Lateral Diffusion of 1,1‘-Dioctadecyl-3,3,3‘3‘- tetramethylindocarbocyanine Perchlorate at the Interfaces of C₁₈ and Chromatographic Solvents

The influence of mobile phase on the lateral diffusion of an amphiphilic dye was studied for four chromatographic interfaces using fluorescence correlation spectroscopy. The fluorescent dye was DiI (1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate); the stationary phase was a covalently bonded monolayer of dimethyloctadecylsiloxane on fused silica; and the four mobile phases were acetonitrile, methanol, tetrahydrofuran, and water. Time-resolved measurements at varying focal positions of the microscope confirmed negligible fluorescence from solution. Single-molecule spectroscopy showed that exchange between mobile and immobile adsorbates was negligible. The lateral diffusion of DiI was the fastest for tetrahydrofuran, D = 5.3 x 10(-5) cm2/s, and slowest for water, D = 2.5 x 10(-6) cm2/s. Methanol and acetonitrile gave intermediate rates of diffusion, D = 3.4 x 10(-6) and 4.8 x 10(-6), respectively. There was no correlation of the lateral diffusion coefficient with solvent viscosity, a weak correlation with interfacial tension, and a strong correlation with 13C NMR line shapes for the terminal methyl group of the hydrocarbon chain. The increased wetting of the C18 interphase in the order water < methanol < acetonitrile < tetrahydrofuran agreed with data for the miscibility of these solvents with n-hexadecane. It is concluded that the wetting of the hydrocarbon interphase by the mobile phase enhances the lateral diffusion of the amphiphile.