During a recent cleaning and chemistry course, a student asked a thought-provoking question about solubility, miscibility, and cocktail hour. I explained that some liquids, like oil and water, are immiscible. That is, they don’t form a “single phase;” the oil is on top of the water. The observant student wondered if this was what went on with fancy mixed drinks, where beverages of different colors are layered. Well, not quite, I said. So, he asked, what’s going on? I’ll share the answer with all of you. Once you understand the chemistry of oil and water, layered drinks, and mayonnaise, you’ll have a better understanding of how your cleaning chemistry does the job (or maybe doesn’t do the job!)
Salad dressing
Water and alcohol mix to form a single solution; chemists refer to them as “miscible” liquids. To illustrate immiscible liquids, make a vinaigrette salad dressing. Pour balsamic vinegar (a polar liquid) and olive oil (a non-polar liquid) into a carafe and shake vigorously; you get a cloudy mixture. Set it down for a minute and the oil rises to the top, forming two phases. Oil and water (the major component of balsamic vinegar) don’t mix, even if you shake them up. They are immiscible liquids.
Chemistry
First, let’s plow through the following nerd-like fact: Chemists use the three Hansen solubility parameters to determine if molecules are likely to mix together. Hansen parameters are a measure of the energy associated with attractive forces. Let’s simplify. For this discussion, think of a Hansen parameter as being sort of like a primary color (red, yellow or blue). The Hansen parameters are: non-polar (sometimes called “dispersive”), polar, and hydrogen bonding. Non-polar forces are “oil-like,” polar and hydrogen bonding forces are “water-like.” Both the absolute number and the ratio of the three numbers influence the behavior of the molecules. Chemists use Hansen solubility parameters sort of like different colors of paint chips to indicate how well molecules will blend together. It’s the same reasoning you would use to create a shade of turquoise; you probably wouldn’t include pink paint!
Hansen and critical cleaning
Why do we care about Hansen parameters? Because Hansen parameters predict how effective a cleaning chemistry will be at removing a soil. Hansen parameters can also predict how damaging a cleaning chemistry might be to the product being cleaned – like a polymer (a plastic). We say “predict” rather than “guarantee for certain” because there are quite a few factors involved.
We won’t get into more detail about Hansen parameters in this discussion. However, if you’d like to learn more, go to SUR/FIN in Las Vegas next month and show up at “Development of Blend Recipes for Vapor Degreasing,” on Tuesday, June 7 in Session 3. Is this a shameless plug for our program? You bet! We want a nice audience; and we’d love to have you participate. We are co-presenting with Dr. Darren Williams of Sam Houston State University.
Mixing oil and water
What if you added another chemical called an emulsifying agent and kept shaking the two immiscible liquids? For salad dressing, egg yolk or mustard are effective emulsifying agents. With emulsions, you have a liquid that doesn’t separate out for a while. Emulsifying agents (not egg yolk or mustard!) in aqueous (water-based) cleaning chemistries help to achieve effective removal of oils.
Now, we mentioned that emulsions don’t separate out for “a while.” The time frame can be quite variable; and how long you want an emulsion to last depends on the application. Commercially-prepared mayonnaise is a very stable emulsion; and you don’t want mayonnaise to “separate out.” If you are cleaning parts, you want the emulsion to last until the soil has been removed from the part – and you probably want it to last longer so that the soil stays away from the part.
The two-phase advantage
Would you always want to use an emulsion cleaner? Not really. A disadvantage to an emulsion is that because the soil and the cleaning chemistry are mixed together, when you dispose of the soil, you also dispose of the chemistry. However, there are what are often termed “oil-splitting” aqueous cleaning chemistries. Such chemistries are purposely designed to release the oil. The oil pops up to the surface; the cleaning chemistry can often be re-used. This is important, because disposing of used cleaning chemistry can be expensive.
One reason we emphasis aqueous cleaning as being a process, not just a chemistry, is that with oil-splitting chemistries, adding a sparger with an overflow weir or a skimmer to the cleaning tank is very important. If you don’t skim off the oil, the process of lifting the parts out of the tank will result in recontamination.
Layered drinks; tea with honey
Finally! We have arrived at “Happy Hour.” The phases in most multi-colored layered drinks don’t happen because the liquids are immiscible. Instead, you create temporary phases or layers by carefully layering liquids with different densities, with the liquid with the highest density at the bottom (1). It’s like the layer of honey at the bottom of a cup of tea. The honey has a much higher density than the tea; to form a uniform solution, you have to stir.
In critical cleaning, you can learn a lot from layered drinks and tea with honey. Even if the soil is miscible in the cleaning chemistry, if you don’t use some sort of force in the cleaning process, you may get a gradient where the soil remains close to the substrate (the substrate is the part you are trying to clean). Gradients are the opposite of what you want to achieve in the wash step; during washing, it is crucial to dislodge the soil from the substrate and then keep the soil away from the substrate. Perhaps you don’t need to discriminate as much as James Bond; frequently “stirred” can be effective as well as “shaken” (2). Ultrasonics may help even more.
Sometimes you want miscible gradients, even in the world of cleaning and contamination control. Miscible gradients are also used as part of some analytical techniques. Gradients are used to separate out or to isolate different kinds of particles, even different kinds of living cells. These gradients can be step gradients – just what that sounds like, the liquids form density steps similar to what happens in layered drinks. The gradients can also be continuous where the density changes gradually. Gradients can be used in analytical techniques (like high pressure liquid chromatography, HPLC) to separate out parts of complicated mixtures. That’s important because soils often contain many different molecules and you may want to identify which ones are of most concern.
References:
1. “Density Gradients and Making them QUICKLY”
http://www.science-projects.com/GradCent.htm
2. “Shaken not Stirred,” Royal Society of Chemistry
http://www.rsc.org/chemistryworld/Issues/2010/December/ShakenNotStirred.asp
