydroxyphenylazobenzoate) anions, the transfer process is enthalpically controlled.