title: Understanding Viscosity Changes due to Polyelectrolyte Surfactant Complexes
Creators: name: Del Sorbo, Giuseppe Rosario
Publisher: name: UNSPECIFIED
Year: 2022

description: Aqueous mixtures of polyelectrolytes and oppositely charged surfactants can give rise to a large variety of self-assembled structures, depending on the concentration and chemical nature of both components. These mixtures form clear and homogeneous solutions if either the polyelectrolyte or the surfactant is present in excess. Around charge equilibrium, macroscopic precipitates are formed. For some of these systems, near the phase boundary on the polyelectrolyte-rich side a drastic increase in solution viscosity is observed, due to the formation of nanometric mixed rod-like aggregates. These polyelectrolyte/surfactant complexes act as cross-links between several polyelectrolyte chains, conferring high viscosity but also shear thinning behavior to the solution. In this thesis, different aqueous mixtures of the cationically modified cellulose-based polyelectrolyte JR 400 and anionic surfactants were studied with various characterization techniques to shed light on unexplored aspects of their unique rheological behavior. (I) Solutions of JR 400 and sodium dodecyl sulfate (SDS) were investigated by small angle neutron scattering under rheological deformation (rheo-SANS) to study the origin of the shear thinning behavior while simultaneously inspecting the structural evolution of the rodlike aggregates under steady shear. Results have shown that the shear thinning is correlated to a slight orientation of the aggregates under shear. (II) To investigate the influence of the surfactant tail length on the rheological behavior, complexes consisting of JR 400 and two different surfactants, namely sodium octyl sulphate (SOS, with C8 tail) and sodium tetradecyl sulphate (STS, with C14 tail) were analyzed. SANS and neutron spin-echo (NSE) spectroscopy experiments reveled that the pronounced increase in viscosity for JR 400/STS solutions is due to the presence of mixed aggregates which interconnect different polyelectrolyte chains. Solutions with SOS are not viscous since distinct polyelectrolyte chains are not cross-linked. To gain complementary insights into the mesostructure of the mixed aggregates observed in the experiments, atomistic molecular dynamics (MD) simulations were performed using two charged polypeptides as generic representations of JR 400. MD and experimental results show good qualitative agreement. (III) In order to elucidate the influence of the surfactants headgroup charge, mixtures of JR 400 and the anionic surfactant sodium monododecyl phosphate (SDP) were investigated. At pH &gt; 12, the surfactant headgroup bears two charges and solutions exhibits a water-like behavior, while at lower pH the same mixtures become highly viscous. The change of the mesoscopic structure of the system was again studied using SANS and NSE, revealing that the double-charged form of the surfactant is not hydrophobic enough for the formation of network-forming mixed aggregates. (IV) Finally, further information on the role of the mixed rod-like aggregates in gluing together two or more polyelectrolyte chains was obtained by performing MD simulations involving defined forces. Taking all these results together, the physical mechanisms leading to the unique viscosity behavior of PESCs are now better understood.