Gibbs Free Energies Of Transfer Research Articles

Oxygen reduction at a water-1,2-dichlorobenzene interface catalyzed by cobalt tetraphenyl porphyrine – A fuel cell approach

Oxygen reduction at the polarized water-1,2-dichlorobenzene interface, catalyzed by 5,10,15,20-tetraphenyl 21H,23H-porphine cobalt(II), is utilized in a novel type of flow fuel cell. In this fuel cell, hydrogen is oxidized at the anode as usual, but oxygen reduction takes place at the water-1,2-dichlorobenzene interface by a redox mediator, which is regenerated at the cathode. Oxygen reduction is coupled with proton transfer from water to the organic phase to form hydrogen peroxide, which is extracted into an aqueous phase flowing through the cell. The advantage of the cell is that no platinum catalyst is required at the cathode for O 2 reduction. Furthermore, recombination of H + and O 2 at the cathode, like in a conventional fuel cell, is not possible, because the Gibbs free energy of transfer of protons from water to an organic phase is very high, 50–60 kJ/mol. The experiments reported here demonstrate that proton transfer is possible only by the facilitation of the catalyst.

Thermodynamic aspects of solubility, solvation and partitioning processes of some sulfonamides

The thermodynamic aspects of sublimation processes of three sulfonamides with the general structures C 6H 5–SO 2NH–C 6H 4–R (R = 4-NO 2) and 4-NH 2–C 6H 4–SO 2NH–C 6H 4–R (R = 4-NO 2; 4-CN) were studied by investigating the temperature dependence of vapor pressure using the transpiration method. These data together with those obtained earlier for C 6H 5–SO 2NH–C 6H 4–R (R = 4-Cl) and 4-NH 2–C 6H 4–SO 2NH–C 6H 4–R (R = 4-Cl; 4-OMe; 4-C 2H 5) were analyzed and compared. A correlation was derived between sublimation Gibbs free energies and the sum of H-bond acceptor factors of the molecules. Solubility processes of the compounds in water, phosphate buffer with pH 7.4 and n-octanol (as phases modeling various drug delivery pathways) were investigated and corresponding thermodynamic functions were calculated as well. Thermodynamic characteristics of the sulfonamides solvation were evaluated. Also in this case a correlation between solubility/solvation Gibbs free energy values and the sum of H-bond acceptor factors was observed. For the sulfonamides with various substituents at para-position the processes of transfer from one solvent (water or buffer) to n-octanol were studied by a diagram method combined with analysis of enthalpic and entropic terms. Distinguishing between enthalpy and entropy, as is possible through the present approach, leads to the insight that the contribution of these terms is different for different molecules (entropy- or enthalpy-determined). Thus, in contrast to the interpretation of only the Gibbs free energy of transfer (extensively used for pharmaceuticals in the form of the partition coefficient, log P), the analysis of thermodynamic functions of the transfer process provides additional mechanistic information. This may be important for further evaluation of the physiological distribution of drug molecules and may provide a better understanding of biopharmaceutical properties of drugs.

Partitioning of 2,6-Bis(1 H-Benzimidazol-2-yl)pyridine fluorophore into a phospholipid bilayer: Complementary use of fluorescence quenching studies and molecular dynamics simulations

