An Empirical Approach to Modeling Ion Size Effects in Diffuse Double Layer Theory

Structure and Orientation of the Imidazolium Cation at the Room-Temperature Ionic Liquid/SiO2 Interface Measured by Sum-Frequency Vibrational Spectroscopy

In this study, we have examined both the effect of alkyl chain length and anion composition on the 1-alkyl-3-methylimidazolium (C(n)mim, n = 4, 6, 8, 10, and 12) structure and orientation at the room-temperature ionic liquid (RTIL)/SiO(2) interface by sum-frequency vibrational spectroscopy (SFVS). Four different anions were investigated in this study: tetrafluoroborate (BF(4)), hexafluorophosphate (PF(6)), bis(trifluoromethylsulfonyl)imide (BMSI), and bis(pentafluoroethylsulfonyl)imide (BETI). It was found that the alkyl chain in BMSI and BETI RTILs showed a decrease in gauche defects with an increase in chain length, whereas the alkyl chains of the BF(4) and PF(6) RTILs have virtually no gauche defects regardless of chain length. The tilt of the alkyl chain lies predominantly perpendicular to the surface for all the RTILs examined. A strong correlation between the HCCH vs tilt angle and alkyl chain length was observed; as the alkyl chain is lengthened the HCCH vs lies more perpendicular to the SiO(2) surface. The results of this study suggest that the length of the alkyl chain dictates to a large degree the orientation of the imidazolium cation at the surface, regardless of anion composition. To a lesser extent, the HCCH vs tilt of the imidazolium ring of the cation also appears to be correlated to the surface charge density of the SiO(2). As the SiO(2) surface charge density becomes more negative the HCCH vs tilt angle lies more parallel to the surface.

Series Approach to Modeling Ion Size Effects for Symmetric Electrolytes in the Diffuse Double Layer

An analytical model is developed for the potential drop and differential capacity across the diffuse layer which considers the effects of ion size on these properties. For symmetric electrolytes, this potential drop is expressed in terms of a cubic polynomial in the corresponding estimate in the Gouy-Chapman theory. Optimal polynomial coefficients and model validation for 1:1 and 2:2 electrolytes are provided by fits of Monte Carlo data obtained for a restricted electrolyte in a primitive solvent. Simple relationships between these coefficients and parameters commonly associated with the mean spherical approximation are obtained. It is shown that the series approach accurately describes potential drops and differential capacities of the diffuse layer for 1:1 and 2:2 electrolytes for the chosen assumptions.

Ion distribution around electrostatically stabilized polystyrene latex particles studied by ellipsometric light scattering

The ion distribution around electrostatically stabilized polystyrene latex spheres for different ionic strengths is investigated by ellipsometric light scattering. This method is sensitive to the refractive index profile around colloidal particles, which is affected by the local salt content. At an average salt concentration of c* = 10(-4) mol L(-1), the ion concentration at the particle interface increases discontinuously, and a layer of high salt content with 20-30 nm thickness is built up. The observation cannot be explained within the framework of the Poisson-Boltzmann equation; it rather resembles a prewetting transition. Interactions that could possibly lead to a stabilization of the observed layer of high salt content are discussed.

New developments in the theory of the diffuse double layer

The role of ion size effects in determining diffuse layer properties is considered. Monte Carlo data relevant to this question are reviewed. Then the integral equation approach to the problem is considered with an emphasis on attempts to derive an analytical equation for the potential drop across the diffuse layer. An empirical model for ion size effects is described that allows one to estimate not only the diffuse layer potential drop but also the diffuse layer capacity, the ionic surface excesses, and the potential distribution in the diffuse layer. It is demonstrated that the resulting model can easily be applied by experimentalists to the analysis of experimental data for double layer phenomena in electrochemistry and colloid science.