Supramolecular Assembly of Self‐Labeled Amphicalixarenes

Shape‐persistent micelles consisting of exactly 12 molecules (see picture), which coexist with rodlike micelles, are formed by the self‐labeled amphicalixarene at pH 7. In contrast to other examples of structurally defined micelles, this new type of amphiphile serves as a fluorescence reporter and provides an extended cavity structure in the hydrophilic part, thereby facilitating the inclusion of guest molecules.

     

Nanoscale Control Over Interfacial Properties in Mixed Reverse Micelles Formulated by Using Sodium 1,4‐bis‐2‐ethylhexylsulfosuccinate and Tri‐n‐octyl Phosphine Oxide Surfactants

The interfacial properties of pure reverse micelles (RMs) are a consequence of the magnitude and nature of noncovalent interactions between confined water and the surfactant polar head. Addition of a second surfactant to form mixed RMs is expected to influence these interactions and thus affect these properties at the nanoscale level. Herein, pure and mixed RMs stabilized by sodium 1,4‐bis‐2‐ethylhexylsulfosuccinate and tri‐n‐octyl phosphine oxide (TOPO) surfactants in n‐heptane were formulated and studied by varying both the water content and the TOPO mole fraction. The microenvironment generated was sensed by following the solvatochromic behavior of the 1‐methyl‐8‐oxyquinolinium betaine probe and ³¹P NMR spectroscopy. The results reveal unique properties of mixed RMs and we give experimental evidence that free water can be detected in the polar core of the mixed RMs at very low water content. We anticipate that these findings will have an impact on the use of such media as nanoreactors for many types of chemical reactions, such as enzymatic reactions and nanoparticle synthesis.     

Design of Stimuli-Responsive Dynamic Covalent Delivery Systems for Volatile Compounds (Part 2): Fragrance-Releasing Cleavable Surfactants in Functional Perfumery Applications

Amphiphilic imines prepared by condensation of a hydrophobic fragrance aldehyde with a hydrophilic amine derived from a poly(propylene oxide) and poly(ethylene oxide) diblock copolymer were investigated as cleavable surfactant profragrances in applications of functional perfumery. In water, the cleavable surfactants assemble into micelles that allow solubilization of perfume molecules that are not covalently attached to the surfactant. Dynamic headspace analysis on a glass surface showed that solubilized perfume molecules evaporated in a similar manner in the presence of the cleavable surfactant as compared with a non-cleavable reference surfactant. Under application conditions, the cleavable surfactant imine hydrolysed to release the covalently linked fragrance aldehyde. The profragrances were stable during storage in aqueous media, and upon dilution showed a blooming effect for the hydrolytical fragrance release and a more balanced performance of a solubilized perfume by retaining the more volatile fragrances and boosting the evaporation of the less volatile fragrances.     

Classical Amphiphilic Behavior of Nonclassical Amphiphiles: A Comparison of Metallacarborane Self‐Assembly with SDS Micellization

The self‐assembly of metallacarboranes, a peculiar family of compounds exhibiting surface activity and resembling molecular‐scale Pickering stabilizers, has been investigated by comparison to the micellization of sodium dodecylsulfate (SDS). These studies have shown that molecules without classical amphiphilic topology but with an inherent amphiphilic nature can behave similarly to classical surfactants. As shown by NMR techniques, the self‐assembly of both metallacarboranes and SDS obey a closed association model. However, the aggregation of metallacarboranes is found to be enthalpy‐driven, which is very unusual for classical surfactants. Possible explanations of this fact are outlined.     