Successful use of fluorescence sensing in elucidating the biophysical properties of lipid membranes requires knowledge of the distribution and location of an emitting molecule in the bilayer. We report here that 2,6-bis(1 H-benzimidazol-2-yl)pyridine (BBP), which is almost non-fluorescent in aqueous solutions, reveals a strong emission enhancement in a hydrophobic environment of a phospholipid bilayer, making it interesting for fluorescence probing of water content in a lipid membrane. Comparing the fluorescence behavior of BBP in a wide variety of solvents with those in phospholipid vesicles, we suggest that the hydrogen bonding interactions between a BBP fluorophore and water molecules play a crucial role in the observed “light switch effect”. Therefore, the loss of water-induced fluorescence quenching inside a membrane are thought to be due to deep penetration of BBP into the hydrophobic, water-free region of a bilayer. Characterized by strong quenching by transition metal ions in solution, BBP also demonstrated significant shielding from the action of the quencher in the presence of phospholipid vesicles. We used the increase in fluorescence intensity, measured upon titration of probe molecules with lipid vesicles, to estimate the partition constant and the Gibbs free energy (ΔG) of transfer of BBP from aqueous buffer into a membrane. Partitioning BBP revealed strongly favorable ΔG, which depends only slightly on the lipid composition of a bilayer, varying in a range from − 6.5 to − 7.0 kcal/mol. To elucidate the binding interactions of the probe with a membrane on the molecular level, a distribution and favorable location of BBP in a POPC bilayer were modeled via atomistic molecular dynamics (MD) simulations using two different approaches: (i) free, diffusion-driven partitioning of the probe molecules into a bilayer and (ii) constrained umbrella sampling of a penetration profile of the dye molecule across a bilayer. Both of these MD approaches agreed with regard to the preferred location of a BBP fluorophore within the interfacial region of a bilayer, located between the hydrocarbon acyl tails and the initial portion of the lipid headgroups. MD simulations also revealed restricted permeability of water molecules into this region of a POPC bilayer, determining the strong fluorescence enhancement observed experimentally for the membrane-partitioned form of BBP.

Salting-out phenomenon and 1-octanol/water partition coefficient of metalaxyl pesticide

In this paper, we present the effect of inorganic cations such as Na +, K +, Ca 2+, Mg 2+ on the salting-out phenomenon of metalaxyl from pure water to aqueous salt solutions. Moreover the 1-octanol/water partition coefficient in pure water is presented. To accomplish this, aqueous solubility of metalaxyl was determined in pure water, in different salt solution (NaCl, KCl, CaCl 2 and MgCl 2), and at different concentration level ranging from 0.01 to 1.5 M. The 1-octanol/water partition coefficient was determined using the static shake-flask method. Solubility was determined using dynamic saturation method for pure water in the range of 298.15–325.15 K and at 298.15 K for different salt solutions. The solubility value in pure water for studied interval was found constant ( m = 3.118 × 10 −2 mol kg −1). Solubility values were used to calculate the standard molar Gibbs free energy of dissolution (Δ sol G°) and transfer (Δ tr G°) at 298.15 K. The values of Δ tr G° from pure to all studied aqueous salt solutions did not exceed 2 kJ mol −1, the value of Δ sol G° of dissolution is 18.5 ±0.72 kJ mol −1. The 1-octanol/water partition coefficient in pure water log K o/w is equal to 1.69. The obtained results confirm the classification of the neutral metalaxyl as a slightly hydrophobic molecule.

Micellar chromatographic determination of partition coefficients and associated thermodynamic data for pharmaceutical compounds

Micelle/water partition coefficients were obtained for procaine hydrochloride using micellar liquid chromatography (MLC) to illustrate the potential application of this technique to compounds of pharmaceutical interest. Experiments were conducted over a temperature range which led to calculation of a Gibbs free energy, enthalpy and entropy of transfer for procaine hydrochloride. Successful application of this technique was confirmed using a second compound over a range of temperatures, namely caffeine. Overall, this work confirms that MLC can be used to determine precise and accurate partition coefficients that possibly more closely mimic biological membranes than traditional in vitro systems, namely octanol/water.

Molecular dynamics study of the behavior of selected nanoscale building blocks in a gel-phase lipid bilayer.

