Shoichi Ikeda

Formation of spherical and rod-like micelles of cetyltrimethylammonium bromide in aqueous NaBr solutions

Abstract Static light scattering from aqueous NaBr solutions of cetyltrimethylammonium bromide (CTAB) has been measured at 35°C with 488-nm laser light, and the molecular weight, radius of gyration, and the other parameters of its micelles have been evaluated. In water, spherical micelles are formed above a critical micelle concentration, 0.04 × 10 −2 g cm −3 , and each micelle consists of 91 monomers of CTAB. In aqueous solutions having NaBr concentrations higher than 0.06 M , rod-like micelles are formed, besides spherical micelles. The molecular weight of rod-like micelles increases rapidly with increasing NaBr concentration. The rod-like micelle formed in 0.5 M NaBr has molecular weight as high as 3,470,000, and is considerably flexible. The persistence length of micelle is evaluated to be 44 nm. Electron micrographic observations have been carried out for specimens prepared from CTAB solutions in water and in 0.5 M NaBr at 35°C. A specimen prepared from aqueous solution of CTAB and negatively stained with uranyl acetate displays many globular images, which have a uniform diameter around 5–6 nm and can be assigned to spherical micelles. A specimen from 0.5 M NaBr solution shows tortuous thread-like images with a uniform diameter of 4.5–6 nm, which can be regarded as flexible rod-like micelles. Thus, the size and shape of spherical and rod-like micelles of CTAB observed by the light-scattering measurement are confirmed by electron microscopy.

Micelle molecular weight of dodecyldimethylammonium chloride in aqueous solutions, and the transition of micelle shape in concentrated NaCl solutions

Abstract Light scattering of aqueous NaCl solutions of dodecyldimethylammonium chloride has been measured at different surfactant concentrations for different series of NaCl concentrations, and the micelle size and shape have been determined. At low NaCl concentrations the micelle molecular weight is of the order of 104 and slightly increases with NaCl concentration. The micelle is spherical or globular. At high NaCl concentrations the micelle molecular weight increases with increasing surfactant concentration and reaches a maximum: at the critical micelle concentration (CMC) small spherical micelles would be formed, but beyond the CMC they associate into large micelles. The molecular weight of large micelles increases sharply with increasing NaCl concentration and exceeds 106. The large micelle is rod-like but is flexible. Reduced repulsion among polar head groups induces the transition of micelle shape from sphere to rod. The effect of size of polar head groups on the micelle shape is discussed with reference to the micelle molecular weight of dodecylammonium chloride and dodecyltrimethylammonium chloride.

The sphere—rod transition of micelles and the two-step micellization of dodecyltrimethylammonium bromide in aqueous NaBr solutions

Abstract Light scattering was measured on aqueous NaBr solutions of dodecyltrimethylammonium bromide in the presence of 1.00 to 4.00 M NaBr, and these results are discussed together with the previous data at lower NaBr concentrations. When the NaBr concentration is lower than 1.8 M , spherical micelles alone are formed. At 2.00 and 3.00 M NaBr, the first and second critical micelle concentrations (CMC) exist, but at 4.00 M NaBr they coincide with each other. The primary micelle formed above the first CMC has the molecular weight about 23,500–24,400 in 1.00–4.00 M NaBr and is spherical. The secondary micelle formed above the second CMC has the molecular weight about 29,000–57,500 in 2.00–4.00 M NaBr, and its shape would be rodlike. Combined with the previous data at lower NaBr concentrations, it is found that a linear Corrin-Harkins relation holds for the first CMC up to 3.00 M NaBr, and that a linear double logarithmic relation between molecular weight and ionic strength holds for the spherical micelles. Another linear double logarithmic relation between molecular weight and ionic strength holds for rodlike micelles. The sphere-rod conversion of ionic micelles appears as a sharp transition with respect to the salt concentration, while it occurs as a chemical equilibrium with respect to the surfactant concentration. The sphere-rod transition of dodecyltrimethylammonium micelles induced by the change in counterion species from Cl − to Br − means that such a transition of ionic micelles is caused by the counterion binding, which reduces the electrostatic repulsion among charged head groups.

