rate constants ( /2 "= 8 s) and thus the literature values quoted in this paper should be considered lower limits to the reaction rate for the conditions employed here (1 N NaOH).Acid-Catalyzed Hydrolysis of PE-COgCHg. Hydrolysis was carried out by floating PE-C02CH3 (ester side down) on the surface of 50% (w/w) H2S04 at 25 ± 2 °C for the specified time.The film was rinsed 4 times in water and once in acetone and air dried. Literature values for the hydrolysis rate were determined from the pseudo-first-order rate constants established under identical conditions.26Registry No. polyethylene,

In this work, the role of nanoparticle surface charge in surface-enhanced Raman scattering (SERS) is examined for the common case of measurements made in colloidal solutions of Ag and Au. Average SERS intensities obtained for several analytes (salicylic acid, pyridine, and 2-naphthalenethiol) on Ag and Au colloids are correlated with the pH and zeta potential (zeta) values of the nanoparticle solutions from which they were recorded. The consequence of the electrostatic interaction between the analyte and the metallic nanoparticle is stressed. The zeta potentials of three commonly used colloidal solutions are reported as a function of pH, and a discussion is given on how these influence SERS intensity. Also examined is the importance of nanoparticle aggregation (and colloidal solution collapse) in determining SERS intensities, and how this varies with the pH of the solution. The results show that SERS enhancement is highest at zeta potential values where the colloidal nanoparticle solutions are most stable and where the electrostatic repulsion between the particles and the analyte molecules is minimized. These results suggest some important criteria for consideration in all SERS measurements and also provide important insights into the problem of predicting SERS activities for different molecular systems.

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A low-cost optical fiber sensor to detect volatile organic compounds has been developed. Changes of up to 13.5 dB in the transmitted optical power have been detected with different concentrations of acetone and dichloromethane vapors. The device uses a standard single-mode fiber. The sensing mechanism relies on a vapor-induced refractive index change in a film of a vapochromic material deposited on the thinner region of a tapered fiber.

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Summary Accessibility of adsorbates to internal active sites of soils depends mainly on the porous structure of the material. We aimed to determine the distribution of pore sizes in soils with varied physico‐chemical properties, by combining adsorption of gases and mercury porosimetry. Microporosity was studied by physical adsorption of N2 at 77 K and CO2 at 273 K; mercury intrusion porosimetry allowed us to evaluate the macroporosity; and mesoporosity was determined by capillary condensation of N2 and mercury porosimetry. The soils investigated were essentially macroporous, with volumes between 0.33 and 0.73 cm3 g–1; the maxima in the differential pore‐size distribution were in the range 1500–4000 nm. Volumes of meso‐ or micropores were always less than 10% of macropore volumes. Calculations based on the theory of Dubinin and the αs‐method (for N2 at 77 K) provided, generally, coincident results. In a soil containing much organic matter, N2 adsorption was only one‐ninth that of CO2 adsorption, showing that N2 adsorption into the narrow micropores of organic matter was kinetically restricted. When accessibility to micropores was not restricted, the total volume of micropores could be deduced from N2 adsorption, whereas CO2 measured exclusively the narrowest microporosity.