The cellular membrane functions as a regulating barrier between the intracellular and extracellular regions. For a molecule to reach the interior of the cell from the extracellular fluid, it must diffuse across the membrane, via either active or passive transport. The rigid structure of lipid bilayers, which are a key component of cellular membranes, prohibit simple diffusion of most particles, while vital nutrients are transported to the interior by specific mechanisms, such as ion channels and transport proteins. Although the cellular membrane provides the cell with protection against unwanted toxins that may be in the extracellular medium, some foreign particles can reach the interior of the cell, resulting in irregularities in cellular function. This behavior is particularly noted for permeants with compact molecular structure, suggesting that common nanoscale building blocks, such as fullerenes, may enter into the interior of a cell. To gauge the propensity for such particles to cross the membrane, we have computed the Gibbs free energy of transfer along the axis normal to the bilayer surface for two nanoscale building blocks, C(60) and a hydrogen-terminated polyhedral oligomeric silsequioxane (H-POSS) monomer, in a hydrated dipalmitoylphosphatidylcholine (DPPC) bilayer using molecular dynamics simulations and potential of mean force calculations. The studies show that C(60) has a substantial energetic preference for the soft polymer region of the lipid bilayer system, below the water/bilayer interface, with a transition energy from bulk water of -19.8 kcal/mol. The transition of C(60) from the bulk water to the center of the bilayer, while also energetically favorable, has to overcome a +5.9 kcal/mol energetic barrier in the hydrophobic lipid tail region. The H-POSS simulations indicate an energy minimum at the water-bilayer interface, with an energy of -10.9 kcal/mol; however, a local minimum of -2.7 kcal/mol is also observed in the hydrophobic dense aliphatic region. The energy barrier seen in the hydrophobic core region of the C(60) study is likely due to the significant penalty associated with inserting the relatively large particle into such a dense region. In contrast, whereas H-POSS is found to be subject to an energetic penalty upon insertion into the bilayer, the relatively small size of the H-POSS solute renders this penalty less significant. The energy barrier seen in the soft polymer region for the H-POSS monomer is primarily attributed to the lack of favorable solute-bilayer electrostatic interactions, which are present in the interfacial region, and fewer van der Waals interactions in the soft polymer region than the dense aliphatic region. These results indicate that C(60) may partition into the organic phase of the DPPC/water system, given the favorable free energies in the soft polymer and dense aliphatic regions of the bilayer, and H-POSS is likely to partition near the water-bilayer interface, where the particle has low-energy electrostatic interactions with the polar head groups of the bilayer.

Activity coefficients of LiCl in (PEG 4000 + water) at T = (288.15, 298.15, and 308.15) K

The electromotive force of the cell containing two ion-selective electrodes (ISE), Na-ISE | LiCl ( m ) , PEG 4000 ( w ) , H 2 O ( 1 - w ) | Cl-ISE has been measured at temperatures of (288.15, 298.15, and 308.15) K as a function of the mass fraction of PEG 4000 ( w) in the mixture. w was varied between 0 and 0.25 in 0.05-unit steps and the molality of the electrolyte ( m) was between c.a. (0.03 and 5.8) mol · kg −1. The values of the standard electromotive force, E ∘ , were determined using routine methods of extrapolation, together with extended Debye–Hückel and Pitzer equations. The results obtained produced good internal consistency for all the temperatures studied. Once E ∘ was determined, the mean ionic activity coefficients for LiCl, the Gibbs free energy of transfer from the water to (PEG 4000 + water), and the primary LiCl hydration number were calculated.

Gibbs free energy of transfer of a methylene group on {UCON + (sodium or potassium) phosphate salts} aqueous two-phase systems: Hydrophobicity effects

The Gibbs free energy of transfer of a suitable hydrophobic probe can be regarded as a measure of the relative hydrophobicity of the different phases. The methylene group (CH 2) can be considered hydrophobic, and thus be a suitable probe for hydrophobicity. In this work, the partition coefficients of a series of five dinitrophenylated-amino acids were experimentally determined, at 23 °C, in three different tie-lines of the biphasic systems: (UCON + K 2HPO 4), (UCON + potassium phosphate buffer, pH 7), (UCON + KH 2PO 4), (UCON + Na 2HPO 4), (UCON + sodium phosphate buffer, pH 7), and (UCON + NaH 2PO 4). The Gibbs free energy of transfer of CH 2 units were calculated from the partition coefficients and used to compare the relative hydrophobicity of the equilibrium phases. The largest relative hydrophobicity was found for the ATPS formed by dihydrogen phosphate salts.

