Reverse Micelle Size and Stability During Electrolyte Encapsulation and Implications for the Synthesis of Nanomaterials
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Abstract
A model for reverse micelle size with additions of salts, based on the formation of a pair of overlapping electrical double layers, has been developed. Specific ion effects demonstrate that reverse micelle size is proportional to ion hydration complex size and the electrical double layer thickness. Limited salt solubility has been observed due to one of two destabilization mechanisms: (1) one based on too small of an average reverse micelle size and (2) one based on too large of a hydrated ion size. Precursor limits for obtaining stable solutions once a precipitation reaction is initiated by mixing two reverse micellar solutions does not necessarily correlate to the salt solubility limits. Analytical solutions of the model provide several parameters that are either not reported by other researchers at all or are not experimentally assessed by the methods presented within: (1) the critical potential, or potential minimum between the two overlapping electrical double layers, (2) the local ion concentration generating the critical potential, (3) the extent of Stern layer occupation in reverse micelles, and (4) an estimate of dielectric constant of the reverse micelle interior phase. Changing the water to surfactant ratio, partial substitution of polar solvents for water, and partial substitution of cationic or nonionic surfactant for the anionic surfactant were investigated. None of these changes affect the critical potential, but do affect the local ion concentration at the critical potential. The estimated dielectric constant can also vary. Reduced water to surfactant ratio had the biggest effect on increasing local ion concentration, increasing dielectric constant for counterions of high valence, and is the only modification that increased reactant limits for obtaining stable precipitate solutions.