Standard Free Energies Of Transfer Research Articles

Micellization pattern of cationic surfactants in presence of azo dye in methanol mixed media

Conductivity measurements at three different temperatures (298.15, 308.15, and 318.15 K) were performed to determine the micellization nature of cationic surfactants, i.e., dodecyl trimethylammonium bromide (DTAB) and cetylpyridinium chloride (CPC), in the absence and presence of the azo dye methyl red (MR) in four mixed solvent systems (0.1 to 0.4) volume fraction of methanol in water. We calculated the critical micelle concentration (CMC) of DTAB and CPC and the degree of ionization (α) for both in the absence and presence of MR using William’s method. The interaction of surfactants DTAB and CPC with MR has been identified separately through the dye-surfactant aggregation process, with reduced CMC values due to the molecular reorientation of methyl red in the systems. Micellization becomes more likely with MR due to the formation of a molecular complex in mixed systems. We calculated different thermodynamic properties both in the absence and presence of methyl red. Such parameters are the standard Gibbs free energy of micellization (ΔGmo), the standard enthalpy of micellization (ΔHmo), the standard entropy of micellization (ΔSmo), the standard free energy of transfer (ΔGtranso) and heat capacity of micellization (ΔmCp0). In accordance with ΔGmo values, the mixtures show the spontaneous nature of micellization of DTAB and CPC with MR as well.

Micellization of sodium dodecyl sulphate in presence and absence of alkali metal halides at different temperatures in water and methanol-water mixtures

The micellization of sodium dodecyl sulphate (SDS) in presence and absence of KCl, KBr, NaBr and NaCl in water and methanol-water mixtures containing 8, 16, 25 and 34wt% of methanol at 298.15 to 323.15K have been assessed from conductometric measurements. The concentrations of SDS were varied from ~0.001 to ~0.05molL−1 and salts concentration was 0.01molL−1. The critical micelle concentration (CMC) of SDS in presence and absence of salts in water as well as in methanol-water mixtures are found to increase with the increase in temperature and found to decrease with the addition of salts. The CMC of SDS in water and methanol-water mixtures in presence of studied salts is found in the order NaCl>NaBr>KCl>KBr.Thermodynamic properties of micellization such as the standard free energy of micellization, standard enthalpy of micellization, the standard entropy of micellization, the standard free energy of transfer and heat capacity of micellization of SDS in presence and absence of salts have been examined. The micellization of SDS with presence and absence of salts has been assessed in terms of different solvent parameters. The solvophobic parameters have been presented.

Influence of methanol and dimethyl sulfoxide and temperature on the micellization of cetylpyridinium chloride

The micellar behavior of cetylpyridinium chloride (CPC) in water and polar non-aqueous solvents has been investigated in the temperature range of 298.15–323.15K using conductivity methods. The specific conductivity versus concentration plot determines the micellization parameters using critical micelle concentration (CMC) and degree of counter-ion dissociation (α) of the micelle. It was observed that the micellization tendency of the surfactant decreases in the presence of solvents.Thermodynamic properties such as the standard free energy of micellization (∆Gmo), the standard enthalpy of micellization (∆Hmo), the standard entropy of micellization (∆Smo), the standard free energy of transfer (∆Gtranso) and the heat capacity of micellization (∆mCp°) of CPC are significantly affected by the addition of methanol and dimethyl sulfoxide (DMSO). The solvophobic parameter characterized by the Gibbs energy of transfer of hydrocarbons from gas into a given solvent can be used to account for the effect of methanol and DMSO on the formation and growth of the CPC aggregate in water.

