Aqueous Solutions Of Sodium Octanoate Research Articles

Modification of the Two-Point Scaling Theory for the Description of the Phase Transition in Solution. Analysis of Sodium Octanoate Aqueous Solutions

On the basis of conventional scaling theory, the two-point scaling theory was modified in order to describe the influence of composition on the partial molar heat capacity and volume during the micellization process. To verify the theory, isobaric heat capacities and densities of aqueous sodium octanoate solutions were measured over wide composition and temperature ranges and the modified approach was used to analyze the calculated partial molar heat capacities and volumes of the surfactant in water. The results obtained indicate that the micellization process is subject to the scaling laws. The results were compared with those for other systems. Peculiar behavior of the critical indices was observed and correlated with the structure of the micelles.Electronic Supplementary MaterialThe online version of this article (doi:10.1007/s10953-012-9795-6) contains supplementary material, which is available to authorized users.

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.

Heteronuclear Overhauser effect measurements in surfactant systems. 3. Average location of water with respect to surfactant polar head in micellized sodium octanoate and sodium dodecanoate

By means of heteronuclear Overhauser effect measurements between water protons and the carbon-13 of the carboxylate head group in aqueous solutions of sodium octanoate and sodium dodecanoate, an estimate of the average distance between water and the surfactant polar head has been determined as a function of surfactant concentration. The concentration range investigated goes from monomers to micelles in the case of sodium octanoate and is limited, for sensitivity reasons, to the micellar state for sodium dodecanoate

Raman spectra of aqueous sodium octanoate solutions: solute and solvent study

Aqueous solutions of sodium octanoate between 3 and 40% of NaCe have been studied by Raman scattering. The spectral behavior is followed as a function of concentration in the low-temperature solid phase and in the liquid phase for all samples. We have focused our study on the modifications that appear in the spectrum of the solutions when the concentration is lower of higher than the critical micellar concentration, which is quite high for this surfactant (6% in weight)

The influence of pH on gas solubilities in aqueous solutions of sodium octanoate at 25°C

Measurements have been made to determine the solubilities of oxygen, methane, ethane, and propane in aqueous solutions of sodium octanoate at 25°C and at pH values ranging from 7 to 13. The solubility of each gas obeys Henry's law at all surfactant concentrations and pH values. The solubilities measured for each gas are effectively independent of sodium octanoate concentration as well as pH at concentrations well below the CMC, and, within experimental error, are the same as the respective gas solubilities in pure water at 25°C. Above the CMC, the solubility of each gas increases with surfactant concentration indicating micellar solubilization. The degree of solubilization is greatest for propane and decreases in the order: propane > ethane > methane > oxygen. However, unlike the situation at low surfactant concentrations, the individual gas solubilities vary with pH at higher surfactant concentrations. The degree of solubilization is found to be quite constant for each gas at pH values greater than 9, and intramicellar solubilities derived from gas solubility data taken at pH 12 are identical to those determined for a similar surfactant having a seven carbon alkyl chain, sodium 1-heptane sulfonate. However, the extent of solubilization for each gas increases rapidly as the solution pH decreases from 9 to 7, suggesting that mixed micelles composed of octanoic acid and sodium octanoate are more effective solubilizing agents than those of the pure soap.

A comparison of the conformation of micellar and non-micellar sodium octanoate and octanoic acid using 13C–1H and 1H–1H vicinal spin–spin coupling constants

1 H–1H and 1H–13C vicinal spin–spin coupling has been studied at 21 °C for micellar and non-micellar aqueous solutions of sodium octanoate, aqueous solutions of sodium butanoate and pentanoate, and CDCl3 and CD3OD solutions of octanoic acid. For the octanoyl subjects, there is no detectable variation in the proportion of trans conformer with solvent or degree of association except for the CH2–CH2 bond adjacent to the carboxy group which shows an increase in fractional trans content of ca. 0.1 on micellisation. In sodium butanoate, the trans fraction is ca. 0.1 lower than in sodium pentanoate owing to the absence of four-bond steric conflicts.

Enthalpies of solution in sodium octanoate + water + alcohol mixtures

Enthalpies of solution of sodium octanoate in water, 1-propanol and aqueous mixtures of 1-propanol, 1-butanol, 1-pentanol and 1-hexanol, and of the alcohols in aqueous solutions of sodium octanoate at various concentrations were determined calorimetrically at 35 °C. MostΔH(soln) values are exothermic and strongly dependent on the solute concentration. The main energetic factor governing the process of dissolution of the surfactant is associated with changes in the water structure caused by the presence of alcohol. That governing the process of the alcohol dissolution in surfactant solutions is due to the effect alcohols have on the CMC of the octanoate. There is no indication of the alcohol being either solubilized in the interior of the aqueous micelle, or becoming part of the micellar film.

Solubility and thermodynamics of solution of argon in aqueous solutions of sodium octanoate and sodium dodecylsulfate

The ostwald absorption coefficient of argon was measured in aqueous solutions of two surfactants; sodium n-octanoate and in sodium dodecylsulfate, at several concentrations and at a few temperatures between 10–25°C. The free energies, entropies and enthalpies of solution were computed. A tentative interpretation of the results is given, based on a competition between the solubilization and the salting-out effects of the surfactants.

