Ingvar Danielsson

Anion association and micelle formation in solutions of hydrotropic and short-chain carboxylates☆

Abstract A survey is given of studies on the association processes, the changes in structure and the formation of micelles in solutions of alkali metal salts of various carboxylic acids. The substances investigated are normal and branched alkanoates with 1–8 carbon atoms, and typical hydrotropes such as hydroxybenzoate. The experimental methods have been measurement of activities of carboxylate ions, counterions, and solvent by potentiometry, vapor pressure osmometry and gas chromatography; calorimetric determination of enthalpies of mixing; NMR studies of relaxation times for protons and line widths of signals from counterions; dilatometric measurements of changes in density on mixing and potentiometric investigations of hydrolytic equilibria. In order to simplify the evaluation of the results, the solutions were given a high content of electrolyte, mostly by adding sodium chloride to an ionic strength of 3 c. It has been established that association occurs in solutions of all alkanoates except acetate and propionate; the results are less conclusive for the benzoates. The initially formed aggregates contain from 2 to 4 anions. Larger aggregates (micelles) are formed by alkanoates with more than four carbon atoms. The results support the view that these association processes are mainly results of structural changes in the solvent, i.e., the aggregates are held together by hydrophobic bonding.

Assoziations- und Phasengleichgewichte bei der Einwirkung von Paraffinkettenalkoholen an wässrigen Lösungen von Assoziationskolloiden

ZusammenfassungDie Phasengleichgewichte im Drei-Komponentensystem Natriumkaprylat-Dekanol-Wasser wurden untersucht und die Resultate in einem Dreieckdiagramm dargestellt. Es treten zwei Gebiete mit isotropen Lösungen auf, und zwar das eine mit Wasser, das andere mit Dekanol als Lösungsmittel.Außer dekanolhaltiger Kernseife (Neat soap) (GebietG) im seifenreichsten Teil des Systems, treten mesomorphe dekanolhaltige Klumpseife (Middle soap) (E) sowie vier bis fünf weitere mesomorphe Kaprylat-, dekanol- und wasserhaltige Phasen (A– D, B, C, F) auf. Der Wassergehalt einiger mesomorpher Phasen kann recht erheblich sein. Diese Wasseraufnahme wird durchDonnan- Gleichgewicht geregelt.Donnan-Verteilung tritt ein, sobald die Ladungsdichte an den inneren ionisierten Oberflächen der mesomorphen Phasen so weit abgenommen hat, daß die Natrium-Ionen nicht mehr an die Oberfläche fest gebunden sind. Die wäßrigen Kaprylatlösungen stehen im Gleichgewicht mit fünf mesomorphen Phasen oder Phasenabschnitten. Die Lösungs- bzw. mesomorphen Phasenabschnitte sind durch 3-Phasendreiecke getrennt, die von der Grenzkonzentration (LAC) bzw. der sogenannten Zwischenkonzentration (2nd LAC), der kritischen Konzentration (CMC), der zweiten kritischen Konzentration (2nd CMC) und der Alkohol-Klumpseifen-Ecke (AMSC) ausgehen. Die betreffenden Lösungsabschnitte enthalten verschiedene Assoziationsstufen von Kaprylationen und Dekanol. Eine allgemeine Theorie für die Bildung wasserhaltiger mesomorpher Substanzen aus wäßrigen Seifenlösungen wurde entwickelt. Die Theorie geht von der Annahme aus, die verschiedenen Aggregate, seien es Mizellen oder kleinere Aggregate, die in den Seifenlösungen vorkommen, wirkten bei der Abscheidung aktiv mit.

A study of the micelle formation of short—chain carboxylates by laser—ramon spectroscopy

Abstract The C-C and C-H vibration regions at 1000–1200 cm −1 and 2800–3000 cm −1 , respectively, have been investigated by Laser-Raman spectroscopy for the sodium salts of normal aliphatic fatty acids (C 2 -C 8 ) in aqueous solutions at constant and variable ionic strength. The spectra are compared with the corresponding regions for the crystalline salts, with spectra of an aliphatic alcohol, and with spectra of perdeuterated sodium octanoate and normal sodium octanoate in deuterium oxide. It is shown that the relative intensities of the peaks in these regions are dependent on the concentration. The results can be reconciled with the following model of micelle structure. As the concentration of monomers increases in aqueous solution, the hydrocarbon chain (h.c.) structure changes toward a larger proportion of trans configurations. When micelles are formed, the configuration of the hydrocarbon chains in the micelles remains similar to that of monomers or premicelles in equilibrium with the micelles. The polar character of the environment of the hydrocarbon chains changes gradually toward a nonpolar character; this change continues up to concentrations several times higher than the critical micelle concentration (CMC), indicating considerable binding of water in the micelles. Thus, in the concentration range below the CMC the most important structural change with increasing concentration seems to be the change in the h.c. chain configuration.