The synthesis of 1,8-naphthalimide groups containing imidazolium salts/ionic liquids using I −, PF 6−, TFSI − anions and their photophysical, electrochemical and thermal properties

1,8-Naphthalimide groups containing imidazolium iodide salts of different alkyl chain length on the imidazole group were synthesized. Hexafluorophosphate and bis(trifluoromethanesulfonyl)imide salts were obtained by ion exchange reactions. The synthesized salts were characterized using a variety of spectroscopic techniques. Fluorescence emission quenching of iodide and bis(trifluoromethanesulfonyl)imide salts were studied using a ruthenium dye, whilst that of hexafluorophosphate salts were performed using tris(8-hydroxyquinoline)aluminum. Calculated fluorescence quenching rate constants were as high as 10 14 M −1 s −1 and values of the Gibbs free energy of electron transfer were [−32.04/−33.68], [−17.52/−20.74] and [−26.50/−30.89] kcal mol −1 for the iodide, hexafluorophosphate and bis(trifluoromethanesulfonyl)imide salts, respectively. The thermal stability of the synthesized salts was high especially in the case of bis(trifluoromethanesulfonyl)imide salts for which values of 470–498 °C were achieved. Photostability studies of hexafluorophosphate and bis(trifluoromethanesulfonyl)imide salts were also performed; calculated photodegradation half-life values ranged from 19.25 to 48.12 h.

Investigation of the driving forces for retention in reversed-phase liquid chromatography: Monte Carlo simulations of solute partitioning between n-hexadecane and various aqueous–organic mixtures

With the aim of learning about the solubility characteristics of retentive and mobile phases often encountered in reversed-phase liquid chromatography (RPLC) and to examine the driving forces for retention, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out for systems consisting of three phases: an n-hexadecane retentive phase (serving as a model of the hydrophobic RPLC stationary phase), a mobile phase with varying water–acetonitrile or water–methanol composition, and a helium vapor phase (serving as an ideal gas reference state). Gibbs free energies of transfer between each of these phases were computed for a series of small alkane and alcohol solutes and from these the methylene and hydroxyl increments were determined. The results indicate that both nonpolar and polar solutes have favorable interactions with both the mobile and stationary phase thus showing that neither the solvophobic theory nor lipophilic theory alone can explain the driving forces for retention in RPLC. In addition, an analysis of the solvation environments of methylene and hydroxyl groups in the mobile and stationary phases was carried out. The methylene group is shown to be preferentially solvated by the organic component of the aqueous–organic mixture, however, clustering of the organic solvent molecules in the absence of the solute was not observed. For the hydroxyl group, hydrogen bonding was shown to be important in both the mobile and stationary phases.

Mass Transfer of Carbaryl from Pure Water to Salt Aqueous Solution: Result Comparison Between Sea and Lab-Made Water

