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The concept of molecular packing parameter is widely invoked in the literature to explain, rationalize and even predict molecular self-assembly in surfactant solutions. The molecular packing parameter is defined as vo/aelo, where vo is the surfactant tail volume, lo is the tail length, and ae is the equilibrium area per molecule at the aggregate surface. A particular value of the molecular packing parameter can be translated via simple geometrical relations into specific shape and size of the equilibrium aggregate. This is the predictive application of the concept of molecular packing parameter, as discussed in the literature. In calculating the packing parameter, the dominant notion in the literature is that the surfactant headgroup determines the surface area per molecule of equilibrium aggregates ae. It follows that, given a headgroup, the molecular packing parameter vo/aelo is fixed, because the volume-to-length ratio (vo/lo) of the tail is a constant independent of the tail length for common surfactants. Therefore, in this view, the surfactant tail has no role in determining the size and shape of equilibrium aggregates. We show that this is contrary to fact, by focusing on the neglected role of the surfactant tail. Illustrative calculations are presented in this paper to demonstrate that the surfactant tail does also control equilibrium aggregate structures. It is shown that the role of the tail can be either explicit via modification of the area ae and thus of the packing parameter, or implicit via other means, without modifying ae or the packing parameter. † Paper prepared to mark the 60th birthdays of Mats Almgren, Josef Holzwarth, Ray Mackay, and Evan Wyn-Jones, honoring their contributions to the fundamentals and applications of surfactant self-assembly.

A theory of micellization of AB diblock copolymer molecules in a selective solvent S is developed here. The micelles are assumed to have a completely segregated core region consisting only of the A block and a shell region consisting of the solvent S and the solvent compatible B block. The theory allows one to predict the critical micelle concentration, the micelle size distribution, the average aggregation number, as well as the core radius and the shell thickness of the micelle. The novel outcome of the present theory, in contrast to the treatments of micellization pioneered by de Gennes, Leibler, Orland, and Wheeler, and Noolandi and Hong, is its prediction that the solvent compatible B block plays an important role in determining the micellization behavior. The influence of the B block becomes relatively more prominent in systems where the solvent S constitutes a very good solvent for the B block. Further, scaling relations that are not system specific have been developed relating explicitly the micellar size parameters to the characteristics of the block copolymer and the solvent.

Solutions of block copolymer micelles can be designed to achieve highly selective solubilization of components from a mixture of solubilizates. Illustrative experimental results are presented for poly(ethylene oxide-propylene oxide) and poly(N-vinylpyrrolidone-styrene) copolymer solutions and for aromatic and aliphatic hydrocarbon solubilizates. The results show substantial solubilization of aromatics in contrast to negligible solubilization of aliphatics. The solubilizate that is more compatible with the polymer block that constitutes the core of the micelle is solubilized to a greater extent. The experimental data on the amounts of hydrocarbon solubilized are correlated with the molecular volume of the solubilizate, with a volume-polarity parameter characterizing the solubilizate as well as with the Flory-Huggins interaction parameter between the solubilizate and the polymer block constituting the micellar core.

It was recently reported that water soluble conducting polyaniline may be prepared using a new template-guided enzymatic approach. To address the mechanistic role of the template in this reaction, various macromolecular and surfactant templates were investigated. It was found that the template provides a necessary type of “local” environment where the pH and charge density near the template molecule is different from that of the bulk solution. 13C and 1H NMR studies showed that this “local” environment serves as a type of nano-reactor that is critical in anchoring, aligning, and reacting the aniline monomers and ultimately controls what form of polyaniline (conducting or insulating) is obtained during reaction. Strong acid polyelectrolytes, such as sulfonated polystyrene (SPS), are the most favorable because they provide a lower, local pH environment that serves to both charge and preferentially align the aniline monomers through electrostatic and hydrophobic interactions to promote the desired head-to-tail coupling. Interestingly, it was found that micelles formed from aggregating, strong acid surfactant molecules such as sodium dodecylbenzenesulfonic acid (SDBS) also provide suitable local template environments that lead to the formation of conducting polyaniline. 1H NMR spectral data showed that the aniline monomers in these micelle systems intercalate between the sulfonated styrene headgroups of the micelles. However, if the reaction media was such that micelles were not formed or if the distance between the sulfonated headgroups in the mixed micelle systems was too large, then the conducting form of polyaniline could not be obtained. The information gained from this study strongly supports the existence and importance of “local” template environments in guiding the enzymatic synthesis of polyaniline. A fundamental understanding of these types of mechanisms should lead to the design and optimization of a broad range of other interesting template-guided reactions.

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The aggregation behavior of surfactants in mixed solvents composed of water and ethylene glycol is predicted using an extension to our theory of aqueous surfactant solutions. The extension accounts for the dependence of (i) the surfactant tail transfer free energy, (ii) the aggregate core-solvent interfacial free energy, and (iii) the headgroup interaction free energy on the composition of the mixed solvent. As the proportion of ethylene glycol in the mixed solvent increases, the model predicts an increase in the critical micelle concentration (cmc), a decrease in the average aggregate size, an increase in the aggregate polydispersity, and a stronger dependence of the average aggregation number on the total surfactant concentration. The model reveals that (a) large values for cmc originate primarily from the smaller magnitude of the surfactant tail transfer free energy in the mixed solvent, (b) the aggregation numbers are small mainly because of the smaller magnitude of the hydrocarbon-mixed solvent interfacial tension, and (c) neither the cmc nor the aggregate size are greatly affected by the lower dielectric constant of the mixed solvent. The predicted aggregation characteristics of cetyl pyridinium bromide, cetyl trimethylammonium bromide, and sodium dodecyl sulfate at various compositions of the mixed solvent are presented for illustrative purposes and are compared with available measurements.

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