Shape Transition in Micelles of Undecylammonium Chloride in the Presence of Sodium Chloride and n-Butanol: A Study by Dynamic Light Scattering

Model shape transitions of micelles: spheres to cylinders and disks

We present a microscopic analysis of shape transitions of micelles of model linear nonionic surfactants. In particular, symmetric H(4)T(4) and asymmetric H(3)T(6) surfactants have been chosen for the study. In a previous work, it has been observed that symmetric surfactants have a strong tendency to prefer spherical micelles over a wide range of chemical potentials, while asymmetric surfactants undergo shape transitions between a spherical micelle at low concentration to other forms, mainly finite cylindrical micelles. This study combines the application of a two-dimensional single-chain mean-field theory (SCMFT) with Monte Carlo (MC) simulations of exactly the same systems. On the one hand, the characteristics of the SCMFT make this method suitable for free energy calculations, especially for small surfactants, due to the incorporation of relevant microscopic details in the model. On the other hand, MC simulations permit us to obtain a complete picture of the statistical mechanical problem, for the purpose of validation of the mean-field calculations. Our results reveal that the spherical shape for the symmetric surfactant is stable over a large range of surfactant concentrations. However, the asymmetric surfactant undergoes a complex shape transition that we have followed by calculating the standard chemical potential as a function of the aggregation number. The results indicate that the system forms prolate spheroids prior to developing short capped cylinders that gradually grow in length, with some oscillations in the energy of formation. The most important result of our work is the evidence of a bifurcation where, together with the elongated objects, the system can develop oblate aggregates and finally a torus shape similar to a red blood cell.

Effect of Catechol on the Size and Shape of CTAB Micelles in Aqueous Solutions

The effects of catechol on the change of cetyltrimethylammonium bromide (CTAB) micelles in aqueous solutions have been studied by Ultramicroelectrode (UME) cyclic voltammetry and Time-Resolved Fluorescence Quenching (TRFQ). It has been shown that the diffusion coefficient decreases and the micelle aggregation number increases with the addition of catechol. This indicates that catechol has influence on the size or shape of CTAB micelles. In order to analyze the change of the micelles, other micelle parameters like the effective radius (a), surface area of micelle (S), area of single CTAB molecule (A) and packing parameter (P) were calculated. The values of P which is relative to the shape of the micelles are consistent with the change of the diffusion coefficient. All the results show that catechol can accelerate the increase of the aggregate size and sphere-to-rod morphology change of CTAB micelles even with lower concentration of CTAB. This is because that hydrophobic force makes catechol insert into CTAB micelles between CTAB molecules, which leads to the increase of the radius of curvature. The increase of aggregate size and the sphere-to-rod morphology change are also promoted as a result.

Solubilization by different-sized surfactant mixtures

The solubilization phenomenon was investigated in mixed surfactant systems. The solubilization power of a mixed surfactant reaches its maximum at a particular temperature at each mixing ratio of surfactants. When the mole fraction of C4E1 in the total surfactant (w1 value) was varied in a water/C12E5/C4E1/decane system, the minimum mole fraction of total surfactant in the system necessary to obtain a single microemulsion phase (xi value) was almost unchanged for w1<0.3, whereas it increased remarkably for w1>0.8. The molar solubilization capacity (Cs=(1-xi)/xi) of the mixed surfactant decreased remarkably for w1<0.3, whereas it decreased gradually for w1>0.8. The result [Formula: see text] is due largely to the characteristic of the function xi(Cs)=1/(1+Cs), specifically, [Formula: see text] , where dxi/dw1=(dxi/dCs)(dCs/dw1). The partial molar solubilization capacity (Cs) of C4E1 was negative at almost all w1, but the Cs value of C12E5 went through a maximum on the addition of C4E1. Propanol (a cosurfactant) has the same effect on the solubilization phenomenon in the water/C12E6/propanol/heptane system. In the water/C12E5/C12E7/decane system, the Cs value of each surfactant did not vary greatly as the mixing ratio of surfactants was varied. The Cs and xi values were close to molar additivity for each mixing ratio.