# Hygroscopic behaviour of DMSO - how bad is it?

For a fluid flow experiment I am using DMSO (dimethylsulfoxide), because of its low volatility, reasonably high surface tension, low viscosity and relative safety.

In the MSDS of DMSO I find that the liquid is hygroscopic, meaning it adsorbs moisture from the surrounding environment. Unfortunately, I cannot find anywhere how much water will be absorbed (mass%) or how quickly this will happen, is it saturated in minutes, hours, days?

Therefore, my question is twofold: (i) how much water will DMSO at maximum absorb and what is the rate at which this absorption occurs? (ii) with the absorbed water, how badly do the physical properties (viscosity, surface tension, density) of the (now) mixture change?

Being curious about the hygroscopicity I have tried the experiment using pure, dried DMSO in our lab where the temperature was $21^\circ \text{C}$ and the relative humidity about $60\%$.

Have a look at the two images below. This is a DMSO droplet originally with $V=50\; \mu\text{L}$. You can clearly see that the volume of the droplet has increased substantially in 20 minutes from the different height.

Using a crude approximation, assuming that both shapes are caps of a sphere, I calculated that the change in height from 100 px, to 112 px is equivalent to a volume increase of $15\%$, which is much too big for an accurate flow experiment.

I only used DMSO in other contexts and I have no idea how fast the water uptake is or how it changes the physical properties, but

• DMSO is miscible with water in any ratio
• it does not form an azeotrop with water

The typical protocol for drying is:

1. Keep over night over anhydrous $\ce{CaSO4}$ or powdered $\ce{BaO}$
2. Decant off
3. Distill over $\ce{CaH2}$ (about 10 g/liter) at reduced pressure (bp is around 75 °C at 16 hPa)
4. Store over 4 A molecular sieve in a dark and tightly closed bottle
• And before you do that you should be very certain, that you need it "that" dry. Did it once, hated it, turned theoretician. :D – Martin - マーチン Apr 2 '14 at 8:27
• @Martin :D I mostly did it for Swern oxidations or in fluorescence spectroscopy. It is a bit tedious and I don't know if it is necessary in Michiel's case, but it might be helpful do have somewhat defined conditions initially. – Klaus-Dieter Warzecha Apr 2 '14 at 8:52
• For these reactions it is absolutely necessary. For everyday reactions stirring it over night over $\ce{CaSO4, BaO, NaH, CaH2}$ is sufficient. btw, you are missing an $\ce{H}$. – Martin - マーチン Apr 2 '14 at 8:58
• It doesn't necessarily have to be very dry. My main concern is that the properties (in particular mass) of the liquid shouldn't change significantly over the course of about 1 hour of exposure to air at about 50% RH. – Michiel Apr 2 '14 at 9:21
• The liquid in my case, is a droplet of 50 $\mu L$ forming a roughly hemispherical cap. – Michiel Apr 2 '14 at 9:34