Anuj Shukla

A New Family of Nonionic Dendritic Amphiphiles Displaying Unexpected Packing Parameters in Micellar Assemblies

In this paper we report on the synthesis of a new family of nonionic dendritic amphiphiles that self-assemble into defined supramolecular aggregates. Our approach is based on a modular architecture consisting of different generations of hydrophilic polyglycerol dendrons [G1-G3] connected to hydrophobic C(11) or C(16) alkyl chains via mono- or biaromatic spacers, respectively. All amphiphiles complex hydrophobic compounds as demonstrated by solubilization of Nile Red or pyrene. The structure of the supramolecular assemblies as well as the aggregation numbers are strongly influenced by the type of the dendritic headgroup. While the [G1] amphiphiles form different structures such as ringlike and fiberlike micelles, the [G2] and [G3] derivatives aggregate toward spherical micelles of low polydispersity clearly proven by transmission electron microscopy (TEM) measurements. In the case of the biaromatic [G2] derivative, the structural persistence of the micelles allowed a three-dimensional structure determination from the TEM data and confirmed the aggregation number obtained by static light scattering (SLS) measurements. On the basis of these data, molecular packing geometries indicate a drastic mass deficit of alkyl chains in the hydrophobic core volume of spherical micelles. It is noteworthy that these highly defined micelles contain as little as 15 molecules and possess up to 74% empty space. This behavior is unexpected as it is very different from classical detergent micelles such as sodium dodecyl sulfate (SDS), where the hydrophobic core volume is completely filled by alkyl chains.

Zeta Potentials and Debye Screening Lengths of Aqueous, Viscoelastic Surfactant Solutions (Cetyltrimethylammonium Bromide/Sodium Salicylate System)

In a series of experiments, we studied the dynamic properties of aqueous surfactant solutions of cetyltrimethylammonium bromide (CTAB) at conditions after adding different amounts of sodium salicylate (NaSal). The aggregates, present in these solutions, are elongated, wormlike micelles, which tend to form entanglement networks. The viscoelastic, gel-like samples were analyzed by means of static, dynamic, and electrophoretic light scattering techniques. We separately investigated the effects of surfactant concentration and added salt on intermicellar interactions. The electrostatic interactions between the anisometric micelles were analyzed by considering the effective dimensions of the aggregates. We calculated the Debye-Huckel lengths from experimental data of the osmotic second virial coefficient and from the diffusion second virial coefficient. It turned out that the results were in good agreement with theoretically estimated values. We also measured the zeta potential and intensity of scattered light in a large range of different salt concentrations keeping the CTAB concentration constant. We observed an isoelectric point and charge reversal of the threadlike micelles at an excess salicylate concentration of about 100 mM. The observed decrease of the zeta potential points to striking processes of counterion condensation. In these solutions, the salicylate ion acts as a cosurfactant, due to its discrepancy between polar and hydrophobic groups. We also detected a simple linear correlation between the zeta potentials and the Debye screening lengths of the surfactant solutions.