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.     

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.     

Volumes and compressibilities of pentanol in aqueous dodecyltrimethylammonium bromide solutions at 15, 25 and 35°C

Ultrasonic velocities and densities of the water-dodecyltrimethylammonium bromide (DTAB)-pentanol (PentOH) ternary system were measured at 15, 25 and 35°C as a function of the surfactant and alcohol concentrations. The apparent molar volumes and isentropic compressibilities of PentOH were calculated. The standard partial molar volumes increase with surfactant concentration continuously whereas the standard partial molar isentropic compressibilities show sharp changes in slope at about 0.25 mol-kg^–1 DTAB, which can be ascribed to a micellar structural transition. The volume data for alcohol in micellar solutions were treated by a model reported for the distribution of polar additives between aqueous and micellar phases. In the application of the model to compressibility, the contributions due to the pressure effect on the shift of both the micellization equilibrium and the alcohol distribution constant cannot be neglected. This is in contrast to what is found in the case of heat capacity. The distribution constant and the partial molar volumes and compressibilities of PentOH in the micellar phase have been derived by linear regression. Also, the apparent molar volumes and isentropic compressibilities of DTAB in water-pentanol mixed solvents at fixed composition have been calculated. These properties as a function of the surfactant concentration show maxima depending on the temperature and the mixed solvent composition. The decrease beyond the maximum can be attributed to the extraction of PentOH from the aqueous into the micellar phase, where its concentration tends to zero with the progressive increase of the surfactant concentration. As a consequence, by increasing the surfactant concentration, the apparent molar properties of the surfactant in the mixed solvent shifts towards the value in water.     

Heat capacities of butanol and pentanol in aqueous dodecyltrimethylammonium bromide solutions

Heat capacities of the ternary systems water-dodecyltrimethylammonium bromide (DTAB)-butanol and water-DTAB-pentanol were measured at 25°C. The standard partial molar heat capacities of pentanol in micellar solutions show a maximum at about 0.35 mol-kg^–1 DTAB that has been attributed to a micellar structural transition. This maximum tends to vanish by increasing the alcohol concentration and by decreasing the alcohol alkyl chain length; in the case of butanol it was not detected. The behavior of the standard partial molar heat capacities of alcohols in micellar solutions in the region above the cmc and below the structural transition was explained using a previously reported mass-action model for the alcohol distribution between the aqueous and the micellar phase and the pseudophase transition model for micellization. In the resulting equation the contributions due to the temperature effect on the shift of both the micellization equilibrium and the distribution are shown to be negligible so that only the distribution effect and the shift of the micellization equilibrium due to the added alcohol remain. The distribution constant and the partial molar heat capacities of alcohols in the aqueous and micellar phases have been derived by linear regression. The distribution constant for both alcohols agree well with those previously obtained using different techniques. Since the best fit below the structural transition correlates as well with the experimental points above the structural transition, it seems that no difference exists in the standard partial molar heat capacities of alcohols in the two shapes of the micelles. Also, from the present data and those for alkanols in sodium dodecylsulfate reported in the literature it seems that the standard heat capacity of alcohols in the micellar phase does not depend on both the alcohol alkyl chain length and the nature of the hydrophilic moiety of the head group of the micelles.     

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.     

Heat capacities,volumes and solubilities of pentanol in aqueous alkyltrimethylammonium bromides

Apparent molar heat capacities and volumes of pentanol, 0.05m in decyl-, tetradecyl- and hexadecyltrimethylammonium bromides micellar solutions, were measured at 25°C. They were assumed to approach the standard infinite dilution values and rationalized by means of previously reported equations following which the distribution constant between the aqueous and the micellar phase, heat capacity, and volume of pentanol in both phases are simultaneously derived. The present results show that the volume of the micellar core does not seem to have a significant effect on the apparent molar volume and heat capacity of pentanol in the micellar phase and on the free energy of transfer of pentanol from the aqueous to the micellar phase. We report an equation correlating the free energy of transfer of alcohols in alkyltrimethylammonium bromides as a function of the number of carbon atoms in the alcohol and surfactant alkyl chain. Also, the apparent molar heat capacities of pentanol in micellar solutions as a function of surfactant concentration show evidence of two maxima, which, by increasing the alkyl chain length of surfactant display an opposite dependence on concentration. The second maximum can be attributed to a sphere to rod transition. The second transition was also found in the case of butoxyethanol in hexadecyltrimethylammonium bromide. It is more difficult to explain the nature of the first maximum although an attempt is made.     

