We report on a series of SANS experiments on the structure of binary water−nonionic surfactant systems accompanied by complementary ultralow shear experiments and depolarized light scattering. The analysis gives a clear picture of the temperature dependence of aqueous solutions of nonionic surfactants of the n-alkyl polyglycol ether type (C i E j ) when approaching the cloud point curve. The series is based on temperature variations from 3 °C up to a temperature of about 1.5 K below the critical point T c and concentration variations around the critical concentration c c by a factor of 3−9. Six different surfactants were studied, changing the alkyl chain length i as well as the number of ethylene oxide groups j. Excluded volume effects were taken into account in the evaluation procedure by a generalized indirect Fourier transformation procedure recently developed for the evaluation of scattering data from semidilute and dense systems. The bottom line is that all systems examined show a sphere-to-rod transition, the degree of growth and the transition temperature depending on the concentration and hydrophobicity of the surfactant. Superimposed on this transition is the onset of attractive interactions as the cloud point curve is approached, the range depending on the overall surfactant size.

Self-assembly in mixtures of cationic and anionic surfactants occurs synergistically because of attractive interactions between the oppositely charged headgroups. Here, such effects are exploited to obtain highly viscoelastic fluids at low total surfactant concentration. The systems considered are mixtures of the C18-tailed anionic surfactant, sodium oleate (NaOA), and cationic surfactants from the trimethylammonium bromide family (C n TAB). In particular, mixtures of NaOA and C8TAB show remarkably high viscosities:  for 3% surfactant, the zero-shear viscosity η 0 peaks at ca. 1800 Pa·s for a weight ratio of 70/30 NaOA/C8TAB. The high viscosities reflect the growth of giant, entangled wormlike micelles in the solutions. Mixtures of NaOA with a shorter-chain analogue (C6TAB) have much lower viscosities, indicating a weak micellar growth and hence a weak attraction between the surfactants. On the other hand, increasing the C n TAB tail length to n = 10 or 12 leads to much stronger interactions between these surfactants and NaOA. Consequently, both micellar and bilayer structures are formed in these mixtures, and the samples separate into two or more phases over a wide composition range. Thus, the synergistic growth of wormlike micelles in cationic/anionic mixtures is maximized when there is an optimal asymmetry in the surfactant tail lengths.

The electrosteric stabilization of model colloidal dispersions is quantified through high-frequency rheometry and complementary techniques. Model aqueous dispersions with a poly(butyl acrylate)polystyrene core and a layer of poly(methacrylic acid) grafted onto the surface are prepared and characterized. The influence of pH, electrolyte concentration, and amount of polymer in the stabilizing layer on dispersion stability and rheology is investigated. Dynamic light scattering, electrophoretic mobility, and rheology are used to quantify thickness, hydrodynamic permeability, and charge density of the stabilizing shell. A collapsed layer at low pH leads to aggregation after addition of salt, while a swollen layer at high pH induces stability. The colloidal interaction potential is deduced from measurements of the high-frequency elastic modulus using torsional resonators. The complex electrosteric forces are shown to be dominated by the excess osmotic pressure created by overlap of the electrosteric layer for particles in contact. The measured moduli G′ ∞ can be predicted quantitatively based on a simple model for the osmotic repulsion introduced by Vincent et al. [J.

The structure of colloidal particles can be studied with small-angle X-ray and neutron scattering (SAXS and SANS). In the case of randomly oriented systems, the indirect Fourier transformation (IFT) is a well established technique for the calculation of model-free real-space information. Interaction leads to an overlap of inter-and intraparticle scattering effects, preventing most detailed interpretations. The recently developed generalized indirect Fourier transformation (GIFT) technique allows these effects to be separated by assuming various models for the interaction, i.e. the so-called structure factors. The different analytical behaviour of these structure factors from that of the form factors, describing the intraparticle scattering, allows this separation. The meandeviation surface is de®ned by the quality of the ®t for different parameter sets of the structure factor. Its global minimum represents the solution. The former non-linear least-squares approach has proved to be inef®cient and not very reliable. In this paper, the incorporation of the completely different Boltzmann simplex simulated annealing (BSSA) algorithm for ®nding the global minimum of the hypersurface is presented. This new method increases not only the calculation speed but also the reliability of the evaluation.

