Apparent molar quantities of sodium octanoate in aqueous solutions

Densities and sound velocities of aqueous solutions of sodium octanoate were determined in a range of molalities between 0.0352 and 0.8105 mol kg^–1 at 25, 30, 35, 40 and 45 °C. The isotherms of molality dependence of both density and sound velocity were used to determine the cmcs. Apparent molar volumes and compressibilities were determined from measurements of ultrasound velocity and density. The values of apparent molar volumes and compressibilities at infinite dilution and the apparent molar quantities in the micellar range were obtained and studied as a function of temperature. Values of the critical micelle concentration and the apparent molar quantities in the premicellar and postmicellar range are discussed and compared with the values of the corresponding fluorinated compound.An erratum to this article can be found at     

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.     

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.     

Use of the semi-equilibrium dialysis method in studying the thermodynamics of solubilization of organic compounds in surfactant micelles. Systemn-hexadecylpyridinium chloride-phenol-water

The semi-equilibrium dialysis method has been used to infer solubilization equilibrium constants or, alternatively, activity coefficients of solutes solubilized into micelles of aqueous surfactant solutions. Methods are described for inferring the concentrationa of monomers of the organic solute and of the surfactant on both sides of the dialysis membrane, under conditions where the organic solute is in equilibrium with both the high-concentration (retentate) and low-concentration (permeate) solutions. By using a form of the Gibbs-Duhem equation, activity coefficients of both phenol (the solubilizate) and n-hexadecylpyridinium chloride (the surfactant) are obtained for aqueous solutions at 25°C throughout a wide range of relative compositions of surfactant and solubilizate within the micelle. The apparent solubilization constant, K=[solubilized phenol]/([monomeric phenol][micellar surfactant]), is found to decrease significantly as the mole fraction of phenol in the micelle increases.     

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.     

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.     

Surface Tension, Heat Capacity, and Volume of Amphiphilic Compounds in Formamide Solutions

Density measurements of sodium dodecyl sulfate (SDS), sodium decyl sulfate (SDeS), sodium octyl sulfate(SOS), and sodium hexyl sulfate(SHS) in formamide (FA) as functions of the surfactant concentrations were carried out at 25°C. For SDS in FA, additional density measurements at 35 and 60°C and surface tension and specific heat capacity measurements at 25°C were also performed. From density and specific heat capacity data, the apparent molar volume and heat capacity of the surfactants as functions of concentration were calculated. The surface excess of SDS at the solution–air interface was also determined from the surface tension measurements using the Gibbs adsorption equation. Under our experimental conditions, none of the experimental results evidence micelle formation. In addition, volumetric studies of the hexanol–SDS–FA ternary system at 25°C evidence only interactions between the dispersed surfactant and alcohol.     

Thermodynamic properties of some N-alkyl-N-methylpiperidinium chlorides and N-alkylpiperidine hydrochlorides in water

Densities and heat capacities at 25°C were measured for N-octyl-, N-decyl- and N-dodecyl-N-methylpiperidinium chlorides and for N-octyl- and N-dodecylpiperidine hydrochlorides in water as functions of concentration. Enthalpies of dilution at 25°C and osmotic coefficients at 37°C of the N-methyl-N-alkylpiperidinium chlorides were also measured as functions of concentration. The partial molar volumes, heat capacities, relative enthalpies, nonideal Gibbs energies and entropies at 25°C were derived as functions of the surfactant concentration. By increasing the alkyl chain length of the surfactant, both the apparent molar volume vs. concentration curves are shifted toward greater values while the corresponding ones for the heat capacity are moved toward more negative values. These results are consistent with the higher hydrophobicity the longer the alkyl chain of the surfactant is. In the micellar region, the entropy and enthalpy vs. log m/m _cmc curves increase in a parallel manner by decreasing the alkyl chain length of the surfactant. Consequently, the negligible effect of the hydrophobicity of the surfactant on the Gibbs energy vs. log m/m _cmc trends is due to the enthalpy-entropy compensative effect. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. The absolute values of both the volume and heat capacity of micellization increase with an increasing number of carbon atoms in the alkyl chain (n _c ). The enthalpy and entropy of micellization vs. n _c are convex curves. Comparisons are also made between the present data and those of some alkylpyridinium chlorides reported elsewhere.     