It is well known that mass transfer of a solid compound into a liquid phase is characterized by mutual properties of both sides. Solubilization capacity of the liquid phase is primary affected by its composition in inorganic species inducing salting in/out effect evaluated by Setschenow constant, generally defined at 298.15 K. On the other hand, the equilibrium process is highly influenced by polar compatibility issue. Therefore, the study of such transfer is essential in order to set the role of each party participating in the solid–liquid equilibrium. Thus, a comparison of transfer magnitude and feasibility of carbaryl from pure water, to seawater, and lab-made seawater was held. To do so, solubility of the compound was experimented at different temperature in multiple media. Solubility determination is based on the saturation of a specific heated fluid passing through a saturation cell. The solute transported is subsequently trapped in a specific extraction column. Back flashing method is then applied to dissolve the compound. After validation, aqueous solubility of carbaryl was studied as function of temperature ranging from (273.15 to 318.15 K) at atmospheric pressure. In addition, solubility was determined in pure, seawater, and corresponding lab-made water, thus solubility values ranged from 3.57E-06 to 3.49E-05 in pure water, from 3.04E-06 to 2.53E-05 in seawater and from 6.51E-06 to 3.61E-05 in lab-made water. As a result, thermophysical properties of transfer and usage of lab-made water on the mass transfer properties divergence was spotted. The Salting out phenomenon observed for carbaryl was interpreted by the internal pressure theory that suggests the reduction of internal cavities in the presence of salt, making solubilization process more difficult to achieve. Thus, showing an overall endogenic process with a positive Gibbs free energy of transfer that is highly affected by the magnitude of salting out effect and the temperature. The molar entropy of transfer increases versus temperature caused by the disorder of solvent molecules due to the dissolution process.

Note on the Free Energy of Transfer of Fullerene C60 Simulated by Using Classical Potentials

Diverse atomistic parameters of C60 have been developed and utilized to simulate fullerene solutions in biological environments. However, no thermodynamic assessment and validation of these parameters have been so far realized. Here, we employ extensive molecular dynamics simulations with the thermodynamic integration method in the isothermal-isobaric ensemble to investigate the transfer of a single fullerene C60 between different solvent environments using different potential models. A detailed analysis is performed on the structure and standard Gibbs free energy of transfer of C60 from benzene to ethanol. All of the interactions concerned in the transfer process are included via atomistic models. We notice that having only structural and dynamical properties is not decisive to validate reliable atomic parameters capable of describing a more realistic thermodynamic process. Thus, we employ the calculated free energy of transfer to validate more accurate atomic parameters for the solvation thermodynamics of fullerenes by direct comparison with the solubility experimental data.

Thermodynamic determination of solvation potentials of divalent metal chlorides (MCl 2) in iso–dielectric media by EMF measurements

Cell potentials (EMF) of metal chlorides (CoCl 2 and CuCl 2) were measured at T = (303.15 and 313.15) K on the cell: M X Hg|MCl 2 ( m) in solvent S|AgCl–Ag, where S = water or iso–dielectric media with water of FD + DMF or FD + D. Stock solutions of metal chlorides were prepared by mass in water/iso–dielectric solvents. There was a change in the EMF values in these iso–dielectric media from its pure components. This change of EMF was associated with the change of ion–solvent interactions and was explained on the basis of kinetics of electrode processes. Equilibrium constants ( K 1 and K 2) as well as the degrees of dissociation ( α 1 and α 2) values for two step dissociation of metal chlorides in these solvents were obtained by iterative procedures. The standard electrode potential ( E°) values of the above cell were calculated in these solvents, and same were used to get the mean ionic activity coefficients ( γ ±) and standard thermodynamic functions (Δ G°, Δ H°, and Δ S°) of MCl 2. The E° values were also used to compute the standard molar Gibbs free energy of transfer ( Δ G tr ∘ ) and other transfer thermodynamic parameters ( Δ H tr ∘ and Δ S tr ∘ ) of MCl 2 from water to iso–dielectric media.

