Sodium Octanoate Solutions Research Articles

Laser diagnostics of self-organization of amphiphiles in aqueous solutions on the example of sodium octanoate

In this study, a method of non-contact laser diagnostics of self-organization processes of surfactants in aqueous solutions is proposed. The study of solutions of sodium octanoate using laser Raman and near-IR spectroscopy has shown that spectral bands of stretching vibrations of CC-, CH- and OH-groups (including second overtone of the OH-vibrations) of the surfactant molecules and water, as well as bands of intermolecular vibrations of hydrogen bonds O-H⋯O are sensitive to the change the conformation of the hydrocarbon radical of the surfactant and the dynamics of hydrogen bonds during self-organization of surfactants in solution. It was found that the dependence of the spectral characteristics of these bands on concentration of surfactants has peculiarities in the region of critical premicelle concentration (CPMC), first critical micelle concentration (CMC1) and second critical micelle concentration (CMC2). A method of determining the critical concentrations of surfactants in water based on the revealed peculiarities is proposed. A comparative analysis of the obtained CMC values using laser Raman and IR spectroscopy and those measured by independent contact methods – pH-metry, conductivity, surface tension measurement – has shown a good match of the determined parameters. Advantages of the method of laser diagnostics of surfactant self-organization in water were discussed.

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.

Structural micellar transition for fluorinated and hydrogenated sodium carboxylates induced by solubilization of benzyl alcohol.

The solubility of benzyl alcohol in micellar solutions of sodium octanoate and sodium perfluorooctanoate was studied. From the isotherms of specific conductivity versus molality at different alcohol concentrations, the critical micelle concentration and the degree of ionization of the micelles were determined. The cmc linearly decreases upon increasing the amount of benzyl alcohol present in aqueous solutions with two distinct slopes. This phenomenon was interpreted as a clustering of alcohol molecules above a critical point, around 0.1 mol kg(-1). Attending to the equivalent conductivity versus square root of molality, the presence of a second micellar structure for the fluorinated compound was assumed. The thermodynamic parameters associated with the process of micellization were estimated by applying Motomura's model for binary surfactant mixtures, modified by Pérez-Villar et al. (Colloid Polym. Sci 1990, 268, 965) for the case of alcohol-surfactant solutions. A comparison of the hydrogenated and fluorinated compounds was carried out and discussed.

Phase equilibria and phase structure in the ternary systems sodium or potassium octanoate-octanoic acid-water

Two isotropic solution regions and several liquid crystalline regions occur in the ternary system sodium octanoate-octanoic acid-water at 20°C The solution regions are an aqueous solution and a solution of sodium octanoate and water in liquid octanoic acid. A region displaying one-dimensional lamellar structure is located in the center of the phase diagram. A region along the soap-water axis has a two-dimensional normal hexagonal structure. Another region at high octanoic acid content has a reversed hexagonal structure. Along the soap-fatty acid axis the acid-soap 2NaC8:1HC8 in crystalline state is found. X-ray and density findings for the various phases are presented, and structural parameters for the different liquid crystalline phases are estimated. The phase behavior of the potassium soap system is similar to that of the sodium system. The isothermal ternary phase diagram of a soap, the corresponding fatty acid and water provides information about the ionization state of the system, from the unionized fatty acid to the fully ionized soap.

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 structure and properties of medium-chain surfactant solutions: a case study of sodium octanoate

In the paper the most characteristic properties of binary sodium octanoate solutions of variable ion strengths are discussed. Both pre- and postmicellar association is investigated with reference to available scattering, spectroscopic and thermodynamic data. The focus is then placed on references dealing with, or referring to, results including sodium octanoate. A picture is offered illustrating the sodium octanoate micelle as a rather unstable entity including micelle sizes from oligomeric pre-associates up to some 20 monomers. Due to this circumstance the molecular features obtained with different scattering and spectroscopic techniques are rather divergent. However, the structure emerging describes the micelle as consisting of a small “dry” hydro-carbon core with a radius shorter than the fully extended heptyle chain of the octanoate. Instead a “rough shell” encompassing the hydrated polar head groups, some “wet” methylene/methyle groups and the hydrated counterions separates the micelle from the surrounding solvent. On purely geometric grounds this hydrated shell represent, however, more than 50% of the micellar volume. The numerous distinct properties of medium-chain surfactants should thence not, as is frequently done, be related to those of long-chain surfactants! At high concentrations of octanoate or salt, or upon solubilization, the average degree of water contact seem to diminish due to an increased aggregation number or a lack of available “free” hydration water. The latter effect is suggested to produce observable effects at the second critical concentration as well as close to the phase boundary. The influence of solubilizates are dealth with only when the results provide significant information on the properties of the binary system.

