Wolfgang Wachter

Adsorption of Anionic and Cationic Surfactants on Anionic Colloids: Supercharging and Destabilization

We present herein a study on the adsorption of anionic (SDS), cationic (CTAB), and nonionic (C(12)E(5)) surfactants onto anionic silica nanoparticles. The effects of this adsorption are studied by means of the static structure factor, S(q), and the collective diffusion coefficient, D(c), obtained from small-angle X-ray scattering and dynamic light scattering measurements, respectively. The effective charge on the particles was determined also from classical electrophoresis and electroacoustic sonic-amplitude measurements. The surface tension of the sample was also investigated. Of particular note is the adsorption of SDS onto the silica nanoparticles, which leads to supercharging of the interface. This has interesting repercussions for structures obtained by the layer-by-layer (LbL) technique, because emulsions stabilized with supercharged and hydrophobized silica are perfect candidates for use in a multilayer system.

Interactions between large colloids and surfactants

The interfacial adsorption of an anionic SDS and a nonionic surfactant C12E5, above the cmc onto submicron-sized, negatively charged silica particles in aqueous solution has been investigated by using electrokinetics, conductometry and static light scattering. It was found that both surfactants are prone to being adsorbed onto the silica/water interface. Addition of C12E5 to the silica dispersion leads to a decrease in mobility. This reduced surface charge causes a decrease in the stability of the silica particles. Surprisingly, the addition of SDS brings about an increase in the negative electrophoretic mobility of the anionic silica particles, leading to a super-charging effect. Subsequent addition of C12E5 gives rise to even higher negative electrophoretic mobilities. This unexpected hyper-charging effect can only be understood as a cooperative effect based on mixed micelles of C12E5 and non-adsorbed SDS. Not so unexpectedly, the sequence of surfactant addition was found to be decisive, as quite different results are obtained when C12E5 is added before SDS.

Dynamics of water confined in self-assembled monoglyceride–water–oil phases

The dynamics of water confined in various self-assembled liquid-crystalline phases of the system Dimodan U/J–R(+)-limonene–water have been investigated by dielectric relaxation spectroscopy (DRS) over a wide range of frequencies (0.2 ≤ ν/GHz ≤ 50). For all phases, two water-related Debye processes were detected, one slightly slower, the other considerably slower, than bulk water. Besides these bulk-like and slow-water relaxations, a further fraction of water does not appear in the dielectric spectra due to strong interactions with the interface (bound water). Increasing the water content of the inverse hexagonal (HII) phase leads to an increase in both bulk-like and bound water, whereas the amount of slow water remains constant. Furthermore, the bulk-like water relaxation times accelerate until they reach the pure-water value when approaching the saturation line. This finding is supported by differential scanning calorimetry (DSC), which also confirms the presence of three types of water in liquid-crystalline confinement.

Scandium Sulfate Complexation in Aqueous Solution by Dielectric Relaxation Spectroscopy

Ion association in aqueous solutions of scandium sulfate has been investigated at 25 degrees C and at concentrations from 0.01 to 0.8 M by broadband dielectric spectroscopy over the frequency range 0.2 <or= nu/GHz <or= 89. Detailed analysis of the spectra reveals the presence of both inner- and outer-sphere 1:1 [ScSO 4] (+)(aq) complexes, similar to solutions of other high-valent metal sulfates. Outer-outer-sphere 1:1 complexes are probably also formed, but their contribution is swamped by the presence of higher-order inner-sphere complexes. The latter predominate in the more concentrated solutions, causing major changes to the low-frequency end of the spectrum. The data, while not definitive, are consistent with fac-[Sc(SO 4) 3(OH 2) 3] (3-) as the major species present. The speciation is strikingly different from that recently reported for aluminum sulfate solutions and indicates that the often-postulated similarity between the aqueous chemistry of Al(III) and Sc(III) has to be treated with caution.