title: Lateral diffusion of 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate at the interfaces of C18 and chromatographic solvents
abstract: 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.
authors.affiliation: Department of Chemistry and Biochemistry, University of Delaware, Newark 19716, USA.
authors.initials: DJ
authors.lastname: Swinton
authors.initials: MJ
authors.lastname: Wirth