Enthalpies Of Transfer Research Articles

Solvation in Mixed Aqueous Solvents from a Thermodynamic Cycle Approach

A novel approach is presented for interpreting and potentially predicting values of the isothermal, isobaric transfer free energy, entropy, and enthalpy (Deltamicrotr2, Deltastr2, and Deltahtr2) for a solute between water and water-cosolvent mixtures. The approach explicitly accounts for volumetric properties of the solvent and solute (the equation of state, EoS) and casts the overall transfer process as a thermodynamic cycle with two stages: (1) isothermal solvent exchange from pure water to the cosolvent composition of interest at fixed mass density; (2) isothermal expansion or compression at the final solvent composition to recover the pressure of the initial state. Using molecular simulations with methane as the solute, the analysis is illustrated over a wide range of cosolvent concentrations for sorbitol-, ethanol-, and methanol-water binary mixtures. The EoS contribution semiquantitatively or quantitatively captures Deltamicrotr2, Deltastr2, and Deltahtr2 in almost all cases tested, highlighting the importance of considering the effects of changes in solvent density on the overall transfer process. The results also indicate that apolar solvation at these length scales is dominated by the work of cavity formation across a range of cosolvent species and concentrations.

Calorimetric study of the α-tocopherol solubility in reversed AOT micelles

The experimental data of heat of mixing ( Q) for heterogeneous system α-tocopherol/AOT/ n-heptane with and without water at 25 °C are presented. The Q dependence on AOT (sodium bis (2-ethylhexyl) sulfosuccinate) concentration, and R parameter defined as R=[H 2O]/[AOT] with flow calorimetric method were investigated. Using the D'Aprano model (which is formally identical to that used earlier by Magid et al.) the binding constant ( K), the distribution constant of α-tocopherol ( K distr ) between hydrocarbon and the micellar phase, and the standard enthalpy of transfer ( Δ H tr 0 ) of α-tocopherol from the hydrocarbon to AOT reversed micelles were calculated. The solubility of α-tocopherol in AOT reversed micelles explored with the calorimetric technique was compared to the literature data obtained respectively with UV spectrophotometry for reversed micelles and by other techniques for the phospholipid bilayer.

Physicochemical properties of an insensitive munitions compound, N-methyl-4-nitroaniline (MNA)

Accurate information on physicochemical properties of an organic contaminant is essential for predicting its environmental impact and fate. These properties also provide invaluable information for the overall understanding of environmental distribution, biotransformation, and potential treatment processes. In this study the aqueous solubility ( S w), octanol–water partition coefficient ( K ow), and Henry's law constant ( K H) were determined for an insensitive munitions (IM) compound, N-methyl-4-nitroaniline (MNA), at 298.15, 308.15, and 318.15 K. Effect of ionic strength on solubility, using electrolytes such as NaCl and CaCl 2, was also studied. The data on the physicochemical parameters were correlated using the standard Van’t Hoff equation. All three properties exhibited a linear relationship with reciprocal temperature. The enthalpy and entropy of phase transfer were derived from the experimental data.

Enthalpies of transfer of amino acids from water to aqueous solutions of N-methylformamide and N, N-dimethylformamide at T = 298.15 K

Enthalpies of solution of glycine, l-alanine and l-serine in aqueous solutions of N-methylformamide and N, N-dimethylformamide were measured at 298.15 K. Enthalpies of transfer (Δ tr H) of amino acids from water to aqueous solutions of amides were derived and interpreted in terms of structural interaction. The results were compared with those in aqueous formamide solutions.

The influence of isomerism on the enthalpies of solution of dipyrrolylmethanes

The enthalpies of solution of 3,3′,4,4′-tetramethyl-5,5′-dicarbethoxydipyrrolylmethane-2,2′ and its α,β-and β,β isomers in organic solvents of various natures were measured calorimetrically at 298.15 K. The enthalpies of transfer of the compounds from benzene into chloroform, pyridine, dimethylsulfoxide, and dimethylformamide were calculated. Pyridine, dimethylsulfoxide, and formamide (proton acceptor solvents) specifically solvated the NH groups of dipyrrolylmethanes. Solvation interactions could be sterically complicated depending on the type of the isomer.

