Forces within Single Pairs of Charged Colloids in Aqueous Solutions of Ionic Liquids as Studied by Optical Tweezers

Forces of interaction within single pairs of negatively charged microsized blank colloids in aqueous solutions of water miscible room temperature ionic liquids (RTILs) have been measured at varying concentrations and pH by using optical tweezers (OT). Three different water miscible RTILs (1-butyl-3-methylimidazolium tetrafluoroborate [BMIM-BF4], 1-butyl-3-methylimidazolium trifluoromethanesulfonate [BMIM-TfO], and 1-butyl-3-methylimidazolium chloride [BMIM-Cl]) having the same organic cation [BMIM](+) and different inorganic anions ([BF4](-), [TfO](-), and Cl-) are used and compared with the high temperature molten salt (KCl). The experimental data are well described by a size-corrected screened Coulomb interaction approach which originates from the linearized Poisson-Boltzmann (PB) equation. The effective surface charge density sigma derived from the fitted force-separation data is found to be concentration and dependent.