Solubilization of paraffin gases in aqueous solutions of sodium octanoate

The solubilities of methane, ethane, propane and n-butane were measured in aqueous solutions of sodium octanoate of molar concentrations between 0–0.8M. From these measurements we have computed the standard free energies, entropies and enthalpies for the process of transferring the solute molecules from the gaseous phase into the solutions. We found that propane and n-butane behave as expected at, and beyond, the cmc, namely the solubility takes a steep turn upwards. On the other hand, methane and ethane seem to cross the cmc with almost no change in their solubility. A tentative interpretation of this behavior is suggested, based on the competing effects of salting out and solubilization.

Thermodynamics of micellization and solubilization in the system water + sodium n-octanoate + n-pentanol at 25°c. Part 1.—Partial molar enthalpies

The molar enthalpies of mixing have been determined for aqueous solutions of sodium octanoate (NaC8) and pentanol in the aqueous solution region as well as for solutions of water in pentanol. From these results, the partial molar enthalpies of the three components have been calculated. The behaviour of sodium octanoate, even at extremely high dilution, is different from that of a simple binary electrolyte, which can be attributed to the interaction between the hydrocarbon chain and water. Ionic solute effects have a dominating influence on the enthalpies in solution. The partial molar enthalpies of pentanol and octanoate increase steeply below the c.m.c. while that of water decreases. Above the c.m.c. there are slower changes in the opposite direction. The transfer of sodium octanoate or pentanol from infinite dilution to a micelle is an endothermic process involving heats of a few kJ mol–1. When pentanol is added to a solution of NaC8 below the c.m.c., micelle formation is induced. When it is added to NaC3 solutions with low concentrations of micelles, the pentanol dissolves almost ideally while the addition at high micellar concentrations appears to be accompanied by changes in the binding of water to the micelles.

The effect of solubilized decanol on some properties of aqueous sodium octanoate solutions

The influence of decanol solubilization on the viscosity and the water vapor pressure of sodium octanoate solutions has been studied. The incorporation of decanol in the micelles causes an increase in the viscosity as well as in the vapor pressure at all concentrations above the CMC. The increase in viscosity is caused by the increased volume fraction of the micellar substance and is particularly obvious above the 3rd critical concentration, where the amount of hydrated micellar substance is so high that crowding between the micelles occurs. Under some circumstances the viscosity increase is somewhat retarded by the decrease of the charge density in the boundary layers of the micelles caused by the incorporation of decanol. The increase of water vapor pressure is due to the fact that the concentration of octanoate and sodium ions in the intermicellar solution is lower in the decanol-containing system than in the pure octanoate solutions and to the fact that the charge density in the boundary layer of the micelles decreases by the incorporation of decanol. The latter is more significant the higher the micellar concentration.

A carbon-13 NMR shielding study of the water-sodium octanoate-pentanol, and water-sodium octanoate-decanol systems

The 13 C NMR chemical shifts on both soap and alcohol carbons have been studied as a function of solution composition for the isotropic solution phases of the three-component systems sodium octanoate-pentanol-water and sodium octanoate-decanol-water. For aqueous sodium octanoate solutions the influence on the 13 C chemical shifts of solubilization of cyclohexane, p -xylene, and N,N -dimethylaniline was investigated as well as the effect of addition of sodium chloride. In general the 13 C shielding is sensitive to concentration changes and from a discussion of the origin of 13 C chemical shift changes it is found that the results provide information inter alia on alkyl chain conformational and polar end environmental changes. For water-rich solution phases, micelle formation, alcohol solubilization, and sodium chloride addition are all found to lead to an increased fraction of trans conformers in the soap alkyl chains. The soap alkyl chain flexibility seems to be greater for solutions containing reversed micelles than in normal micellar solutions for the nonpolar end, while there is no great difference between the two types of micelles in regard to the polar end. Studies of the carbonyl resonance of octanoate reveal considerable environmental alterations in both normal and reversed micelles and chemical shifts from the solubilizates give some insight into their solubilization state. An interaction between alcohol hydroxyls and soap carboxylate at the micellar surface is indicated.

The effect of sodium chloride on the solubilization of decan-1-ol in sodium octanoate solutions and the formation of mesophases

The influence of sodium chloride addition upon the ability of aqueous sodium octanoate solutions to solubilize decan-1-ol has been studied. At low octanoate concentrations an addition of neutral salt increases the solubilizing capacity; this depends on the displacement of cmc toward lower concentrations, thereby causing an increase in the content of micellar matter. At high octanoate concentrations the neutral salt exerts the opposite effect; this is obviously a result of the increased number of counterions bound to the mixed micelles and the earlier formation of mesophases caused thereby. Between the limiting association concentration and the second critical concentration, within which region of the neutral salt-free system water-rich mesophases are formed (water-rich D, B, and C phase), the addition of sodium chloride causes a decrease in the water content of the mesophase. Above the second critical concentration where even in the absence of sodium chloride a water-poor mesophase (D phase) is formed, there will primarily be an increase in the octanoate content of the phase. The composition of the middle phase (E phase) seems to be rather unaffected by sodium chloride addition. At high neutral salt addition the formation of mesophases is replaced by the formation of crystalline sodium octanoate; simultaneously, the aqueous solubility of octanoate markedly decreases.

