Vitor Francisco

Novel catanionic vesicles from calixarene and single-chain surfactant

The mixed system between p-sulfonatocalix[4]arene and tetradecyltrimethylammonium bromide forms unilamellar vesicles after sonication of the aqueous dispersion. Furthermore these vesicles can be stored, without use of lyoprotectants, by lyophilization and then rehydration without change in size or shape.

Aggregation of p-Sulfonatocalixarene-Based Amphiphiles and Supra-Amphiphiles

p-Sulfonatocalixarenes are a special class of water soluble macrocyclic molecules made of 4-hydroxybenzenesulfonate units linked by methylene bridges. One of the main features of these compounds relies on their ability to form inclusion complexes with cationic and neutral species. This feature, together with their water solubility and apparent biological compatibility, had enabled them to emerge as one the most important host receptors in supramolecular chemistry. Attachment of hydrophobic alkyl chains to these compounds leads to the formation of macrocyclic host molecules with amphiphilic properties. Like other oligomeric surfactants, these compounds present improved performance with respect to their monomeric counterparts. In addition, they hold their recognition abilities and present several structural features that depend on the size of the macrocycle and on the length of the alkyl chain, such as preorganization, flexibility and adopted conformations, which make these molecules very interesting to study structure-aggregation relationships. Moreover, the recognition abilities of p-sulfonatocalixarenes enable them to be applied in the design of amphiphiles constructed from non-covalent, rather than covalent, bonds (supramolecular amphiphiles). In this review, we summarize the developments made on the design and synthesis of p-sulfonatocalixarenes-based surfactants, the characterization of their self-assembly properties and on how their structure affects these properties.

Self-Aggregation Properties of Ionic Liquid 1,3-Didecyl-2-methylimidazolium Chloride in Aqueous Solution: From Spheres to Cylinders to Bilayers

The self-aggregation behavior of the double-chained ionic liquid (IL) 1,3-didecyl-2-methylimidazolium chloride ([C10C10mim]Cl) in aqueous solution has been investigated with a number of different experimental techniques. Two cmc values (cmc1 and cmc2) are obtained from conductivity measurements. The fraction of neutralized charge on the micellar surface suggests that cmc1 corresponds to the formation of spherical micelles and cmc2 to the transition from spherical to cylindrical micelles. Data obtained from fluorescence spectroscopy (using pyrene and Nile red as chemical probes), fluorescence anisotropy (using rhodamine B as probe), and chemical shift (1)H NMR (in D2O) provide a picture that is also consistent with a sphere-to-cylinder transition. This structural change is further confirmed by diffusion-ordered NMR spectroscopy (DOSY), from the self-diffusion coefficients for surfactant unimer and aggregates. Furthermore, a third evolution from cylindrical micelles to bilayer aggregates is proposed from the analysis of diffusion coefficients at high surfactant concentration ([IL] > 0.2 M). Phase scanning experiments performed with polarized light microscopy clearly demonstrate the presence of a lamellar liquid crystalline phase at very high IL concentration, thus confirming the coexistence of bilayer structures with elongated micelles, found at lower concentration. Additionally, [C10C10mim]Cl micelles are proposed as novel reaction media, as evidenced by the solvolysis reaction of 4-methoxybenzenesulfonyl chloride (MBSC).

Using Calixarenes To Model Polyelectrolyte Surfactant Nucleation Sites

The formation of mixed micelles composed of dodecyltrimethylammonium bromide (C(12)TAB) and a hexamethylated p-sulfonatocalix[6]arene (SC6HM) was studied by several techniques. It was found that above the critical aggregation concentration the concentrations of free and micellized surfactant are strongly related to that of SC6HM. When there is free SC6HM in solution, the addition of C(12)TAB mainly results in an increase in the concentration of micellized surfactant, but when all SC6HM has been aggregated, the addition of C(12)TAB results in a substantial increase in the concentration of free surfactant in solution. When the concentration of free surfactant is equal to the critical micelle concentration of the pure system, a second independent aggregation process is observed. This aggregation behavior has many features that are similar to those of more complex systems that involve surfactants in the presence of oppositely charged polyelectrolytes. In this way, calixarenes can serve as simple models to mimic polyelectrolytes and to gain insight into the complex behavior displayed by these macromolecules.

The “True” Affinities of Metal Cations to p‐Sulfonatocalix[4]arene: A Thermodynamic Study at Neutral pH Reveals a Pitfall Due to Salt Effects in Microcalorimetry

A microcalorimetric study on the inclusion of monovalent and divalent metal cations by p-sulfonatocalix[4]arene was performed. The thermodynamic parameters for the complexation of alkali metal cations and Ag(+) were obtained for the first time at neutral pH. The Na(+) cation is routinely present as counterion of the calixarene in neutral aqueous solution, and this must be taken into account in the determination of the thermodynamic parameters for the complexation of Na(+) and the other cations by considering a sequential or a competitive binding scheme. The ΔH° and ΔS° values show that the inclusion process is entropically driven, although an influence of the temperature on the complexation reaction indicates that the enthalpic term is also an important contributor. The results also reveal that enthalpy/entropy compensation balances the gain in one contribution against a corresponding loss in the other. The obtained thermodynamic data are in contrast to the results from previous microcalorimetric studies, which showed binding constants that were orders of magnitude smaller and complexations, which were in part enthalpically driven but which neglected the influence of the alkali metal counterions.