Surface tension and UV–visible investigations of aggregation and adsorption behavior of NaC and NaDC in water–amino acid mixtures

Aqueous micellar solutions of bile salts, sodium cholate (NaC) (1–30mmolkg−1) and sodium deoxycholate (NaDC) (1–17mmolkg−1) in the absence and presence of amino acids viz. glycine, leucine, methionine, and histidine (0.1molkg−1) have been examined by surface tension and UV–visible measurements at different temperatures (293.15–318.15K). From the surface tension study, critical micelle concentration (CMC) values of bile salts have been calculated and reviewed in terms of effect of amino acids on the hydrophobic character of bile salt–amino acid complex. Further, the adsorption behavior of NaC and NaDC in aqueous solution of amino acids has been analyzed with the help of interfacial parameters like maximum surface excess concentration (Γmax), minimum area per surfactant molecule (Amin), surface pressure at CMC (ΠCMC), efficiency in reducing surface tension (pC20), and adsorption at the air/water interface relation to micellization (CMC/C20). The thermodynamic parameters, standard free energy of micellization (ΔGm°), standard Gibb’s free energy of adsorption (ΔGad°), and standard free energy of transfer (ΔGtr°) have also been calculated. UV–visible studies, by using pyrene as a probe, have also been carried out in order to substantiate the CMC values derived from surface tension studies.

The effect of structural modifications on the solution and interfacial properties of straight and branched aliphatic alcohols: The role of hydrophobic effects

The effect of structural modifications, such as branching of the hydrocarbon chain on the solution and interfacial properties of short-chain aliphatic alcohols has been investigated. Surface tension measurements have been used to study the adsorption of the alcohols at the aqueous solution/air interface from water/alcohol mixtures, and to determine the aqueous solubilities of the alcohols. The related process of the partitioning behaviour of the alcohols between two immiscible phases has also been studied. Standard free energies of adsorption at the aqueous solution/air interface, standard free energies of transfer between water and hexane, and standard free energies of solution were obtained for the alcohols. A linear “Traube” relationship, an elegant demonstration of the hydrophobic effect, was found to exist between the various free energies for the normal alcohols and the number of carbon atoms in the alcohol molecule. The free energies showed that structurally modified alcohols have less negative free energy of adsorption, transfer or solution compared to the normal alcohol with the same number of carbon atoms. We assign effective numbers of carbon atoms for each branched alcohol for each transfer process. The position of the hydroxyl group relative to the branched part of the molecule was found to be a factor which influences the hydrophobic contribution to the free energy of each transfer process.An attempt has been made to ascertain whether there is a correlation between the molecular surface area, or the molecular volume, and the interfacial and solution thermodynamic properties of an alcohol in aqueous solution. The standard free energies of some of the branched alcohols have been found to be inconsistent with the values expected from the energetic of the adsorption and the water/hexane transfer processes. This is thought to reflect the different modes of association of the branched and normal alcohols in the liquid state as revealed by small and wide angle X-ray scattering which identifies correlation peaks attributable to intermolecular hydrogen bonding and interchain associations consistent with non-polar segmentation.

Effect of Head Groups, Temperature, and Polymer Concentration on Surfactant—Polymer Interactions

AbstractThe micellization behaviour of sodium dodecyl sulphate, sodium dodecylbenzenesulfonate, hexadecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and cetylpyridinium chloride in water and in aqueous solutions of polyethylene oxide (PEO, molecular weight = 100,000) having concentrations (0.005–0.04 %, w/v) has been studied at different temperatures (288.15–318.15 K) using conductivity, surface tension, and viscosity methods. From conductivity measurements various micellar parameters, like critical micellar concentration (CMC), critical aggregation concentration (CAC), polymer saturation point (PSP), degree of ionization (β), and standard free energy of transfer (), have been calculated. CAC values have been found to decrease with polymer concentration and increase with temperature. However, the PSP values increase with both polymer concentration and temperature for all surfactants. Similar parameters have also been calculated from surface tension data (CMCσ, CACσ, PSPσ) along with other parameters such as maximum surface excess concentration at the air/water interface (), minimum area per molecule (Amin), and packing parameter (p). The CMCσ, CACσ, and PSPσ values are smaller than the corresponding CMC, CAC, and PSP values, but both show similar behaviour with temperature and concentration of polymer. Various parameters indicate that the presence of the aromatic ring in the head group of surfactant decreases its interaction with PEO, whereas the increased hydrophobicity in the tail leads to stronger interactions with PEO. Viscosity studies further supplement the conclusions drawn from the above results.

