Surfactants in Hard Water
The theme of this exercise is hard water and how problems with hard water can be circumvented. Three simple exercises compare soap with a synthetic detergent in hard water, look at the effect of builders on soap in hard water, and demonstrate the action of water softeners.
Students compare formation of curd by soap and synthetic detergent. They are instructed to add a drop of soap or of synthetic detergent to 2 mL of either distilled water, tap water, or hard water. After shaking the tubes, they are to describe what happened.
The exercise is followed by questions asking them to (1) write the equation for curd formation, (2) conclude whether the tap water or the hard water had more calcium, and (3) explain, from structures I put on the board, why the synthetic detergent (with an uncharged headgroup) behaved differently than soap.
Hard water is 5 mM calcium chloride (200 ppm calcium). Soap is 10% Ivory soap in 70% isopropyl alcohol. I used Brij 58 for the synthetic detergent, also a 10% solution in 70% isopropyl alcohol. (Sodium dodecyl sulfate was not much better than soap in terms of sudsing in the water samples.) I don't have any sodium alkylbenzene sulfonate, the most widely-used synthetic, but I have several polyoxyethylene-type synthetics, and Brij 58 worked well, forming no curd in any solution. It didn't make as much suds as soap; other congeners in the Brij series, such as Brij 35, might make better suds.
Students observe the effect of detergent builders on curd formation. They are instructed to add 15 drops of a builder solution to 2 mL of hard water, then add one drop of soap, shake, and compare with soap in distilled water alone.
For builders, I have used the sodium salts of citrate (1 M), tripolyphosphate (10%), NTA, and EDTA (both 0.5 M, pH about 9). EDTA worked the best; there was no sign of curds. Both citrate and polyphosphate were cloudy but gave some suds.
The exercise is followed by questions asking them to (1) explain why chelation is an apt term for the action of builders (given the structures of the chelates on the board) and (2) explain why curd doesn't form in water containing builders.
Students observe the results of mixing soap with hard water that has passed through a miniature makeshift water softener. They are instructed to put 2 mL of hard water into their device and test the eluate with a drop of soap.
The water softener is made using a disposable polyethylene transfer pipet, the graduated 1-mL kind. The stem is cut off half way up and a glass wool plug blocks it. The top of the bulb is cut off and Dowex-50 resin is poured into the stem until it reaches the bottom of the bulb, about 0.5-mL bed volume. The resin I used was in the hydrogen form, which I washed with 1 M NaCl until the eluate was no longer acidic. 8% cross-linked, 100-200 mesh works very well; higher mesh sizes (e.g. 20-50) give flow rates too high for efficient ion exchange. (A tip: it's important that the resin bed extend upward into the bottom of the bulb, otherwise a bubble will form and flow will stop.) Dowex resin is a little expensive, but these devices are infinitely reusable.
The exercise is followed by questions asking them (1) why curd didn't form in the softened water, (2) what holds the calcium ions to the ion exchange resin, and (3) what new ion is in the softened water that wasn't there before.
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