Light Scattering in Solutions of a Linear Polyelectrolyte

Solutions of arabic acid have been investigated by the light scattering technique. The molecular weight for a standard preparation was found to be 1.00 ± 0.05 X 10⁶in contrast to the 300,000 value determined by earlier osmotic pressure measurements. The molecule appears to be a stiff coil due to side branching, varying in extension from 1050 Å. at zero charge to 2400 Å. at maximum charge. Viscosity measurements are in essential agreement with the scattering data. The scattering function, Kc/R90, increases with increasing polyion charge at a given value of the concentration, c. It decreases with increasing ionic strength to a limiting value of that in un-ionized systems. The scattering function is third order at high values of the charge. The dissymmetry, z, exhibits a distinct minimum at c = 0.75 X 10¯3 g./cc. for all values of µ for the ionized acid and rises at higher c to a maximum value. The maximum is lower as µ increases and occurs at lower values of c. Some models are described which may explain the free energy relationships giving rise to the above-mentioned Kc/R90 vs. c and z vs. c data.