Excess enthalpies of solution of primary and secondary alcohols in dodecyldimethylamine oxide micellar solutions

The excess enthalpies of solution with respect to water of some primary and secondary alcohols in dodecyldimethylamine oxide (DDAO) micellar solutions were measured by mixing aqueous solutions of alcohols with surfactant solutions. Standard free energies, enthalpies and entropies were obtained from the distribution of alcohols between aqueous and micellar phases. It is shown that thermodynamics of transfer of secondary alcohols from aqueous to the DDAO micellar phase differ slightly from those of their corresponding primary alcohols, that the additivity rule holds for free energies of transfer and that enthalpy and entropy display convex curves. The present data are compared with those from the aqueous to the dodecyltrimethylammonium bromide (DTAB) micellar phases and to the literature data for transfer from water to octane. The role of the hydrophilic interactions between OH group and the micellar head groups and of the hydrophobic interactions between the methylene group and its apolar environment is evidenced.     

Binding constants and partial molar volumes of primary alcohols in sodium dodecylsulfate micelles

The densities of methanol, ethanol, 1-propanol, 1-butanol and 1-hexanol were measured in aqueous solutions of sodium dodecylsulfate at 25°C. The partial molar volumes of the alcohols at infinite dilution in the aqueous surfactants solutions were calculated and discussed using a mass-action model for the alcohol distribution between the aqueous and the micellar phase. The partial molar volumes of the alcohols in the aqueous and in the micellar phases, and the ratios between the binding constant and the aggregation number, were calculated. The partial molar volume for all the alcohols in micellar phase is 10 cm³-mol^–1 smaller than that in octane. This can be related to the strong hydrophilic interaction between the head groups of the alcohol and the micellized surfactant. From the extrapolated values of the distribution constant and the partial molar volumes in the aqueous and micellar phases, the standard partial molar volume of heptanol in micellar solutions was found to decrease with increasing surfactant concentration. The standard free energy of transfer of alcohols from water to micelles was rationalized in terms of hydrophilic and hydrophobic contributions. A model is proposed in which the empty space around each solute is assumed to be the same in the gas and liquid phases, and is used to explain the behavior of micelles in the presence of amphiphilic solutes.     

Thermodynamics of transfer of polar additives from the aqueous to the dodecylsurfactant micellar phases

The enthalpies of transfer from water to aqueous surfactant solutions, H(WW+S), of polar additives have been determined as a function of the surfactant concentration at fixed additive concentration. The surfactants used are sodium dodecylsulfate (NaDS), dodecyltrimethylammonium bromide and dodecyldimethylamine oxide (DDAO). The additives used are iso-butanol t-butanol, butoxyethanol, phenol, benzene, tributylphosphine oxide (TBPO), octyldimethylphosphine oxide (ODPO), octydimethylamine oxide (ODAO), DDAO and NaDS. A maximum was observed in the plots of H(WW+S) vs. f_sm_s curves for ODPO and ODAO in NaDS while a small minimum was observed for TBPO. The experimental data are rationalized on the basis of the pseudo-phase transition model for the micellization process and a mass action model for the distribution of the additive between aqueous and micellar phases. The standard free energies, enthalpies and entropies of transfer of the additives from the aqueous to the micellar phases are reported. The effect of different butanol isomers on the thermodynamics of solubilization in the micellar phase has been derived. The enthalpies of transfer of benzene are always negligible with respect to those of phenol while the free energies of transfer are always comparable. Studies of symmetrical and asymmetrical additives show that asymmetry causes an increase of the free energy of transfer due to the decrease of the entropy. The thermodynamics of transfer of NaDS from the aqueous to the DDAO micellar phases and of DDAO from the aqueous to the NaDS micellar phases are compared to the thermodynamics of micellization of the two surfactants; the formation of mixed micelles seems to be energetically unfavored with respect to the pure micelles.     

