NUCLEATION LABORATORY

EFFECTS OF BACKGROUND GASES ON THE

HOMOGENEOUS NUCLEATION OF VAPORS I.

J. Phys. Chem. 98, 4443 (1994)

Richard H. Heist, M. Janjua, and J. Ahmed

RESEARCH SUMMARY

      In this paper we report new results indicating a strong dependence of the measured Scrit (critical supersaturation) of 1-propanol on total pressure.  These results were obtained using a diffusion cloud chamber specially designed to operate at high pressures and temperatures.  The observed Scrit of 1-propanol increases with total pressure (accomplished by increasing the amount of background or carrier gas).  This implies that the homogeneous nucleation rate of 1-propanol decreases strongly as the total pressure increases.

    We have observed a significant dependence of this effect upon the nature of the background gas.  The effect of increasing total pressure on the nucleation of 1-propanol is greater using helium than using hydrogen as the background gas.  Further, we observe that the dependence of Scrit on total pressure (as manifested by the slope of the Scrit vs Ptot data at constant temperature) is temperature dependent.  The measured slope of the Scrit vs Ptot dependence increases as the temperature decreases.  Finally, by combining data obtained in our laboratory with data from the literature, we present Scrit vs temperature data for 1-propanol and helium that appear to span the widest range of temperature to date.

    This is the first time nucleation data obtained over a wide range of temperatures has been obtained for a wide range of total pressure as well.  The data, as presented, suggests that the role of a background in the nucleation process may be significant.  If these effects are due to the nature and the amount of the background gas and are confirmed, it would appear that commonly used descriptions of the nucleation process will need to be revised to include the presence of secondary gases. 

    The results of this investigation support the need for additional studies on a wider class of materials to more fully characterize and, eventually, understand the dependence of a secondary gas on homogeneous nucleation.  Since in most realistic situations there will be at least one additional gas present during the nucleation process, it is essential we investigate how to incorporate the presence of a secondary gas into our understanding of nucleation.

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