The formation of triblock copolymer/surfactant complexes upon mixing a nonionic Pluronic polymer (PEO-PPO-PEO) with a cationic surfactant, hexadecyltrimethylammonium chloride (CTAC), has been studied in dilute aqueous solutions using small-angle X-ray scattering, static and dynamic light scattering, and self-diffusion NMR. The studied copolymer (denoted P123, EO(20)PO(68)EO(20)) forms micelles with a radius of 10 nm and a molecular weight of 7.5 x 10(5), composed of a hydrophobic PPO-rich core of radius 4 nm and a water swollen PEO corona. The P123/CTAC system has been investigated between 1 and 5 wt % P123 and with varying surfactant concentration up to approximately 170 mM CTAC (or a molar ratio n(CTAC)/n(P123) = 19.3). When CTAC is mixed with micellar P123 solutions, two different types of complexes are observed at various CTAC concentrations. At low molar ratios (>/=0.5) a "P123 micelle-CTAC" complex is obtained as the CTAC monomers associate noncooperatively with the P123 micelle, forming a spherical complex. Here, an increased interaction between the complexes with increasing CTAC concentration is observed. The interaction has been investigated by determining the structure factor obtained by using the generalized indirect Fourier transformation (GIFT) method. The interaction between the P123 micelle-CTAC complexes was modeled using the Percus-Yevick closure. For the low molar ratios a small decrease in the apparent molecular weight of the complex was obtained, whereas the major effect was the increase in electrostatic repulsion between the complexes. Between molar ratios 1.9 and 9 two coexisting complexes were found, one P123 micelle-CTAC complex and one "CTAC-P123" complex. The latter one consists of one or a few P123 unimers and a few CTAC monomers. As the CTAC concentration increases above a molar ratio of 9, the P123 micelles are broken up and only the CTAC-P123 complex that is slightly smaller than a CTAC micelle exists. The interaction between the P123/CTAC complexes was modeled with the hypernetted-chain closure using a Yukawa type potential in the GIFT analysis, due to the stronger electrostatic repulsion.

Small-angle x-ray and neutron scattering are widely used techniques to study the structure of colloidal particles in the size range up to 100 nm. The indirect Fourier transformation technique is well established to obtain model free real space information, but the interpretation of the results is limited to cases where particle interaction can be neglected. The extended generalized indirect Fourier transform (GIFT) allows one to separate inter- and intraparticle effects, but needs models for the particle interaction. We present the application of three different models for the calculation of interaction effects of charged particles, represented by the structure factor. With this extension, useful real space information can be obtained by the GIFT method for solutions with volume fractions up to about 0.3 without any assumption for the shape of the particles. Only the interaction effects need a model assumption, and the parameters determined from this model can give some additional information. Simulations show that it is impossible to determine charge and ionic strength simultaneously. There exists another ambiguity between the parameter sets for charge, radius, and volume fraction, but we show how this problem can be overcome in most cases. The practical applicability of the method is demonstrated by means of the micellar system CTAB in different concentrations from 1% up to 20% and with varying amounts of added salt to screen the charges and change the particle shape.

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The generalized indirect Fourier transformation (GIFT) technique is a versatile tool for the evaluation of small angle scattering data. It does not depend on models for the size and shape of the particles and requires model assumptions only for the interaction effects that are typically not as sensitive to the details of the assumptions. We review here the development of the technique from its inception, focusing on the included interaction models for hard, charged and attractive spheres, and lamellae. A considerable number of applications has also been reported ranging from surfactants, emulsions, microemulsions, food science, and ceramics to melts and block-copolymers.