Fluid mixtures at high pressures: Phase behavior and critical phenomena for binary mixtures of water with aromatic hydrocarbons

Phase behavior and critical phenomena for 26 binary mixtures and two ternary mixtures of aromatic hydrocarbons with water were measured with a static, windowed cell at temperatures from 420 to 772 K and pressures to 54 MPa. Liquid–gas (lg) and liquid–liquid (ll) critical curves, three-phase liquid–liquid–gas (llg) lines and upper critical end points (UCEPs) were all measured. Both the experimental apparatus and technique were designed so that measuring times could be kept to less than an hour; thus, it was possible to investigate the phase behavior of substances with limited thermal stability at elevated temperatures.The alkyl-substituted benzenes and decahydronaphthalene (decalin) all exhibited type III phase behavior with water, with one leg of the discontinuous critical curve extending from the critical point of water to higher pressures (we call this configuration type IIIa). However, all polycyclic aromatic hydrocarbons (PAHs) investigated (including naphthalene, 1-methylnaphthalene, 1,2,3,4-tetrahydronaphthalene (tetralin), anthracene, phenanthrene and pyrene) exhibited type II phase behavior with water. Surprisingly, then, even 3- and 4-ring PAHs become completely miscible with compressed liquid water. For the indene + water system, type IIIb phase behavior was observed for the first time for an aromatic + water binary, with one leg of the discontinuous critical curve extending from the critical point of the hydrocarbon to higher pressures. Thus, for aromatic + water mixtures we have documented for the first time transitions between three types of phase behavior: types IIIa, II and IIIb, with the type observed dependent on factors such as the critical temperature, molecular size and structure, and aromaticity of the hydrocarbon.