Kinetics of particle growth. III. Particle formation in the photolysis of sulfur dioxide-acetylene mixtures

The photolysis of SO₂ with radiation >3000 Å in the presence of C₂H₂ was studied at 29°. The reaction is unaffected by the absence or presence of 1 atm of N₂ or CO₂. The sole reaction products are CO and a solid compound of the trimer of C₃H₄S₂O₃. The rate of CO production, R(CO), is proportional to the SO₂ pressure at constant [C₂H₂]/[SO₂] and increases slightly with an increase in C₂H₂ pressure. Initially as the reaction mixture is irradiated there is an induction period of about 45 sec, before which particles are not produced, though CO is. Then suddenly nucleation occurs, particles are produced for a few seconds, and then particle production ceases. The particles grow rapidly by condensation of the C₃H₄S₂O₃ trimer, which is continually produced during the radiation. The particle density drops slowly because of coagulation. Ordinary diffusional loss of particles to the wall is negligible, but many of the particles are lost, probably because of convective diffusion and gravitational settling. In some experiments, after about 1-hr radiation, second nucleation occurs and the particle density rises rapidly. The reactions of importance in the system are T → wall removal (6), rT → T_r (9), T + T_n → T_n+1, (8), and T_n + T_m → T_n+m (7), where T is the C₃H₄S₂O₃ trimer, r is the average number of trimers needed to produce a nucleus capable of growth, and n and m are integers >r. The value found for r was 3.6 and the critical trimer concentration needed for particle nucleation was 4.9 × 10¹⁰ per cc. The values of the rate coefficients were k₆ = 3.0 × 10^-3 sec^-1, k₉ = 1.2 × 10^-34 cc^2.6/sec, and k₇ = 1.5 × 10^-7 cm³/sec for particles between 100 and 1000 Å in diameter. For larger particles, k₇ is smaller, as expected from theory. The value of 1.5 × 10^-7 cm³/sec for k₇ corresponds to coagulation on every collision for particles with diameters between 100 and 1000 Å. The value for k₈ increased with the size of T_n, and about as expected if every collision were effective. For particles <200 Å diameter, every collision is effective, and k₈ has collision efficiencies of >0.3-0.5 for particle diameters of ∼3000 Å.