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

Controlled Formation of Hydrated Micelles by the Intervention of Cyclodextrins

The interaction between surfactants and cyclodextrins (CDs) is well known. Studies have focused mainly on destruction of micelles with CDs to release the encapsulated drugs. However, less emphasis has been given on understanding the formation of micelles with the CD encapsulated surfactants. We have used fluorescence spectroscopy to study the impact of CDs on micelles using a fluorophore that has been tactically designed as a reporter. This molecule has a pyrene moiety on one end and a cationic head group on the other so that the orientation of the compound can be prefixed on micelle formation in aqueous environment. We have observed that the CD encapsulated surfactants can form "hydrated micelles" that allow extensive penetration of water molecules toward the core. The mechanism for such a process involves inclusion of the hydrophobic surfactant tails within the CD core and participation of these inclusion complexes in micelle formation. The process could be controlled by tuning the concentration of CD. The degree of hydration varies as the micelles get more opened up due to the residence of the CDs inside them.

Cyclodextrins and surfactants in aqueous solution above the critical micelle concentration: where are the cyclodextrins located?

Cyclodextrins (CDs) are known to bind surfactant molecules below the surfactant critical micelle concentration (CMC); however, interactions of CDs with surfactant micelles (above the CMC) are not well understood. In particular, direct structural evidence of the location of CDs in the different subphases found in micellar solutions is lacking. We have utilized small-angle neutron scattering (SANS) with contrast matching to probe the localization of α-cyclodextrin (α-CD) and 2-hydroxypropyl-β-cyclodextrin (HPβ-CD) in sodium dodecyl sulfate (SDS) micelles in aqueous (D2O) solutions. SANS data from solutions containing either hydrogenated or deuterated surfactants were analyzed by considering three different scenarios pertaining to the localization of cyclodextrin, either all in solution or some in the micelle shell or some in the micelle core, and were simultaneously fitted using the core-shell prolate ellipsoid form factor and the Hansen-Hayter-based structure factor. The scenario that considered a fraction of CD to localize in the micelle core well described the SANS data from both hydrogenated and deuterated SDS-CD-D2O solutions, while the other two scenarios did not. Among the various structural and interaction parameters obtained from this analysis, it emerged that the micelle core consisted of up to ∼10% HPβ-CD or ∼16% α-CD with respect to the total number of molecules (surfactants and CDs) present in the micelle at 25 mM SDS, and up to 14% HPβ-CD or 28% α-CD at 50 mM SDS. This is the first study that provides direct evidence on the location of cyclodextrin in the core of surfactant micelles. An improved understanding of CD interactions with surfactants and lipids would enable better strategies for drug encapsulation and delivery with CDs.

The formation of host-guest complexes between surfactants and cyclodextrins

Cyclodextrins are able to act as host molecules in supramolecular chemistry with applications ranging from pharmaceutics to detergency. Among guest molecules surfactants play an important role with both fundamental and practical applications. The formation of cyclodextrin/surfactant host-guest compounds leads to an increase in the critical micelle concentration and in the solubility of surfactants. The possibility of changing the balance between several intermolecular forces, and thus allowing the study of, e.g., dehydration and steric hindrance effects upon association, makes surfactants ideal guest molecules for fundamental studies. Therefore, these systems allow for obtaining a deep insight into the host-guest association mechanism. In this paper, we review the influence on the thermodynamic properties of CD-surfactant association by highlighting the effect of different surfactant architectures (single tail, double-tailed, gemini and bolaform), with special emphasis on cationic surfactants. This is complemented with an assessment of the most common analytical techniques used to follow the association process. The applied methods for computation of the association stoichiometry and stability constants are also reviewed and discussed; this is an important point since there are significant discrepancies and scattered data for similar systems in the literature. In general, the surfactant-cyclodextrin association is treated without reference to the kinetics of the process. However, there are several examples where the kinetics of the process can be investigated, in particular those where volumes of the CD cavity and surfactant (either the tail or in special cases the head group) are similar in magnitude. This will also be critically reviewed.

Tuning the viscoelasticity of nonionic wormlike micelles with β-cyclodextrin derivatives: a highly discriminative process