Effect of the Cationic Surfactant Moiety on the Structure of Water Entrapped in Two Catanionic Reverse Micelles Created from Ionic Liquid‐Like Surfactants

The behavior of water entrapped in reverse micelles (RMs) formed by two catanionic ionic liquid‐like surfactants, benzyl‐n‐hexadecyldimethylammonium 1,4‐bis‐2‐ethylhexylsulfosuccinate (AOT‐BHD) and cetyltrimethylammonium 1,4‐bis‐2‐ethylhexylsulfosuccinate (AOT‐CTA), was investigated by using dynamic (DLS) and static (SLS) light scattering, FTIR, and ¹H NMR spectroscopy techniques. To the best of our knowledge, this is the first report in which AOT‐CTA has been used to create RMs and encapsulate water. DLS and SLS results revealed the formation of RMs in benzene and the interaction of water with the RM interface. From FTIR and ¹H NMR spectroscopy data, a difference in the magnitude of the water–catanionic surfactant interaction at the interface is observed. For the AOT‐BHD RMs, a strong water–surfactant interaction can be invoked whereas for AOT‐CTA this interaction seems to be weaker. Consequently, more water molecules interact with the interface in AOT‐BHD RMs with a completely disrupted hydrogen‐bond network, than in AOT‐CTA RMs in which the water structure is partially preserved. We suggest that the benzyl group present in the BHD⁺ moiety in AOT‐BHD is responsible for the behavior of the catanionic interface in comparison with the interface created in AOT‐CTA. These results show that a simple change in the cationic component in the catanionic surfactant promotes remarkable changes in the RMs interface with interesting consequences, in particular when using the systems as nanoreactors.     

Molecular Logic Operations Based on Surfactant Nanoaggregates

Two molecular logic gates, FS1 and FS2, which display a UV and fluorescence behavior that is dependent on the pH value and the sodium dodecyl sulfate (SDS) surfactant concentration, are demonstrated based on the intramolecular charge‐transfer mechanism. They are constructed according to the inorganic salts that induce transformation from premicelle to micelle. The absorption band of FS1 at 480 nm is significantly enhanced only when both SDS and Na₂SO₄ are the input at high concentrations, in accordance with an AND logic gate. The OR logic function can be realized in a 3.5 mM SDS/FS2 aqueous solution with SDS and Na₂SO₄ as inputs along with the emission intensity as output. Furthermore, half addition and half subtraction can be incorporated in FS1. This is facilitated by the surfactant, due to its versatility.     

A Surfactant Concentration Model for the Systematic Determination of the Critical Micellar Concentration and the Transition Width

The critical micellar concentration (cmc) is a fundamental property of surfactant solutions. Many proposed methods for the definition and determination of the cmc from property-concentration plots yield values, which depend on the studied property, on the specific technique used for its analysis and in many cases on the subjective choice of the chosen type of plot and concentration interval. In this focus review, we revise the application of a surfactant concentration model we proposed earlier that defines the cmc directly based on the surfactant concentration. Known equations for the concentration-dependence of different surfactant properties can then be combined with this concentration model and fitted to experimental data. This modular concept makes it possible to determine the cmc and the transition width in a systematic and unambiguous way. We revise its use in the literature in different contexts: the determination of the cmc of surfactants and their mixtures from different properties (electrical conductivity, NMR chemical shift, self-diffusion, surface tension, UV-Vis absorption, fluorescence intensity and fluorescence correlation). We also revise the dependence of the width of the transition region on composition, detailed studies of the properties of fluorescent probes and the aggregation of non-surfactant systems, namely amyloid peptides.     

Surfactant Diffusion Near Critical Micelle Concentrations

Taylor dispersion and differential refractometry are used to measure mutual diffusion coefficients (D) for binary aqueous solutions of octylglucopyranoside, dodecylsulfobetaine, and sodium dodecyl sulfate (nonionic, zwitterionic and ionic surfactants, respectively). Aggregation causes a sharp drop in D as the concentration of each surfactant is raised through the critical micelle concentration (cmc). Differential mutual diffusion coefficients are determined in this composition region by using small initial concentration differences (3 mmol-dm^–3) and by extrapolating the measured D values to zero initial concentration difference relative to the carrier stream. The drop in D for each surfactant is more gradual than the concentration dependence predicted by the chemical equilibrium model of surfactant diffusion. Micelle polydispersity and nonideal solution behavior are discussed as possible explanations for this discrepancy. Intradiffusion coefficients (D*) for aqueous octylglucopyranoside and dodecylsulfobetaine are evaluated by integrating the relation d(cD*) = Ddc previously derived for dilute solutions of self-associating nonelectrolyte solutes.     

