Chain conformation: A key parameter driving clustering or dispersion in polyelectrolyte – Colloid systems

Abstract

The work presents the characterization of Pluronic F127 micellar solutions in presence of hyaluronic acid in semi-dilute regime. The effects of the nature and salt concentration are investigated by differential scanning calorimetry and small angle neutron scattering. Hyaluronic acid reduces the critical micellar temperature to the same extend as an increase of the ionic strength. Within the investigated HA concentration range, the size and shape of the micelles are not modified by the addition of HA but their dispersion state depends on the salt concentration. By increasing the ionic strength we observe the formation of small micellar clusters which organize into a face-centered cubic liquid crystalline phase at high salt concentration. This behavior is reinforced by increasing the HA concentration or molecular weight. The nature of the salt plays also a role and divalent cations such as Ca2+ promote the clustering of micelles and their crystallization. The origin of the aggregative behavior is the change of the HA chain conformation –from stretched to coil- by addition of salt which in turn induces an excluded volume around the micelles and exerts a depletion interaction.