Enthalpy of solution of phenoxyalcohols solubilized in aqueous sodium dodecylsulfate and hexadecyltrimethylammonium bromide solutions

The enthalpy of solution of phenoxy 2-ethanol, 1 phenoxy,-3 propanol and benzylalcohol was determined at 25 °C in aqueous sodium dodecylsulfate and hexadecyltrimethylammonium bromide solution, up to 0.2 mol/kg surfactant concentration. Using the pseudo-phase model, the standard enthalpy of transfer and the partition coefficient of the alcohols between micelle and water are calculated. The latter quantity is found to be systematically larger when derived from enthalpy than from free energy measurements. Using the so-called compensation plot, the solution thermodynamics of aromatic and aliphatic alcohols in aqueous sodium dodecylsulfate and in the octane+water systems are compared. Aromatic alcohols display an anomalous behavior in the octane+ water system but not in the micellar one.The standard enthalpy of solution of various alcohols presents, when plotted against hexadecyltrimethylammonium bromide concentration, a shoulder in the region around 0.05 mol/kg; a discussion is presented on the evidence for alleged micellar structural changes in aqueous micellar systems.     

Effect of sodium salicylate and other electrolytes on the partition of 1-pentanol between sodium dodecylsulfate micelles and water: A gas-chromatographic study

The salt effect of sodium salicylate (NaS) on the micellization and micellar solubilization of sodium dodecylsulfate (NaDS) has been studied. The experimental and theoretical conditions for the determination of the thermodynamic partition coefficient P of 1-pentanol between the micellar pseudo-phase and water in presence of added salt is discussed in the case of a precise gas-chromatographic method. In Particular, it is shown that P decreases with solute concentration in aqueous NaDS and sodium perfluorooctanoate surfactant solutions in opposition to the classical behavior in water-organic immiscible phases. As a reference salt effect, it is shown that P is constant with added NaCl in a large salt concentration domain where NaDS micelles are known to undergo dranatic structural changes. In the case of added NaS, P decreases slightly at very high salt concentration. It is suggested that this behavior might be the consequence of partial mixed micelle formation between the salicylate ion and NaDS micelles.     

Enthalpies of mixing of some nitriles aqueous solutions with dodecylsurfactants micellar solutions

The enthalpies of mixing of some n-nitriles (from acetonitrile to valeronitrile) aqueous solutions with dodecyltzimethylammonium bromide, sodium dodecylsulfate and dodecyltzimethylammonium oxide micellar solutions were determined. The measurements were performed by systematically changing the surfactant concentration at a given solute concentration. The experimental enthalpies were rationalized in terms of the standard enthalpy of transfer of solute from the aqueous to the micellar phase and of the distribution constant between the two phase. Information on the effect of the nature of the surfactant on the standard thermodynamic quantities of transfer(G _t ^o , H _t ^o , TS _t ^o ) is reported. The present data are compared to those previously reported for primary alcohols and the solubilizing properties shown by the different types of micelles are discussed.     

Enthalpies of transfer of pentanol from water to sodium dodecylsulfate-dodecyl-dimethylamine oxide-water mixtures

At a given surfactant-surfactant ratio, the enthalpies of transfer ΔH (W→W+S) of pentanol 0.03m from water to sodium dodecylsulfate (NaDS)-dodecyldimethylamine oxide-water mixtures as functions of the surfactants mixture concentration (m _t) were determined. ForX _NaDS=0.9, ΔH (W→W+S) increases monotonically withm _t such as observed for pure surfactants. ForX _NaDS=0.12 and 0.3, ΔH (W→W+S) increases withm _t up to 0.12m beyond which it decreases withm _t. AtX _NaDS=0.6, two monotonic curves can be distinguished in the ΔH (W→W+S)vs. m _t trend. Experimental data were fitted through an equation previously reported for additives in pure surfactants derived by assuming the pseudo-phase transition model for the micellization and a mass action model for the distribution of the additive between the aqueous and the micellar phases. This method did permit to simultaneously obtain the distribution constant of the alcohol between the aqueous and the micellar phase (and, then, the standard free energy of transfer) and its enthalpy of transfer from the aqueous to the micellar phases. By combining these properties the standard entropies of transfer were calculated. From these results, the excess properties of pentanol in the mixed micelles were calculated as a function of the mixture composition. The excess enthalpies and entropies are positive and compensate with each other leading to null values for the excess free energies in the whole range of the mixed micelles composition.     

