Bill's Comments

It's time to take a break from all the seriousness of politics, religion, and philosophy.  Tonight we are going to have a science lesson.  I haven't done one of these in quite a few years.  The topic tonight is soft water. 

So-called hard water creates soap scums and doesn't rinse the soap off well.  It leaves all sorts of white to rusty colored deposits on plumbing fixures and shower stalls, and creates the ring around the tub.  This is due to two things, the nature of soap and the calcium and other ions in water.  Calcium is the primary culprit in hardness, but one can also get magnesium and iron in the mixture as well. 

Soap is an organic acid that has been neutralized with lye, so that soap is the sodium salt of an organic acid, usually stearic, the primary component of beef fat.  Stearic acid is a chain of carbon atoms eighteen atoms long, with two oxygens on one end that make it an acid.  When it is an acid there is a hydrogen shared by the two oxygens.  When it is soap, it is a sodium ion that is shared. 

It is important to understand that soap has a split personality, the carbon chain part loves fats, and the acid end loves water when it has been neutralized by sodium.  This is what makes soaps work--the carbon end attaches to fats, and other water-hating things in the environment, and the other end then attracts to water.  What results is an emulsion of particles of water hating material surrounded by water-loving acid groups sticking off the ends of the carbon chains attached to or immersed in the dirt. 

So what does calcium have to do with this?  Well let's backtrack a little bit.  Stearic acid is what is called a weak acid, and lye is a strong base.  When the two combine the result is a salt that is somewhat soluable in water.  Calcium makes a fairly weak base, but more importantly it can combine with two stearic acids instead of just one like sodium.   The combination of calcium and stearic acid is much more stable than the combination of sodium and stearic acid.  Calcium sticks tightly to two stearic acids and makes a molecule that is a center of calcium and acid ends with the two carbon chains sticking out.  The carbon chains on separate molecules are attracted to each other and dislike water.  They combine and form a scum. 

So softening water means one either gets rid of or neutralizes the effects of calcium and similar ions.  It is possible to use additives that will bind calcium stronger than the stearic acid does, but this can become very expensive, very quickly.  The more effective way is to remove the calcium from the water.  This can be done two ways, ion-exchange which is what the salt-using home water softeners do, or reverse osmosis, which is what commercial establishments do, such as hotels.  The thing is, the results are different in terms of behavior of soap, and are quite noticeable.  In fact, taking a shower this morning at the hotel was what prompted this blog post.

Home water softeners have materials called zeolites, which are sort of like clays that have lots of pores in them and the lining of the pores like to hang onto ions.  Fortunately they prefer calcium ions more than sodium ions, but calcium can be removed with high concentrations of sodium.  So what happens is that the home owner fills a big tank with salt, sodium chloride, and the programming on the water softener runs water over the salt to make a saturated solution and then pumps that through the zeolite to make all the ions attached to the zeolite sodium ions.  Then the machine rinses out the excess brine, and the softener is ready to use.  Water from the supply, city water or well water, is run through the zeolite, and all the calcium displaces sodium from the zeolite and is trapped.  The calcium is removed, and the water is now soft. 

In reverse osmosis, water is forced through a membrane that has holes smaller than the calcium ions, and the calcium is filtered out of the water.  Every so often the input side of the system is flushed to remove all the calcium and other materials that didn't make it through the membrane.  The softened water is kept in a large tank on the roof of the building.  This is because reverse osmosis runs constantly, except when flushing, but the demands are intermittent. 

There is a notable difference between the two types of soft water in the way they feel when taking a shower.  Home water softeners make a water that requires the barest amount of bar soap, and never seems to rinse the soap off.  Actually it has rinsed the soap off, but the nature of the water now makes everything feel slippery.  Reverse osmosis water seems to use the regular amount of soap, though actually one could use less, but will rinse off completely.  The skin looses the slipperyness of soap. 

So why does home-softened water leave one feeling slippery?  It has to do with the other stuff that is left in the water after it passes through the zeolite.  Calcium in water is actually in the form of dissolved calcium carbonate, or limestone.  When the water passes through the zeolite, the carbonate remains and we now have an equivalent amount of sodium carbonate dissolved in the water.  Carbonate in water actually exists as a mixture of carbonate, bicarbonate (carbonate with a hydrogen attached), and hydroxide.  The hydroxide, which is the other half of lye along with sodium, is formed by carbonate grabbing a hydrogen from water to become bicarbonate and leaving the hydroxide ion behind.  Carbonate solutions are weakly basic and add to the basicity of the soap.  (Soap is weakly basic because some of the acid ions prefer to grab a hydrogen from water leaving a hydroxide ion just like carbonate does.)  This basic solution removes the free hydrogen ions from the acid groups on the proteins of the skin, as well as the hydroxyl ions tending to bond weakly to the skin.  The result is that the skin has a net negative charge which causes it to slip on itself.  Running more water over it does not help since the water running over it is basic, maintaining the net negative charge.  You can demonstrate the truth of this by running vinegar over your fingers and finding out they lose their slipperyness. 

So why don't homes have reverse osmosis units?  Economics.  RO units are very large and very expensive.  They are only justified by large volumes of water being consumed.  At the same time the home system would be prohibitively expensive for a hotel, just from the cost of the salt needed.  Actually most people use and enjoy water that is slightly hard.  Over long periods of time it can build up scums and deposits, but generally it rinses well, and the slight amounts of calcium make it seem like it rinses the soap off better.  Mildly hard water is the market motivator behind the shower and bathroom cleaners that advertise their ability to remove scum. 

Now won't you enjoy your shower more knowing all this?

Goodnight from Dr. Science.