SOLUBILIZATION OF P-ALKYLPHENOLS IN PLURONICS F-68 AND F-127 MICELLES: PARTITION COEFFICIENTS AND EFFECT OF SOLUTE ON THE AGGREGATE STRUCTURE

The partition of a series of substituted phenols between water and polymer micelles formed by Pluronic F-68 and F-127 has been studied. The formation of micelles by these block copolymers has been evidenced by steady-state fluorescence and dynamic light scattering. The data show that micelles and larger aggregates are formed above the critical micelle concentration. In presence of p-alkylphenols a micelle rearrangement occurs that leads to the formation of just one and large aggregate. This effect depends both on polymer and phenols structures. The partition coefficients and the standard free energies of transfer from the aqueous to the micellar phases were determined. The incremental free energy for a methylene group are 2.88 kJ mol-1 and 1.65 kJ mol-1 for F-127 and F-68, respectively. This difference is explained in terms of the core sizes that are determined by the length of the PPO block.

Thermodynamics and micellization of cetyltrimethyl ammonium bromide in the presence of lysozyme

To study the structural changes in lysozyme (cationic protein) on the addition of N-cetyl-N,N,N-trimethyl ammonium bromide (CTAB) and to understand the mechanism underlying aggregation of resulting protein–surfactant complex, surface tension, density and sound velocity measurements of lysozyme–CTAB solutions were carried out over a temperature range of 25–40°C. The critical micelle concentration (cmc) of CTAB has been calculated from surface tension measurements and is found to increase with increase in lysozyme concentration as well as with temperature. Surface tension data have been further used to calculate the interfacial parameters; maximum surface excess concentration (Γmax), minimum area per molecule (Amin), standard free energy of adsorption (ΔGoad), surface pressure at cmc (Πcmc) and standard free energy of transfer (ΔGotr), that have direct bearing on the consequences of such interactions at the molecular level. The negative values of standard Gibbs energy changes indicate spontaneity of micellization. Apparent molar volume (ϕv) and apparent molar adiabatic compressibility (ϕκ) have also been calculated using density and velocity of sound. These parameters have been examined as a function of surfactant concentration, lysozyme concentration and temperature to understand the consequences of interactions between these two components. A good qualitative correlation is found to exist with regard to the surfactant–lysozyme interaction obtained from the experimental data from different techniques.

Physico-chemical studies of oppositely charged protein–surfactant system in aqueous solutions: Sodium dodecyl sulphate (SDS)–lysozyme

Sound velocities (v) and densities (d) of sodium dodecyl sulphate (SDS) in various aqueous solutions of lysozyme have been measured over a temperature range of 25–40°C. The data was used to calculate apparent molar volume (ϕv) and apparent molar adiabatic compressibility (ϕk). These parameters have been examined as a function of surfactant concentration, lysozyme concentration and temperature to understand the consequences of interactions between these two components. Surface tension (γ) measurements were also carried out for the same systems using Man Singh Survismeter. The surface tension data have been used further to calculate the interfacial parameters like, maximum surface excess concentration (Γmax), minimum area per molecule (Amin), standard free energy of adsorption (ΔGad°), surface pressure at CMC (Πcmc) and standard free energy of transfer (ΔGtr°), that have direct bearing on the consequences of such interactions at the molecular level. The work has been extended to include UV-studies for different SDS–lysozyme solutions. A reasonably good qualitative correlation is found to exist with regard to the surfactant–lysozyme interaction obtained from the experimental data, from different techniques.