Photocrosslinked poly(vinylbenzophenone)‐core micelles via mild Friedel–Crafts benzoylation of polystyrene amphiphiles

Photocrosslinkable poly(vinylbenzophenone)‐containing polymers were synthesized via a one‐step, Friedel–Crafts benzoylation of polystyrene‐containing starting materials [including polystyrene, polystyrene‐block‐poly(tert‐butyl acrylate), polystyrene‐block‐poly(ethylene oxide), polystyrene‐block‐poly(methyl methacrylate), and polystyrene‐block‐poly(n‐butyl acrylate)] with benzoyl trifluoromethanesulfonate as a benzoylation reagent. The use of this mild reagent (which required no added Lewis acid) permitted polymers with well‐defined compositions and narrow molecular weight distributions to be synthesized. Micelles formed from one of these benzoylated polymers, [polystyrene_0.25‐co‐poly(vinylbenzophenone)_0.75]₁₁₅‐block‐poly(acrylic acid)₁₄, were then fixed by the irradiation of the micelle cores with UV light. As the irradiation time was increased, the pendent benzophenone groups crosslinked with other chains in the glassy micelle cores. Dynamic light scattering, spectrofluorimetry, and Fourier transform infrared spectroscopy were all used to verify the progress of the crosslinking reaction. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2604–2614, 2006     

The Supramolecular Association of Polyelectrolytes to Complementary Charged Surfactants and Protein Assemblies

Cohesion matters! The correlation between the conformational rigidity of the polyelectrolyte and the size and stability of the globular assembly is discussed in this review article. Some examples of models for the association of polyelectrolytes to globular assemblies are shown here.

     

Role of the Surfactant Structure in the Behavior of Hydrophobic Ionic Liquids within Aqueous Micellar Solutions

The behavior of an ionic liquid (IL) within aqueous micellar solutions is governed by its unique property to act as both an electrolyte and a cosolvent. The influence of the surfactant structure on the properties of aqueous micellar solutions of zwitterionic SB‐12, nonionic Brij‐35 and TX‐100, and anionic sodium dodecyl sulfate (SDS) in the presence of the “hydrophobic” IL 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([bmim][PF₆]) is assessed along with the possibility of forming oil‐in‐water microemulsions in which the IL acts as the “oil” phase. The solubility of [bmim][PF₆] within aqueous micellar solutions increases with increasing surfactant concentration. In contrast to anionic SDS, the zwitterionic and nonionic surfactant solutions solubilize more [bmim][PF₆] at higher concentrations and the average aggregate size remains almost unchanged. The formation of IL‐in‐water microemulsions when the concentration of [bmim][PF₆] is above its aqueous solubility is suggested for nonionic Brij‐35 and TX‐100 aqueous surfactant solutions.     

A Water Molecule Triggers Guest Exchange at a Mono-Zinc Centre Confined in a Biomimetic Calixarene Pocket: a Model for Understanding Ligand Stability in Zn Proteins

In this study, the ligand exchange mechanism at a biomimetic Zn^II centre, embedded in a pocket mimicking the possible constrains induced by a proteic structure, is explored. The residence time of different guest ligands (dimethylformamide, acetonitrile and ethanol) inside the cavity of a calix[6]arene-based tris(imidazole) tetrahedral zinc complex was probed using 1D EXchange SpectroscopY NMR experiments. A strong dependence of residence time on water content was observed with no exchange occurring under anhydrous conditions, even in the presence of a large excess of guest ligand. These results advocate for an associative exchange mechanism involving the transient exo-coordination of a water molecule, giving rise to 5-coordinate Zn^II intermediates, and inversion of the pyramid at the Zn^II centre. Theoretical modelling by DFT confirmed that the associative mechanism is at stake. These results are particularly relevant in the context of the understanding of kinetic stability/lability in Zn proteins and highlight the key role that a single water molecule can play in catalysing ligand exchange and controlling the lability of Zn^II in proteins.