Characteristics of rodlike micelles of cetyltrimethylammonium chloride in aqueous NaCl solutions: Their flexibility and the scaling laws in dilute and semidilute regimes

Static light scattering has been measured for aqueous NaCl solutions of cetyltrimethylammonium chloride (CTAC) at 25 °C. While spherical micelles are formed above the critical micelle concentration for 0–1.5 M NaCl solutions, rodlike micelles are formed at NaCl concentrations higher than 1.18 M.The aggregation number of rodlike micelles increases markedly with increasing NaCl concentration, and it is as large as 11400 in 4.0 M NaCl. Long rodlike micelles are semiflexible and behave like wormlike chains. Their contour length and persistence length have been calculated as 630 and 46.4 nm, respectively, in 4.0 M NaCl.Rodlike micelles overlap and entangle together to form a network in semidilute solutions above a threshold micelle concentration. The radius of gyration of the blob can be scaled for its molecular weight with the exponent, 0.55, coinciding with that for isolated rodlike micelles in dilute solutions. The scaling laws for the reciprocal envelope of light scattered in the semidilute regime and for the molecular weight and the radius of gyration of the blob are also discussed with reference to the micelle concentration.

The effects of ionization on micelle size of dimethyldodecylamine oxide

Abstract Light scattering from aqueous NaCl solutions of dimethyldodecylamine oxide has been measured at different degrees of neutralization adjusted by addition of HCl, and molecular weights of differently charged micelles are determined from the Debye plot. In the absence of added salt, the micelle molecular weight decreases with increasing ionization, indicating the electrostatic effect in micelle formation. In the presence of NaCI above 0.01 M , however, the largest micelle is formed at half-ionization, and its size steeply increases with increasing NaCI concentration. In 0.20 M NaCl the large micelle is composed of about 300 ionized and 300 nonionic molecules, and it grows further with increasing surfactant concentration.

The viscosity behavior of aqueous NaCl solutions of dodecyldimethylammonium chloride and the flexibility of its Rod-Like micelle

Abstract The relative kinematic viscosity of aqueous solutions of dodecyldimethylammonium chloride in the presence of NaCl of different concentrations has been measured by an Ubbelohde-type capillary viscometer having four bulbs of successive heights. When referred to the solution of critical micelle concentration, the relative viscosity of the solutions is independent of shearing stress, if the NaCl concentration is lower than 2.40 M . However, non-Newtonian viscosity is manifest for solutions of higher NaCl concentrations. Extrapolated to zero shearing stress, the reduced viscosity and the logarithmic reduced viscosity of micellar solutions are obtained at different surfactant concentrations. The intrinsic viscosity for the micelle is derived as the limit of reduced viscosity or logarithmic reduced viscosity to zero micelle concentration at different NaCl concentrations. The double logarithmic plot of intrinsic viscosity for micelle against micellar molecular weight gives two linear sections. The steeper section has a slope of 1.3 and covers the region of molecular weights from 10 5 to 10 6 , which corresponds to NaCl concentrations from 1.5 to 3.0 M . The less steep section has, however, a slope of 0.61 and covers the region of higher molecular weights or of higher NaCl concentrations. This observation indicates that dodecyldimethylammonium chloride forms rod-like micelles in aqueous NaCl solutions of concentrations higher than 1.5 M , but that the micelles are not rigid but considerably flexible.

