Investigating dispersion regimes for effective mass transfer in single-step silica nanofluids for improved CO$_{2}$ utilization

Abstract

Nanofluids are novel colloidal dispersions of particles (size $\ge 100$ nm) dispersed in base fluids which have widespread industrial applications. However, the suspended particles in the nanofluids are highly prone to agglomeration and sedimentation. Thus, it becomes highly essential to understand the “dispersion regime” inside the nanofluid to ascertain its viability for desired applications. The dispersion regime is the state of particle suspension inside the base fluid and it has not been widely explored in past literature. Hence, in this work, the dispersion regimes inside silica nanofluids synthesized via single-step method have been explored by the simultaneous UV–Vis, CO$_2$ absorption, electrical conductivity, and viscosity measurements. Unlike past studies, a comprehensive comparative study (with respect to the base fluid) was carried out to denote NP agglomeration and dispersion regimes. The parameters varied were pH (2–12) and salinity (0–4 wt%), and the optimum conditions in which silica nanofluids exhibited a well-dispersed dispersion regime (i.e. negligible NP agglomeration) have been identified. Increasing salinity beyond $>0.5$ wt% induced agglomeration in the silica nanofluid (evident by the change in dispersion regime). Scanning electron microscope (SEM) images were used to verify the presence of anticipated dispersion regime inside the fluid. Based on these observations, single-step silica nanofluids showed improved heat/mass transfer capacity in the pH range of 7.6–9.4 and salinity $< 2$ wt% NaCl. At higher salinity ($> 2$ wt%) and at low pH conditions (denoting an acidic environment), NP agglomeration was severe and use of nanofluid is not recommended.