Effects of addition of short-chain alcohol solvents on micellization and thermodynamic properties of anionic surfactants sodium dodecyl sulfate and sodium dodecyl sulfonate in aqueous solutions

Micellar and thermodynamic properties of anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl sulfonate (SDSn) in aqueous solutions of 5 wt% short-chain alcohols methanol, ethanol, and 1-butanol were investigated by experimental electrical conductivities, densities and sound velocities at 298.15 K. It was found that methanol behaves like a cosolvent and increases the critical micelle concentration (CMC) of both surfactants in aqueous solutions. However, the other investigated alkanols act as a cosurfactant and decrease the CMC by their presence. The values of the degree of counterion association on the micelles of both surfactants in aqueous methanol solution are same as those in pure water, and they decrease with increasing the alkyl chain length of alcohol. Furthermore, the values of the apparent molar volume and isentropic compressibility of the monomeric and micellar forms of the investigated surfactants were obtained from the experimental density and sound velocity data. It was found that the values of the apparent molar properties of both micellar and monomeric forms of the studied surfactants increase by increasing the alkyl chain of the alcohols.     

First evaluation of the thermodynamic properties for spheres to elongated micelles transition of some propanediyl-alpha,omega-bis(dimethylalkylammonium bromide) surfactants in aqueous solution

The apparent and partial molar enthalpies, apparent molar volumes, and adiabatic compressibilities at 298 K of the aqueous solutions of the cationic gemini surfactants propanediyl-alpha,omega-bis(octyldimethylammonium bromide) (8-3-8) and propanediyl-alpha,omega-bis(dodecyldimethylammonium bromide) (12-3-12) have been measured as a function of concentration. The trends of the partial molar enthalpies versus concentration are the first well documented thermodynamic evidence of sphere to rod transition in the micellar phase, involving a detectable quantity of heat, and allow the determination of the change in enthalpy associated with this transition. The changes in enthalpies upon micellization and for the sphere to elongated micelles transition, DeltaH(s)(-->)(r), have been obtained from the experimental data by using a pseudo-phase transition approach: -1.5 kJ mol(-1) for 8-3-8 and -3.9 kJ mol(-1) for 12-3-12. No evidence of the above transition is found in the trends of volumetric properties versus m. The apparent adiabatic molar compressibilities for the compounds under investigation are also reported here for the first time: a negative group contribution for the methylene group is evaluated, when the surfactants are present in solution as a single molecule, reflecting its solvation structure. In the micellar phase, the -CH(2)- group contribution becomes positive. A value of 1.17 x 10(-3) cm(3) bar(-1) mol(-1) for the change in adiabatic molar compressibility upon micellization is obtained. The lower values of the methylene group contributions to the volumetric properties for the monomers support the hypothesis of partial association of the chains before the cmc.     

Thermodynamics of micellization of tetraethylammonium perfluorooctylsulfonate in water

Conductivity, density, and sound velocity measurements as functions of temperature were made on tetraethylammonium perfluorooctylsulfonate solutions to determine the Krafft point, the dependence on temperature of the critical micelle concentration, the micellar ionization degree, and several thermodynamic properties: Gibbs free energy, enthalpy and entropy of micellization, apparent molar partial volume, thermal expansion coefficient, and the adiabatic compressibility factor of both micellized and unmicellized surfactants. Important changes occur at about 30 degrees C. Results are interpreted on the basis of dehydration of surfactant on micellization and on temperature increase.     

A Volumetric Study of Water–Decyltrimethylanimonium Bromide–Pentanol Ternary System from 25 to 130°C at 2 and 19 MPa

Density measurements on decyltrimethylammonium bromide (DeTAB)–water and pentanol (PentOH)–DeTAB–water systems as functions of both alcohol and surfactant m _S concentrations were carried out at 2 and 19 MPa from 25 to 130°C. From experimental data for the water–DeTAB binary system, the standard (infinite dilution) partial molar volumes, expansibilities, and compressibilities of DeTAB, and the corresponding properties in the micellar phase are calculated. The trends of the standard partial molar volumes of PentOH V _R ^o in DeTAB micellar solutions as functions of m _S reflect the transfer of PentOH from the aqueous to the micellar phase, except at 130°C and 19 MPa. On the basis of an equation previously used, the distribution constant of PentOH between the aqueous and the micellar phases and the standard partial molar volume of alcohol in the aqueous and the micellar phases are obtained from V _R ^o data. Comparisons with data for PentOH in dodecyltrimethylammonium bromide are made.     