Counterion exchange as a decisive factor in the formation of host:guest complexes by p-sulfonatocalix[4]arene.

Calorimetric and NMR titration experiments have been done to measure the binding constant between p-sulfonatocalix[4]arene and a quaternary ammonium ion. Our results show that the binding constants depend both on the calixarene concentration and on the presence of added Na(+). These results have been interpreted by considering the ion-exchange equilibrium between sulfonatocalixarene counterions and the added organic cation. Our results show that it is necessary to extrapolate the binding constants to zero calixarene concentration and zero added salts in order to get the true equilibrium constant.

Host–Guest Chemistry of a Water-Soluble Pillar[5]arene: Evidence for an Ionic-Exchange Recognition Process and Different Complexation Modes

The complexation of an anionic guest by a cationic water-soluble pillararene is reported. Isothermal titration calorimetry (ITC), (1)H NMR, (1)H and (19)F DOSY, and STD NMR experiments were performed to characterize the complex formed under aqueous neutral conditions. The results of ITC and (1)H NMR analyses showed the inclusion of the guest inside the cavity of the pillar[5]arene, with the binding constant and thermodynamic parameters influenced by the counter ion of the macrocycle. NMR diffusion experiments showed that although a fraction of the counter ions are expelled from the host cavity by exchange with the guest, a complex with both counter ions and the guest inside the pillararene is formed. The results also showed that at higher concentrations of guest in solution, in addition to the inclusion of one guest molecule in the cavity, the pillararene can also form an external complex with a second guest molecule.

Independent Pathway Formation of Guest–Host in Host Ternary Complexes Made of Ammonium Salt, Calixarene, and Cyclodextrin

The interaction between γ-cyclodextrin and amphiphilic p-sulfonatocalix[4]arenes was studied using NMR and isothermal titration calorimetry techniques. The results indicate that these calixarenes are able to form 1:1, 1:2, and 2:1 host-guest complexes with the cyclodextrin. The ROESY spectra suggest that the cyclodextrin binds the calixarenes through the hydrophobic alkyl chains. p-Sulfonatocalix[4]arenes, which are traditionally used as host molecules, act as guests in the presence of γ-cyclodextrin. However, their recognition site remains active upon complexation with the cyclodextrin, and ternary complexes can be devised. Here, we also demonstrate the formation of such complexes using tetramethylammonium chloride as a model guest. Moreover, it is also demonstrated that the recognition properties of the calixarene are unaffected upon complexation with the cyclodextrin.

Ionic Exchange in p‑Sulfonatocalix(4)arene-Mediated Formation of Metal−Ligand Complexes

The effect of alkali and transition metal cations in the formation of host-guest complexes with the water-soluble p-sulfonatocalix[4]arene (SC4) was studied using 2-chloropyridine and Na(+) and Cu(2+) as model guest and model cations, respectively. The results obtained from isothermal titration calorimetry and NMR experiments provide evidence for the formation of 1:1:1 ternary complexes for both cations with Cu(2+) showing positive cooperativity and Na(+) negative cooperativity. The formation of ternary complexes comprising transition metal cations has been scarcely explored but present high potential for devising catalytic systems/models or for enhancing the stability and selectivity of SC4 complexes. Because transition metal cations are usually present in solution together with other SC4 countercations (e.g., Na(+)), a general binding model that considers the dynamic formation of all possible complexes (including ionic exchange between ternary complexes) is presented. This model allows the optimization of the conditions required to selectively form target complexes.

Interaction of bolaform surfactants with p-sulfonatocalix[4]arene: the role of two positive charges in the binding.

The inclusion binding manners of bolaform surfactants of type C(n)R6(2+) 2Br(-) with different spacer lengths (n = 6, 12) and terminal headgroup volumes (R = methyl, ethyl) by p-sulfonatocalix[4]arene were studied. The combination of ITC parameters (binding constants and complexation enthalpy and entropy) and NMR chemical shifts and NOE cross-peaks obtained upon complexation allows us to propose different binding modes. The results point out that the spacer length has an influence on the binding stoichiometry. The bolaforms with larger spacer lengths between polar head groups enable the formation of 2:1 complexes in addition to 1:1, while with the shorter spacer they form only 1:1 complexes. On the other hand, the formation of 1:1 complexes is not affected by the headgroup volume or the spacer length of the bolaform. Unexpectedly, a complex binding mode was observed where both positive charged terminal groups of the bolaform are accommodated in the cavity of the calixarene. The inclusion of both terminal groups of the guests can be related to its own structure but also evidence the high flexibility of the calixarene.