Polyelectrolyte (PE) Collapse

Interest on the PE collapse was aroused by new predictions of theoreticians, proposing anisotropic intermediate states like cigars or pearl necklaces along this shrinking process. Structural changes of PE can be induced by e.g. alkali and alkaline earth halides. Furthermore, the interaction between PE and metal cations plays a central role in many industrial applications as well as in biological systems (e.g. the dispersion of inorganic pigments, scale inhibition in steam pipes or protein supported biomineralisation).

The specific binding of Ca² to the dissociated carboxylate groups of sodium polyacrylate (NaPA) induces a shrinking process (coil to globule transition) prior to the precipitation of CaPA. The phase diagram of NaPA in the presence of Ca², dissolved in 0.01 M NaCl solution, is presented in figure 1. The subtle interplay of NaPA, NaCl with Ca² gives access to well defined and extremely small increments and allowed us to address any intermediate state along the shrinking process of NaPA.

SANS measurements of different NaPA intermediates, obtained by varying the ratio of Ca² per carboxylate group, were carried out at D11. The scattering curves show a q^-4 dependency at high q (indicating densely packed objects with sharp boundaries) and a maximum with adjacent minimum at lower q. Experimental scattering curves are compared with theoretical models. Neither a sphere nor a cylinder form factor can render the measured curve and are thus clearly ruled out as possible PE shapes. Furthermore, the minimum/maximum does not stem from an interparticular interference, as proven by a reference measurement of pure, fully ionized NaPA.

Eventually, the minimum could be attributed to the distance of neighbouring pearls in a PE chain. We succeeded in recovering all features of the scattering curves and consider the results as indication for the appearance of pearl-necklace shapes during PE collapses.