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The thermal decomposition of 1,2 butadiene has been studied behind reflected shock waves over the temperature and total pressure ranges of 1300-2000 K and 0.20-0.55 atm using mixtures of 3% and 4.3% 1,2 butadiene in Ne. The major products of the pyrolysis are C2H,, C4Hz, C2H4, CH, and CsH6. Toluene was observed as a minor product in a narrow temperature range of 1500-1700 K. In order to model successfully the product profiles which were obtained by time-of-flight mass spectrometry, it was necessary to include the isomerization reaction of 1,2 to 1,3 butadiene. A reaction mechanism consisting of 74 reaction steps and 28 species was formulated to model the time and temperature dependence of major products obtained during the course of decomposition. The importance of C3H3 in the formation of benzene is demonstrated.C3H3 similar to that in the allene pyrolysis experiments. The C -CH3 bond in 1,2 butadiene is the weakest bond [41. Almost equal amounts of benzene were detected over comparable temperature ranges. However, the respective benzene profiles displayed different shapes. The purpose of this study is to understand the differences in the rates of benzene formation from 1,2 butadiene and allene pyrolyses and to provide additional evidence for the role of C3H3. A previous study of Collin and Lossing [51 concentrated on the ionization and dissociation of 1,2 butadiene and the subsequent determination of the C3H3 heat of formation. The emphasis herein is on the identity of the reaction products, their temporal behavior, and the mechanism attendent to the thermal decomposition.

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The thermal decomposition of acetylene has been studied in the temperature and pressure regimes of 1900-2500 K and 0.3-0.55 atm using a shock tube coupled to a time-of-flight mass spectrometer. A series of mixtures varying from 1.0-6.2% CzHz diluted in a Ne-Ar mixture yielded a carbon atom density range of 0.24-2.0 x 1017 atoms cm-3 in the reflected shock zone. Concentration profiles for CzH2, C4H2, and CsHz were constructed during typical observation times of 750 ps. CsH2 and trace amounts of C4H3 were found in relatively low concentrations at the high-temperature end of this study. A mechanism for acetylene pyrolysis is proposed, which successfully models this work and the results obtained by several other groups employing a variety of analytical techniques. Two values of the heat of formation for C,H(134 ? 2 and 127 t 1 kcal/mol) were employed in the modeling process; superior fits to the data were attained using the latter value. The initial step of acetylene decomposition involves competition between two channels. In mixtures (<200 ppm) where the acetylene concentrations are less than 2.18 x lo-' mol ~m -~, the decay is predominantly first order with respect to CZH,; in mixtures >200 ppm, the dominant initial step is second order. The rate constant for the second-order reaction is described by the equationBenzene concentrations predicted by the model are below the TOF detectability limit.C4H3 was observed in the 6.2% C,H2 mixture in accordance with the proposed mechanism.

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An Evaporimeter and a ventilated chamber technique have been compared in their ability to measure transepidermal water loss (TEWL) through rat skin. These techniques measure TEWL under very different conditions; the Evaporimeter measures the net TEWL under ambient relative humidity (RH) whereas the ventilated chamber employs a constant atmosphere, usually of low RH and thus measured the uni-directional diffusion of water. Paired Evaporimeter and ventilated chamber measurements were made of TEWL through normal skin and through skin whose barrier properties had been altered by tape-stripping (15 applications) or single applications of n-hexadecane (28.4 mumol cm-2). Both measuring techniques indicated the same level of TEWL through normal skin (mean 0.3 mg cm-2 h-1) and during increases in TEWL induced by n-hexadecane (max TEWL c 3.5 mg cm-2 h-1). However, the Evaporimeter was found to underestimate the higher rates of TEWL induced by tape-stripping, ie above TEWL raters of 7.5 mg cm-2 h-1. The Evaporimeter is portable, easy to use and suitable for measurements of net water loss up to 7.5 mg cm-2 h-1; it can only be used for comparative assessment of epidermal barrier function if used at a particular ambient RH. The more cumbersome ventilated chamber is to be preferred for accurate assessments of barrier function where high rates of TEWL occur.

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