Ryohei Matuura

Thermodynamic studies on adsorption at interfaces: IV. Dodecylammonium chloride at water/air interface

Abstract The thermodynamic treatment of the adsorption at water oil interface developed in the previous papers has been extended so as to be applied to the adsorption at water/air interface. The surface tension of the aqueous solution of dodecylammonium chloride (DAC) has been measured as a function of temperature and concentration. By applying the thermodynamic relations, the entropy of surface formation Δs and the surface density of DAC Γ 1 H have been calculated. Further, we have shown that the energy of surface formation Δ u can be evaluated numerically. It has been observed that the values of Δs and Δ u decrease, while the value of Γ 1 H increases, with increasing concentration of DAC. It has been found that the adsorbed film transforms from a gaseous to an expanded state at a relatively low concentration, which has been proved thermodynamically. Such a behavior seems to be similar to that at water hexane interface except for the phase transition. The mutual interaction between DAC molecules in the adsorbed film appears to be weakened by the presence of hexane.

Micelle formation of sodium alkylsulfate under high pressures

Abstract Under high pressures up to 3000 atm, the effects of added salt, chain length, and temperature on the critical micelle concentration (CMC) of sodium dodecylsulfate (SDS) were investigated. The linear relationship between the logarithm of the CMC and of the gegenion concentration was confirmed at high pressures up to 3000 atm. From the behavior of gegenion under pressure, the variation of the aggregation number of micelles with pressure was estimated; initial compression seems to cause the micelle to disaggregate, resulting in the formation of a number of small micelles. The well known fact, at atmospheric pressure, that the CMCs of SDS and its homologous surfactants decrease logarithmically with an increase in the number of carbon atoms in the alkyl chain was confirmed even at pressures up to 3000 atm. From these results, the contributions of hydrocarbon tail and ionic head to the standard free energy and partial molal volume changes on micellization are discussed. Finally, the CMC of SDS was determined at various pressures and temperatures of 17 to 40°C. The temperature at which the CMC becomes minimum tends to shift toward the lower temperature as the pressure increases. The changes of partial molal entropy and enthalpy on micellization, which decrease as the temperature is raised and increase as the pressure increases to a certain value, could be qualitatively explained by the formation of an “iceberg” structure of water around the hydrocarbon tail in the monomer state. Further, the micelle-forming properties are discussed in terms of the temperature and pressure dependence of the CMC.

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.

Thermodynamic studies on adsorption at interfaces. II. One surface-active component system: Tetradecanol at hexane/water interface

Abstract In order to show that the thermodynamic [ J. Colloid Interface Sci. 64 , 348 (1978)] treatment developed in Part I is useful in studying the adsorption of surface-active substances at interfaces, comparison has been made with a system consisting of water and dilute solution of tetradecanol in hexane. As the system is trivariant, we have chosen the temperature, pressure, and mole fraction of tetradecanol in hexane for the thermodynamic independent variables and measured the interfacial tension as their functions. By means of the thermodynamic equations derived in Part I, the interfacial concentration of tetradecanol and thermodynamic quantities of interface formation have been determined from the experimental results. Further, the partial molar thermodynamic quantity changes at the adsorption have been calculated. It has been observed that the thermodynamic quantities of interface formation decrease, while the interfacial concentration increases with increasing the mole fraction of tetradecanol. The results are explicable in terms of negative values of the partial molar quantity changes.

Phase transition in the adsorbed films at water/air interface

Abstract The surface tension at the aqueous solution/air interface has been measured for eight surface active substances at 298.15°K under atmospheric pressure. It has been found that each surface tension versus concentration curve has a distinct break at a relatively low concentration and a relatively high surface tension. This result implies that the two states of adsorbed monolayer coexist in equilibrium at the break point. The surface excess numbers of moles of surface active substances and the surface pressure versus mean area per adsorbed molecule curves confirmed that the phase transformation takes place between the gaseous and the expanded states. It has also been shown that the hydrocarbon chain length, and the ionic nature and size of polar head group of a surface active substance have a great influence on the properties inherent in the phase transition.

Thermodynamic studies on adsorption at interfaces

Abstract The thermodynamic treatment of adsorption developed in Part I has been extended so as to include the case of strong electrolytes. By applying the thermodynamic relations to experimental results obtained for the system consisting of aqueous solution of sodium dodecyl sulfate (SDS) and hexane, the thermodynamic quantities of interface formation and partial molar quantities of the adsorption have been evaluated. Their values have been compared with the corresponding ones of tetradecanol described in Part II. It has been observed that SDS has a negative entropy of interface formation which is smaller than tetradecanol, while it has a larger interfacial concentration. This value is closely related to a larger positive value of the volume of interface formation. The above view has been confirmed by changes in the mean partial molar entropy and volume at adsorption. It has also been found from the energy of interface formation and partial molar energy of the adsorption that the adsorption of SDS from aqueous solution is inferior energetically to that of tetradecanol from hexane solution.

Interaction between ionic surfactants and polyethylene oxide in relation to mixed micelle formation in aqueous solution

Abstract An essential difference between the interactions of anionic and cationic surfactants with polyethylene oxide was investigated from the point of mixed micelle formation of anionic- and cationic-nonionic surfactants (polyoxyethylene alkyl ether type). The free-energy difference of the ionic head group of the surfactant between a bulk and a mixed micellar state is much greater for anionic-nonionic than for cationic-nonionic surfactant mixtures. The former is −6 to −9 kcal/mole, and the latter is 2 to 3 kcal/mole. This indicates that the complex comes mainly from the interaction between the ionic head group and the ethylene oxide group of polymer and is hardly affected by the hydrophobic interaction between the surfactant tail and the ethylene oxide group. From this concept it can be shown that the mixed micelle concentration of anionic (SDS)-nonionic surfactant mixtures is approximately equal to the SDS concentration at the first inflection point of an electroconductivity vs concentration curve for complex formation.

Thermodynamic studies on adsorption at interaces: V. Adsorption from micellar solution

Abstract The thermodynamic treatment of the preceding paper has been extended so as to be applied to the adsorption of ionic surfactant from its micellar solution at the water/air interface. The surface tension of dodecylammonium chloride solution has been measured as a function of temperature at concentrations around its critical micelle concentration (CMC). The derivative of surface tension with respect to temperature has been observed to change abruptly at the CMC and then decrease very slowly with increasing concentration. It has been asserted by comparing the thermodynamic relations with experimental results that the chemical potential of micelle depends only on temperature and pressure in a limited concentration range, and the derivative of the surface tension of micellar solution with respect to temperature is related directly to the entropy change associated with the adsorption of surfactant from the micellar state. The micelle has been concluded to be similar in thermodynamic behavior to the adsorbed film.

Thermodynamic study on the adsorption of dodecylammonium chloride at water/hexane interface

Abstract The adsorption of dodecylammonium chloride (DAC), which is a cationic surfactant, at water/hexane interface has been studied by measuring the interfacial tension as functions of temperature and pressure at various concentrations of DAC in the aqueous solution. By applying the thermodynamic treatment developed previously to the experimental results, the thermodynamic quantity changes associated with interface formation and corresponding partial molar quantity changes of DAC have been evaluated. These values have been compared with those of sodium dodecyl sulfate (SDS), which is an anionic surfactant. It has been observed that the dependence of the entropy change on temperature and that of the volume change on pressure are remarkable. It has also been observed that DAC and SDS differ appreciably in the values of the partial molar entropy and energy of adsorption while they have similar values in the partial molar volume change of adsorption under atmospheric pressure.