Micellization of Dissymmetric Cationic Gemini Surfactants and Their Interaction with Dimyristoylphosphatidylcholine Vesicles

The micellization process of a series of dissymmetric cationic gemini surfactants [CmH2m+1(CH3)2N(CH2)6N(CH3)2C6H13]Br2 (designated as m-6-6 with m = 12, 14, and 16) and their interaction with dimyristoylphosphatidylcholine (DMPC) vesicles have been investigated. In the micellization process of these gemini surfactants themselves, critical micelle concentration (cmc), micelle ionization degree, and enthalpies of micellization (DeltaHmic) were determined, from which Gibbs free energies of micellization (DeltaGmic) and entropy of micellization (DeltaSmic) were derived. These properties were found to be influenced significantly by the dissymmetry in the surfactant structures. The phase diagrams for the solubilization of DMPC vesicles by the gemini surfactants were constructed from calorimetric results combining with the results of turbidity and dynamic light scattering. The effective surfactant to lipid ratios in the mixed aggregates at saturation (Resat) and solubilization (Resol) were derived. For the solubilization of DMPC vesicles, symmetric 12-6-12 is more effective than corresponding single-chain surfactant DTAB, whereas the dissymmetric m-6-6 series are more effective than symmetric 12-6-12, and 16-6-6 is the most effective. The chain length mismatch between DMPC and the gemini surfactants may be responsible for the different Re values. The transfer enthalpy per mole of surfactant within the coexistence range may be associated with the total hydrophobicity of the alkyl chains of gemini surfactants. The transfer enthalpies of surfactant from micelles to bilayers are always endothermic due to the dehydration of headgroups and the disordering of lipid acyl chain packing during the vesicle solubilization.

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.

Thermodynamic Study of Partitioning and Solvation of (+)-Naproxen in Some Organic Solvent/Buffer and Liposome Systems

Naproxen (NAP) partitioning thermodynamics was studied in different solvent/buffer systems such as cyclohexane (CH/W), octanol (ROH/W), isopropyl myristate (IPM/W), and chloroform (CLF/W), as well as in dimyristoyl phosphatidylcholine (DMPC) and dipalmitoyl phosphatidylcholine (DPPC) liposome systems. In all cases, the mole fraction partition coefficients (K X o/w ) were greater than unity; therefore, the standard Gibbs energies of transfer were negative indicating a high affinity of NAP for all the organic media. K X o/w values were approximately 1000-fold higher in the ROH/W system with respect to the CH/W system, thus indicating a high degree of hydrogen-bonding contribution to partitioning, whereas in the case of the IPM/W and CLF/W systems, the K X o/w values were approximately only 2-fold or 5-fold lower than those observed in ROH/W. On the other hand, K X o/w values were approximately 55-fold or 39-fold higher in the liposomes compared to the ROH/W system, for DMPC and DPPC, respectively, thus indicating a high degree of bilayer immobilization and/or an electrostatic contribution to partitioning. In almost all cases, standard enthalpies of transfer of NAP from water to organic solvents were negative (except for CH), whereas the standard entropies of transfer were positive (except for IPM). For liposomes, the standard enthalpies and entropies of transfer were positive. These results indicate some degree of participation of the hydrophobic hydration on the NAP partitioning processes. By using the reported data for solvation of NAP in water, the associated thermodynamic functions for NAP solvation in all tested organic phases were also calculated. Finally, all the results obtained for NAP were compared with those presented previously for ketoprofen.

Prediction of gas to water partition coefficients from 273 to 373 K using predicted enthalpies and heat capacities of hydration

In previous works, we have developed methods for the prediction of gas to water partition coefficients at 298 K, log K w (298), and for the prediction of enthalpies of transfer from the gas phase to water, that is enthalpies of hydration, Δ H w ° , using the set of Abraham descriptors. In the present work we collect data on heat capacities of hydration, Δ C p w , for 321 varied compounds and obtain equations that can be used to predict this quantity, again in terms of Abraham descriptors. If the three thermodynamic input quantities, log K w (298), Δ H w ° and Δ C p w , are all available it is trivial to predict log K w at any temperature between 273 and 373 K through equations given by Plyasunov and Shock. However, if one or more of these input quantities are unavailable, we show that predictions of these input quantities using our equations based on Abraham descriptors can be used to predict log K w at any temperature between 273 and 373 K, log K w ( T). In the most rigorous test of this method, log K w (373) values were calculated using predicted values of all three input quantities, log K w (298), Δ H w ° and Δ C p w . For 40 varied compounds covering a range of 5.5 log units in log K w (373), values of log K w (373) were predicted with an average error of −0.02, and a standard deviation of 0.16 log unit. Many other examples of predictions are given. A spreadsheet is available for the prediction of log K w ( T) values; all that is required is insertion of Abraham descriptors for a compound and the temperature. We consider polyaromatic hydrocarbons and polychlorobiphenyls separately, and show that it is possible to predict log K w ( T) for these compounds as well.

