Effect of Organic Solvent on the Mass Transfer Mechanism of Coumarin 102 in a Single Octadecylsilyl Silica Gel/Organic Solvent–Water System by Laser Trapping and Fluorescence Microspectroscopy

Understanding mass transfer kinetics within individual porous particles is crucial for theoretically explaining the retention and elution behaviors in chromatography and drug delivery. Using laser trapping and fluorescence microspectroscopy, we investigated the diffusion mechanism of coumarin 102 (C102) into single octadecylsilyl particle in acetonitrile (ACN)/water, N, N -dimethylformamide (DMF)/water, and 1-butanol (BuOH)/water solutions. The intraparticle diffusion behavior of C102 was evaluated using the spherical diffusion equation, allowing us to determine the intraparticle diffusion coefficients ( D intra ): (8–10) × 10 –9 cm 2 s –1 for ACN, (10–16) × 10 –9 cm 2 s –1 for DMF, and (4–6) × 10 –9 cm 2 s –1 for BuOH. The obtained D intra values were further analyzed using a pore and surface diffusion model. Thus, we revealed that the diffusion mechanism of C102 differed depending on the organic solvent: surface diffusion for ACN and DMF and pore and surface diffusions for BuOH were observed. This difference is attributed to the formation of a concentrated liquid phase of ACN and DMF at the interface of the alkyl chain and the bulk solution in the pore.