RESUMEN
Molecular transport through thin polymer films has become a subject with a variety of challenges and opportunities for chemists, physicists, and material scientists in recent years. The diffusion of probe molecules in and out of macromolecular environments plays a major role in the response of polymer-based sensor materials or the design of time-released drug delivery systems. Obtaining an improved understanding of the relevant dynamic phenomena, like transport of molecular probes, in boundary layers represents a crucial step to develop a clearer picture of the molecular transport processes taking place at interfaces modified with macromolecular assemblies. In this work, we present a new approach based on the derivation of the theoretical impedance transfer function to unambiguously describe the impedance response of gold electrodes modified with poly(methacryloyloxy)-ethyl-trimethyl-ammonium chloride (PMETAC) brushes. We demonstrate that this methodology not only enables the description of the experimental data but also provides insightful information about the dynamics of the diffusion of probe molecules inside the brush. More important, we show the capabilities of electrochemical impedance spectroscopy to gather information on a molecular transport process inside the brush under experimental conditions in which other electrochemical techniques are no longer applicable. As such, we consider that this experimental approach constitutes a new and powerful tool to estimate diffusion coefficients of probe molecules into interfacial macromolecular assemblies.
RESUMEN
Airborne particulate trace metals have been measured bimonthly during day and night hours in four permanent stations located in residential, industrial and commercial sectors of La Plata City region, to characterize the sources and variability of atmospheric inputs. Airborne trace metal regional averages (Pb 64 +/- 62, Cu 30 +/- 27, Mn 26 +/- 20, Zn 273 +/- 227, Fe 1183 +/- 838, Ca 5343 +/- 3614, Mg 1472 +/- 967, Cr 4.3 +/- 2.4, Ni 3.2 +/- 3.5 and Cd 0.41 +/- 0.42 ng/m3) are comparable to the values reported for not grossly polluted cities and below the general trend described for urban particulates. Two- and three-way analysis of variance and variance components tests (P < 0.05) were performed to assess the contribution of the diurnal (day vs. night), spatial (inter-station) and temporal (inter-month) components of variability. Trace metal concentrations followed the behavior of total suspended particles with higher concentrations during the day and at the Downtown station and lower at the Residential site. In general, spatial and temporal variations prevailed over diurnal differences. Spatial differences were clearly most significant for Pb, which presented higher values at the Downtown site reflecting the importance of motor exhaust inputs. In contrast, diurnal differences were more important for Mn due to increased dust resuspension during day hours. A seasonal trend with concentrations usually increasing during winter months and decreasing in spring-summer was also detected. Enrichment factors (EF) were calculated to evaluate anthropogenic versus natural element sources. High EF (21-376) were obtained for Pb, Zn, Cd and Cu reflecting the importance of anthropic inputs. The comparison with EF calculated for high-emitting vehicle particle emissions indicated that motor exhausts are the most important source of these elements in La Plata region. In contrast, the EF calculated for Mn, Cr, Ni, Ca and Mg were low (1.3-7.5) suggesting chiefly natural sources, i.e. soil-derived dusts.