Solubilization of methane,ethane, propane andn-butane in aqueous solutions of sodium dodecylsulfate

The solubilities of methane, ethane, propane, and n-butane were measured in aqueous solutions of sodium dodecylsulfate (NaDS) (0–0.1M) from 15 to 27°C. From these measurements the standard Gibbs energies, entropies, and enthalpies for the process of transferring the solute molecules from the gaseous phase into the solutions were calculated. An approximate relationship was found relating the volume fraction of the micelles to NaDS concentration.     

Thermodynamic study of polyethylene-glycols in micellar solutions water+sodium dodecylsulfate

Interactions of a series of polyethyleneglycols (PEG) in aqueous micellar solutions of sodium dodecylsulfate (SDS) were investigated through thermodynamic properties. Volumes, heat capacities and enthalpies of dilution were measured at 25°C. The resulting transfer properties of PEG are reflecting hydrophobic interactions which increase with the polymer length and the hydrophilic interactions occurring in the aqueous polar layer of the micelles. Typically heat capacities clearly evidence various structural changes taking place in the micellar solutions.     

Application of a chemical equilibrium model to thermodynamic functions of transfer of alcohols from water to aqueous surfactants

In ternary aqueous solutions, hydrophobic solutes such as alcohols tend to aggregate with surfactants to form mixed micelles. These systems can be studied by meas of the functions of transfer of hydrophobic solutes from water to aqueous solutions of surfactant. These thermodynamic functions often go through extrema in the critical micellar concentration (CMC) region of the surfactant. A simple model based on interactions between surfactant and hydrophobic solute monomers, on the distribution of the hydrophobic solute between water and the micelles and on the shift in the CMC induced by the hydrophobic solute, can simulate the magnitude and trends of the transfer functions using parameters which are mostly derived from the binary systems. In order to check the model more quantitatively, volumes and heat capacities of transfer of alcohols from water to aqueous solutions of a nonionic surfactant, octyldimethylamine oxide, were measured. A quantitative agreement was achieved with three adjustable parameters. Good fits are also obtained for the transfers to the ionic surfactants, octylamine hydrobromide and sodium dodecylsulfate. When the equilibrium displacement contribution is small, the distribution constants and the partial molar properties of the alcohols in the micellar phase agree well with the parameters obtained with similar models.     

Effect of Temperature on the Binding and Distribution Characteristics of Thionine in Sodium Dodecylsulfate Micelles

The interaction between thionine (a cationic thiazine dye) and anionic surfactant sodium dodecylsulfate in aqueous solution at different temperatures has been studied spectrophotometrically. The absorption spectra were used to quantify the dye/surfactant binding constants and surfactant/water partition coefficients of the dye by applying mathematical models that consider partitioning of the dye between the micellar and aqueous pseudo-phases. The Benesi-Hildebrand equation was applied to calculate the binding constants of thionine to sodium dodecylsulfate micelles over a temperature range of 293 to 333 K. To evaluate the thermodynamic aspects of the interaction of thionine with sodium dodecylsulfate micelles, Gibbs energy, enthalpy and entropy changes were determined. The effect of temperature on the critical micelle concentration of sodium dodecylsulfate in the presence of thionine was also studied and discussed. The binding affinity of thionine to the sodium dodecylsulfate micelles significantly decreased with increasing temperature because of the thermal agitation.     