Cooperative Association of p-Alkylbenzene Sulfonates Sodium Salts to Poly-2-(dimethylamino) Ethylmethacrylate-N-alkyl Quaternized at the Water/Chloroform Interface

The association of p-alkyl benzene sulfonate sodium salts with several poly-2-(dimethylamino) ethylmethacrylate-N-alkyl quaternized bromides at the water/chloroform interface was studied. High association percentages were found which increase with both the size of the sulfonate molecule and the length of the polyelectrolyte side chain: octyl, decyl, dodecyl and tetradecyl. The results fit well to the Hill's equation for the association of anionic surfactant to polyelectrolytes. Benzene sulfonate shows an anticooperative behavior for the association, whereas 4-methyl benzene sulfonate and 4-ethylbenzene sulfonate are increasingly cooperative for the association. From the association constants, the standard free energies of transfer from water to the interfacial polyelectrolytes were determined. Their values were a linear function of the number of carbon atoms of the aliphatic residue of the benzene sulfonate molecule ranging from −15.7 kJ/mol of CH2 groups for the octyl polyelectrolyte to −18.9 kJ/mol of CH2 groups for the tetradecyl derivative. These values are similar to those reported for the association of some of these sulfonates molecules with ammonium type cationic micelles. The results for the incremental free energy of transfer by mol of methylene group from water to micelles are in the same order of magnitude as those reported for p-alkyl phenoxides and p-alkylbenzoates to hexadecyltrimethyl ammonium bromide micelles. The results shows that amphipathic counterions as small as 4-ethylbenzenesulfonate are enough to induce a cooperative effect in their association to cationic polyelectrolytes.

Potential drug delivery system: study of the association of a model nitroimidazole drug with aggregates of amphiphilic polymers on aqueous solution

This study evaluated the association of N-hexyl-2-methyl-4-nitroimidazol, a model drug, to aggregates formed by anionic polyelectrolytes on aqueous solution. The alternating copolymers of maleic anhydride and N-vinyl-2-pyrrolidone were synthesized and then modified by reaction of the anhydride groups with aliphatic amines and alcohols of varying length of the alkyl chain. The partition of the model drug between water and the hydrophobic microdomains provided by the copolymers was studied using the pseudo-phase model to determinate the distribution coefficient K S, and the standard free energy of transfer ∆µ°t. The results indicate that all copolymers assessed are potential pharmaceutical reservoirs of the model drug. Nevertheless, the solubility of N-hexyl-2-methyl-4-nitroimidazol on the polymeric solutions is independent from the length of the alkyl chain of the copolymer.

The influence of medium on the stability of coordination compounds 4. The system In(tiron)−-NaC1O4-C2H5OHH2O

The stability constants of complex anion In(tiron)− in mixed solvents of ethanol-water with weight percentages of ethanol being 0,5,15,25,35 and 50 have been measured spectrophotometrically at 293 K, using NaClO4 as supporting electrolyte with ionic strength ranging from 0.1 to 3.3mol·dm−3. The thermodynamic stability constants of In(tiron)− in various mixed solvents have been obtained by a polynomial approximation equation based on Pitzer's theory. As a comparison, the traditional Debye-Huckel equation has also been used to treat the same data. The results show that this polynomial approximate method is obviously superior to the old one. It was discovered that there is a linear relation between the thermodynamic stability constants of In (tiron)− and the mole fraction of organic component in mixed solvents or the reciprocal of dielectric constants of mixed solvents. The medium effect and the standard free energy of transfer, ΔG°tr, for the coordination reaction: In3+ + tiron4- = In(tiron)− have also been discussed.

Study of Micellization Behavior of Some Alkyldimethylbenzyl Ammonium Chloride Surfactants in the Presence of Polymers

The micellization behavior of homologous series of cationic surfactants tetradecyl- (C14BCl), hexadecyl- (C16BCl) and stearyldimethyl-benzyl ammonium chloride (C18BCl) in water and in aqueous solutions of polymers such as polyvinyl pyrrolidone (PVP) and polyethylene glycols (PEG-20 and PEG-06) has been studied at 298.15 K using the conductometric method. The same method has also been used to study the mixed systems, that is, C16-C14BCl, C18-C16BCl, and C18-C14BCl over the whole mole fraction range of the higher chain length surfactant at 298.15, 308.15, and 318.15 K in water and in presence of 0.5 and 1.0% PVP by volume at 298.15 K. From these measurements various micellar parameters like critical aggregation concentration (CAC), polymer saturation point (PSP), degree of ionization (ψ), and standard free energy of transfer () have been calculated. The CAC and values were found to decrease with increase in polymer concentration whereas PSP values were found to remain constant except in the case of C14BCl where PSP values were found to increase with polymer concentration. The regular solution approach has been applied to determine the mixed micelle mole fraction (X) and the interaction parameter (β). These parameters have also been discussed in terms of micelle-polymer interactions. The combination C16-C14BCl shows synergism at all the studied temperatures in aqueous solution whereas remaining combinations show antagonism (except for C18-C16BCl at 318.15 K). In presence of PVP, the binary mixtures show antagonistic behavior which increases with increase in PVP concentration.

