Studies of the kinetics of cluster redistribution in carbon trichloride monofluoride vapours.

Abstract

A shock tube coupled to a laser-schlieren detection system in conjunction with computer simulation was used to study the formation of small clusters in nearly saturated Freon-11 vapour. When a shock wave passed through Freon-11 vapour a temperature and pressure jump was produced. A new equilibrium cluster distribution at the new temperature and pressure was reached by formation and stabilization of clusters, this being an exothermic reaction. This process is accompanied by a temperature and density change. The laser-schlieren technique records a signal proportional to the density gradient. In the experimental studies the initial pressure of CCl$\sb3$F was varied from 26 mm Hg to 762 mm Hg and the speed of the shock wave was changed from 141.1 m/s to 320.7 m/s. Several carefully chosen gases (Ar, CH$\sb4$, CCl$\sb4$ and SF$\sb6$) were used for comparison studies. The time scale for the observed process was 10-100 $\mu$s. Characteristic of bond formation, the observed process displayed a negative activation energy suggesting a strong competition between cluster formation, dissociation and stabilization. Oscillatory signals were observed in those cases where the initial pressure was high, the postshock temperature was low and the postshock pressure was high. It was found that the oscillatory signal was biperiodic at an early stage. Then this oscillatory signal became a chaotic one. The experimentally reproducible chaotic signals are proof that the processes occurring in our system are nonlinear kinetic processes. A computational model for the rate of formation of clusters (cluster size 5) based on the scheme $\rm A\sb{n-1} + A = A\sbsp{n}{*}$ and $\rm A\sbsp{n}{*} + M = A\sb{n} + M$ was built and constrained by experimental observation, thermodynamics and nonlinear kinetics. With the help of computer simulation it was confirmed that the negative signal observed is caused by the cluster formation process followed by exothermic cluster collisional stabilization. The oscillation is a direct result of thermal feedback affecting the equilibrium cluster distribution.