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.     

Partial molar volumes and compressibilities of alkyltrimethylammonium bromides

Density and ultrasound measurements were performed for dodecyl- and tetradecyltrimethylammonium bromide at 15, 25 and 35°C and for hexadecyltrimethylammonium bromide at 25, 35 and 45°C over a wide concentration region. From these and previously reported data, partial molar volumes and isentropic and isothermal compressibilities were derived as a function of the surfactant concentration. It is shown that by increasing the surfactant concentration the apparent molar volumes and compressibilities increase according to the expected behavior of surfactant solutions. However, anomalies are displayed in plots of apparent molar compressibility of tetradecyltrimethylammonium bromide and of the speed of sound for all the surfactants studied as a function of concentration. These peculiarities can be ascribed to micellar structural transitions. The standard thermodynamic properties and the CH₂ group contributions have been obtained by the additivity rule. The results obtained for the compressibility and volume properties are different from those reported in the literature. The volumes and compressibilities of micellization were graphically evaluated on the basis of the pseudo-phase transition model.     

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.     

Thermodynamic and Dynamic Properties of Micellar Aggregates of Nonionic Surfactants with Short Hydrophobic Tails

The densities and viscosities of binary aqueous mixtures of poly(ethylenoxide)hexanols [C₆H₁₃(OCH₂CH₂)_mOH, C₆E_m] (m= 3, 4, and 5) have been studied in the micellar composition range. For the same surfactants the self-diffusion coefficients in mixtures with heavy water have been determined by the spin-echo pulsed field gradient method. The volumetric data are interpreted by means of the phase separation model, and values of the CMC, volume change, and standard free energy change of micellization are obtained. From the viscosity data the hydration numbers of the surfactant hydrophilic head in the micellar state are computed; they are in agreement with those obtained from HDO self-diffusion data. The surfactant self-diffusion data are used to calculate the apparent micelle radius and the aggregation number. The micellization parameters obtained for the different surfactants are compared and discussed.     

Standard partial molar volumes of alcohols in aqueous dodecyltrimethylammonium bromide solutions

Density measurements of water-dodecyltrimethylammonium bromide (DTAB)-alcohol ternary systems as a function of alcohol and surfactant concentrations were carried out at 25°C. The alcohols were propanol (PrOH), 2-propanol (2-PrOH) and hexanol (HexOH). The apparent molar volume V_,R of alcohols have been calculated and the standard (infinite dilution) partial molar volumes of alcohols V _R at each surfactant concentration were obtained by means of a least squares fit of V_,R vs. the alcohol concentration. The V _R vs. surfactant concentration curves have been rationalized in terms of the partial molar volume of alcohol in the aqueous V _f and the micellar V _b phases and the distribution constant of alcohol between the aqueous and the micellar phases K. The V _b values for PrOH and HexOH together with those of butanol and pentanol previously reported satisfy the additivity rule giving a methylene group contribution of 16.7 cm³-mol^–1 which is identical to that reported in the literature from the study of pure liquid alcohols. No difference between V _b for PrOH and 2-PrOH has been found. From density data of water-alcohol and water-surfactant binary systems and of water-surfactant-alcohol ternary system, the apparent molar volume of the surfactant in the water-alcohol mixed solvent V_,S have been calculated as a function of the surfactant concentration and of the mixed solvent composition. The effect of the alkyl chain length of the alcohols and the effect of isomerization of the alcohols on the V_,S vs. surfactant concentration trends have been analyzed.     

Heat capacities,volumes and solubilities of pentanol in aqueous surfactant solutions

