Measurement of the molecular content of binary nuclei. II. Use of the nucleation rate surface for waterndash;ethanol

摘要

In a preceding paper we determined the molecular contents of binary nuclei from homogeneous nucleation rate measurements studing ethanolndash;hexanol as a nearly ideal model system. Here we report the analogous measurements for the nonideal waterndash;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 105J/cmminus;3thinsp;sminus;1109. We determined the homogeneous nucleation rates of mixed droplets in supersaturated waterndash;ethanol vapor mixtures with argon as the carrier gas. The experiments were performed as functions of the water and ethanol gas phase activities,a1anda2, respectively, ranging from pure water to pure ethanol with ten intermediate activity ratios atT=260 K. The experimental data shape a nucleation rate surface in threehyphen;dimensionalJndash;a1ndash;a2space from which the critical activities for a rate ofJc=107cmminus;3thinsp;sminus;1were determined. The critical activities are compared with the predictions of the classical binary nucleation theory. The observed large discrepancies can be attributed to very waterhyphen;rich compositions of the nuclei predicted by the theory. The results support previous findings for the waterhyphen;nhyphen;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.