Perturbation of Second and Farther Hydration Shells of Alkali Cations and Bromide in Concentrated Aqueous Protein as a Water-Shortage Medium

The Gibbs free energies of transfer of selected ions from water to concentrated aqueous ovalbumin and albumin (DeltaW(W') G degrees j) have been determined by ion-transfer voltammetry. Negative values for the tetrabutylammonium ion suggest its direct binding to ovalbumin. In contrast, for alkali cations and bromide, the DeltaW(W') G degrees j values are positive and increase with increasing ovalbumin concentration. Positive values are confirmed for concentrated aqueous albumin and poly(styrenesulfonate) as well. The largest value (ca. 10 kJ mol(-1)) is found for the transfer of K(+) from water to 30 wt % ovalbumin. To reveal the solvation structure of these ions in ovalbumin solutions, X-ray absorption fine structure (XAFS) measurements have been performed at the K, Rb, and Br K-edges. Interestingly, the spectra obtained in 30 wt % ovalbumin solutions are identical to those for the corresponding hydrated ions. This strongly suggests that the first coordination shell structures of these ions are not affected by a large concentration of ovalbumin. The detected positive free energy of transfer is slightly lower than the hydrogen bonding energy of a water molecule and should thus come from the perturbation of the second and farther hydration shells of the ions under a water-shortage condition caused by a high concentration of ovalbumin.

Thermodynamic transfer study of various metal chlorides from water to formamide and dimethylformamide by EMF measurements

Cell potentials (EMF) were measured at two different temperatures (303.15 and 313.15 K) of three different metal chlorides (CoCl 2, CuCl 2 and ZnCl 2) on the cell M X Hg|MCl 2 ( m) in solvent S = W, FD and DMF|AgCl–Ag, where W = water, FD = formamide and DMF = dimethylformamide. For ZnCl 2, the investigations were carried out at 313.15 K only. Stock solutions of metal chlorides were prepared by weight in pure solvents (W, FD and DMF) and further varied the concentration by adding more of pure solvents to these stock solutions. There was a significant change in EMF values with a change of concentration of electrolyte and metal ion. This change of EMF was associated with the change of ion–solvent interactions and was explained on the basis of kinetics of electrode processes. The standard electrode potentials ( E°) values of the above cell were calculated in W, FD and DMF. Further the E° values were used to compute standard molar Gibbs free energy of transfer ( Δ G tr ° ) and other thermodynamic parameters of transfer ( Δ H tr ° , Δ S tr ° ) of MCl 2 from W to FD and DMF. Equilibrium constants ( K 1 and K 2) as well as the degrees of dissociation ( α 1 and α 2) for two step dissociation of metal chlorides in these solvents were obtained by iterative procedures. The data were analyzed in terms of the standard thermodynamic functions and standard transfer thermodynamic parameters depending on electrolytes and their concentration, nature of solvents and temperature to interpret the solute–solvent interactions.

Solubility of some phenolic compounds in aqueous alkali metal nitrate solutions from (293.15 to 318.15) K

This paper is continuation of the study concerning the solubility-temperature dependence data for some phenolic compounds (PhC), contained in olive mill wastewater (OMWW), in two nitrate salts (KNO 3 and NaNO 3) aqueous solutions. The solubilities of PhC were determined in the temperature ranging from (293.15 to 318.15) K. It has been observed that the solubility, in aqueous nitrate solutions, increases with increasing temperature. Results showed that alkali metal nitrate has a salting-out effect on the solubility of PhC. The effect of the anion of the electrolyte on the solubility of PhC is observed by comparing these results with values reported in the previous papers for the effect of LiCl, NaCl and KCl. For each cation, the solubilites of the phenolic compounds are higher with nitrate anion than with chloride anion. Results were interpreted in terms of the salt hydration shells and the ability of the solute to form hydrogen-bond with water. The solubility data were accurately correlated by a semi empirical equation. The standard molar Gibbs free energies of transfer of PhC (Δ tr G ∘) from pure water to aqueous solutions of the nitrate salts have been calculated from the solubility data. The decrease in solubility is correlated to the positive Δ tr G ∘ value which is mainly of enthalpic origin.

