In short, can solvents lower water’s surface tension?
Related to well water problems, I hit on a suggestion that the salt crystals added as a water softener can cause a “soapy” feel to the water. At first, I suspected a high pH was causing this as it is typical of alkalines. However, the pH tested fine with a digital meter, no less! Anyhoo, it was suggested that the salt crystals lower surface tension like a surfactant does.
So, what do those in chemistry or environmental science say?
Bottom line: viscocity. While nothing like oil, water does in fact have a measurable viscocity, the reason why drops of water will bead instead of/before spreading to cover a surface. Various additives will increase or reduce that viscocity, with equivalent impacts on surface tension. Beyond that factual-but-not-much-use general statement, we need a good physical chemist to discuss this further.
OK… first, salt would be a “solute”, not a “solvent”. The solvent is the liquid that dissolves the solute: water dissolves salt.
Second, a digital meter is not necessarily more accurate than paper strips.
Third, depends on which salt you use. One of the salts used as softener is Na4(EDTA). The Na is just regular sodium… the same as in Sodium Chloride. The EDTA is EthylenDiaminTetraAcetate (I think I got the spelling right); a largish organic molecule that’s also used as an anticoagulant for the same reason it’s used as a softener.
EDTA consists of two carbon atoms in the center (ethylen); each carbon is in turn united to a nitrogen (amin); each nitrogen to a -COOH group (acetate). The acetates and amines have free electron pairs which “grab” Calcium and Magnesium in the water (think of those claws in fair booths, that you can use to fish a plush doll; the plush doll would be a Ca or Mg). Water with EDTA added has a much Ca/Mg as before the EDTA got added; but, because the Ca and Mg “like” being with the EDTA more than being part of scale buildup, no scale builds up.
Now, would this affect surface tension? Yes. In the case of EDTA not much, but yes.
Soap works by having a piece that’s hydrophobic (“hates water”) and one that’s hydrophilic (“loves water”). The philic part gets in contact with water; the phobic part gets all together, forming “bubbles” where fat gets (fat is hydrophobic).
The two C’s in the center of EDTA would be hydrophobic. But so long as the molecule is free, all that happens is that the four hydrophilic “arms” get all around the CC. Once the EDTA has formed a claw around a Ca or Mg, the CC is suddenly exposed to the water. It doesn’t like this… so when an EDTA-Ca runs into another, they get hump-to-hump.
Not as efficient as a real soap (where a lot of molecules can form each soap bubble) but it’s the same principle. Also note that, since it’s not the EDTA that’s “soapy” but the EDTA-Ca and the EDTA-Mg, you’ll see more of an effect if the water was hard… harder water would produce more EDTA-Ca/Mg for the same amount of EDTA poured into it, than soft water.
I hope that was clear - and also, that my colleagues take into account that I do know how to do all the math but didn’t want to bore the questioner!
Ended up leaving out the crux of the question…
how does soap (or EDTA-Ca) affect surface tension? Air is by definition not-water. So the suds (and the EDTA-Ca claws) like having their phobic sides in contact with air.
So, you go from having a water surface that’s
WWWWWWWWWWWWWWWWWWWW
to having one that’s
WWWWSSSWWWWSWWWWSWWWWW
Since the very strong tension only happens where you have many molecules of water together*, the soap is lowering the tension by breaking down the groups of water molecules.
I feel like I’m trying to pour out three years of Phys Chem into two notes… sorry if I don’t make much sense, it’s just one of those fields where one thing takes to another, and one of my prof’s specialized in the Physical Chemistry of water.
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EDTA is ethylenediaminetetraacetic acid (Na4EDTA would be sodium ethylenediaminetetraacetate). Also, each nitrogen is bonded to TWO acetate (CH2COOH) groups. Each nitrogen and four of the oxygens can donate electron pairs to the metal, making EDTA a hexadentate ligand.
Wikipedia on EDTA
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Are you sure that the water has a soapy feel, and not that the softness of the water makes it difficult to wash off soap? My softened water feels normal, but it takes longer to rinse off soap, because there’s no calcium to tie up the soap. I’ve noticed the same thing when rinsing using deionized water in a laboratory.
In answer to the literal question, solvents (not solutes) can lower water’s surface tension. For example many people would say that ethanol can be called a solvent, and adding ethanol to water lowers the surface tension of the mixture.
Surface tensions are a short way of thinking about how interfaces will act. Really, there is an energy associated with interfaces, and it depends on the species on both sides of the interface, though a liquid and gas interface is pretty much dominated by the liquid only. However, in talking for example about miscible or imiscible liquids, and emulsions and colloids and so forth, you have to consider the relative affinities each species has for itself and for the other.
BTW viscosity doesn’t enter into any of this. Viscosity tells the ratio between shear stress and strain. In nonmoving situations like a drop sitting on a surface, where shear stress and strain are both zero, viscosity doesn’t matter for anything.
Thanks to all for their great insight. Bill Door raises an interesting point. I will have to try and notice the next time we experience this with our well water. But, since this past summer (2006), heavy rains made our well water significantly less acidic. As such, i have not neede to add any water neutralizing solution (soda ash dissolved in water). I believe my first suspicion was correct that over-neutralzing was the root cause of this raising the pH as high as 11.
As for the water softening part of the OP, someone suggested to me that they believe the water softening salt was the cause, from their experience.
Very interesting study!