Acceleration of butane vapor nucleation by carbon dioxide gas

Carbon dioxide (CO₂) gas is known to strongly accelerate nucleation of other gas phase components, such as water and toluene. The acceleration was attributed to the formation of transient heteromolecular dimers and referred to as the chaperon mechanism. In this work, we investigate this phenomenon for butane-CO₂ gas mixtures with mass spectrometry in the post-nozzle flow of a Laval expansion at a temperature of 51 K and a pressure of 40 Pa. At moderate CO₂ and butane concentrations, we observed an acceleration of butane nucleation by the chaperon mechanism, albeit only by a factor of about two compared to unary butane nucleation. The fact that the chaperon mechanism is less important for butane than for water and toluene can be rationalized by the weaker intermolecular interactions between butane and CO₂. At higher CO₂ and butane concentrations, nucleation and cluster growth overlap in time, which leads to saturation of the measured total butane concentration. Using a kinetic model, we show that saturation is caused by the formation of heteromolecular butane-CO₂ clusters of different sizes and compositions. Studies on nucleation at low temperatures in such systems are relevant for flue or natural gas separation.