We report the influence of five β-cyclodextrin (β-CD) derivatives, namely: randomly methylated β-cyclodextrin (MBCD), heptakis (2,6-di-O-methyl)-β-cyclodextrin (DIMEB), heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin (TRIMEB), 2-hydroxyethyl-β-cyclodextrin (HEBCD) and 2-hydroxypropyl-β-cyclodextrin (HPBCD), on the self-assembly of mixtures of nonionic surfactants: polyoxyethylene cholesteryl ether (ChEO10) and monocaprylin (MCL). Mixtures of ChEO10/MCL in water form highly viscoelastic wormlike micelle solutions (WLM) over a range of concentrations; herein, the composition was fixed at 10 wt % ChEO10/3 wt % MCL. The addition of methylated β-CDs (MBCD, DIMEB, TRIMEB) induced a substantial disruption of the solid-like viscoelastic behavior, as shown from a loss of the Maxwell behavior, a large reduction in G' and G″ in oscillatory frequency-sweep measurements, and a drop of the viscosity. The disruption increased with the degree of substitution, following: MBCD < DIMEB < TRIMEB. Cryo-TEM images confirmed a loss of the WLM networks, revealing short rods and disc-like aggregates, which were corroborated by small-angle neutron scattering (SANS) measurements. Critical aggregation concentrations (CAC), measured by fluorescence spectroscopy, increased in the presence of DIMEB for both ChEO10 and MCL, suggesting the existence of interactions between methylated β-CDs and both surfactants involved in WLM formation. Instead, hydroxyl-β-CDs had a very different effect on the WLM. HPBCD only slightly reduced the solid-like behavior, without suppressing it. Quite remarkably, the addition of HEBCD reinforced the solid-like characteristics and increased the viscosity 10-fold. Cryo-TEM images confirmed the subsistence of WLM in ChEO10/MCL/HEBCD solutions, while SANS data revealed a slight elongation and thickening of the worms, and an increase of associated water molecules. CAC data showed that HPBCD had little effect on either surfactant, while HEBCD strongly affected the CAC of MCL and only slightly affected the ChEO10. For both DIMEB and HEBCD, time-resolved SANS measurements showed that morphology changes underlying these macroscopic changes occur in less than 100 ms.

Microheterogeneous catalysis

The catalytic effect of micelles, polymers (such as DNA, polypeptides) and nanoparticles, saturable receptors (cyclodextrins and calixarenes) and more complex systems (mixing some of the above mentioned catalysts) have been reviewed. In these microheterogeneous systems the observed changes in the rate constants have been rationalized using the Pseudophase Model. This model produces equations that can be derived from the Brönsted equation, which is the basis for a more general formulation of catalytic effects, including electrocatalysis. When, in the catalyzed reaction one of the reactants is in the excited state, the applicability (at least formally) of the Pseudophase Model occurs only in two limiting situations: the lifetime of the fluorophore and the distributions of the quencher and the probe are the main properties that define the different situations.

Experimental and theoretical approach to the sodium decanoate-dodecanoate mixed surfactant system in aqueous solution

The mixed system consisting of two anionic surfactants of identical headgroups but with 10 and 12 carbon atoms on the hydrophobic tail, sodium decanoate (C(10)Na) and sodium dodecanoate (C(12)Na), has been studied in aqueous solution at 298.15 K by means of conductivity and fluorescence spectroscopy experiments and from a theoretical point of view. The monomeric and micellar phases of the mixed aggregates were analyzed through the experimental determination of the total critical micelle concentration, cmc*, the degree of ionization of the mixed micelle, beta, and the total aggregation number, N*. Results indicate that, compared to the ideal behavior, the mixed system with two anionic surfactants differing only in two methylenes in the hydrophobic tail shows a negative deviation in the cmc* and a positive one in N*. Pure surfactants (C(10)Na and C(12)Na) form spherical micelles, but mixed micelles must aggregate with a rodlike shape to allow more surfactant molecules than expected. In addition, rodlike micelles result in more compacted aggregation (i.e., less area per polar head). From the experimental data in this work, several theoretical models for mixed surfactant systems have been checked: Rubingh's model predicts lower deviations from ideality than Motomura's model. The stability of the micelles has been analyzed by computing the standard Gibbs energy of micelle formation, Delta G(mic,0), of pure and mixed micelles. Results of this work reinforce the feature that mixed systems formed by alkylsurfactants with the same polar head that differ in the hydrocarbon length, usually admitted as roughly ideal systems, may show nonideal behavior. This deviation, being mostly related to the difference in the chain length, Delta n(c), between surfactants can be analyzed only when very accurate experimental techniques as well as adequate theoretical models are used.

Kinetics of Photochemical Reactions under Restricted Geometry Conditions

Kinetics of ground state reactions under restricted geometry conditions have been rationalized generally taking as a basis the Pseudophase Model, whose basic hypothesis is: The distribution of the reactants between the pseudophases is at equilibrium, that is, the rate of the reaction is slow in relation to the rates of entry and exit of the reactant to and from the receptor. However, photochemical reactions are generally very rapid, so the following question emerges: is the Pseudophase Model still applicable to photochemical reactions? To answer this question has been the primary objective of this work in which photochemical reactions in micelles, polymers and cyclodextrins have been reviewed. The lifetime of the fluorophore and the distributions of the quencher and the probe are crucial properties; they must be taken into account to obtain a proper description of the behaviour of a given system. The models of Infelta, Almgren and Quina visualise different scenarios with varied combinations of these properties. It is shown that, in some cases, the Pseudophase Model holds at least formally.