Why Do Methylated and Unsubstituted Cyclodextrins Interact So Differently with Sodium Decanoate Micelles in Water?

Small-angle neutron scattering is used to study the influence that methylated and unsubstituted cyclodextrins might have on sodium decanoate micelles in D2O aqueous solution. When the concentration of the cyclodextrin is varied in a 200 mM sodium decanoate solution (CMC approximate to 116 mM), it is found that methylated and unsubstituted cyclodextrins exhibit distinct behaviors. In particular, permethyl-alpha CD, dimethyl-beta CD, and permethyl-beta CD display linear increases of the specific surface area S/V obtained from the Porod equation with slopes following the degree of methylation of the cyclodextrins, whereas alpha CD and beta CD present close S/V values which are approximately independent of the cyclodextrin concentration at least for the considered concentration range (from 5 mM up to 45 mM). When a cyclodextrin is added to the 200 mM perdeuterated sodium decanoate NaDec(d(19)) solution in D2O ([CD](o) = 30 mM), methylated cyclodextrins show correlation peaks in the I(Q) distributions, whereas alpha CD and beta CD do not originate any correlation maximum. On the whole, the experimental results point to the adsorption of methylated cyclodextrins on the surface of the formed decanoate micelles and are consonant with the existence of an equilibrium between the molecules in aqueous solution and those adsorbed by the micelles. In turn, unsubstituted cyclodextrins are not adsorbed by the decanoate micelles and so are involved in a guest-host equilibrium with the decanoate ion in solution. As the cyclodextrin concentration increases, this equilibrium leads to the shifting of the multiple equilibria involving micelles of successive aggregation numbers toward those of smaller dimensions and to the decrease of the average intermicellar distance. It is suggested that methylated cyclodextrins should be more easily adsorbed on the electric double layer of the decanoate micelle-water interface than the unsubstituted cyclodextrins, as the former tend to sample the environment that is most energetically favorable, where the dielectric permittivity is lower than in bulk water and so is closer to those of the methylated cyclodextrins.