Mass action model for solute distribution between water and micelles. Partial molar volumes of butanol and pentanol in dodecyl surfactant solutions

The densities of 1-butanol and 1-pentanol were measured in aqueous solutions of dodecyltrimethylammonium bromide and dodecyldimethylamine oxide and the partial molar volumes at infinite dilution of the alcohols in aqueous surfactants solutions were obtained. The observed trends of this quantity as a function of the surfactant concentration were rationalized using a mass-action model for the alcohol distribution between the aqueous and the micellar phase. At the same time, the model was revised to account for the alcohol effect on the surfactant micellization equilibrium. The partial molar volume of alcohols in the aqueous and in the micellar phases and the ratios between the binding constant and the aggregation number were calculated. These thermodynamic quantities are nearly the same in the two surfactants analyzed in this paper but differ appreciably from those in sodium dodecylsulfate. The apparent molar volume of surfactants in some hydroalcoholic solutions at fixed alcohol concentration were also calculated. In the micellization region the trend of this quantity as a function of the surfactant concentration shows a hump, which depends on the alcohol concentration and on the alcohol alkyl chain length. The alcohol extraction from the aqueous to the micellar phase due to the addition of the surfactant can account for the observed trends.     

Thermodynamic properties of alcohols in a micellar phase. Binding constants and partial molar volumes of pentanol in sodium dodecylsulfate micelles at 15, 25, and 35°C

Densities of the ternary system water-sodium dodecylsulfate (NaDS)-pentanol and of the binary systems butanol-octane and pentanol-octane were measured at 15, 25, and 35 °C. The apparent molar volume of pentanol in the ternary system was analyzed using a mass-action model for the alcohol distribution in micellar solutions. The partial molar volume of alcohol bound to the micelles and the ratio between the binding constant and the aggregation number of the surfactant are calculated. The partial molar volume binding constant, is discussed in terms of solubilization sites of the alcohol in the micelles whereas the binding constant is compared with that derived from the Nernstian partition constant previously obtained calorimetrically. From the binding constant and Poisson statistics the distribution function of the number of alcohol molecules per micelle, as a function of the concentration of alcohol and of surfactant, are calculated. The derived distribution functions show that a large amount of alcohol can be solubilized in the micelles so that alcoholic mixed micelles can be predicted when the concentration of pentanol is greater than that of NaDS.     

Enthalpies of solution and dilution of butanol and pentanol in dodecyltrimethylammonium bromide micellar solutions

The enthalpies of solution and of dilution of 1-butanol and 1-pentanol were measured in micellar solutions of dodecyltrimethylammonium bromide by systematically changing the concentration of alcohols and surfactant. The enthalpies of solution at infinite dilution of alcohols at each surfactant concentration were evaluated from a linear plot. This quantity increases with surfactant concentration (up to 0.8m) with a curvature which depends on the alcohol alkyl chain length. The difficulties arising for a quantitative treatment of both the enthalpies of dilution and of solution at finite alcohol concentrations are discussed. The dependence on the surfactant concentration of the standard enthalpies of solution and the enthalpies of dilution for m0 are rationalized. From the resulting equations the distribution constant, standard enthalpy of transfer, standard enthalpy of solution, and the alcohol-alcohol interaction parameter in the micellar phase are evaluated. The enthalpies of transfer obtained using this technique agree well with those previously reported from enthalpies of mixing. The distribution constants also agree with those reported in the literature from several approaches: mixing enthalpies, partial molar volumes, and the dependence of the cmc on added alcohol.     

