Complexation Kinetics of Cyclodextrin with Hydrophobic Molecules Confined in an Isolated Droplet in Water

Inclusion complexation kinetics of methyl-β-cyclodextrin (m-β-CD) to hydrophobic molecules confined in an isolated droplet was investigated with optical tweezers. The radius of the droplet shrank over time as m-β-CD was introduced into the sample chamber due to complexation between the hydrophobic molecules and the m-β-CD molecules. The short time behavior of the droplet shrinkage can be described by a model based on Brownian kinetics. The probability of complexation was greater for a branched 2-hexadecanol than for hexadecane. The probability decreased with increasing chain length for short alkanes. The long time behavior of the droplet shrinkage is surprising because the droplets stopped shrinking even when the uncomplexed m-β-CD concentration was still high. We speculate that surface activities of m-β-CD might have slowed the reaction.