Search

In the synthesis of metallic nanoparticles in microemulsions, we hypothesized that the particle size is controlled by the reaction rate and not by the microemulsion size. Thus, the changes observed in the particle sizes as reaction conditions, such as concentrations, temperatures, the type of surfactant used, etc., are varied which should not be correlated directly to the modification of these conditions but indirectly to the changes they produce in the reaction rates. In this work, the microemulsions were formulated with benzene and water as continuous and dispersed phases, respectively, using n-dodecyltrimethylammonium bromide (DTAB) and n-octanol as the surfactant and cosurfactant. Using time-resolved UV-vis spectroscopy, we measured the reaction rates in the production of palladium (Pd) nanoparticles inside the microemulsions at different reactant concentrations and temperatures, 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 the nanoparticle size increases linearly as the reaction rate increases, independently of the actual reactant concentration or temperature. We proposed a simple model for the observed kinetics where the reaction rate is controlled mainly by the diffusion of the reducing agent. With this model, we predicted that the particle size should depend indirectly, via the reaction kinetics, on the micelle radius, the water volume and the total microemulsion volume. Some of these predictions were indeed observed and reported in the literature. [ABSTRACT FROM AUTHOR]

We present an electric birefringence study of aqueous solutions of an intrinsically rigid polyelectrolyte, xanthan, and of mixed solutions with an oppositely charged surfactant, DTAB. The stationary electric birefringence as well as built-up and decay processes give information on the polarization mechanisms of the polyelectrolyte and on the role played by counterions. By adding a cationic surfactant (DTAB), that replaces the small Na+ counterions of the polymer, we could obtain further insights into the role of condensed counterions in these mechanisms. In some cases, anomalous birefringence signals appear, revealing a new polarization mechanism likely due to field-induced interactions within polymer-surfactant aggregates. [ABSTRACT FROM AUTHOR]

In this paper, we describe laboratory and classroom exercises designed to obtain the fundamental equation of a rubber band by combining experiments and theory. The procedure shows students how classical thermodynamics formalism can help to obtain empirical equations of state by constraining and guiding in the construction of the physical models for the system under investigation. It also serves as an "experimental" link to statistical thermodynamics models for a rubber band, which is taught to students in more advanced courses. The rubber band system also serves as an example of a "non-ideal" thermodynamics system to introduce a van der Waals-like equation in a context different from gases. [ABSTRACT FROM AUTHOR]

The article offers instructions for learning physical chemistry.

We studied the dynamics of adsorption at the air-water interface of a cationic surfactant bearing two charges, Gemini 12-2-12, at concentrations below and above the critical micelle concentration (cmc). We used maximum bubble pressure and Wilhelmy plate techniques in order to access all time scales in the adsorption process. We found that the adsorption dynamics are controlled by diffusion at the initial stage of the adsorption process (milliseconds) and it is kinetically controlled by an electrostatic barrier (minute) approaching the equilibrium surfactant surface concentration. Between these two extremes, we found several relaxation phenomena, all following exponential decays with characteristic times spanning from one to hundreds of seconds. By means of time-resolved surface potential measurements, we show that these processes involve charge redistribution within the interfacial region. The surface tension data are analyzed and interpreted in the framework of the free energy approach. [ABSTRACT FROM AUTHOR]

The article discusses an experiment demonstrating the wetting dynamics of liquids on solid surfaces using a Web cam. Surface properties occupy an important place in a wide variety of applications in chemistry, materials science, engineering, and other areas. Majority of the industrial processes that involve liquids deal with situations where the free surface of the liquids meets a solid boundary, thus creating the so-called three-phase-contact-line. The contact line can move along the solid surface, leading to "wetting." Such interactions include the spreading of films over solids, interactions of drops and bubbles with solid walls and particles. In this experiment, students start with an apparent theoretical problem of wetting dynamics, making use of Navier-Stokes equations where the more important surface property, the surface tension, is involved.