A study of correlations between structural properties and phase behaviour of polyelectrolyte systems: Effects of size and chain form factor

This paper presents a correlation study of structural properties and phase behaviour of polyelectrolyte solutions in the presence of added salt. A three-component system (polyion, salt and conterions) is considered with partial structure factors derived from the Random Phase Approximation according to an analysis equivalent to writing the celebrated Zimm's formula in matrix form. Instability lines of micro and macro-phase separations are obtained from the q-dependence of the polyion structure factor. This work incorporates the effects of chain length with an improved description of the internal polyion form factor as an extension of the results reported in [Joanny JF, Châtellier X. J Phys France II 1996;6:1669]. The enhanced translational entropy gained by polyions as their chain length decreased, induced a shift of the lines of instability to lower temperatures hence an improved stability towards phase separation. The theoretical form factor used to describe the internal structure of polyion had a strong influence on the peak position of the structure factor which in turn correlated to the micro-phase instability and Lifshitz point at the junction of micro and macro-phase transition lines. This aspect was developed here by comparing predictions of the above reference with those obtained using an improved version of the form factor. A brief comparison with existing data was made and routes for improving the theoretical modelling were indicated.