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Photocontrolled Reversible Supramolecular Assemblies of an Azobenzene‐Containing Surfactant with α‐Cyclodextrin

Electrochemistry of ferrocene in anionic, cationic and nonionic micellar solutions. Effet of the micelle solubilization of the half-wave potentials

An introduction to surfactant and micellar systems is presented. The unique properties exhibited by micellar systems that are ideally suited and very useful in the field of analytical chemistry are outlined (ie. their solubilization power; their utilization as a novel reaction medium in which the rates, equilibrium position, products, and, in a few instances, stereochemistry can be affected; and their ability to greatly alter the microscopic properties and change the effective microenvironment about solubilized solutes). A summary of published work is presented in which micellar systems have been successfully utilized to either improve existing analytical methods or help overcome problems associated with the development of new analytical procedures. Specific examples of such practical and useful applications of micellar systems are given for spectral methods of analysis such as ultraviolet-visible absorption, fluorescence, and atomic absorption; for electroanalytical methods involving potentiometry and polarography; and for chemical separations including chromatographic and extraction techniques. In each case, the micellar chemistry and/or properties involved in the particular analytical application are elucidated. The advantages of these micellar-improved analytical methods are discussed. Several other miscellaneous applications as well as potential areas for future research involving micellar systems in analytical chemistry are pointed out in the overview.

Use of Surfactant and Micellar Systems in Analytical Chemistry

Amphiphilic compounds consist of a hydrophobic alkyl chain and a hydrophilic head group. Owing to their surface active properties these compounds are also designated as surfactants. Driven by hydrophobic effects these systems can self organize into molecular assemblies, such as micelles or vesicles, which have been widely used as capsules for drug delivery and as microreactors for nanoparticle preparation. The control of the assembly and disassembly of these amphiphilic surfactants is of major importance and has been achieved by the incorporation and chemical modification of functional groups that are responsive to external stimuli, such as pH, temperature, redox state, and light. The formation of host–guest complexes between macrocyclic hosts and surfactant monomers represents a different approach to modulate the physicochemical properties of these systems, with the cyclodextrins being the most widely studied hosts, owing to their numerous commercial formulations, their biodegradability and lack of toxicity. There are a number of important features of cyclodextrin-surfactant mixed systems that should be highlighted. Firstly, the formation of a host–guest complex between the surfactant and the cyclodextrin shifts the critical micelle concentration (cmc) to higher values. In practice this means that the addition of cyclodextrin to a medium containing micelles may lead to the dissolution of the self-assembled aggregates. The second point is that there are no interactions between self-assembled surfactants and cyclodextrins. To the best of our knowledge, the information in the literature concerned with lowering the cmc values of simple surfactants by macrocyclic compounds is scarce and is limited to examples of cryptands, which complex the counterions of anionic micelles, thus making the former more hydrophobic and favoring the micellization process. We present here the results obtained in a study of a mixed system based on 5,11,17,23,29,35-hexasulfonato37,38,39,40,41,42-hexamethoxycalix[6]arene (1) and the cationic surfactant dodecyltrimethylammonium bromide (2). Our results show that the host–guest complex formed by 1 and 2 self-aggregates at lower concentrations than the pure surfactant alone. This result can be explained in terms of the surfactant properties of the host–guest complex and its selfaggregation to form micelles. Calixarenes are flexible hosts that can adopt several conformations depending on the substituents present on the lower and upper rims. The unmethylACHTUNGTRENNUNGated precursor of 1 was shown to adopt a partial cone conformation in the solid state with three adjacent sulfonate groups pointing to one side of the cavity and the remaining three sulfonates pointing to the opposite end. It was shown that it is possible to control the conformational flexibility of this host in the presence of crown ether and lanthanideACHTUNGTRENNUNG(III) chlorides and a handful of solid-state structural examples show the calixarene to be in an “up–up” double cone conformation. The only crystal structure reported for 1 shows the molecule to be in a pseudo-“up–up” double cone conformation, whereby one sulfonate group points “downwards”. This more disorganized structure is a consequence of the lack of OH groups that potentially act as conformational directing groups through the formation of intramolecular hydrogen bonds. Molecular modeling calculations also indicated that 1 is highly flexible, as the authors could not locate a clearly defined energy minimum. Information about the preferred conformation in solution can be achieved by observing the signals of the ArCH2Ar methylene protons in the H NMR spectra. In the absence of 2 the H NMR spectrum of 1 contains one singlet for the ArCH2Ar protons, indicating that the calixarene is exchanging rapidly in the NMR time scale between several possible conformations. In the presence of 2 we also observed one singlet for the methylene protons for all the concentrations studied. This observation suggests that complexation of 2 with 1 and aggregation of the host–guest complex does not make the host rigid to an extent that the H NMR signals can be differentiated. [a] N. Basilio, Prof. L. Garc a-R o Departamento de Qu mica F sica, Facultade de Qu mica Universidade de Santiago, 15782 Santiago (Spain) Fax: (+34) 981-595012 E-mail : luis.garcia@usc.es

Sulfonated calix[6]arene host-guest complexes induce surfactant self-assembly.

A new amphiphilic pillar[5]arene (AP5-glycol) with five oligomeric glycol groups and five alkyl chains was prepared. AP5-glycol spontaneously formed bilayer vesicles in water, and these vesicles were still stable after several weeks. Additionally, when they were exposed to external physical stimuli, these vesicles also showed reversible thermal and dynamic properties. Interestingly, oleic-acid-stabilized magnetic iron oxide nanoparticles could be incorporated into the bilayer of the AP5-glycol vesicles to form hybrid magnetic-responsive supramolecular vesicles.

R. A. Day

Papers

Electrochemistry of substances solubilized in micelles. Polarography of azobenzene in aqueous surfactant solutions.