- Counterion Distribution around a Macroion in Polyelectrolytes Probed by Anomalous Small-Angle X-ray Scattering (2005)
- A systematic and comprehensive study of polyelectrolytes in solution has been carried out using Anomalous Small Angle X-ray Scattering (ASAXS) in order to probe the counterion correlation around the macroion. In the course of this study, different polyelectrolyte systems such as, rod-like polyelectrolytes, spherical polyelectrolyte brushes and star shaped polyelectrolytes have been studied using ASAXS. ASAXS is the method of choice for the study of aqueous polyelectrolytes because it obviates the need of using labelled compounds or counterion exchange, as in the case of SANS or conventional SAXS. Scattering experiments performed near the absorption edge of the element under investigation, allow us to obtain the three partial scattering terms predicted by theory. All the three partial scattering terms are obtained for the first time experimentally and compared with the theoretical predictions of rod-like polyelectrolytes and spherical polyelectrolyte brushes. Rod-like polyelectrolytes bearing a rigid poly(p-phenylene) and comprising of two counterions per monomer unit with a persistence length of 21 nm were studied using ASAXS and osmometry. The results of the ASAXS experiments carried out on rod-like polyelectrolytes are compared with the predictions of the Poisson-Boltzmann Cell Model. The results suggest that the counterions are strongly correlated with the macroion. Osmotic coefficient measurements on rod-like polyelectrolytes show that approximately 20 percent of the counterions are osmotically active. The comparison of the experimentally obtained osmotic coefficient with the theory proceeds without adjusting the charge parameter. The results are in semi-quantitative agreement with the predictions of the Poisson-Boltzmann Cell Model. Spherical polyelectrolyte brushes consisting of a poly(styrene) core of 68 nm with densely grafted linear poly(acrylic acid) chains with Rubidium counterions were studied using ASAXS. In this thesis the first complete analysis of a colloidal polyelectrolyte brush by ASAXS has been presented. It has been demonstrated that there is a strong correlation of the counterions to the grafted polyelectrolyte chains of the spherical polyelectrolyte brushes. The correlation between ions and polymer chains in spherical polyelectrolyte brushes is much stronger than linear polyelectrolytes. The experiments therefore validate the theoretical predictions that most of the counterions are fully trapped within the brush and this leads to a strong stretching of the polyelectrolyte chains. Star-shaped polyelectrolytes bearing 21 arms of poly(acrylic acid) with Rubidium counterions were studied using SAXS and ASAXS. In SAXS experiments, the dependence of the maxima in the scattering experiments was observed to be concentration dependent. The ordering in star-shaped polyelectrolytes was maximum in the vicinity of the critical concentration. The maxima observed in the scattering experiments obey the scaling theory for polyelectrolytes. This is an indication of some order between the star-shaped polyelectrolytes. Evidence for the presence of any crystalline structure was not found. Scattering experiments with varying number of arms may be helpful to determine their role in the formation of crystalline structure. The three scattering terms predicted by ASAXS theory were also obtained for the star-shaped polyelectrolytes. It has been demonstrated that ASAXS is indeed a very robust method to study the polyelectrolytes in solution and to obtain the information regarding the distribution of counterions in solution. ASAXS studies of polyelectrolytes have demonstrated that the counterions are strongly correlated to the macroion.