Kinetics of particle growth-V: Particle formation in the photolysis of SO₂-allene mixtures

The photolysis of SO₂ with radiation > 3000 A ̊ was studied in the presence of allene (C₃H₄) at 29°C. The reactions products were CO, C₂H₄ and an oil tentatively identified as C₅H₈SO. The rate of CO production, R[CO], was proportional to the SO₂ pressure and nearly independent of the C₃H₄ pressure. However, the ratio [C₂H₄]/[CO] varied from 0·15 to 0·23 to 0·30 as the [SO₂]/[C₃H₄] ratio varied from 5·8 to 1·0 to 0·14. Initially as the reaction mixture was irradiated there was an induction period until the C₅H₈SO concentration reached 3·0 × 10¹² molecules/cc before which particles (i.e. droplets) were not produced, though CO and C₂H₄ were. Then suddenly nucleation occurred, particles were produced for a short time, and then particle production ceased. The particles grew rapidly by condensation of the C₅H₈SO which was continuously produced throughout the irradiation. Thus the particles maintained a fairly uniform size. The maximum in particle density, N_max, occurred when the particles had grown to about 72 Å dia corresponding to a condensation rate coefficient of 7·0 × 10^-7 cc/min. After the maximum was reached, the particle density dropped slowly because of coagulation which proceeded on every collision (corresponding to a rate coefficient ∼ 10^-7 cc/min). Diffusional loss of particles to the wall was negligible. In addition to removal by condensation. C₅H₈SO was also lost by diffusion to the wall with a rate coefficient of 0·30 min^-1. Particle nucleation can be represented by the simplified reaction rC → C_r where C represents C₅H₈SO. The value found for r was 6·7 and for κ₉ was 3·0 × 10^-57 cc^5·7/min.