# Mixing solvents, or how does paint thinner work?

I would like to understand how a mixture of solvents works together.

For example, there are two solvents combined, and each one alone can dissolve some material, but one acts faster.

How do they interact when used in combination on the same material?

An example would be paint thinner, for example a mixture of toluene and xylene.

One reason to use multiple solvents in a paint thinner is to make it dissolve more kinds of paints etc. (In German, this is called "universal thinner".)

But does the combination of multiple solvents somehow make it work better dissolving a single compound or mixture?

I had dissolved some polycaprolactone in that universal thinner above, and it seemed to dissolve quicker than in either toluene or xylene alone.
I do not know exactly what is in the thinner, so it does not mean much – but it made me wonder about this!

So, in which ways interact multiple solvents in the context of the example, and are there other mechanisms in general, not covered by the example?

For discussing a simple example, let's just assume the thinner contains only the two solvents above.

But for context, I looked up the Sicherheitsdatenblatt (MSDS) for my example thinner, which shows some more solvents on the list of ingredients:

Compound                                Mass %
----------------------------------------------
Toluol                                  15- 45
Xylol                                   10- 40
Ethanol                                  2- 15
Aceton                                   5- 20
Ethylacetat                              5- 20
n- Butylacetat                          10- 20
aromat. hydrocarbon mixture (C8-C10)     5- 15
aliph. hydrocarbons (C7-C9)              2- 10


Hansen Solubility Parameters (HSP) - is the short answer.

Your observation has very much to do with the energy density of the solvents - with the idea that lesser the difference in the energy densities better is the ability of solvent to dissolve the solute. I am very poor at explaining things and not an expert in the matter at hand (please excuse me for that), so I would instead explain using an example.

In HSP system (Proposed by Dr. Charles Hansen) there are mainly three parameters in consideration, namely: dispersion δd, polarity δp and hydrogen bonding δh. These values exist for solvents as well as solutes. Now we take into consideration two solvents and one polymer as under. The link is here https://www.hansen-solubility.com/HSP-science/solvent-blends.php.

Name                Type        δd      δp      δh
Toluene (T)         Solvent     18      1.4     2
Acetone (A)         Solvent     15.5    10.4    7
Polycaprolactone    Polymer     17.7    5       8.4
T + A (50:50)       Solvent Mix 16.75   5.9     4.5


The energy difference is calculated by the formula: (Ra)² = 4×(δd2-δd1)² + (δp2-δp1)² + (δh2-δh1)²

Why Ra, I will explain in some while.

1. For Polycaprolactone in Toluene the value is: √(4×(18−17.7)²+(1.4−5)²+(2−8.4)²) = 7.367

2. For Polycaprolactone in Acetone the value is: √(4×(15.5−17.7)²+(10.4−5)²+(7−8.4)²) = 7.105

3. However when you mix the above two solvents in 50:50 ratio the value is: √(4×(16.75−17.7)²+(5.9−5)²+(4.5−8.4)²) = 4.430

So individually solvents can be poor solvents for a solute but their mixture can be a better one.

Now about Ra. The d, p & h values can be considered as three coordinates and through three distinct points one unique sphere exists such that the points lie on the surface. The center of this sphere can be considered as the mean of the three values and Ra is the distance from this centre. The lesser the distance the better is the ability of solvent to dissolve the polymer in question (like dissolves like).

The value of Ra beyond which dissolution is not possible is denoted by R0 (unique to every polymer and calculate by trial and error - reading more on the topic will give you complete detail), so Ra should always be < R0.

Hope I have brought some clarity on the topic.

I had dissolved some polycaprolactone in that universal thinner above, and it seemed to dissolve quicker than in either toluene or xylene alone.

Some of these universal thinners do contain a lot more than just toluene and the xylenes.

If you want to find out about the composition of these mixtures, a material safety data sheet (MSDS, in German: Sicherheitsdatenblatt), provided by the manufacturer gives some information:

As an example, take this MSDS by a swiss manufacturer, which lists the following ingredients:

\begin{array}{lr} \textrm{compound} & \textrm{amount [%]}\\ \hline \textrm{toluene} & 25-50\\ \textrm{xylene} & 10-25\\ \textrm{acetone} & 10-25\\ \textrm{propan-2-ol} & 2.5-10\\ \textrm{4-methyl-pentan-2-one} & 2,5-10\\ \textrm{2-methoxy-1-methylethylacetate} & 2,5-10\\ \end{array}

Particularly the latter two might be much better solvents for the polycaprolactone than toluene or xylene.

• Indeed, my example thinner also contains more solvents. I'll add details (But regarding the example, I think it's good enough to just pretend there are only two solvents.) – Volker Siegel Apr 15 '15 at 19:59
• The answer does explain why the thinner worked quicker - but the question is more about all these solvents interact, like when ethylacetate dissolves the compound quickly, and toluene does so slowly, but there is very little ethylacetate, and a lot toluene. (I'm not asking about the actual interactions with these listed solvents - just simplified examples.) – Volker Siegel Apr 15 '15 at 20:33