Cyclodextrin-Surfactant Mixed Systems as Reaction Media

Abstract

In recent years our reseach group has investigated the chemical behaviour of β-cyclodextrin (CD)/surfactant mixed systems and their characteristics as reaction media. The results have been interpreted in terms of a pseudophase model that takes into account the formation of both CD-surfactant and CD-substrate complexes and also, in some cases, the exchange of X- and OH- ions between the micellar and aqueous pseudophases. from the experimental results it was concluded that the presence of CD has no effect on existing micelles but raises the critical micellar concentration (cmc). on the other hand, at surfactant concentrations above the cmc, competition between the micellisation and complexation processes leads to the existence of a significant concentration of free CD in equilibrium with the micellar aggregates. The percentage of uncomplexed β-CD in equilibrium with the micellar system increases on increasing the hydrophobicity of the surfactant molecule. This behaviour was justified taking into account the existence of two simultaneous processes: complexation of surfactant monomers by CD and the process of self-assembly to form micellar aggregates. The autoaggregation of surfactant monomers is more important than the complexation process in this mixed system. Varying the hydrophobicity of the surfactant monomer enabled us to determine that the percentages of uncomplexed CD in equilibrium with the micellar system were in the range of 5-95%. When the surfactant self-assembly structure is a vesicle, the free CD in the CD/surfactant mixed system yields a percentage of 100%.

Participation of Inclusion Complexes in the Surface Adsorbed Layer in Mixtures of α-Cyclodextrin and Cationic−Anionic Hydrogenated and Fluorinated Surfactants: A Surface Tension Proof

“…Because this SPFO:OTEAB composition ratio was not very far from 1:1, although the cmc*'s were obviously increased [(2.46 and 2.43) mmol·kg −1 , respectively] in comparison with the cmc of equimolar OTEAB/SPFO (1.64 mmol·kg −1 ), the total concentrations of surfactant monomers [(1.94 and 1.61) mmol·kg −1 , respectively] were close to the cmc of equimolar OTEAB/SPFO. It was also consistent with the general situation that the presence of CD has no effect on existing strong micelles but raises the cmc …”

“…Although CD (free and complexed) might show affinity toward the micelles because of ion−dipole interactions between the carboxylate headgroups in the hydrophilic shell and the hydroxyl groups of the CD, ,, this kind of interaction between CD and micelles might be weak because the cationic−anionic surfactant headgroups in micelles strongly interact with each other. Generally it is concluded that the presence of CD has no effect on existing strong micelles but increases the cmc . Here, with the assumptions that α-CD included only surfactant monomers in solution and that the complexes did not participate in the micelles, the concentrations of SPFO and OTEAB in Table could be regarded as the critical composition of micelles: the sums of C SPFO and C OTEAB were 1.94 mmol·kg −1 (critical composition of micelles SPFO:OTEAB = 2.95) and 1.61 mmol·kg −1 (critical composition of micelles SPFO:OTEAB = 1.60) for systems 1 and 2, respectively, which was consistent with the discussion in our previous work that SPFO might be the much more contributive component in mixed OTEAB/SPFO micelles .…”

“…The aggregation of surfactant monomers is more important than the complexation process in this mixed system. When the surfactant self-assembly structure is a vesicle, CD could even remain totally uncomplexed …”

“…When the surfactant self-assembly structure is a vesicle, CD could even remain totally uncomplexed. 24 In a word, recent studies have shown the equilibrium of micellization and its influence on the process of complexation to be more important than we used to think in systems involving CD and a single surfactant. important in industrial applications.…”

Pseudophase Model in Microemulsions

“…The pseudophase model applied to micelles has also been satisfactory for the analysis of the kinetic results in more complex micellar systems such as mixed micellar-cyclodextrin systems [31][32][33][34][35][36][37] or pseudo-micellar humic acids aggregates [38][39][40][41][42].…”