# Miscibility gap of heptane-methanol mixture

To explain my question a bit better: I added heptane and methanol together in equal ratios (1:1) and observed a separation in phase where there appeared to be an oily layer sitting on top of the other liquid. But when I increased the amount of methanol in a ratio of 1:2 and 1:3 (heptane to methanol), this appearance disappeared.

I understand that heptane and methanol are immiscible, but what led to the change? I apologize if this is something really obvious, but I would greatly appreciate any help.

Methanol is a polar solvent; heptane is very non-polar. If you start with methanol as solvent, you can dissolve a little heptane into it; if you keep adding heptane, you reach the solubility limit of the solute heptane in methanol, and the excess heptane floats.

If you start with heptane as solvent, you can dissolve a little bit of solute methanol into it; at its solubility limit, excess methanol will sink.

Now, when you take methanol and heptane in a 1:1 ratio, both solutes have exceeded their solubility in both solvents, and you should wind up with a heptane layer on top, with a little bit of methanol dissolved in it, and a methanol layer below, with a little heptane dissolved in it. In some cases, a metastable intermediate layer forms, but will separate into the upper and lower layers. (Those molecules just can't get it all figured out instantly!)

The phenomenon is called a miscibility gap, where it seems that two liquids go together but only so far. Data on methanol-heptane were not available, but a phase diagram of methanol-cyclohexane is shown below:

The x-axis goes from zero methanol (i.e., all cyclohexane) to all methanol (no cyclohexane). The large central area is the two-phase region. At each extremity, whether methanol-rich or cyclohexane-rich, there is some solubility, which increases if you raise the temperature, until at about 320K (47$$^o$$C), the mutual solubility equals complete miscibility, and at this temperature and higher, there is only one phase.

A somewhat different plot (of enthalpies of hexane and methanol mixtures) shows the same kind of miscibility gap. Hexane and methanol are completely miscible above about 45$$^o$$C, but as the temperature is lowered, mixtures with ratio of 1:1 will separate into two phases. Mixtures with only about 20% of the other solvent are stable at room temperature. But from the chart, it would seem to be a good bet that reducing the temperature will make the miscibility gap even wider.

• If I were to give a simple definition of the miscibility gap, would this work: "when two liquids with partial miscibility are mixed and become completely miscible at a specific temperature or higher". I'm having difficulty comprehending the concept, so I'm wondering whether this is an appropriate way to describe it. Jul 5 '21 at 9:39
• @alwaysconfused_kat: Sure, but two liquids with partial miscibility (i.e., limited solubility) are mixed, there is no absolute requirement that there be some temperature above which both are completely miscible and still liquid (i.e., below the boiling point). Of course, above the boiling points of both, the vapors (gases) will be freely mixable. Jul 5 '21 at 13:37
• There is another two-phase vs temperature novelty in practical chemistry: ethoxylated surfactants frequently have cloud points, which are temperatures above which a solution will separate into two phases. The polyoxyethylene group become less soluble in water as the temp rises, and the surfactant, which is homogeneously dispersed in micelles at lower temps, de-solves, rises and become a distinctly separate phase. Dishwasher detergents use this phenomenon to limit foaming inside the dishwasher; not a property of dishwashing (sink) detergents, which favor foam. Jul 5 '21 at 13:44

The disappearance of boundary is an optical illusion. Whenever we see two boundaries in a liquid, it means that their refractive index is different. If the composition of two phases is such that their refractive indices are very close or match, the phase boundary disappears. It does not mean that separate phases do not exist.

Heptane and methanol are partially miscible though, in heptane methanol system, the mole fraction of methanol is stated to be 0.270 at room temperature.*

*Ref: Savini, C. G., D. R. Winterhalter, and H. C. Van Ness. "Heats of Mixing for Partially Miscible Systems: Methanoi-n-Hexane and Methanol-n-Heptane." Journal of Chemical and Engineering Data 10.2 (1965): 171-172.

Also note that you did not mention the purity of heptane. Low grade heptane from cheaper sources is always contaminated. If you really care with this phenomenon, you have to use the highest purity solvents like LC-MS grade solvents.

• Mr. down voter would you elighten us with your wisdom please rather than pressing buttons and walking away? Jul 3 '21 at 16:59
• I agree with you that downvoters should make an explanatory comment. (I didn't downvote your answer.) Jul 3 '21 at 23:08
• Down voting is not a bad thing if done properly with a constructive tone. The problem is this button is available in every hand and we ave no clue if grade 5 student did it or an Einstein was not satisfied. Jul 3 '21 at 23:42