Yoshikiyo Moroi

The critical micelle concentration of sodium dodecyl sulfate-bivalent metal dodecyl sulfate mixtures in aqueous solutions

Abstract Critical micelle concentrations (CMC) were determined at 30°C for aqueous sodium dodecyl sulfate (SDS)-bivalent metal dodecyl sulfate [M (DS) 2 ] mixtures over the entire surfactant composition range. Bivalent metals used were Mg 2+ , Mn 2+ , Co 2+ , Ni 2+ , and Cu 2+ . The CMC of surfactant mixtures decreased rapidly with the increase of M (DS) 2 content from the value 8.25 × 10 −3 mole/liter for pure SDS to the value 2.7 × 10 −3 mole/liter at 0.2 in mole fraction of M (DS)2, and then asymptotically to the value 1.20 × 10 −3 mole/liter for pure M (DS) 2 . Variation of the CMC with the composition can be explained in terms of an improved Shinodas equation, in which the charge density at the micelle surface and the effective coefficient of an electrostatic contribution to micellar formation defined as K g were modified. An excellent agreement was obtained between experimental and theoretical curves. The same treatment was also successful for the sodium dodecyl sulfate-potassium dodecyl sulfate mixture.

An interrelationship between heat of micelle formation and critical micelle concentration

A discussion whether or not the contribution of water must be introduced into a thermodynamic equation of micellization was made on the basis of a phase separation model. As a result, an enthalpy change on micelle formation at very low CMC can be fairly approximated by the conventional expression, ΔHm = − nRT2(∂ ln CMC/∂T)p. The contributions of hydrophilic and hydrophobic groups to the enthalpy change were investigated for the case of four kinds of sodium alkyl sulfates with different chain lengths from C8 to C14 over the temperature range from 10 to 55°C. It was found that the hydrophilic part has a major contribution at lower temperatures but at higher temperatures it gives a minor contribution, and that the hydrophilic part of ΔHm is always positive having a minimum around 40°C, while the hydrophobic part is always negative and decreases monotonically with temperature. Furthermore, the water around the hydrophilic head group was found to have a great effect on the CMC; when a parameter due to the water is selected properly, the degree of dissociation of micelle and other electrochemical properties at the micellar surface can be calculated to be the reasonable values.

Micellization of fluorinated amphiphiles

Abstract New cationic fluorinated surfactants and new types of fluorinated surfactants having fluorocarbon–hydrocarbon hybrids, dimeric and polymeric structure have been synthesized recently. Their synthesis requires many steps and consequently requires much time and high expense. Since the fluorinated surfactants have unusual molecular aggregation properties, 19F-NMR, novel fluorescence probes and cryo-transmission electron microscope techniques have been applied to study their aggregation behaviour in aqueous systems. Their unique characteristics are summarized as follows: (1) the dissolution process from solid state to dissolved aggregate state requires a very long time for the long chain fluorinated surfactants under thermodynamic equilibrium. The equilibration time can be reduced at higher temperatures; (2) interfacial properties and critical micelle concentration (CMC) are influenced by the nature of the hydrophobic terminal groups (CF3− or HCF2−); (3) the fluorocarbon functionality can make it possible even for single-chain amphiphiles to form vesicles or lamellar structures; (4) the hybrid surfactant made of both hydrocarbon and fluorocarbon chains showed a life time of 2.0×10−3 s for the exchange rate between the monomeric and the micellar states at the CMC and moreover, these detergents can cosolubilize fluorocarbon–hydrocarbon mixed solubilizates.

Micelle formation of sodium cholate and solubilization into the micelle.

The micellization of sodium cholate (NaC) was studied at 298.2 K by aqueous solubility at different pH values. Using a stepwise association model of cholate anions without the sodium counterion, the aggregation number (n) of the cholate micelle was evaluated and found to increase with the total concentration, indicating that the mass action model worked quite well. The n value at 60 mM was found equal to 16. The membrane potential measurement of sodium ion with a cation exchange membrane was made in order to confirm the low counterion binding to micelle. The solubilization of alkylbenzenes (benzene, toluene, ethylbenzene, n-propylbenzene, n-butylbenzene, n-pentylbenzene, n-hexylbenzene) and polycyclic aromatic compounds (naphthalene, anthracene, pyrene) into the aqueous micellar solution of sodium cholate was carried out. Solubilizate concentrations at equilibrium were determined spectrophotometrically at 298.2 K. The first stepwise association constants (K1) between solubilizate monomer and vacant micelle were evaluated from the equilibrium concentrations and found to increase with increasing hydrophobicity of the solubilizate molecules. From the Gibbs energy change for solubilization at the different mean aggregation numbers and from molecular structure of the solubilizates, the function of sodium cholate micelle for solubilization was discussed and was compared with data from conventional aliphatic micelles.

Micelle formation of sodium glyco- and taurocholates and sodium glyco- and taurodeoxycholates and solubilization of cholesterol into their micelles

Micelle formation of sodium glyco- and taurocholates and sodium glyco- and taurodeoxycholates was studied at 308.2 K for the critical micelle concentration at various NaCl concentrations by pyrene fluorescence and scattered light intensity, and the degree of counterion binding to micelle was calculated using the Corrin–Harkins plots. The change of I1/I3 values of the fluorescence spectrum with the conjugate bile salt concentration suggested two steps for the bile salt aggregation, which was supported by the scattered light intensity. The first step is a commencement of smaller aggregates, the first CMC, and the second one is a beginning of stable aggregates, the second CMC. Between the first and the second CMCs, the aggregates grow in size with the conjugate concentration. The aggregation number above the second CMC was determined at 308.2 K and 0.15 mol dm−3 NaCl concentration by the static light scattering: 8.7 and 6.0 for sodium glyco- and taurocholate, respectively, and 15.7 and 15.9 for sodium glyco- and taurodeoxycholate, respectively. The solubilization of cholesterol into the bile salt micelles in the presence of coexisting cholesterol phase or the maximum additive concentration (MAC) of cholesterol was determined against the bile salt concentration, and then, the standard Gibbs energy change for the solubilization was evaluated, where the micelles were regarded as a chemical species. The solubilization was stabilized in the order of sodium taurocholate

