Why is old soap useless?

14 January 2019
Presented by Eva Higginbotham.

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When a bar of soap gets used a lot and gets smaller, it seems to struggle to form suds properly. Is something other than just a smaller surface area going on? Eva Higginbotham has been scubbing up to answer this question with the help of Phillip Broadwith, Business Editor at Chemistry World, and Paul Dauenhauer from the University of Minnesota...

In this episode

Hand Washing

00:00 - QotW: Why won't old soap make suds?

When a bar of soap gets used a lot and gets smaller, it seems to struggle to form suds properly. Is something other than just a smaller surface area going on?

QotW: Why won't old soap make suds?

Eva has been scrubbing up to answer this question from William...

Eva - Alan Calverd from the forum said "my guess is that most soaps include volatile and easily soluble components that aid lathering, as the bar shrinks its surface to volume ratio increases. So these components are lost more rapidly". But what do the experts think. Phillip Broadwith, business editor at chemistry world for the Royal Society of Chemistry, gave us this answer.

Phillip - The reason a small soap bar doesn't make such a good lather is mostly down to surface area. To form bubbles you need to dissolve soap in water and then agitate it and with a small bar of soap you can only dissolve the soap molecules at the surface of the bar. With a large bar the water on your wet hands is in contact with quite a large area, so can dissolve quite a lot of soap quite quickly and make a generous lather. But with the smaller bar you surface area in contact with your hand is smaller. So even if you have the same amount of water on your hands the soap is released more slowly.

Eva- Paul Dauenhauer, associate professor of chemical engineering and material science at the University of Minnesota, and soap enthusiast, had this to add.

Paul - A three dimensional object like a bar of soap that shrinks half its original size actually only has one eighth of the original surface area. People are not scrubbing with the bar that much longer and the perception is that they are not getting as much soap transferred to their hands.

Eva - Right. So by that math to use a bar of soap that's gone down to half its original size you'd need to be scrubbing your hands for eight times longer. No wonder we get impatient.

Paul - A second impact is a perception of the bar by the hand washer with time, soap bars are made of heavy molecules such as soap molecules but they also contain light components that enhance the handwashing experience. A person washing their hands is aware of some of the compounds that add color and odor but other components are added to have additional benefits such as stabilizing a foam or eliminating the effects of hard water.

Eva - Hard water, like we suffer in Cambridge, contains metals which bind to the soap molecules and stop them from being able to clean properly. Soap manufacturers add special compounds called chelants which will bind to the metals and protect the soap molecules. But what does this have to do with getting a good lather from a small bar of soap.

Paul - Overtime these lighter compounds will evaporate into the air or preferentially leach into the water faster than the heavier soap compounds. The end result is that the smaller soap bar is less effective later in its life at forming stable lathery foams that are good for washing your hands.

Eva - And Philip has an extra tip for the frustrated hand washes out there.

Phillip - You might be able to get a better lather by carefully adding more water to your hands. But that's quite difficult to do without washing off the existing lather. Alternatively when the bar is small enough try rubbing it over the backs of your wet hands as well as just the palms so it's exposed to more water to dissolve more soap.

Eva - So there you have it, next time we'll be answering this question from Marcus.

Marcus - How can oak trees and others grow so huge without making a great whole in the earth? Where does their mass come from if not from the dirt?Eva - Alan Calvert from the forum said my guess is that most soaps include volatile and easily soluble components that aid lathering, as the bar shrinks its surface to volume ratio increases. So these components are lost more rapidly. But what do the experts think. Phillip Broadwith, business editor at chemistry world for the Royal Society of Chemistry, gave us this answer.

Phillip - The reason a small soap bar doesn't make such a good lather is mostly down to surface area. To form bubbles you need to dissolve soap in water and then agitate it and with a small bar of soap you can only dissolve the soap molecules at the surface of the bar. With a large bar the water on your wet hands is in contact with quite a large area, so can dissolve quite a lot of soap quite quickly and make a generous lather. But with the smaller bar you surface area in contact with your hand is smaller. So even if you have the same amount of water on your hands the soap is released more slowly.

Eva- Paul Dauenhauer, associate professor of chemical engineering and material science at the University of Minnesota, and soap enthusiast, had this to add.

Paul - A three dimensional object like a bar of soap that shrinks half its original size actually only has one eighth of the original surface area. People are not scrubbing with the bar that much longer and the perception is that they are not getting as much soap transferred to their hands.

Eva - Right. So by that math to use a bar of soap that's gone down to half its original size you'd need to be scrubbing your hands for eight times longer. No wonder we get impatient.

Paul - A second impact is a perception of the bar by the hand washer with time, soap bars are made of heavy molecules such as soap molecules but they also contain light components that enhance the handwashing experience. A person washing their hands is aware of some of the compounds that add color and odor but other components are added to have additional benefits such as stabilizing a foam or eliminating the effects of hard water.

Eva - Hard water, like we suffer in Cambridge, contains metals which bind to the soap molecules and stop them from being able to clean properly. Soap manufacturers add special compounds called chelants which will bind to the metals and protect the soap molecules. But what does this have to do with getting a good lather from a small bar of soap.

Paul - Overtime these lighter compounds will evaporate into the air or preferentially leach into the water faster than the heavier soap compounds. The end result is that the smaller soap bar is less effective later in its life at forming stable lathery foams that are good for washing your hands.

Eva - And Philip has an extra tip for the frustrated hand washes out there.

Phillip - You might be able to get a better lather by carefully adding more water to your hands. But that's quite difficult to do without washing off the existing lather. Alternatively when the bar is small enough try rubbing it over the backs of your wet hands as well as just the palms so it's exposed to more water to dissolve more soap.

Eva - So there you have it, next time we'll be answering this question from Marcus.

Marcus - How can oak trees and others grow so huge without making a great whole in the earth? Where does their mass come from if not from the dirt?

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