Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: vahid on 22/04/2018 12:26:47
-
Hello guys
here is a question about removing hydrocarbons using a mixture of polar and non - polar solvents.
I've found experimentally that the mixture of toluene+methanol (30%methanol+70%toluene) is more effective than pure toluene in removing oil deposits from reservoir Rock.Toluene+propanol was worse than pure toluene. Similar to what happens to toluene, dichloromethane+Ethanol yields a better result compared to pure DCM. DCM+Methanol was worse than pure DCM.
I’ve been trying these things for Tetrahydrofuran and the best result was for THF+Propanol and THF+methanol.
So what do you think guys? Is there a scientific explanation for this?
In general, polar solvent removes polar hydrocarbons (like aromatic) and non-polar solvent tends to remove non-polar hydrocarbons (like paraffin). but why toluene is better with methanol and DCM with ethanol and THF with propanol?
What is the reason for this preference?
-
Can you rephrase the title of your post as a question please.
-
Yes, mixtures of solvents often have better solubilizing properties than either of the individual solvents (not always).
The key to dissolving a compound is to have favorable intermolecular interactions between the solvent and solute (ideally stronger interactions between solute and solvent than solute-solute or solvent-solvent, but entropy of dissolution goes a long way to favoring dissolution, even when the interactions are not stronger.)
Common intermolecular interactions are (in order from strongest to weakest) ion-ion, dipole-ion, dipole-dipole, dipole-induced dipole, and induced dipole-induced dipole. Hydrogen bonding is an extreme case of dipole-dipole interaction, and molecules can be distinguish by how many hydrogen bond donors (hydrogens capable of forming hydrogen bonds), and hydrogen bond acceptrs (lone pairs on electronegative atoms) capable of interacting with hydrogens to form hydrogen bonds. And π-π interactions are a special case of induced dipole-induced dipole interactions (also having quadrupole-quadrupole interactions, but these are not very strong).
If the compound (or mixture of compounds) you wish to dissolve is complex, having portions that are very polar, and portions that re very polarizable, then having a solvent that is polar but not polarizable (like ethanol) and a non-polar but polarizable solvent (like toluene) is likely to be more effective than either solvent by themselves.
Dichloromethane is an excellent solvent because it is both very polar and highly polarizable (chloroform and 1,2-dichloroethane also share both of these properties, and are similarly excellent solvents, while carbon tetrachloride is nonpolar and despite its extreme polarizability is not a good solvent for most compounds). However, these chlorinated solvents are not able to participate in hydrogen bonding, so they will be noticeably poor solvents for compounds like sugars and other poly-ols (with many OH groups), or carboxylic acids. However, by adding just a few drops of a simple alcohol like methanol or ethanol (or isopropanol or t-butanol) the strong hydrogen bonding between solute molecules can be disrupted, in favor of forming hydrogen bonds with the alcohols (adding hydrogen bond acceptors like acetone, ethyl acetate or pyridine can sometimes have a substantial effect, but not bearly as dramatic as with alcohols which are both hydrogen bond donors and acceptors.)
There are some cases where this doesn't work. For instance, when dissolving salts (which are held together by extremely strong ion-ion interactions), one must focus on maximizing the dielectric constant of the solvent. Water has one of the highest dielectric constants of all known solvents (the only one I know of that's higher is formamide... see here: https://depts.washington.edu/eooptic/linkfiles/dielectric_chart%5B1%5D.pdf), so it is common to use water to dissolve salts, and any solvent that is added decreases the solubility of the salt.
The other notable caveat when mixing solvents, is that solvents have to be miscible in each other (or at least somewhat soluble). For instance, adding ethylbenzene to water (or vice versa) will not lead to any improvement in solubilizing properties... see here: https://www.csustan.edu/sites/default/files/groups/Chemistry/Drake/documents/solvent_miscibility_table.pdf
-
Thanks @chiralSPO
That was a valuable Comment and I appreciate it.
Would you please mention your references?
I 'd like to study more about this issue. Particularly where one solvent disrupted another solvent.
-
Most of what I discussed can be found in this textbook: https://www.amazon.com/Modern-Physical-Organic-Chemistry-Anslyn/dp/1891389319
(it's probably too expensive to buy for just this one question), but if there is a college campus nearby, you may be able to find it in the library (or you can buy a used version for not that much)
There are probably also some good online resources, but I can't direct you to any that I know of personally...