Transient small-angle neutron scattering experiments on micellar solutions with a shear-induced structural transition

Abstract

Transient small-angle neutron scattering (SANS) experiments on a sheared 5 mM aqueous surfactant solution of trimethyltetradecylammonium salicylate (TTMA-Sal) are reported. The radially symmetric scattering pattern without shear shows a weak ringlike correlation "peak". For steady shear rates above a threshold value of r t h = 50 s-', the scattering pattern becomes rapidly anisotropic and the ring is compressed into two sharp, symmetric peaks. These changes are related to a shear-induced structural transition. The analysis of the data reveals the existence of two different types of micelles: Short rodlike micelles (called type I) are always present, and they are only weakly aligned. Very long rodlike micelles (named type 11) can be observed only above the threshold value of I?, and they are strongly aligned. Both types of micelles are assumed to be of cylindrical shape, with a radius R = 1.94 f 0.07 nm. We report the results on three time-dependent SANS experiments, where the shear rate was raised stepwise from zero to a value of rex = 100, 200, and 400 s-', respectively. It is found that the long micelles of type 11, which are not present at time t = 0, begin to grow a short time after switching on the shear. The number of surfactant molecules bound in type I micelles decreases, whereas the number of molecules bound in micelles of type I1 increases. The sum of both remains constant. The growth of the large micelles proceeds according to the Avrami law, which was originally designed to describe nucleation and growth reactions in metals and alloys.

Structure modifications of AOT reverse micelles due to protein incorporation

“…The transition from nonisotropic to isotropic scattering patterns was quantified using the anisotropy parameter A(q), which is defined as the ratio between the scattering intensities in perpendicular and parallel to the flow direction in the detector plane H-V and is given by [31] where I V (q) and I H (q) are the scattering intensities measured in the vertical and in the horizontal axes of the detector plane, in which the scattering vector is perpendicular and Fig. 11.…”

Structure of AOT reverse micelles under shear

“…6). It was previously reported that cylindrical micelles under shear forces could go through shear-induced alignment [28], with an orientation distribution of the micelles respectively to the flow direction and the incident X-ray beam. The instantaneous angular velocity of the micelles is determined by their orientation relative to the local streamlines [29].…”

“…The most important observation that can be made based on the fits is that no change has occurred in the structural parameters of the micelles: the core radius and polydispersity, the micelle length, and the cylinders/sphere ration remained practically constant with increasing shear rate. Following the work of Münch et al [28], we have also calculated the anisotropy parameter A(h), defined as…”

“…The anisotropy degree indicates the degree of spatial order of the cylindrical reverse micelles in the flow direction due to induced shear force [28]. Table 4 and Fig.…”

Kinetics of Alignment and Decay in a Highly Entangled Transient Threadlike Micellar Network Studied by Small-Angle Neutron Scattering

“…Since 1984, SANS has been used to study various aqueous micellar solutions under shear below and slightly above c *. , More recently, time-dependent SANS has been used to study the kinetics of alignment and decay. Studies on two systems containing shear-induced structures (SIS, so-called type II micelles) have found characteristic times on the order of seconds. , …”