CH3CH2OD/D2O binary condensation in a supersonic Laval nozzle: Presence of small clusters inferred from a macroscopic energy balance

We determined the heat released in the condensing flow of a CH3CH2OD/D2O/carrier gas mixture (EtOD/D2O for brevity) through a supersonic Laval nozzle by integrating the equations for supersonic flow with condensation, using the static pressure, temperature, and mole fractions of EtOD and D2O monomers [S. Tanimura, B. E. Wyslouzil, M. S. Zahniser, , J. Chem. Phys. 127, 034305 (2007)] as inputs. By considering the depletion of the monomer species, the deviation of the pressure from the isentropic value, and the heat released, we estimated that similar to 10% of the EtOD molecules are present as pure clusters (dimer to tetramer) upstream of the onset point of condensation. In contrast, clustering was not detected when only pure EtOD was present under the same conditions (temperature and the partial pressure of EtOD) for which clustering was observed in the EtOD/D2O flow. This suggests that the formation of EtOD clusters is facilitated by D2O in the EtOD/D2O flow. A comparison of the heat released to the flow and the expected heat of dissociation of the EtOD/D2O droplets suggests that small EtOD clusters persist downstream of the onset point. Both upstream and downstream of the onset point of condensation, the concentration of these clusters in the nozzle is higher than that expected at equilibrium. A possible mechanism for the overabundance of pure EtOD clusters is that they form in the mixed EtOD/D2O particles (droplets or clusters) and evaporate from them.