Study of dynamic and structural properties of complex fluids for green applications: effects of ambient conditions and nature of solutes Studio delle proprietà dinamiche e strutturali di fluidi complessi per applicazioni ecologiche: effetto delle condizioni ambientali e della natura dei soluti

This research project focused on the study of different complex fluid systems, formulated by using green chemical compounds and designed for potentially more environmental-sustainable applications. The study was performed mainly from a rheological and structural perspective because these are two strictly connected aspects in complex fluids systems. Moreover, rheology represents an important parameter in both many industrial processes and quality of final manufactured products. In the context of the wider subject of complex fluid, the present study mainly deals with polymer solutions, surfactants solutions, carbonaceous suspensions, and glassy forming liquid. More precisely, the project can be divided into three sub-topics of research: the study of salt/co-solute effects on aqueous polysaccharide systems; the modification of rheological behaviours, thermal properties, and responsivity to electrical treatment induced by specific additives and observed in polysaccharide formulations; and the examination of salt effects on the molecular association of glycerol carbonate, a structured glassy forming liquid. Firstly, a systematic study on the interactions between ten distinct anions or two different neutral co-solutes and three polysaccharides was carried on with the purpose of examining how specific salt can significantly affect biopolymers rheological and thermal properties. The three polysaccharides investigated at the begin are guar gum (GG), sodium alginate (SA) and sodium hyaluronate (SH), while the added salts or co-solutes belong to the following list: NaF, NaCl, NaBr, NaI, Na2SO4, NaClO4, NaSCN, Na3PO4, Na2HPO4, NaH2PO4, trehalose and urea. From an applicative point of view, the study was aimed at evaluating the control over viscosity of polysaccharide-based green formulations through simple salt triggers. The results were discussed in terms of changes in the polymer chains hydration, conformation and structure of the network. Since the SA based dispersions revealed low viscosity values and unchanged rheological behaviours in response to the salt addition, we drove our research towards a fourth polysaccharide that is hydroxypropyl cellulose (HPC) as a replacement for SA. Successively, the polysaccharide aqueous systems were tested in high salinity conditions, obtained through a mixture of monovalent and bivalent ions in different concentrations, in order to acquire info about their resistance in high- salinity environment. In the second topic of research, the polysaccharide aqueous systems were more deeply investigated through the analysis of the effects induced by specific additives on their mechanical, thermal, anti-scale precipitation and electro-responsive properties. More precisely, among the investigated additives, sodium citrate and the two bio-surfactants saponins ad rhamnolipids were evaluated as agents capable of modifying viscosity and rheology of GG, SH and HPC dispersions. Carbon Black (CB) was studied as a multifunctional additive that increases the viscosities, enhances the thermal stabilities and provides electro- responsive features to SH, GG and HPC formulations. Polyglutamate and polyaspartate were evaluated as anti-scale agents, able to reduce the precipitation of scales, constituted by CaSO4 or SrSO4, and modify the crystalline structure of the observed precipitate. The effectiveness of additive inclusion in the network, and their consequent capability of modifying the polysaccharide properties, were evaluated through rheology, optical microscopy, and conductivity measurements. Finally, in the third topic of research, it was investigated the solubility of various potassium salts in glycerol carbonate (GC, 4-hydroxymethyl-1,3- dioxolan-2-one), a structured organic solvent. More specifically, this study focused on the specific ion capability of significantly affecting the solvent-solvent molecular interactions, of promoting considerable changes in both molecular association mechanism and glassy liquid- forming tendency. The chronicle of this investigation began with the peculiar features observed on GC solutions of KF in a wide concentration range, varying from 10-3 M up to the saturation threshold. Indeed, the progressive addition of the salt promotes the formation of a glassy liquid by the solvent molecules, whose arrangement was evaluated through conductivity, rheology, differential scanning calorimetry (DSC) and infrared spectroscopy experiments. Successively, the study was extended to other seven potassium salts (K3PO4, KOCN, K2CO3, KCl, K2SO4, KBr and KI) and their effects on the GC structure were evaluated through NMR, DSC, solubility and ATR-FTIR experiments. From the combination of the results related to the two parts of the study, a dual mechanism of solvation, based on the cation and anion combined actions, was described.