The thermochemical characteristics of solution of DL-α-alanylglycine and DL-α-alanyl-DL-α-alanine in water-organic solvent mixtures at 298.15 K

The integral enthalpies of solution of DL-α-alanylglycine and DL-α-alanyl-DL-α-alanine in water-organic solvent (acetonitrile, 1,4-dioxane, acetone, N,N-dimethylformamide, and N,N-dimethylsulfoxide) mixtures were measured at organic component concentrations x 2 = 0–0.4 mole fractions and T = 298.15 K. The standard enthalpies of solution (Δsol H°) and transfer (Δtr H°) of the peptides from water into mixed solvents were calculated. The influence of the structure and properties of solutes and mixture composition on the thermochemical characteristics of the peptides was considered. The enthalpy coefficients of pair interactions (h xy ) of DL-α-alanylglycine and DL-α-alanyl-DL-α-alanine with organic solvent molecules were calculated. The h xy values were correlated with the properties of organic solvents using the Kamlet-Taft equation.

Enthalpies of solution of tetramethyl-bis-urea ( Mebicarum) in amides and acetone at 298.15 K

The enthalpies of solution of 2,4,6,8-tetramethyl-2,4,6,8-tetraazabicyclo[3.3.0]octane-3,7-dione or tetramethyl-bis-urea (the drug Mebicarum) in formamide, N-methylformamide, N, N-dimethylformamide, N, N-dimethylacetamide, N, N, N′, N′-tetramethylurea, and acetone were measured at 298.15 K. Standard enthalpies of solution and transfer from one solvent to another were computed. The enthalpies of solution of the solute were found to be endothermic and weak depending on the nature of methylation in an amide molecule. It was concluded that the solvent proton-donor ability and existing steric hindrances for H-bonding and other interparticle interactions play the key role in solvation of tetramethyl-bis-urea.

Enthalpies of Transfer of Tetraalkylammonium Bromides and CsBr from water to Aqueous DMF at 298.15 K

Enthalpies of transfer of tetraalkylammonium bromides and CsBr from water to aqueous DMF mixtures are reported and analyzed in terms of a new solvation theory. It was found that a previous equation could not reproduce these data over the whole range of solvent compositions. Using a new solvation theory to model the enthalpies of transfer shows excellent agreement between experimental and calculated values over the entire range of solvent compositions. The analyses show that tetrapropylammonium bromide, Pr4NBr, and tetrapentylammonium bromide, Pen4NBr, are preferentially solvated by water; in contrast tetrabutylammonium bromide, Bu4NBr, is preferentially solvated by DMF. The solvation of tetramethylammonium bromide, Me4NBr, and cesium bromide, CsBr, is random. The extent to which the tetraalkylammonium bromides disrupt solvent–solvent bonds increases systematically in going from Me4NBr to Pen4NBr.

Interactions between Gemini Surfactants and Polymers: Thermodynamic Studies

Aqueous mixtures containing a homopolymer, poly(vinylpyrrolidone) (PVP), or a hydrophobically modified graft copolymer, HM-pullulan, (PULAU9, where 9 stands for the nominal substitution degree), and different Gemini surfactants have been investigated at 25.0 degrees C. A wide variety of experimental conditions were addressed by changing the amount of polymer and of surfactant. The Gemini surfactants were synthesized, purified, and characterized by routine methods. They differ from each other in polar head groups (two sulfonate-, two quaternary ammonium-, or two arginine-based groups), in alkyl chain length (11 or 12 carbon atoms), and in the distance between the polar head groups. The spacers consist of 2, 3, and 6 methylene units or 3 oxyethylene units. Surface activity and solution calorimetry measurements yield some physicochemical features inherent to micelle formation and polymer-surfactant interactions. The data are supported by ionic conductivity, detecting the critical thresholds and quantifying the modifications in binding associated with critical association (CAC) and micelle formation (CMC*). The Gibbs energy of transfer from the micelles to a polymer-binding site, DeltaGtrans, was evaluated from the CAC/CMC* ratios versus the amount of added polymer. A similar procedure determined the enthalpy of transfer, DeltaHtrans. DeltaGtrans decreases with added polymer, whereas DeltaHtrans becomes more negative on increasing the amount of polymer in the medium. According to the selected data presented here, cationic Geminis do not interact with PVP, while significant interactions have been observed in other surfactants. In mixtures with PULAU9, the interaction is significant for all Geminis. This effect is due to interactions between the surfactants and the hydrophobic alkyl groups on the main polymer chain. The pendent groups facing away from the polysaccharide chain act as binding sites for aggregates onto such polymers.

