Thermophoresis of cyclic oligosaccharides in polar solvents

Cyclodextrins are cyclic oligosaccharides which are interesting as drug delivery systems, because they can be used as containers for pharmaceutical substances. We studied the Ludwig-Soret effect of $\alpha$-, $\beta$-, $\gamma$- and methyl-$\beta$-cyclodextrin in water and formamide by infrared thermal diffusion forced Rayleigh scattering (IR-TDFRS).In water the Soret coefficient, $S_{\mathrm{T}}$, of $\alpha$-, $\beta$- and $\gamma$-cyclodextrin increases with increasing temperature and shows a sign change from negative to positive around $T=35^\circ$C, while $S_{\mathrm{T}}$ of methyl-$\beta$-cyclodextrin is positive in the entire investigated temperature. In formamide $S_{\mathrm{T}}$-values of all cyclodextrins coincide and show a slight decrease with temperature. We discuss the obtained results and relate the $S_{\mathrm{T}}$-values to the different hydrogen bonding capabilities of the cyclodextrins and the used solvents. It turns out that the change of $S_{\mathrm{T}}$ with temperature correlates with the partition coefficient, log~$P$, which indicates that more hydrophilic substances show a more pronounced temperature sensitivity of $S_{\mathrm{T}}$. Additionally we obtained a surprising result measuring the refractive index contrast factor with temperature, $(\partial n/\partial T)_{c,p}$ of cyclodextrins in formamide, which might be explained by a complex formation between cyclodextrins and formamide.