This is the first study to quantitatively examine the miscibility of butanol and compare with miscible alcohols by employing molecular dynamics simulations and graph theoretical analysis of three water-alcohol mixtures at various concentrations. We show how distinct alcohol aggregates are formed, thereby affecting the water structure, which established the relationship between the morphological structure of the aggregates and the miscibility of the alcohol in aqueous solution.
The aggregates of methanol and ethanol in highly concentrated solutions form an extended H-bond network that intertwines well with the H-bond network of water. Graph theoretical analysis revealed that the alcohol aggregates of methanol and ethanol solutions have a morphological structure different from that of n -butanol, although there is no significant difference in morphology between the three pure alcohols.
These two distinct alcohol aggregates are classified as water-compatible and water-incompatible depending upon their interaction with the water H-bond network, and their effect on the water structure was investigated. Our study reveals that the water-compatible network of alcohol aggregates in methanol and ethanol solutions disrupts the water H-bond networks, while the water-incompatible network of n -butanol aggregates does not considerably alter the water structure, which is consistent with the experimental results.
Furthermore, we propose that miscible alcohols form water-compatible networks in binary aqueous systems while partially miscible alcohols form water-incompatible networks. The bifurcating hypothesis on the alcohol aggregation behavior in liquid water is of critical use to understand the fundamental issues such as solubility and phase separation in solution systems.
Choi, S. Parameswaran and J. Choi, Phys. To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.
If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given.
If you pour them into the same container, they will form separate liquid layers, one on top of the other. Other liquids, for example rubbing alcohol and water, can be mixed with each other. But did you know that once both of these liquids have mixed you can separate them again into two different layers?
How can you do that? The answer might surprise you—with salt! In this activity you will find out how this works. To be able to mix, the molecules of both liquids have to be able to attract one another.
Molecules that are polar meaning their electric charge is distributed unevenly so they have a more positive side and a more negative side tend to form hydrogen bonds whereas nonpolar molecules which have an equal charge balance do not tend to form such bonds. Because water molecules are polar, any liquid that does not have polar molecules—such as oil—is usually immiscible with water. Rubbing alcohol molecules have a polar and nonpolar part, which means they are able to form hydrogen bonds with water and therefore able to mix with it.
But how can you break these bonds in order to separate both liquids once they are mixed? You have to add something to the mixture that competes with the alcohol in binding to the water molecules. One substance that can do that is salt.
Salt is an ionic compound, meaning it is a substance made up of electrically charged molecules called ions. When ionic compounds dissolve in water, the individual ions separate and get surrounded by water molecules—a process called solvation. Because the salt ions are charged, they dissolve much better in a polar solvent, which is also slightly more charged than a nonpolar solvent. For this reason, salt ions attract the water molecules much more strongly than alcohol molecules do because alcohol is less polar than water.
This means that when there is a lot of salt, all the water molecules will bond to the salt ions, leaving none to form hydrogen bonds with the alcohol molecules. As a result, the alcohol becomes immiscible with water and starts to form a separate layer. Historically this method has been used in the soap-making process to remove ingredients that should not be in the final soap product.
Salting out is also commonly used in biochemistry laboratories to purify proteins, because different protein molecules become immiscible at different concentrations of salt solutions. Chemists use this technique to extract liquids out of a solution, which is what you are going to do in this activity: You will separate a rubbing alcohol and water mixture using just a teaspoon of table salt!
Observations and results You should have seen that the salt easily dissolved in the water in cup 1. After shaking it the salt seemed to disappear. Remember that this occurs because the ionic salt molecules easily bond to the polar water molecules. The salt, however, did not dissolve as easily in the rubbing alcohol in cup 2. Even after shaking it you will still be able to see the salt. This occurs because the alcohol molecules are less polar than water is, so the salt ions do not bond with them as easily.
With the permanent marker ink you should have observed the exact opposite phenomenon. The ink does not dissolve well in water but it does easily in the alcohol, giving the latter much more color.
This is due to the fact rubbing alcohol also has a portion of its molecule that has no charges, and is nonpolar. This portion is more compatible with nonpolar molecules such as the marker ink. The alcohol dissolves in the water to form a homogenous solution, so you cannot distinguish the alcohol and the water anymore. As water is polar it attracts OH group. Carbon chain on the other hand as nonpolar is repelled. Solubility of alcohols is therefore determined by the stronger of the two forces.
Because of the strength of the attraction of the OH group, first three alcohols methanol, ethanol and propanol are completely miscible.
0コメント