Conformations of poly(allylamine hydrochloride) in electrolyte solutions: Experimental measurements and theoretical modeling

In this work, the conformations of poly(allylamine hydrochloride) (PAH) molecules in electrolyte solutions were determined experimentally for various pH and ionic strength by dynamic light scattering (DLS), microelectrophoresis and dynamic viscosity measurements. The molecular weight of the polymer exhibited a narrow size distribution and the average value of 15 kDa. Using the diffusion coefficient data obtained by DLS, the hydrodynamic radius RH of PAH for various conditions was calculated. It was found that that RH varied between 4.80 nm for I = 5 × 10−3 and 6.15 nm for I = 0.15 M at pH 6.5. These RH values were consistent with predictions stemming from Brenner's theory, approximating the real particle shape by prolate spheroids, bent to the various forms. Such molecule shape was also confirmed by the molecular dynamics numerical simulations. Using these RH values and electrophoretic mobility data, the average number of uncompensated (free) charges Nc and the effective ionization degree of PAH molecules in solutions were calculated. The maximum number of the free charges Nc was 27 at pH = 6.0, which gives the effective ionization degree of 17%. Supplementary shape information was derived from the dynamic viscosity measurements of dilute PAH solutions using a capillary viscometer. The intrinsic viscosity derived from these measurements varied between 105 and 35 for the ionic strength from 5 × 10−3 to 0.15 M. It was shown, after introducing the correction for hydration, that the experimental results were well accounted for by the Brenner's viscosity theory for slender particle suspensions. From these measurements, the effective lengths of the molecule for various ionic strength and pH were calculated. This parameter varied between 32 nm for I = 5 × 10−3 M and pH = 6.5 (high ionization of the molecule) to 17.3 nm for I = 0.15 M and pH = 9.5 (low ionization). For comparison, the contour length Lext predicted for fully extended polymer chain was 40.7 nm. However, these values, consistent with the hydrodynamic radius measurements, were significantly higher than the data derived from numerical simulation. This discrepancy was interpreted as due to the extensive hydration of PAH molecules, increasing their stiffness, which was not considered in simulations. Therefore, confirming the major role of hydration for determining polyelectrolyte shape, especially their effective length, was considered the major finding of this work.