Funciones termodinámicas relativas a la transferencia del ketoprófen desde el agua hasta algunos sistemas orgánicos

The thermodynamics of molal partitioning of ketoprofen (KTP) was studied in cyclohexane/buffer (CH/W), octanol/buffer (ROH/W), and dimyristoyl phosphatidylcholine (DMPC), dipalmitoyl phosphatidylcholine (DPPC), and egg lecithin (EGG/W) liposome systems. In all cases the partition coefficients (Kmo/w)were greater than unity; therefore the standard free energies of transfer were negative indicating affinity of KTP for organic media. The Kmo/w values were approximately seventy-fold higher in the ROH/W system compared with the CH/W system. On the other hand, the Kmo/w values were approximately ten or fifty-fold higher in the liposomes compared with the ROH/W system. In all cases, the standard enthalpies and entropies of transfer of KTP were positive indicating some degree of participation of the hydrophobic hydration on partitioning processes.

Solubility of proline–leucine dipeptide in water and in aqueous sodium chloride solutions from T = (288.15 to 313.15) K

Solubility of proline–leucine dipeptide, in water and in aqueous sodium chloride solutions, was measured at T = (288.15, 298.15, 308.15 and 313.15) K as a function of electrolyte concentration m = (0.1, 0.25, 0.5, 0.75 and 1) mol · kg −1 of water. Solubility data has been evaluated from density measurements using a vibrating tube densimeter. It has been observed that sodium chloride renders the dipeptide proline–leucine more soluble in water. Salting-in coefficients and standard free energies of transfer of proline–leucine, from water to aqueous sodium chloride solutions, have been calculated from the solubility data. Standard enthalpies and entropies of transfer have also been estimated and interpreted in terms of electrostatic and hydrophobic perturbed domains in the hydration shells of the dipeptide and of the cation and anion of the salt, as a function of temperature and salt concentration.

Estimation of the Gibbs free energy of transfer of electrolytes from aqueous to organic solvents – a novel application of the quadratic activation–driving force relationship

The Gibbs free energy of transfer of electrolytes from a reference solvent to the target solvent has been presented employing the Marcus–Saveant quadratic activation–diving force relationship. The electroreduction of carbon tetrachloride has been used as the probe reaction in 12 organic solvents, in which the parabolic variation of logarithmic heterogeneous electron transfer rate constant with potential has been observed in eight solvents. Using the standard electrode potentials for the reduction of carbon tetrachloride, the standard free energies of transfer of silver chloride from water to eight solvents have been estimated.

Effect of phenols on the potentiometric response of a nitrate-ion-selective electrode

The effect of phenols containing different electron-withdrawing substituents on the potentiometric responses of several liquid PVC membranes containing the complex trioctylmethylammonium–nitrate, TOMA +NO 3 −, is analyzed. The results make it possible to separate these phenols into two groups; those phenols containing electron-releasing groups, which produce almost Nernstian slopes, and those containing electron-withdrawing substituents, which generate sub-Nernstian slopes. The highly negative standard free energy of transfer of the aryl phenolic group from water to a cationic polar head suggests that these phenols are mainly located in the membrane phase associated with TOMA + via a cation– π interaction. It seems that the strength of this interaction, and hence of the nitrate dissociation, is affected by the presence of phenols in an extension which correlates well with the kind of phenol present in these membranes and, consequently, with the type of their potentiometric responses.