Enthalpy of Interaction and Binding Isotherms of Non-ionic Surfactants onto Micellar Amphiphilic Polymers (Amphipols)

The interactions in water between short amphiphilic macromomolecules, known as amphipols, and three neutral surfactants (detergents), dodecylmaltoside (DM), n-octylthioglucoside (OTG), and n-octyltetraethyleneoxide (C8E4), have been assessed by static and dynamic light-scattering (SLS and DLS), capillary electrophoresis (CE), and isothermal titration calorimetry (ITC). The amphipols selected are random copolymers of the hydrophobic n-octylacrylamide (25-30 mol %), a charged hydrophilic monomer, either acrylic acid ( approximately 35 mol %) or a phosphorylcholine-modified acrylamide (40-70 mol %), and, optionally, N-isopropylacrylamide (30-40 mol %). In water, the copolymers form micelles of small size (hydrodynamic radius: approximately 5 nm). Neutral surfactants, below their critical micellar concentration (cmc), form mixed micelles with the amphipols irrespective of the chemical structure of the detergent or the polymer. The fraction of detergent in the surfactant/polymer complexes increases significantly (cooperatively) as the surfactant concentration nears the cmc. The ITC data, together with data gathered by CE, were fitted via a regular mixing model, which allowed us to predict the detergent concentration in equilibrium with complexes and the heat evolved upon transfer of detergent from water into a mixed surfactant/polymer complex. The enthalpy of transfer was found to be almost equal to the enthalpy of micellization, and the regular mixing model points to a near-ideal mixing behavior for all systems. Amphipols are promising tools in biochemistry where they are used, together with neutral surfactants, for the stabilization and handling of proteins. This study provides guidelines for the optimization of current protein purification protocols and for the formulations of surfactant/polymer systems used in pharmaceutics, cosmetics, and foodstuffs.

The use of isothermal titration calorimetry to assess the solubility enhancement of simvastatin by a range of surfactants

Surfactants are commonly used to increase the solubility of poorly water soluble drugs but the interactions between drug and surfactant can be complex and quantitative relationships can be hard to derive. One approach is to quantify the thermodynamics of interaction and relate these parameters to known solubility or dissolution rate enhancement data. Isothermal titration calorimetry (ITC) was used to measure the enthalpy and free energy of transfer of a model drug (simvastatin) to a number of surfactant (SDS, HTAB, SDCH and Brij 35) micelles. These data were then compared with the solubility enhancements determined for each surfactant using HPLC assays. As expected, there was correlation between the free energy of transfer for the drug to each surfactant and the solubility enhancement of that surfactant. Although the data set is limited, the results suggest that ITC screening of a range of surfactants against a poorly water soluble drug may allow the selection of the best potential solubilising surfactants.

Physicochemical Characterization and Dissolution Study of Solid Dispersions of Lovastatin with Polyethylene Glycol 4000 and Polyvinylpyrrolidone K30

Solid dispersions in water-soluble carriers have attracted considerable interest as a means of improving the dissolution rate, and hence possibly bioavailability, of a range of hydrophobic drugs. The aim of the present study was to improve the solubility and dissolution rate of a poorly water-soluble drug, Lovastatin, by a solid dispersion technique. Solid dispersions were prepared by using polyethylene glycol 4000 (PEG 4000) and polyvinylpyrrolidone K30 (PVP K30) in different drug-to‐carrier ratios. Dispersions with PEG 4000 were prepared by fusion-cooling and solvent evaporation, whereas dispersions containing PVP K30 were prepared by solvent evaporation technique. These new formulations were characterized in the liquid state by phase solubility studies and in the solid state by differential scanning calorimetry, X-ray powder diffraction, and FT-IR spectroscopy. The aqueous solubility of Lovastatin was favored by the presence of both polymers. The negative values of the Gibbs free energy and enthalpy of transfer explained the spontaneous transfer from pure water to the aqueous polymer environment. Solid-state characterization indicated Lovastatin was present as amorphous material and entrapped in polymer matrix. In contrast to the very slow dissolution rate of pure Lovastatin, the dispersion of the drug in the polymers considerably enhanced the dissolution rate. This can be attributed to improved wettability and dispersibility, as well as decrease of the crystalline and increase of the amorphous fraction of the drug. Solid dispersion prepared with PVP showed the highest improvement in wettability and dissolution rate of Lovastatin. Even physical mixture of Lovastatin prepared with both polymers also showed better dissolution profile than that of pure Lovastatin. Tablets containing solid dispersion prepared with PEG and PVP showed significant improvement in the release profile of Lovastatin compared with tablets containing Lovastatin without PEG or PVP.

