Structuration in the interface of direct and reversed micelles of sucrose esters, studied by fluorescent techniques

Abstract

Background: Reactors found in nature can be described as micro-heterogeneous systems, where media involved in each micro-environment can behave in a markedly different way compared with the properties of the bulk solution. The presence of water molecules in micro-organized assemblies is of paramount importance for many chemical processes, ranging from biology to environmental science. Self-organized molecular assembled systems are frequently used to study dynamics of water molecules because are the simplest models mimicking biological membranes. The hydrogen bonds between sucrose and water molecules are described to be stronger (or more extensive) than the ones between water molecules themselves. In this work, we studied the capability of sucrose moiety, attached to alkyl chains of different length, as a surface blocking agent at the water-interface and we compared its properties with those of polyethylenglycol, a well-known agent used for this purposes. Published studies in this topic mainly refer to the micellization process and the stability of mixed surfactant systems using glycosides. We are interested in the effect induced by the presence of sucrose monoesters at the interface (direct and reverse micelles) and at the palisade (mixtures with Triton X-100). We believe that the different functional group (ester), the position of alkyl chain (6-O) and the huge capability of sucrose to interact with water will dramatically change the water structuration at the interface and at the palisade, generating new possibilities for technological applications of these systems.

Results: Our time resolved and steady state fluorescence experiments in pure SEs micelles show that sucrose moieties are able to interact with a high number of water molecules promoting water structuration and increased viscosity. These results also indicate that the barrier formed by sucrose moieties on the surface of pure micelles is more effective than the polyoxyethylene palisade of Triton X-100. The fluorescence quenching experiments of SEs at the palisade of Triton X-100 micelles indicate a blocking effect dependent on the number of methylene units present in the hydrophobic tail of the surfactant. A remarkable blocking effect is observed when there is a match in size between the hydrophobic regions forming the apolar core (lauryl SE/ Triton X-100). This blocking effect disappears when a mismatch in size between hydrophobic tails, exists due to the disturbing effect on the micelle core.

Fig 1. Chemical structure of Triton X-100 and Sucrose esters.

Fig 2. Chemical structure of sodium 8-hydroxy-1,3,6-pyrenetrisulfonate, Pyranine.

Fig 3. Diagrammatic scheme of the equilibria of Pyranine in both, ground and excited state.

Absorption and emission maxima wavelength are also included.

Fig 4. Singlet oxygen phosphorescence decay of O₂(¹Δ_g) in D₂O (air equilibrated), employing Bengal Rose as sensitizer.

Signal growth corresponds to dye triplet lifetime (3,4μs) and decay is excited oxygen lifetime (66μs). Lower panel shows fitting residuals.

Fig 5. Emission spectra of Pyranine included in the pool of MPS/CHCl₃ reversed micelles with increasing amounts of water.

Arrows indicate the effect observed upon water increase.

Fig 6. Intensities ratio for Pyranine emission as a function of water amount in the aqueous pool of reversed micelles ([H₂O]/[surf]) formed with sucrose monoesters (A) and AOT (B), arrows indicate corresponding Y axis.

Fig 7. Bengal Rose triplet excited state lifetime in reversed micelles of MPS at different proportions of water.

Line corresponds to the lifetime in pure water.

Fig 8. Rough diagrammatic representation of Triton X-100 micelles, considering chains fully extended.

The inner core (white) corresponds to the hydrophobic nucleus, and the grey region corresponds to the palisade. Black cursor below indicates sucrose moiety position in the palisade for MCS (a), MLS (b) and MPS (c). Also DPC localization is indicated.