NMR studies of counterion binding in the water—sodium octanoate-pentanol system

The nuclear magnetic relaxation rate and quadrupole splitting of 23Na + counterions have been studied as a function of temperature and composition for all the different phases of the three-component system sodium octanoate pentanol-water. For isotropic aqueous sodium octanoate solutions the effect on 23Na relaxation of adding decanol, N,N-dimethylaniline, p-xylene, cyclohexane, and sodium chloride was investigated. The results are used to obtain information on the ionic interactions in the solutions. For aqueous sodium octanoate solutions a strengthened interaction of the micellarly bound counterions is observed as the amphiphile concentration is increased and addition of sodium chloride is found to lead in general to a change, not only in the fraction of bound counterions, but also in the interactions of these ions. Solubilization of pentanol and decanol in octanoate micelles leads to an enforced counterion relaxation. For the sodium octanoate-pentanol-water system, counterion binding appears to be mainly determined by the ratio of octanoate to water while the effects of phase structure (normal and reversed micellar solutions and hexagonal and lamellar liquid crystals) as well as of pentanol concentration are small. Very rapid 23Na relaxation is observed for the highest alcohol contents of the alcohol-rich solution phases of the sodium octanoate-decanol-water and sodium octanoate-pentanol-water systems. This is ascribed to a partial dehydration of the sodium ions due to a dissociation of the reversed micelles and a formation of ion-pairs. The activation energies of the relaxation process support the conclusions drawn from the relaxation rates as do the quadrupole splittings. The latter permit some insight into the geometrical arrangement of counterions and amphiphile polar end-groups at the micellar surface. This is achieved by using the temperature dependence to deduce the sign of the order parameter. Both the relaxation rates and the quadrupole splittings indicate a considerable specificity of the ionic interactions and, over a wide concentration region in the lamellar phase, the quadrupole splittings suggest some penetration of the counterions between the polar end-groups. At the highest and lowest water contents the counterions appear to take up positions in which the ion-ion vectors are more nearly perpendicular to the lamellae.

An investigation of the size and structure of the micelles in sodium octanoate solutions by small-angle X-ray scattering

Micellar radii and the electron densities of the polar and nonpolar regions in micelles have been determined by small-angle X-ray scattering. This was done in aqueous solutions of sodium octanoate (NaC 8), in 1.2 M (=mole/liter) NaC 8 with added NaCl and in 1.2 M solutions of NaC 8 to which were added the solubilizates p-xylene, N,N-dimethyl aniline (DMA) and decanol. The micellar model developed by Luzzati, which describes the micelle in terms of different electron densities at different radii, was employed. Interpretation of the results according to this model results in a thick polar layer, indicating considerable hydration of the micelles and penetration of water between the hydrocarbon chains. Support for this interpretation is also given by the relatively low electron density in the polar layer. Solubilized decanol and DMA appears to be located in the polar region of the micelle, while p-xylene is mainly solubilized in the inner, nonpolar part. It was also shown that there is a slightly increased adsorption of counterions as their concentration in the solution increases.

Translational motion in aqueous sodium n-octanoate solutions

The translational self-diffusion coefficients of water molecules, sodium ions and octanoate ions in aqueous sodium octanoate solutions have been measured at 25°C with the open-ended capillary tube method using radioactive labeling. Below the critical micelle concentration (CMC), the self-diffusion coefficients of the ions vary only moderately with concentration. Above the CMC, however, a rapid decrease in translational mobility with increasing concentration is observed. The variation of water diffusion with sodium octanoate concentration is stronger before the CMC than at higher concentrations. Assuming that the ions can be found in three different environments, i.e., nonassociated, small complexes, and micelles, the self-diffusion coefficients are used to derive information on premicellar aggregation, CMC, counterion binding, and micelle hydration. From sodium ion and octanoate ion self-diffusion, the CMC is determined to 0.39 moles kg −1. The degree of counterion binding is about 60% for the first formed micelles and increases at higher concentrations. The first formed micelles contain about 8.7 water molecules/octanoate ion. A change in hydration is indicated at roughly 1.1 moles kg −1. These results are found to be in good agreement with conclusions based on studies employing other experimental techniques.

CONDUCTANCES OF AQUEOUS SOLUTIONS OF SODIUM OCTANOATE AT 25° AND 35° AND THE LIMITING CONDUCTANCE OF THE OCTANOATE ION

Equivalent conductances, densities, and viscosities of aqueous solutions of sodium octanoate have been determined at 25° and 35 °C, at concentrations ranging from 0.0002 M to 2.8 M. The limiting equivalent conductances of the octanoate ion have been determined as 23.08 mhos and 29.09 mhos, at 25° and 35 °C respectively.Comparison has been made of our experimental conductances with those calculated, using the equations of Robinson–Stokes, of Falkenhagen–Leist, and of Fuoss.No evidence has been found of micelle formation in solutions of sodium octanoate.