Solubilization of cholesterol and polycyclic aromatic compounds into sodium bile salt micelles. (Part 2)

The aqueous solubility of cholesterol was determined over the temperature range from 288.2 to 318.2 K with intervals of 5 K by the enzymatic method. The solubility was (3.7+/-0.3)x10(-8) mol dm(-3) (average +/- S.D.) at 308.2 K. The maximum additive concentrations of cholesterol into the aqueous micellar solutions of sodium deoxycholate (NaDC), sodium ursodeoxycholate (NaUDC), and sodium cholate (NaC) were spectrophotometrically determined at different temperatures. The cholesterol solubility increased in the order of NaUDC<NaC<NaDC; for example, 0.10 for NaUDC, 0.61 for NaC, and 2.99 mmol dm(-3) for NaDC at the concentration of 60 mmol dm(-3) and at 308.2 K. The same solubilization experiments were made at 308.2 K using polycyclic aromatic compounds (benzene, naphthalene, anthracene, pyrene) as a reference. Their solubility increase for the bile salts was in the same order as above. Thermodynamic analysis was made for the solubilization, where a micelle was regarded as a chemical species. The average number of solubilizate per micelle was less than unity throughout the experiments. From the Gibbs energy change for solubilization at the different mean aggregation numbers, the function of bile salt micelles was discussed from the viewpoint of molecular structure of solubilizates. The DeltaG(0) value for cholesterol was most negative among the solubilizates studied, which reflected that solubilization of cholesterol into bile salt micelles brought about largest thermodynamic stabilization.

Mass action model of micelle formation: Its application to sodium dodecyl sulfate solution

Abstract The mass action model was applied to micelle formation of an ionic surfactant in aqueous solution. A method was developed to evaluate the micellization constant, the micelle aggregation number, and the number of counterions per micelle from the bulk concentrations of surfactant ion and/or counterion at definite surfactant concentrations, where the monodispersity of micelles was assumed. Application of the method to the micellization of sodium dodecyl sulfate demonstrated excellent agreement between the calculated and the reported values of the bulk concentrations from the CMC up to a concentration of 10 times the CMC. The slope of the logarithm of CMC against the logarithm of total counterion concentration turned out to be approximately equal to the degree of counterion association with the micelle.

Temperature effect on formation of sodium cholate micelles

The micellization of sodium cholate (NaC) at 293.2, 298.2, 303.2, 308.2, and 313.2 K by cholate anion concentration was studied over the pH range from 6.0 to 7.2. Using a stepwise association model of cholate anions without bound sodium counterions, the aggregation number (nmacr;) of the cholate micelles was evaluated and found to increase with the total concentration, indicating that the stepwise association model is applicable. The nmacr; values go up and down with increasing temperature; 17 at 298.2 and 12 at 313.2 K and at 60 mM of the sodium cholate. The fluorescence of pyrene was measured in sodium cholate solution to determine the critical micelle concentration (CMC), indicating a narrow concentration range for CMC. A sodium-ion-specific electrode was used to determine a relatively low degree of counterion binding to micelles, supporting the validity of the present association model of cholate anions. The aggregation numbers evaluated at a constant ionic strength of 0.15 and at lower but variable ionic strengths were similar except for higher cholate concentrations.

Dissolution and micellization of sodium n-alkylsulfonates in water

Abstract Kraft points, critical micelle concentrations (CMC), and solubility of sodium n -alkylsulfonates from 10 to 22 carbon atoms were determined in an aqueous solution. Kraft points increase from 22 (C 10 ) to 74°C (C 22 ), while the CMC values decrease with increasing alkyl chain indicating a linear log CMC against the carbon number. Solubilities also decrease with increasing alkyl chain with the rate of decrease in chain length diminishing as carbon numbers increase. The thermodynamical parameters for dissolution, Δ G 0 , Δ H 0 , and Δ S 0 , were evaluated from the solubility change with the temperature at 20°C. Since it was found that the Δ S 0 values decrease almost linearly with alkyl chain length, water molecules around the alkyl chain play a very important role in the dissolution behavior, and the dissolved alkyl chain seems to remain stretched, not coiled. Comparison was made on micellar and dissolution behavior between sodium n -alkylsulfonates and corresponding sulfonic acids.

Phase equilibria of anionic surfactant mixtures in aqueous solution

Abstract Krafft points were determined for aqueous sodium dodecyl sulfate (SDS)-bivalent metal dodecyl sulfate [M(DS)2, where M is Ca, Mn, Cu, or Zn] mixtures by electrical conductivity. Determination of the composition of aqueous solutions in equilibrium with solids and chemical analyses of the solid phases at equilibrium obtained by cooling were performed in order to construct a phase diagram including the water component. The phase diagram of the Mn system with a constant water component and a different composition of the surfactant mixture was constructed. It represents the behavior of the other bivalent ions and is very similar to that of a usual two-component mixture above the critical micelle concentration (CMC), giving a single eutectic point at about 5°C located at high sodium contents (∼80%). Below the CMC of a mixture, with higher water content, the line of the eutectic point goes down to the ternary point where the mixture has a much higher sodium content. It was found from the phase diagram that the water component plays an important role in the whole system and that the Krafft point is not the temperature at which solubilization of the crystal comes to an end but the temperature at which micelle formation is initiated.