On the Impact of Ambient Humidity and Evaporation on Demixing of Binary Mixtures

As per known data, hexane and diethylene glycol monoethyl ether (DGME) are perfectly miscible at temperatures above some 6 °C (critical solution temperature, CST), manifesting a miscibility gap below this temperature. Yet, when deposing hexane-DGME layers or sessile droplets, we unexpectedly observe demixing already at room temperature. As hexane is volatile, one may be tempted to explain this in terms of evaporative cooling. However, apart from some extreme cases, estimations and direct measurements reveal that such cooling cannot be as drastic as to bring the temperature down to CST. Then, we hypothesize that such anomalous demixing could be caused by moisture in the ambient atmosphere. After all, even if hexane is practically immiscible with water, DGME is hygroscopic. To verify this conjecture, a series of experiments were carried out in a chamber with well-controlled temperature and relative humidity (RH), where a layer of the hexane-DGME mixture was observed by reflective shadowgraphy. In this way, we could measure the "apparent" CST as a function of RH, which indeed proves to be higher than 6 °C and tends to the classical value only at vanishing RH. Our picture of the phenomenon is also well backed up by a heuristic model of the ternary mixture (also including water) based on regular-solution and van Laar fits of the known binary-pair properties.