Volumes and compressibilities of pentanol in aqueous alkyltrimethylammonium bromide solutions at different temperatures

Speed of sound and density properties of ternary water-tetradecyltrimethylammonium bromide-pentanol system at 15, 25 and 35°C and of water-hexadecyltrimethylammonium bromide-pentanol system at 25, 35 and 45°C were measured at fixed alcohol concentration as a function of surfactant concentration. The apparent molar volumes V_,R and isentropic compressibilities K _,R ^S of pentanol in micellar solutions as a function of the surfactant concentration show irregular behavior which depends on the alkyl chain length of the surfactant and tends to disappear with increasing temperature. These anomalies are ascribed to micellar transitions. For both surfactants at high concentrations, V_,R decrease and the magnitude of the change seems to depend on the type of densimeter used. This observation is tentatively explained in terms of a correlation between the micellar structure and features of the densimeter. From this work and literature data, the apparent molar isothermal compressibilities K _,R ^T of the alcohol in micellar solutions were calculated at 25°C. V_,R , K _,R ^S and K _,R ^T are interpreted in terms of the distribution constant of the alcohol between the aqueous and the micellar phases and of the apparent molar property of the alcohol in the micellar and the aqueous phases. For a given surfactant increasing the temperature increases V_,R and K _,R ^S in the micellar phase while the distribution constant is weakly dependent. At a given temperature, an increase in the alkyl chain length of the surfactant increases the apparent molar volume and slightly changes the apparent molar compressibility of the alcohol in the micellar phase.     

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.     

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.     

Evaluation of surfactant ion activity in micellar solutions of dodecylpyridinium chloride from partition data and counter ion activity measurements

The partition isotherm of dodecylpyridinium chloride in the water-1,2-dichloroethane system, measured over a broad concentration range, served for the evaluation of the critical micelle concentration and of the effective concentration and mean activity of the surfactant ions in the micellar concentration range. On the basis of the charged phase separation model for the micelles and EMF measurements of counter-ion activity, the surfactant cation activity and the apparent degree of counter ion association to the micelle () of 0.69 were estimated. The results indicate a gradual decrease of surfactant cation activity with increasing total concentration in the micellar range and support literature data derived from EMF measurements that apply surfactant selective plastic exchange membrane electrodes.     

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.     

Role of alcohol in microemulsions as determined from volume and heat capacity data for the water-sodium dodecylsulfate -n-butanol system at 25°C

Densities and heat capacities of the ternary system water-sodium dodecylsulfate — n-butanol were measured at 25°C over the complete alcohol mole-fraction or solubility range. Apparent and partial molar volumes and heat capacities of n-butanol were derived and have been analyzed as a function of the concentration of both the surfactant and the alcohol. Characteristic changes suggest that, at low concentrations, n-butanol is partially solubilized in mixed micelles but, in concentrated alcohol solutions, n-butanol largely exists in the form of microaggregates stabilized by the surfactant. Results would also suggest that at low concentrations of n-butanol another transition zone occurs in the micellar structure around 0.2–0.3 mol-kg^–1 in sodium dodecylsulfate.     