Lamellar mesophase with high contents of water: X-ray investigations of the sodium octanoate-decanol-water system

Abstract The phase equilibria at high weight ratios of water to amphiphilic substance in the three-component system water/sodium octanoate/1-decanol have been reinvestigated. It is concluded that the liquid crystalline regions designated B and C in earlier reports on the equilibria in this system do not represent phases that are separate from the lamellar liquid crystalline phase D. The boundary of the region D in water-rich systems has been redetermined by utilization of the relationship between the water content and the Bragg spacings. This makes possible determination of phase boundaries without separation of samples by ultracentrifugation. The results show the existence of only one lamellar liquid crystalline phase (D) in equilibrium with micellar or submicellar aqueous solution (L 1 ). The lower phase boundary of D is located at lower ratios decanol/octanoate than was reported previously whereas the water content of the D phase in three-phase equilibrium with aqueous and decanolic solutions is about 2 wt% lower than that reported earlier. In the C region a very stable dispersion is formed.

Equilibria and structures of microemulsions and liquid crystals in water-surfactant-oil systems

Microemulsions and lyotropic mesophases in aqueous solutions of association colloids are closely related. The ionic groups are always completely hydrated in micelles and mesoaggregates. Strong evidence of this is provided by the fact that the heat of solution of solid sodium octanoate both in a moderately concentrated micellar aqueous solution and in a reversed micellar solution of sodium octanoate in decanol containing solubilized water is ca 1.8 kJ mole/sup -1/. These results are in agreement with the hypothesis that the electric contributions to the energy of the interfaces are decisive for the formation of different aggregates. Accordingly, the equilibrium between 2 different aggregates should be characterized by an equal number of ionized groups per unit area of the micellar surfaces and equal effects of counterions. 22 references.

Nuclear magnetic relaxation of 85Rb in some aqueous soap solutions

The nuclear quadrupole relaxation rate of 85Rb in aqueous solutions of rubidium caprylate and rubidium caproate has been studied as a function of soap concentration and temperature. At low concentrations, the 85Rb NMR line width is independent of concentration. In the region of the critical micelle concentration (cmc), the line width starts to increase. At moderately high concentrations, data are compatible with a model with two binding sites for the rubidium ions, one site corresponding to a free Rb+ ion and the other site to a micellarly bound ion. In a region above the cmc the counterion binding to the micelles is constant, but at very high concentrations the counterion binding increases. The apparent energies of activation for the relaxation process calculated from the variation of relaxation rate with temperature are considerably greater in the micellar solutions than in a rubidium chloride solution. The marked difference in behavior between the rubidium ions in the present investigation and the bromide ions in solutions of alkylammonium bromides is discussed.

The association of short chain alkanoates as studied by NMR methods

Abstract NMR chemical shifts and nuclear spin-lattice relaxation times for aqueous solutions of short chain alkanoates are reported. For caproate solutions micelles are shown to be formed. For valerate and butyrate solutions the results indicate the formation of smaller aggregates. The chemical shift method is shown to be unreliable for determining the critical micelle concentration.

AGGREGATION STUDIES IN THE SYSTEM WATER - SODIUM OCTANOATE - BUTANOL

ABSTRACT The L-phase of the three-component system water - sodium octanoate -butanol extends over a large composition range. It is well known that micellar aggregates exist in the water rich part (the L1 part) and inverse micellar aggregates in the butanol rich part (the L2 part). The mechanism for the change from a continuous water medium to a continuous butanol medium in the middle part is of fundamental interest. The properties of the L-phase were studied by density, refractive index, ultracentrifugation, electromotive force and conductivity measurements. This investigation indicates in which regions aggregation occurs as well as the changes in aggregate and medium structure which take place with increased butanol concentration.