Ion–surface interactions can alter the properties of nanopores and dictate nanofluidic transport in engineered and biological systems central to the water–energy nexus. The ion adsorption process, known as “charge regulation”, is ion-specific and is dependent on the extent of confinement when the electric double layers (EDLs) between two charged surfaces overlap. A fundamental understanding of the mechanisms behind charge regulation remains lacking. Herein, we study the thermodynamics of charge regulation reactions in 20 nm SiO2 channels via conductance measurements at various concentrations and temperatures. The effective activation energies (E a) for ion conductance at low concentrations (strong EDL overlap) are ∼2-fold higher than at high concentrations (no EDL overlap) for the electrolytes studied here: LiCl, NaCl, KCl, and CsCl. We find that E a values measured at high concentrations result from the temperature dependence of viscosity and its influence on ion mobility, whereas E a values measured at low concentrations result from the combined effects of ion mobility and the enthalpy of cation adsorption to the charged surface. Notably, the E a for surface reactions increases from 7.03 kJ mol–1 for NaCl to 16.72 ± 0.48 kJ mol–1 for KCl, corresponding to a difference in surface charge of −8.2 to −0.8 mC m–2, respectively. We construct a charge regulation model to rationalize the cation-specific charge regulation behavior based on an adsorption equilibrium. Our findings show that temperature- and concentration-dependent conductance measurements can help indirectly probe the ion–surface interactions that govern transport and colloidal interactions at the nanoscalerepresenting a critical step forward in our understanding of charge regulation and adsorption phenomena under nanoconfinement.

Authors introduce an ultrahigh-temperature (i.e., 2500–3000 °C) continuous fluidized bed furnace, in which the key operating variable is specific electrical resistance of the bed. A correlation has been established to predict the specific electrical resistance for the natural graphite-based precursors. Fluid dynamics models have been validated with the data from a fully functional prototype reactor. Data collected demonstrated that the difference between the calculated and measured values of specific resistance is approximately 25%; due to chaotic nature of electrothermal fluidized bed processes, this discrepancy was deemed acceptable. Optimizations proposed allow producing natural graphite-based end product with the purity level of 99.98 + wt. %C for battery markets.

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