The separation issue of methanol, ethanol, and water
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Whether it is methanol or ethanol, their densities are different from that of water; will stratification occur when they are mixed with water? The baijiu that people drink certainly contains water, so why isn’t there a separation between the water and alcohol?
Whether two different liquids mix and form a layer or not depends on their mutual solubility, not on their density! Methanol, ethanol, and water are mutually soluble in any proportion. There is no layering issue!
N2 and O2 also have different densities, so it’s still impossible to see layers in air! Mainly because they can be infinitely miscible with each other!
It seems that this is not only related to the relationship between solubility and density, but also involves an issue of surface tension! For example, methanol and water mix freely with each other, but when methanol leaks and lands on a surface with very little water, you will clearly observe a separation between the methanol and water! It is because there is a force that causes the liquid surface to contract, namely surface tension. The direction of surface tension is tangent to the liquid surface and perpendicular to the boundary between the two phases; if the liquid surface is flat, then the surface tension lies on that plane. If the liquid surface is curved, the surface tension lies on the tangent plane of this surface.
Surface tension is a manifestation of intermolecular forces. It occurs at the boundary where a liquid and a gas come into contact. It is determined by the special conditions of the liquid molecules in the surface layer. The molecules within a liquid are almost in close contact with one another; they maintain a balanced distance apart. When they are slightly farther apart, they attract each other, while when they are slightly closer together, they repel each other. This is why liquid molecules cannot spread infinitely like gas molecules, but can only vibrate and rotate around their equilibrium positions. Molecules near the liquid surface are subjected to uneven forces because they are only significantly affected by the molecules inside the liquid; as a result, molecules with higher speeds can easily escape to the surface and turn into vapor. Consequently, the molecular distribution in the surface layer of the liquid (the thin layer in contact with the gas) is sparser than that in the interior parts of the liquid. Compared to the distribution of molecules within the liquid, they are in a special situation. The repulsive force between molecules in the surface layer decreases as the distance between them increases; in this special layer, the attractive forces between molecules become dominant. Therefore, if a dividing line MN is drawn arbitrarily on the liquid surface to divide it into two parts, a and b, as shown in the figure. F represents the attraction between the molecules in the surface layer of part a and those in part b, while F6 represents the attraction between the molecules in the surface layer of the right-hand part and those in part a. The forces exerted by these two pairs of molecules are always equal in magnitude but opposite in direction. The mutual attraction between any two parts of this surface layer leads to a tendency for the liquid surface layer to contract. Due to surface tension, the liquid surface always strives to minimize its size; as a result, small droplets in the air tend to be spherical in shape.
Pure methanol and ethanol are miscible with water and do not form layers. The reason why an analysis for water solubility is included in the methanol production process is that the synthesis reaction involves side reactions; other components can affect the solubility of methanol, resulting in insoluble substances – which might be what the original poster meant by layer formation
Will methanol separate into layers when mixed with gasoline?