ABSTRACT
In the present paper we have investigated some photo-physical characteristics of different micellar-based fluorescent probes containing a fluorophore (pyrene) and a quencher unit (dodecyl-dioxo 2,3,2). The fluorescent response of the probe in the presence of Cu(II) ions was studied using different micellar substrates, and it was found that the pH at which the On-Off jump occurs is not influenced by the chemical structure of surfactant. In addition, the experimental residual fluorescence is not proportionally affected by microviscosity or by the size of the micellar aggregates. The signal of the native fluorescence of pyrene was observed even when the quencher's occupancy number was greater than one. Moreover, we observed discrepancies between experimental values and calculated residual fluorescence using Laplace data. These results were interpreted suggesting that the residual fluorescence has two main components, one that seems to be independent on micellar properties, while the other is directly related to location of molecules inside the surfactant aggregates that serve as substrate.
ABSTRACT
El efecto de la radiación gamma proveniente de una fuente de 60Co en la estructura del surfactante no iónico Tritón X-100 fue investigado. Tres regiones principales pueden ser distinguidas en el comportamiento del valor medio del número de grupos etóxidos al aumentar la dosis. Sin embargo, el resultado global encontrado fue una pequeña variación en este valor medio al cambiar la dosis entre 0 y 70 KGy.
The effect of gamma radiation from a 60Co source on the structure of a nonionic surfactant, namely TRITON X-100, was investigated. Three main regions can be distinguished in the behavior of the mean value of ethoxy groups with an increase in the absorbed dose. However just a slightly decrease on this mean value was obtained when the dose range from 0 to 70 kGy.
ABSTRACT
The interactions involved in aqueous mixtures of polyethyleneimine (PEI) and cetyltrimethylammonium bromide (CTAB) were studied under dilute conditions. The phase diagram of this polyelectrolyte-surfactant system of similar charge was constructed by determining the CAC and CMC* values at different PEI concentrations, using surface-tension and conductivity measurements, respectively. Formation of thin films at the air-solution interface was detected at concentrations belonging to the interaction region of the phase diagram, using Brewster angle microscopy. These films were formed at low polymer and surfactant concentrations, 0.01% w/v PEI and 0.1 mmol dm CTAB. Results from SAXS determinations indicate that, under these conditions, mesostructure formation occurs exclusively at the surface. The effect of PEI on surfactant micellisation is determined by the polyelectrolyte nature of the polymer. The presence of the polymer in the aggregate significantly affects free micelle formation, even when hydrophobic interactions are mainly determined by the surfactant structure. The films obtained at low surfactant concentration are mesostructured, composed of five layers, each one 49.7 Å thick, as was determined using specular X-ray reflectometry. These results indicate that mesostructured film formation is achievable under more economical and environmentally friendly conditions, suggesting novel routes for surfactant templating in mixed polyelectrolyte systems of similar charge.
ABSTRACT
A micellar approach is proposed to build a series of systems featuring an "off-on-off" fluorescent window response with changes in pH. The solubilizing properties of micelles are used to self-assemble, in water, plain pyrene with lipophilized pyridine and tertiary amine moieties. Since these components are contained in the small volume of the same micelle, pyrene fluorescence is influenced by the basic moieties: protonated pyridines and free tertiary amines behave as quenchers. Accordingly, fluorescence transitions from the "off" to the "on" state, and viceversa, take place when the pH crosses the pK(a) values of the amine and pyridine fragments. To obtain an "off-on-off" fluorescent response in this investigation we use either a set of dibasic lipophilic molecules (containing covalently linked pyridine and tertiary amine groups) or combinations of separate, lipophilic pyridines and tertiary amines. The use of combinations of dibasic and monobasic lipophilic molecules also gives a window-shaped fluorescence response with changes in pH: it is the highest pyridine pK(a) and the lowest tertiary amine pK(a) that determine the window limits. The pK(a) values of all the examined lipophilic molecules were determined in micelles, and compared with the values found for the same molecules in solvent mixtures in which they are molecularly dispersed. The effect of micellization is to significantly lower the observed protonation constants of the lipophilized species. Moreover, the more lipophilic a molecule is, the lower the observed logK value is. Accordingly, changing the substituents on the basic moieties or modifying their structure, tuning the lipophilicity of the mono- or dibases, and choosing among a large set of possible combination of lipophilized mono- and dibases have allowed us to tune, almost at will, both the width and the position along the pH axis of the obtained fluorescent window.