Apparent molar heat capacities and volumes of pentanol (PentOH) 0.05m in dodecyltrimethylammonium chloride (DTAC), dodecyldimethylammonium chloride (DDAC) and dodecylamine hydrochloride (DAC) micellar solutions were measured at 25°C. They were assumed to approach the standard infinite dilution values and rationalized by means of previously reported equations. The distribution constant between the aqueous and the micellar phase and heat capacity and volume of pentanol in both phases were thus derived. The results show that the presence of methyl groups on the surfactant head group does not appreciably influence the apparent molar volume and heat capacity of pentanol in micellar phase and the free energy of transfer of pentanol from the aqueous to the micellar phase. Also, the apparent molar heat capacities of pentanol in micellar solutions as a function of surfactant concentration show evidence of two maxima for DAC and of one maximum for DTAC whereas no maxima were detected for DDAC. According to the literature data for alkyltrimethylammonium bromides these maxima can be ascribed to the presence of structural post-micellar transitions. It is shown that the C_,PentOH vs. surfactant molality curve for DAC lies between that for hexadecyltrimethylammonium bromide and that for tetradecyltrimethylammonium bromide. This evidence, which is similar to that found for solubilities, agrees with the previously reported idea that the removal of a CH₃ group from the head group of surfactant is equivalent to the introduction of a CH₂ group in its hydrophobic moiety. By comparing data for DTAC with those for the corresponding bromide, the role of the nature of the counterion in the thermodynamics of solubilization of pentanol in micellar solutions is derived.     

Effect of counterion on thermodynamic micellar properties of tetradecylpyridinium in aqueous solutions

Electrical conductivity of aqueous solutions of tetradecylpyridinium bromide and chloride has been measured as a function of surfactant molal concentration and temperature. From the molal dependence of conductivity, the critical micelle concentration and the micellar ionization degree were estimated. The temperature dependence of these parameters has been used for calculating the thermodynamic parameters related with the micellization process by using the classical charged pseudophase separation model. The effect of the counterion on the conventional thermodynamic potentials of micellization such as standard Gibbs free energy, enthalpy and entropy has also been a matter of study. Finally, the occurrence of the enthalpy–entropy compensation phenomenon was verified and the relevant parameters discussed.     

Thermodynamics of mixed micellar systems of sodium octanoate and ethoxylated alcohols in water at 25°C

Thermodynamic volume and compressibility properties of binary aqueous systems of sodium octanoate (C₈Na) and polyoxyethylene butyl ether compounds (ethoxylated alcohols), with one to three oxyethylene groups (C₄EO_X), and ternary systems of these compounds, have been determined as a function of surfactant and alcohol concentrations at 25°C from density and ultrasonic velocity measurements. Values of standard state apparent molar volume and apparent molar adiabatic compressibility properties of transfer of ethoxylated alcohols from water to aqueous C₈Na solutions were obtained. The observed trends in the properties of the aqueous binary C₈Na systems and of the transfer functions for C₄EO_x, at a given low concentration of 0.05m, were analyzed using theoretical models. Good agreement between the simulated results and the experimental data was achieved. The results obtained for the distribution coefficients of the alcohols and the thermodynamic properties of both components of the mixed micelle show that the hydrophobicity of the ethoxylated alcohols is enhanced by introducing more oxyethylene groups into the alcohol. The distribution behavior of these compounds between the C₈Na micelles and the aqueous phase also depends on the difference between the hydrophobicity of the surfactant and the alcohols.     

Partial molar volume of dodecyltrimethylammonium hydroxide in water and NaOH aqueous solutions

 The partial molar volume of dodecyltrimethylammonium hydroxide in water and aqueous NaOH solutions was measured. The addition of NaOH did not affect either the micellized or the unmicellized molecules. The expansion on micellization is much larger than in dodecyltrimethylammonium bromide systems, which reflects the stronger ionization of the hydroxide surfactant micelles, when compared with that of the bromide amphiphile. Received: 27 October 1997 Accepted: 4 March 1998     

A volumetric study of two related amphiphilic beta-blockers as a function of temperature and electrolyte concentration

The effects of electrolyte concentration and temperature on aqueous solutions of propranolol and acebutolol hydrochlorides have been investigated using density and ultrasound velocity measurements. The electrolyte range was 0.0–0.5 and 0.4–1.0 m for propranolol and acebutolol, respectively. For each electrolyte concentration the temperature range was 288.15–313.15 K. Critical concentrations were obtained from plots of ultrasound velocity against drug concentration. Experimental results yielded the apparent molar volume and the apparent molar adiabatic compressibility for both beta-blockers, measured over a wide concentration range. Negative deviations of the apparent molar volume from the Debye–Hückel limiting law in dilute solutions indicate the absence of premicellar aggregation. A negative slope was found for ΔV_m against temperature for both drugs. This negative value suggests that the expansibility of the surfactant in the micellar state is less than that in the aqueous phase. Changes in molar volume and adiabatic compressibility accompanying aggregate formation were smaller than those of typical surfactants, suggesting a more tightly packed aggregate.