Lateral diffusion of 1,1 '-dioctadecyl-3,3,3 ' 3 '-tetramethylindocarbocyanine perchlorate at the interfaces of C-18 and chromatographic solvents

The influence of mobile phase on the lateral diffusion of an amphiphilic dy e was studied for four chromatographic interfaces using fluorescence correl ation spectroscopy. The fluorescent dye was DiI (1,1'-dioctadecyl-3,3,3'3'- tetramethylindocarbocyanine perchlorate); the stationary phase was a covale ntly 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 c onfirmed negligible fluorescence from solution. Single-molecule spectroscop y showed that exchange between mobile and immobile adsorbates was negligibl e. The lateral diffusion of DiI was the fastest for tetrahydrofuran, D = 5. 3 x 10(-5) cm(2)/s, and slowest for water, D = 2.5 x 10(-6) cm(2)/s, Methan ol 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 dif fusion coefficient with solvent viscosity, a weak correlation with interfac ial tension, and a strong correlation with C-13 NMR line shapes for the ter minal methyl group of the hydrocarbon chain, The increased wetting of the C -18 interphase in the order water < methanol < acetonitrile < tetrahydrofur an agreed with data for the miscibility of these solvents with n-hexadecane . It is concluded that the wetting of the hydrocarbon interphase by the mob ile phase enhances the lateral diffusion of the amphiphile.