J. Kalus

Small-angle neutron scattering on shear-induced micellar structures

Small-angle neutron scattering (SANS) experiments on sheared aqueous surfactant solutions of tetradecyltrimethylammoniumsalicylate (TTMA-Sal) are reported. A5-mM-solution without shear shows a weak correlation peak at a momentum transfer of 0.09 nm−1 which has its origin in the micellar interaction. For shear rates above a threshold value of Γ=40 s−1 the scattering pattern shows an irregular increase in anisotropy. The analysis of the anisotropic pattern reveals the existence of two types of micelles: Small rodlike micelles which are weakly aligned and very large rodlike aggregates which are strongly aligned and which are present above the threshold value ofΓ. The two micelles are in equilibrium with each other and the equilibrium shifts with increasing shear rate to the side of the large oriented micelles.

Determination of phonon eigenvectors in naphthalene by fitting neutron scattering intensities

The coherent neutron scattering intensities from five phonon modes in naphthalene have been measured at twenty four points in reciprocal space. These intensities are determined by a set of eigenvectors, and a fit to these intensities has been obtained by altering the eigenvectors iteratively. Significantly the best result has been obtained starting from eigenvectors derived from a model calculation, which were found to be close to the final result. This supports our method of assigning neutron scattering results by a comparison with model calculations.

A new shear apparatus for small angle neutron scattering (SANS) measurements

A new shear apparatus for small angle neutron scattering experiments was developed. Compared to other instruments two new experimental degrees of freedom are characteristic for this apparatus: (i) there is only one shear cell and not two as in other shear apparatus, and (ii) the angle α between the incident beam of neutrons and the vector of the velocity gradient of the sheared solution can be changed between 0° and 50°. This gives the possibility to test the anisotropic distribution of nonspherical particles, like micelles, in much more detail than with the other shear apparatuses. For example the symmetry of the scattering pattern of rodlike micelles is reduced from point group 2 mm for α=0 to point group 2 for α≠0, which is shown in an experiment performed with rodlike micelles of Cetylpyridiniumsalicylate.

Orientational relaxation of aligned rod-like micelles on a time scale of 300 ms

Abstract Data on the time dependence of small-angle diffraction patterns of aligned rod-like micelles are presented. The system is a 20 mM solution of cetylpyridinium salicylate in D 2 O in the presence of 20 mM NaCl. Under these conditions rod-like micelles are present which are more than 500 A long. These micelles can be aligned by a shear gradient Γ. The time dependence of the anisotropic neutron small-angle diffraction pattern is followed in time steps of 250 ms after a sudden reduction of Γ to zero. The scattering curves can be fitted with a single fit parameter, the rotational diffusion coefficient D . The rotational diffusion coefficient turns out to be time dependent. This probably indicates an interaction between the charged micelles.

Eigenvectors of low frequency internal phonons in crystalline anthracene-d10

Abstract Coherent neutron inelastic scattering intensifies for two low-frequency Γ 4 internal modes in anthracene- d 10 have been measured at several different Brillouin zone centres. These intensifies have been used to determine the eigenvectors of the two modes involved. It is shown that the motion in both cases is a combination of a butterfly motion (B) and a twisting motion (T). The energy proportions in these two modes are 0.97B and 0.03T (±0.01) for the mode at 3.16 THz and 0.94T and 0.06B (±0.04) for the mode at 4.60 THz. This experimental result strengthens our reliance on model calculations for the assignment of phonon frequencies measured by neutron coherent inelastic scattering.

The shape of hexadecyloctyldimethylammoniumbromide micelles in aqueous solutions

Abstract By means of small-angle neutron and X-ray scattering the shape of hexadecyloctyldimethylammoniumbromide micelles in aqueous solution was studied in a temperature range from 25 to 60°C. The micelles were found to be of rod-like shape. The solutions had concentrations between 10 and 108 mM. Typical lengths and radii of the micelles are around 15 nm and 1.9 nm respectively. With X-rays we were able to determine the position of the Br − ions.