The adsorption of cetyltrimethylammonium bromide on aqueous surfaces of sodium bromide solutions

Abstract Surface tension of aqueous NaBr solutions of dodecyltrimethylammonium bromide has been measured by the drop weight method at different NaBr concentrations from 0 to 6.00 M at 25°C. Surface tension decreases sharply with increasing surfactant concentration below the critical micelle concentration (CMC) at a given NaBr concentration, but it has a constant value above the CMC. By applying the Gibbs adsorption isotherm for the surface tension at surfactant concentrations lower than the CMC, surface excess densities of surfactant cation, Na + and Br − have been derived as functions of surfactant concentration at different NaBr concentrations. Generally, surface excess densities of surfactant cation and Br − increase with increasing surfactant concentration, and they approach saturated values at the CMC. The adsorption of Na + is very low in magnitude and negative. The Corrin—Harkins plot of the CMC consists of two straight lines intersecting at 1.30 M NaBr, which can be assigned to the threshold NaBr concentration for the sphere—rod transition of the micelle.

The pH dependence of dispersion of TiO2 particles in aqueous surfactant solutions

The pH dependence of dispersion of titanium dioxide (TiO2) particles has been examined in the presence of surfactant molecules in water. Whereas particles were dispersed in water at acid and alkaline regions rather than at neutral region, the dispersion was enhanced at neutral region in an aqueous sodium dodecyl sulfate (SDS) solution and at acid and alkaline regions in an aqueous dodecyldimethylamine oxide (C12DAO) solution. Considering the pH dependence of zeta potential, the adsorption models of surfactant molecules on a particle were estimated on the basis of the modes of hemimicelle and double-layer compression. While the particles that adsorbed Al3+ were remarkably dispersed around pH 6, their dispersion does not largely depend on pH in the addition of SDS, indicating the adsorption of SDS molecules to form double-layer compression in the whole pH region. Dynamic light-scattering measurement and electron microscopic observation suggested that the particles were dispersed in water as small flocs.

Adiabatic compressibility of alkyltrimethylammonium bromides in aqueous solutions

Abstract The adiabatic compressibility of aqueous solutions of octyl-, decyl-, dodecyl- and tetradecyltrimethylammonium bromides has been determined from measurements of ultrasound velocity and density at 25°C. A theoretical treatment giving the adiabatic compressibility as a function of concentration has been developed, and the apparent adiabatic compressibility of a surfactant in the monomeric and micellar forms is derived from the experimental results. The apparent adiabatic compressibility of surfactant in either form is constant, independent of concentration, so that it is identified with the apparent adiabatic compressibility in each form. The adiabatic compressibility in the monomeric form decreases from −0.25 × 10 −5 bar −1 for the octyl to −2.52 × 10 −5 bar −1 for the tetradecyl derivative, while that in the micellar form increases from 3.41 × 10 −5 bar −1 for the octyl to 4.17 × 10 −5 bar −1 for the tetradecyl derivative. The former includes the hydration of ionic head group of the surfactant, while the latter is related to the structure of micelle. If the same treatment is applied to the literature data, the apparent adiabatic compressibility of sodium alkyl sulfate in the monomeric form changes conversely with the alkyl chain length, while that in the micellar form is qualitatively similar.

Stability of spherical and rod-like micelles of ionic surfactants, in relation to their counterion binding and modes of hydration

Aqueous salt solutions of ionic surfactants in both spherical and rod-like micelles have been treated on the basis of a statistical thermodynamic theory, and the double logarithmic relationship between micelle molecular weight and ionic strength is derived for each micelle. Counterion binding on both micelles are assumed to occur specifically, and their degrees of dissociation are related to the slopes of the linear double logarithmic relations. It is found from the relationship observed for typical surfactants that the effective charge of spherical micelles is 29±4. The degree of dissociation of rod-like micelles of these surfactants is primarily determined by the counterion species, yielding values 0.8 for Na+, 0.4–0.6 for Cl− and 0.2–0.3 for Br−. Hydrophilic hydration of both micelles can be evaluated from the intercepts of the linear relations. Hydrophilic hydration acts repulsively in spherical micelles, while it is attractive or much less repulsive in rod-like micelles.