The relationship between vapour pressure, vaporization enthalpy, and enthalpy of transfer from solution to gas: An extension of the Martin equation

Martin’s equation, Δ sln g G = Δ sln g G o + z δ sln g G , is extended to cover vaporization free energy ( Δ l g G ) . The extended equation is further expanded in terms of enthalpy and entropy and then used to correlate vaporization enthalpy ( Δ l g H ) and enthalpy of transfer from solution to gas ( Δ sln g H ) . Data available in the literatures are used to validate and support the speculations derived from the proposed equation.

Determination of the Enthalpies of Vaporization of Cyclic Organic Peroxides by Correlation of Changes in Gas Chromatographic Retention Times

Liquid and solid cyclic peroxides derived from aliphatic ketones are explosive materials so their enthalpies of vaporization and other thermodynamic or condensed-phase properties cannot be measured directly. In this work the enthalpies of vaporization of peroxides at 298.15 K were estimated simply from gas chromatographic retention times measured at different temperatures. The technique correlates changes in the retention times of compounds whose enthalpies of vaporization are known (called the reference series), with those of the compounds of interest. If tR′ is the adjusted retention time (retention time of each compound minus the retention time of unretained diethyl ether, used as solvent) a plot of ln tR′ against 1/T for each compound (reference compounds and cyclic peroxides) results in a straight line (r2 > 0.99 for all compounds). The enthalpy of transfer from solution to the vapor state (ΔsolgHm) can be obtained by multiplying the slope by the gas constant (R). A second plot correlates the enthalpies of transfer from solution to the vapor state (ΔsolgHm), as measured by gas–liquid chromatography (GLC), with enthalpies of vaporization of reference materials (ΔvapHm at 298.15 K) available in the literature. C9–C15 fatty acid methyl esters and hydrocarbons were used as reference compounds. The enthalpies of vaporization of the cyclic organic peroxides were calculated from the equation of the line obtained in this second correlation, the slope of which was ΔvapHm (at 298.15 K)/ΔgsolHm. The experiments were performed under isothermal conditions with a DB-5 capillary column, flame-ionization detection (FID), and nitrogen as carrier gas. The column temperature was varied over a range of at least 30–70 K between 403 and 473 K, with chromatograms being acquired at 10 K intervals. Enthalpies of vaporization of cyclic organic peroxides are not available in the literature, and the values given in this paper, obtained by gas chromatography, are the first to be reported.

Calorimetrically Measurable Enthalpic Isotope Effect

Calorimetric techniques have revealed that the enthalpy of reaction with water is more exothermic by about 2.2 kcal/mol, for the perdeuteriated naphthalene anion radical (K+C10D8*-(s) + H2O(liq) --> 1/2C10D8H2(s) + 1/2C10D8(s) + KOH(aq)) than it is for the perprotiated system. These results, when coupled with the known enthalpy of electron transfer between naphthalene and its perdeuteriated analogue imply that the heat of hydrogenation of naphthalene decreases by about 1.8 kcal/mol upon perdeuteriation of the naphthalene.

Thermodynamics of partitioning and solvation of ketoprofen in some organic solvent: buffer and liposome systems

The ketoprofen (KTP) partitioning thermodynamics was studied in different solvent/buffer systems such as cyclohexane (CH/W), octanol (ROH/W), isopropyl myristate (IPM/W), chloroform (CLF/W); as well as in dimyristoyl phosphatidylcholine (DMPC) and dipalmitoyl phosphatidylcholine (DPPC) liposome systems. In all cases, the rational partition coefficients (K X O/w) were greater than unit; therefore standard transfer free energies were negative in sign indicating a high affinity of KTP for organic media. K X O/w values were approximately eightyfive-fold higher in the ROH/W system regarding the CH/W system, thus indicating a high degree of hydrogen bonding contribution to partitioning. While in the case of IPM/W and CLF/W systems, K X O/w values were approximately only three or four-fold lower than those observed in ROH/W. On the other hand, K X O/w values were approximately seventy-five or one hundred fifty-fold higher in the liposomes compared to ROH/W system indicating a high degree of bilayers immobilization and/or an electrostatic contribution to partitioning. In all cases, standard transfer enthalpies and entropies of KTP from water to organic media were positive in sign indicating some degree of participation of the hydrophobic hydration on the partitioning processes. Finally, by using the reported data for solvation of KTP in water, the associated thermodynamic functions for KTP solvation in all tested organic phases were also calculated.