Path-dependent rheology of carbon particle-hydroxyethylcellulose fluids

Polymeric fluids with tunable viscosity find application in different sectors, ranging from enhanced oil recovery to environmental remediation. This study investigates polymeric fluids obtained with hydroxyethylcellulose (HEC) and biocarbon particles prepared at low (650 degrees C, LM) and high (900 degrees C, HM) temperature of pyrolization. High pyrolization temperatures increased the hydrophobicity of biocarbon particles. In the absence of HEC, HM particles partitioned preferentially in non-polar solvents (e.g. toluene, rather than in water). Without particles, the shear loss (G") and storage (G') moduli of aqueous hydroxyethylcellulose solutions slowly increased over time, due to HEC hydration. The progressive increase in molecular crowding and microviscosity due to the hydration of HEC was demonstrated using steady-state fluorescence spectroscopy with the molecular rotor 9(dicyanovinyl)julolidine (DCVJ) as a probe. Particles added to HEC increased the viscoelastic moduli of HEC solutions only slightly (i.e. less than doubled them) when HEC was pre-hydrated, and significantly when HEC was not pre-hydrated. Specifically, addition of LM and HM particles (1-30 g/L) to non-hydrated HEC (16 g/L) increased the viscoelastic moduli of HEC solutions in water from below detection to G">9 Pa and G' >6 Pa (in the whole range of biocarbon particle concentrations analyzed). Our study investigates for the first time the effect of path-dependence of the preparation method on the rheology of HEC-carbon particle fluids. The viscoelastic moduli increase of HEC is ascribed to hydrophobic interactions between biocarbon particles and non-hydrated HEC. The preferential particle partitioning in aqueous HEC solutions (rather than in toluene) is also attributed to these interactions. Such interactions were attributed to the partially hydrophobic nature of HEC, which partitioned at the air-water interface (rather than exclusively in bulk water), lowering the interfacial tension and producing interfacial films with compressional rigidity. HEC interfacial film rigidity changed upon particle addition, supporting the hypothesis of HEC-particle interactions. Compression isotherms modelled using the theory by Marczak et al. were used here for the first time as an indirect tool to gain insights regarding HECcarbon particle interactions. Partial HEC hydrophobicity was also indicated by the higher viscosity measured in ethanol-water solutions than in water, which showed that ethanol-water was a better solvent than pure water. Steady state fluorescence measurements confirmed that the amount of HEC-bound solvent was greater in ethanol-water mixtures than in pure water. Biocarbon particles mixed with non-hydrated HEC were well-dispersed. Instead, biocarbon particles aggregated when they were mixed with hydrated HEC, indicating that the magnitude of attractive forces between particles and HEC decreased when HEC was hydrated. Compression isotherms measured at the air-water interface with non-hydrated HEC alone and with non-hydrated HEC-biocarbon particle mixtures differed, indicating HEC-particle co-adsorption at the air-water interface and the formation of HEC-particle aggregates.