Thermodynamic properties of N-octyl- and N-dodecylnicotinamide chlorides in water

Densities, heat capacities and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for N-octyl- and N-dodecylnicotinamide chlorides in water over an extended concentration region. Partial molar volumes, heat capacities, relative enthalpies and nonideal free energies and entropies at 25°C were derived as a function of the surfactant concentration. For both surfactants, plots of volumes, enthalpies and free energies vs. concentration are regular whereas those of heat capacities and entropies present anomalies at about 0.8 and 0.1m for the octyl and dodecyl compounds, respectively. Changes in the slope of a plot of osmotic coefficients times molality vs. molality were also observed at these same concentrations. These peculiarities are ascribed to micelle structural transitions. The nonideal free energies do not seem to depend on the alkyl chain length when they are plotted vs. m/C _cmc . Also, a plot of the nonideal free energy vs. logm/C _cmc is roughly independent of the nature of the surfactant because of the constant activity of surfactants in micellar solutions. Nonideal free energies, enthalpies and entropies have been calculated at 15 and 35°C. At each concentration the nonideal free energy is temperature independent as a result of a compensatory effect between enthalpy and entropy. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. These data suggest that the nicotinamide group possesses less hydrophilic character than the ammonium group.     

Thermogravimetric analysis of organoclays intercalated with the surfactant octadecyltrimethylammonium bromide

Summary High resolution thermogravimetric analysis has been used to study the thermal decomposition of montmorillonite modified with octadecyltrimethylammonium bromide. Thermal decomposition occurs in 4 steps.The first step of mass loss is observed from ambient to 100°C temperature range and is attributed to dehydration of adsorbed water. The second step of mass loss occurs between 87.9 to 135.5°C temperature range and is also attributed to dehydration of water hydrating metal cations such as Na⁺. The third mass loss occurs between 179.0 and 384.5°C; it is assigned to the loss of surfactant. The fourth step is ascribed to the loss of OH units due to dehydroxylation of the montmorillonite and takes place between 556.0 and 636.3°C temperature range. These TG steps are related to the arrangement of the surfactant molecules intercalating the montmorillonite. Changes in the basal spacing of the clay with surfactant are followed by X-ray diffraction. Thermal analysis provides an indication of the stability of the organo-clay.     

Temperature dependence of second critical micelle concentration of dodecyldimethylbenzylammonium bromide in aqueous solution

The specific conductivity of dodecyldimethylbenzylammonium bromide (C12BBr) in aqueous solutions, in the temperature range of 15 to 40 °C, has been measured as a function of molality. The two breaks which were found on the conductivity against molality plots were attributed to the critical micelle concentration, cmc, and second critical micelle concentration, 2^nd cmc, respectively. The ratio of the slopes, S, of the three linear fragments on the plots, S2/S1 and S3/S1, was attributed to the degree of ionization of the micelles at cmc and 2^nd cmc respectively. It was shown that the values of the 2^nd cmc estimated above 27 °C are only apparent due to thermal disintegration of the micelles. In the temperature range of 15 to 27 °C, the values of the 2^nd cmc increase gradually and the plot of the 2^nd cmc against temperature is concave. The ratio of 2^nd cmc/cmc for C12BBr at 25 °C amounts to 15 and appears to be high compared to the literature values for other surfactants. For comparative purposes the cmc and 2^nd cmc values were also estimated conductometrically for decyldimethylbenzylammonium bromide (C10BBr) at 25 °C. The 2^nd cmc value for this surfactant is higher compared to the value for the C12 homologue by a factor of 2.6.The standard Gibbs free energies of micellization at cmc and at the 2^nd cmc were estimated from the experimental data for both surfactants at 25 °C.     

The flow of concentrated surfactant solutions through narrow capillaries

 The resistance of microfilters in a flow of micellar solutions indicates that the surfactant which is adsorbed in the pores forms a mobile admicelle. The flow in these admicelles can be stopped by Marangoni forces if there is a concentration gradient of surfactant across the filter. On the basis of thermodynamic concepts and experimental data it is argued that the resistance of the filters could be affected by shear-induced surfactant adsorption. To minimize the entropy production in the flow through a narrow pore the micellar solution separates into parallel fluxes of liquid through the lumen of the pore and of mobile surfactant in the flowing admicelle. Received: 14 July 1999 Accepted: 24 November 1999     

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.     

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.