MEASUREMENT OF THE MOLECULAR CONTENT OF BINARY NUCLEI .3. USE OF THE NUCLEATION RATE SURFACES FOR THE WATER-N-ALCOHOL SERIES

In two preceding papers the molecular content of binary ethanol-hexanol and water-ethanol nuclei, respectively, was determined from nucleation rate measurements. While nucleation of ethanol-hexanol mixtures behaved nearly ideal, a strong mutual nucleation enhancement for water-ethanol was observed. Here we extend the investigations to include the longer chain alcohols, that is water -CiH2i+1OH systems with i = 2 to 6. Using the nucleation pulse technique developed in the past few years nucleation rates in the range 10(5)<J/cm(-3) s(-1)<10(9) are measured. Ranging from pure water to pure alcohol the water and alcohol activities, a(1) and a(2), respectively, are varied for each system with about ten intermediate compositions at constant temperature T = 260 K. Aside from a remarkably similar behavior of the various alcohols, one observes that with increasing alcohol chain length the mutual nucleation enhancement decreases. Since all water-alcohol systems behave qualitatively similar, we confine ourselves to present the full experimental nucleation rate surface J(a(1),a(2)) for one system, water-n-pentanol, as an example. From the nucleation rate surface for each system the onset activities corresponding to a reference nucleation rate of J(0) = 10(7) cm(-3) s(-1) are determined. From the slopes of the nucleation rate surfaces one obtains the individual s numbers of molecules in the critical cluster n(i)* because partial derivative ln J/partial derivative ln a(i) approximate to n(i)*. As noted previously, determining the molecular content this way does not involve any particular theoretical model, nor does it depend on the structure of the critical cluster. Accordingly, the average composition of the critical clusters can be obtained. An alcohol enrichment of the nuclei at low alcohol activity fraction is found for all alcohols examined, the degree diminishing with increasing alcohol chain length. The appearance of a macroscopic miscibility gap for the higher alcohols is not reflected in any qualitative change of the composition of the microscopic nuclei. (C) 1995 American Institute of Physics.