Aliphatic carboxylic acids as new modifiers for separation of 2,4-dinitrophenyl amino acids by micellar liquid chromatography

The possibilities of isocratic separation of 2,4-dinitrophenyl derivatives of 12 amino acids that considerably differ in hydrophobicity by micellar mobile phases with different organic modifiers have been discussed. For the first time aliphatic carboxylic acids have been used as modifiers of micellar eluent in micellar liquid chromatography with C18 columns. Elution strength of hybrid micellar phases on the basis of sodium dodecylsulfate and aliphatic carboxylic acids increases in sequence: acetic相似文献   

Water-alcohol interactions in the two-phase system water-alcohol-alkane

The system water-alcohol-alkane has been studied at 25°C as a possible model system for micelles. The solubility of water in pure alcohols and in octane in the presence of alcohol has been determined. The data suggest that water may enter the octane phase hydrogen bonded to the hydroxyl group of the alcohol. If this system has any bearing on aqueous micellar solutions, it seems possible that solubilized alcohol molecules may carry water into the micellar interior. Addition of 1,2-propanediol does not increase the water content in octane, probably because dimerization of the diol in octane is energetically more favorable than diol-water interactions.     

Electrostatic contribution to the interaction of α, β poly (N-hydroxyethyl)-DL-aspartamide with sodium dodecylsulfate micelles

The enthalpic effect due to the interaction between α, β poly(N-hydroxyethyl)-DL-aspartamide (PHEA) and sodium dodecylsulfate (SDS) in aqueous solutions as a function of the surfactant concentration was measured by the calorimetric technique at various NaCl concentrations. A marked influence of the added electrolyte on the PHEA-SDS interaction was observed. An analysis of the experimental enthalpies allows to estimate the electrostatic and the hydrophobic contributions to the enthalpy of interaction between PHEA and SDS micelles. The results were rationalized in terms of effects due to the screening of the charges residing on PHEA and SDS micelles.     

Micellar solubilization thermodynamics of acetophenone in surfactant mixtures: Case of benzyldimethyltetradecylammonium chloride and trimethyltetradecylammonium chloride

The thermodynamics of micellar solubilization of acetophenone in mixtures of two cationic surfactants [benzyldimethyltetradecylammonium chloride +trimethyltetradecylammonium chloride] has been derived from calorimetric measurements at controlled solute activity. The partition coefficient between micelles and water as well as the standard enthalpy and entropy of transfer between micelles and water were calculated. The results were compared to the case of benzylalcohol in the same cationic mixtures. For acetophenone, the variation of all thermodynamic transfer functions with micellar composition may be described by the regular solution formalism. The same conclusion has been achieved for most polar solutes in various surfactant mixtures: favorable interaction between unlike surfactants induces an unfavorable micellar solubilization. Exceptions should be found with the cases where solute solubilization induces profound micellar changes. It seems to be the case with some alcohols in the cationic surfactant mixtures studied.     

Chemical equilibrium model for the thermodynamic properties of mixed aqueous micellar systems: Application to thermodynamic functions of transfer

Mixed micelles can be formed in water between various pairs of hydrophobic solutes such as surfactants, alcohols and hydrocarbons. These systems can often be studied through the thermodynamic functions of transfer of one of the solutes, usually kept near infinite dilution, from water to an aqueous solution of the other solute. When mixed micelles are formed, these functions change significantly, and often go through extrema, in the region where the binary system micellizes or undergoes some microphase transition.Three main effects are responsible for the observed trends: pair-wise interactions between both solutes in the monomeric form, a distribution of the reference solute between the aqueous and micellar phases and a shift in the monomer-micelle equilibrium in the vicinity of the reference solute. Simple equations can be derived for these three effects which can account for the sign and magnitude of the observed trends using parameters which are derived for the most part from the two binary systems.     

Thermodynamics of solute transfer between chloroform and water

The apparent enthalpies, free energies and entropies of transfer from water saturated chloroform to chloroform saturated aqueous buffer (pH 7) were determined for five primary alcohols and six other organic nonelectrolytes using an isoperibol flow microcalorimeter. A linear relationship between the enthalpies and free energies of transfer is found for the homologous series of alcohols indicating that the occurrence of enthalpy-entropy compensation in solute transfer is not restricted to solvent systems of low mutual solubility. The apparent thermodynamics of transfer from chloroform to aqueous buffer were compared with those from 2,2,4-trimethylpentane to aqueous buffer and were rationalized in terms of solvation interactions.