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This work presents a simply methodology for coating small unilamellar liposomes bearing different degrees of positive charge with polyelectrolyte multilayers using the sequential Layer-by-Layer (LbL) deposition method. The liposomes were made of mixtures of DOPC (1,2-dioleyl-sn-glycero-3-phosphocoline) and DODAB (dimethyl dioctadecyl ammonium bromide), and coated by alternated layers of the sodium salt of poly(4-styrenesulfonate), PSS, and poly(allylamine), PAH, as polyanion and polycations, respectively. The results shows that the zeta potential of the liposomes was not very sensitive to the mole fraction of DODAB in the membrane, XD, in the range 0.3< XD<0.8. We were able to coat the liposomes with up to four polymer bilayers. The growth of the capsules size was followed by dynamic light scattering, and in some cases by cryo-TEM, with good agreement between both techniques. The thickness of the layers, measured from the hydrodynamic radius of the coated liposome, depends on the polyelectrolyte used, so that the PSS layers adopt a much more packaged conformation than the PAH layers. An interesting finding is that the PSS amount needed to reach the isoelectric point of the capsules increases linearly with the charge density of the bare liposomes, whereas the amount of PAH does not depend on it. As expected, the preparation of the multilayers has to be done in such a way that when the system is close to the isoelectric point the capsules do not aggregate. For this, we dropped the polyelectrolyte solution quickly, fast stirring and using dilute liposome suspensions. The method is very flexible and not limited to liposomes or polyelectrolyte multilayers, also coatings containing charged nanoparticles can be easily made., Comment: Published

The micellization process of the aqueous mixed system Triton X-100 (TX100)-Dodecyltrimethylammonium Bromide (DTAB) was studied with a battery of procedures: surface tension, static and dynamic light scattering and ion-selective electrodes. Results were also analysed with two thermodynamic procedures. The system shows some changes in its behaviour with changing the mole fraction of DTAB, $\alpha_{DTAB}$, in the whole surfactant mixture. For $ \alpha_{DTAB} < 0.40$ micelles are predominantly TX100 with scarce solubilized DTA+ ions, and TX100 acts as a nearly ideal solvent. In the range $0.50 < \alpha_{DTAB} < 0.75$ it seems that none of the components acts as a solvent, and above $\alpha_{DTAB} > 0.75$ there are abrupt changes in the size and electrophoretic mobility of micelles. These phenomena have been interpreted in the light of the thermodynamic results and some TX100-ionic surfactant mixtures of literature.

In the synthesis of metallic nanoparticles in microemulsions, we hypothesized that particle size is mainly controlled by the reaction rate. Thus, the changes observed on the particle sizes as reaction conditions, such as concentrations, temperature, type of surfactant used, etc., are varied should not be correlated directly to the modification of those conditions but indirectly to the changes they produce on the reaction rates. By means of time resolved UV-vis spectroscopy, we measured the reaction rates in the production of Pd nanoparticles inside microemulsions at different reactant concentrations, keeping all the other parameters constant. The measured reaction rates were then correlated with the particle sizes measured by transmission electron microscopy (TEM). We found that nanoparticle size increases linearly as the reaction rates increases, independently of the actual reactant concentrations. We proposed that the kinetics is controlled mainly by the diffusion of the reducing agent through the surfactant monolayer covering the microemulsion membrane. With this model, we predicted that particle size should depend indirectly, via the reaction kinetics, on the micelle radius (v0 ~ r^-3), the water volume (v0~vw^3) and the total microemulsion volume (v0~vT^-3), and temperature (Arrhenius). Some of these predictions were explored in this article.

We studied a system formed by a mixture of a thermoresponsive negatively charged graft copolymer (Alg-g-PNIPAAm) with a brush-type structure, and an oppositely charged surfactant (DTAB), in bulk and at the air-solution interface. We performed experiments of surface tension, electrophoretic mobility, dynamic and static light scattering and atomic force microscopy in order to characterize the complexes formed as a function of DTAB concentration and temperature. We found that these polymer-surfactant complexes are able to respond by changing their sizes, both in bulk and at the air-solution interface, when T is increased above the coil-globule transition temperature (LSCT) of the copolymer. However, the thermoresponse was found to be dependent on surfactant concentration, cs: for cs < 2.8 mM, the size of the aggregates decreases as T increases but, for cs >= 2.8 mM, the opposite behavior takes place, i.e. the size increases with T. At the interface, the intensity of the effect produced on the surface tension by increasing T above LCST diminishes continuously as cs increases, reducing the ability of the interfacial complex to respond to temperature changes. We studied the stability of aqueous foams formulated with these mixtures as a function of T and cs. We found that the stability of the foam can be modulated by changing T, but we observed that this effect is dependent on the surfactant concentration range. We found a correlation between changes in the aggregates sizes, the surface tension behavior and the responsiveness of foam stability to changes of temperature.

In this article a definition of reversible processes in terms of differences in intensive Thermodynamics properties (Affinities) is proposed. This definition makes it possible to both define reversible processes before introducing the concept of entropy and avoid the circularity problem that follows from the Clausius definition of entropy changes. The convenience of this new definition compared to those commonly found in textbooks is demonstrated with examples.

The adsorption kinetics of the cationic surfactant dodecyltrimethylammonium bromide at the air-water interface has been studied by the maximum bubble pressure method at concentrations below the critical micellar concentration. At short times, the adsorption is diffusion-limited. At longer times, the surface tension shows an intermediate plateau and can no longer be accounted for by a diffusion limited process. Instead, adsorption appears kinetically controlled and slowed down by an adsorption barrier. A Poisson-Boltzmann theory for the electrostatic repulsion from the surface does not fully account for the observed potential barrier. The possibility of a surface phase transition is expected from the fitted isotherms but has not been observed by Brewster angle microscopy., Comment: 13 pages, 5 figures