Solutions of alkali soaps and water in fatty acids XI. Correlation between the acid sodium octanoate in the most water-rich part of the L2-phase and the acid soaps in two adjacent phases

The special nature of the outer-most water-rich region of theL2-phase in the ternary system sodium octanoate-octanoic acid-water is evidenced by its somewhat turbid appearance and by the character of its equilibria with adjacent phases. The phase contains aggregated acid sodium octanoate which is dispersed in a very dilute aqueous solution of sodium octanoate. The acid octanoate has the composition 1 NaC8∶2 HC8∶x H2O and is composed of closely packed amphiphilic units, all with the polar groups in the same direction. This acid soap obviously forms double-layered aggregates with the lipophilic hydrocarbon chains pointing inwards and the polar groups pointing outwards towards the surrounding bulk-water. The phase is formed when octanoic acid is added to theL1-phase of the system just above the l.a.c.; in this aqueous solution, the acid reacts with dissolved acid octanoate 1 NaC8∶1 HC8∶x H2O and that results in the formation of the slightly soluble acid soap 1 NaC8 ∶2 HC8∶x H2O that separates as a new phase, the turbidL2 phase. On further addition of octanoic acid, the content of the mentioned acid soap increases until the solution phase is transformed into a liquid crystalline lamellarD-phase with the same acid soap composition. This formation of acid soap 1 NaA∶2 HA on addition of fatty acid to the dilute soap solution just above the l.a.c., has been known for a long time to occur in various systems containing a long-chain sodium soap. However, at suitably low temperatures, the reaction in these systems does not result in separation of the acid soap in the liquid crystalline, but in the solid crystalline state.

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.

Isotachophoretic analysis of surfactants

AbstractThe qualitative and quantitative determination of anionic surfactants (sodium and lithium alkylsulfate, sodium dodecylbenzenesulfonate, sodium di‐2‐ethylhexyl sulfosuccinate, sodium and lithium perfluoroalkylcarboxylate, and lithium perfluorooctanesulfonate) were investigated by a capillary tube isotachophoresis using a potential gradient detector. The leading electrolyte solution was the mixture of acetonitrile and aqueous solution of histidine buffer containing calcium chloride. The terminating electrolyte solution was the aqueous solution of sodium octanoate. These electrolytes were effective for the analysis of the mixtures of strongly acidic surfactants.

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 micelle formation: Model calculations for alkyl carboxylates

A model for the calculation of the standard free energy of transfer of an amphiphilic monomer from the intermicellar solution to a spherical micelle that previously [9] was applied to solutions of sodium octanoate is shown to be applicable to the homologous series of straight-chain sodium alkyl carboxylates. This model is largely based on the previous works by Ruckenstein and Nagarajan [2], Israelachvili, Mitchell and Ninham [12] and Tanford [11]. The two main factors involved are: the standard free energy of transfer of the hydrocarbon chains from water to the liquid hydrocarbon interior of the micelle with the account taken to the residual hydrocarbon/water contact and the repulsion between the polar end groups. The model is shown to give good agreement with experimental data on mean aggregation numbers, apparent molar volumes and counter ion and water activities for all of the short-chain [C 5—C 6] carboxylates.

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.

Partial molal volumes and compressibilities of n-alkanes in aqueous solution of sodium-octanoate and sodium-decanoate

Partial molal volumes and compressibilities have been determined for n-alkanes (C 7C 10) in aqueous solution of sodium-octanoate and sodium-decanoate at 25°C. These quantities have also been determined for the pure liquid alkanes. The partial molal volumes and compressibilities were determined by density measurements and ultrasound measurements, respectively. The partial molal volumes and compressibilities of the alkanes in the surfactant solutions are higher than those in water, especially the compressibilities, and resemble the partial molal quantities of the liquid alkanes. The partial molal volumes are almost independent of the chain length and the concentration of the surfactant. The partial molal compressibilities decrease slightly with decreasing chain length of the surfactant and with increasing micellar concentrations. These results have been interpreted from differences in the hydration and the structure of the micelles.

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.