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

Abstract

Small-angle neutron scattering is used to study the influence that methylated and unsubstituted cyclodextrins might have on sodium decanoate micelles in D(2)O aqueous solution. When the concentration of the cyclodextrin is varied in a 200 mM sodium decanoate solution (CMC approximately 116 mM), it is found that methylated and unsubstituted cyclodextrins exhibit distinct behaviors. In particular, permethyl-alphaCD, dimethyl-betaCD, and permethyl-betaCD 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 alphaCD and betaCD 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 D(2)O ([CD](o) = 30 mM), methylated cyclodextrins show correlation peaks in the I(Q) distributions, whereas alphaCD and betaCD 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.

Self-assembly control via molecular recognition: Effect of cyclodextrins on surfactant micelle structure and interactions determined by SANS

“…Further, they have assumed that the micelle aggregation number is not affected by the presence of methylated CDs, and have utilized the value of aggregation number derived from the SANS data of surfactants in the absence of CDs. However, the authors [33] did not fit their data to form and structure factors, and thus did not provide structural and interaction information at the level of detail that we report here. Although they have mentioned a decrease in aggregation number qualitatively for unsubstituted CDs, they have not reported actual values of aggregation numbers.…”

“…The scattering intensity is not significant for the case of 8.9 mM SDS which is near the CMC, however, with an increase in SDS concentration, micelles grow in number and size as attested by higher overall scattering intensity, and the development of a pronounced correlation peak. The position of the correlation peak maximum reflects the inter-micelle distance (d = 2 /q max ) and the sharpness of the peak indicates the strength of the interactions (long range electrostatic repulsion) between the micelles [33]. An increase in the surfactant concentration leads to an increase in q max and a decrease in the inter-micelle spacing.…”

“…At q values higher than the inverse of the smallest micelle dimension, in our case 0.14 ≤ q ≤ 0.22, the Porod equation I(q) = 2 2 (S/V)q −4 can be fitted to the experimental data to extract the value of surface-to-volume ratio S/V for the total surfactant in the solution, given the value of the scattering contrast between the micelle core and solvent , −6.73 × 10 −6 (Å −2 ) (assuming negligible scattering contrast between micelle shell and solvent) [33]. The average surface area per micelle S micelle can be estimated from the S/V ratio using the relation S micelle = (S/V)x micelle /N P , where x micelle is the fraction of the micelles in the system estimated by x micelle = (C SDS − C CMC )/C SDS , and N P is the micelle number density.…”

“…Although a number of investigations have examined interactions of CDs with SDS molecules, and the effect of CD type on CD-SDS complexation at surfactant concentrations below the CMC [18,23,27,28], this is the first structural investigation to address the influence of CD type and complexation behavior on the modulation of SDS micelles present at relatively high surfactant concentrations. The only other related study has considered the effects of unsubstituted or methylated cyclodextrins on sodium decanoate micelles [33]. These authors have evaluated the effects of cyclodextrins on micelles in terms of changes in S/V ratios determined by fitting the Porod equation at high q values, and in terms of inter-micelle distances d determined from q max .…”

“…However, other reports support partitioning of CDs in micelles. A recent investigation suggested the adsorption of unsubstituted and methylated cyclodextrins on sodium decanoate micelles, indicating that methylated cyclodextrins localize in the micelle shell [33]. Another study reported the formation of mixed ␤-CD and sodium dodecyl sulfate (SDS) micelles below the CMC, and suggested the incorporation of CDs inside the micelles [34].…”