Combined effect of ambient relative humidity and evaporation on the demixing of a binary mixture*

As per known data, hexane and Diethylene Glycol Monoethyl Ether (DGME) are miscible at temperatures above ~7°C (critical solution temperature, CST), manifesting a miscibility gap at lower temperatures. Yet, if we depose a hexane-DGME layer or sessile droplet, we observe, quite unexpectedly, demixing already at room temperature. As hexane is volatile, one might think this was due to evaporative cooling, bringing the temperature down to CST. However, estimations and direct measurements reveal the associated temperature decrease is by far less drastic. Then, we hypothesize that such an 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 the conjecture, a series of experiments were carried out in a chamber with well controlled temperature and relative humidity (RH), where a layer of hexane-DGME mixture was observed by reflective shadowgraphy. In this way, we were able to measure the 'apparent' CST as a function of RH, which indeed tends to ~7°C in the limit of small RH. Our picture of the phenomenon is also well backed up with an heuristic model of ternary mixture (also including water) based on regular-solution and van Laar fits of the known binary-pair properties.