Thermodynamics of transfer of naphthalene and 2-naphthoic acid from water to (water + ethanol) mixtures at T=298.15 K

Standard thermodynamic functions of transfer of naphthalene and 2-naphthoic acid from water to (water + ethanol) mixtures at T=298.15 K have been determined from solubility measurements at different temperatures. Standard free energies of transfer of both naphthalene and 2-naphthoic acid showed decreasing tendency with the increasing x(EtOH), and the standard entropy and enthalpy of transfer exhibited a change of double peaks with x(EtOH). The Δ tr G 0 of 2-naphthoic acid decreased more rapidly than that of naphthalene when x(EtOH) < 0.746 and lower than that of naphthalene when x(EtOH) >0.746 at T=298.15 K. The double peaks in the curves of standard entropy and enthalpy of transfer illustrated that the microstructure of the series of mixed solvents of (water + ethanol) underwent a variable process from ordered to disordered and then from disordered to ordered. The results mean that there is a relatively ordered structure near x(EtOH)=0.13 in the (water + ethanol) solutions besides the existence of a clathrate structure in the water-rich region.

Medium effect (transfer activity coefficient) of methanol and acetonitrile on beta-cyclodextrin/benzoate complexation in capillary zone electrophoresis.

Association constants, Kc, were derived from the electrophoretic mobilities of the anionic solutes (seven benzoates with hydroxy or chloro substituents) by capillary zone electrophoresis in different solvent systems, consisting of binary mixtures of water with up to 20% (v/v) methanol or acetonitrile, respectively. The association constants expectedly are found to decrease with increasing organic solvent concentration. The effect of organic solvents on the Kc of the benzoates with beta-cyclodextrin was analyzed applying the concept of the transfer activity coefficient (or the medium effect). This concept enables the evaluation of the significance of the contributions of the individual species involved in the complexation equilibrium in the different solvents: the benzoate ion, beta-cyclodextrin, and the anionic benzoate-beta-cyclodextrin complex. The medium effect on benzoate was calculated from the change in acidity constant of benzoic acid in the different mixed solvents and the corresponding transfer activity coefficients of the proton and the molecular acid. The transfer activity coefficients for beta-cyclodextrin results from its solubility at saturation in the different solvents. In this way, an estimation of the standard free energy of transfer, deltaG(t)0, of each species involved in the complexation equilibrium was possible for the transfer from water into the respective mixed solvent. It was found that the organic solvents do not significantly affect deltaG(t)0 for the benzoate anion. However, the organic solvents play a different role concerning the stabilization of beta-cyclodextrin and the complex anion: whereas the addition of acetonitrile has nearly no influence on deltaG(t)0 of the anionic complex, the reduction in Kc is caused by the enhanced stabilization of beta-cyclodextrin (reflected by its better solubility). Addition of methanol, on the other hand, lowers the solubility of beta-cyclodextrin, thus giving positive values for deltaG(t)0. Thus, the overall effect on Kc in methanolic solutions must be related to the pronounced destabilization of the benzoate-beta-cyclodextrin complex.

Chemical Meaning of the Standard Free Energy of Transfer. 2. Use of van der Waals' Equation of State To Evaluate the Enthalpic and Entropic Contributions of Free Volume and Attractive Forces to Chemical Potentials

In a previous paper we derived an equation for the chemical potential of a nonideal, infinitely dilute solute in a van der Waals fluid. From this we also explored the change in the Gibbs energy of transfer of a solute between two van der Waals fluids and explored the effects of using different standard states on the interpretation of that Gibbs energy. In this paper, we extend that treatment by dissecting the equation for the chemical potential of an infinitely dilute solute in a van der Waals fluid to reveal the enthalpic and entropic origins of each term. We also provide a physical interpretation of each term based on its enthalpic and entropic contributions to the overall chemical potential. We conclude the paper with an exploration of the phenomenon of enthalpy/entropy compensation, using the van der Waals equation of state to derive a condition under which enthalpy/entropy compensation must be observed.