Measurement of the molecular content of binary nuclei. II. Use of the nucleation rate surface for water–ethanol

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Abstract
In a preceding paper we determined the molecular contents of binary nuclei from homogeneous nucleation rate measurements studing ethanol–hexanol as a nearly ideal model system. Here we report the analogous measurements for the nonideal water–ethanol system known from previous investigations to display surface enrichment of ethanol in the nuclei. The nucleation pulse technique applied allows accurate measurements of homogeneous nucleation rates in the range 105<J/cm−3 s−19. We determined the homogeneous nucleation rates of mixed droplets in supersaturated water–ethanol vapor mixtures with argon as the carrier gas. The experiments were performed as functions of the water and ethanol gas phase activities, a1 and a2, respectively, ranging from pure water to pure ethanol with ten intermediate activity ratios at T=260 K. The experimental data shape a nucleation rate surface in three‐dimensional J–a1–a2 space from which the critical activities for a rate of Jc=107 cm−3 s−1 were determined. The critical activities are compared with the predictions of the classical binary nucleation theory. The observed large discrepancies can be attributed to very water‐rich compositions of the nuclei predicted by the theory. The results support previous findings for the water‐n‐propanol system, where even larger discrepancies were observed. In a next step the individual numbers of ethanol and water molecules in the nuclei are directly determined from the slopes of the experimental nucleation rate surface. The observed variations of the molecular numbers with vapor phase composition differ substantially from that of an ideal mixture. Comparing the measured nucleus compositions to the predictions of a recently proposed explicit cluster model one observes an almost quantitative agreement.