Gibbs Free Energy of Transfer of a Methylene Group in Buffer + Ionic Liquid Biphasic Systems

Partitioning experiments for a series of dinitrophenylated (DNP) amino-acids have been conducted in three (ionic liquid + sodium phosphate buffer) biphasic systems at 23 °C. The ionic liquids used were 1-hexyl-3-methylimidazolium tetrafluoroborate ([C6mim][BF4]), 1-octyl-3-methylimidazolium tetrafluoroborate ([C8mim][BF4]), and 1-decyl-3-methylimidazolium tetrafluoroborate ([C10mim][BF4]). The pH of the initial aqueous solution has been fixed at 7.4, using a sodium phosphate buffer. DNP−amino acids distribute preferentially to the ionic liquid phase, and this partition increases with the length of the alkyl chain of the amino acid. The Gibbs free energy of transfer for a methylene group is calculated from the partition coefficients of the solutes. This provides a measurement for the relative hydrophobicity of the conjugated phases, and the obtained values are compared to those of other biphasic systems used in biotechnology (e.g., polymer−polymer and polymer−salt aqueous two-phase systems).

Interactions of alkyl triphenyl phosphonium bromides with aqueous solutions of LM200 and JR400 polymers

The interactions of the cationic cellulose ether derivatives JR400 and the hydrophobically modified derivative LM200 with cationic alkyl triphenylphosphonium bromides, were investigated in aqueous media. Conductometric, tensiometric and fluorescence techniques were employed in this study. The presence of polymer induced surfactant aggregation and polymer bound aggregates, were detected for C16 and C14 triphenylphosphonium bromide surfactants with LM200. Gibbs free energy of transfer and the dielectric constant values sensed by the fluorescent probe at the micellar interface were evaluated and discussed in terms of strength of interaction between the polymers and surfactants.

Thermodynamic Studies of Drug−α-Cyclodextrin Interactions in Water at 298.15 K: Promazine Hydrochloride/Chlorpromazine Hydrochloride + α-Cyclodextrin + H2O Systems

Data on osmotic coefficients have been obtained for a binary aqueous solution of two drugs, namely, promazine hydrochloride (PZ) and chlorpromazine hydrochloride (CPZ) using a vapor pressure osmometer at 298.15 K. The observed critical micelle concentration (cmc) agrees excellently with the available literature data. The measurements are extended to aqueous ternary solutions containing fixed a concentration of alpha-cyclodextrin (alpha-CD) of 0.1 mol kg(-1) and varied concentrations (approximately 0.005-0.2 mol kg(-1)) of drugs at 298.15 K. It has been found that the cmc values increase by the addition of alpha-CD. The mean molal activity coefficients of the ions and the activity coefficient of alpha-CD in binary as well as ternary solutions were obtained, which have been further used to calculate the excess Gibbs free energies and transfer Gibbs free energies. The lowering of the activity coefficients of ions and of alpha-CD is attributed to the existence of host-guest (inclusion)-type complex equilibria. It is suggested that CPZ forms 2:1 and 1:1 complexed species with alpha-CD, while PZ forms only 1:1 complexed species. The salting constant (ks) values are determined at 298.15 K for promazine-alpha-CD and chlorpromazine-alpha-CD complexes, respectively, by following the method based on the application of the McMillan-Mayer theory of virial coefficients to transfer free energy data. It is noted that the presence of chlorine in the drug molecule imparts better complexing capacity, the effect of which gets attenuated as a result of hydrophobic interaction. The results are discussed from the point of view of associative equilibria before the cmc and complexed equilibria for binary and ternary solutions, respectively.

Determination of micelle/water partition coefficients and associated thermodynamic data for dialkyl phthalate esters

Micelle/water partition coefficients for three dialkyl phthalate esters — dimethyl phthalate ester (DMP), diethyl phthalate ester (DEP) and dipropyl phthalate ester (DPP) were obtained by micellar liquid chromatography (MLC). Experiments were conducted over a temperature range which led to calculation of a Gibbs free energy, enthalpy and entropy of transfer for the phthalate esters. In addition, small angle neutron scattering (SANS) experiments were conducted with no substantial change observed in micelle size before and after phthalate ester incorporation.