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While washing clothes, we are used to thinking that more soapy water would clean better. Adding more water feels wrong because it reduces the 'soapiness'. Does adding more water actually decrease the cleansing action, or is it just an illusion?

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    $\begingroup$ My newish front-loader uses much less water (but the same amount of detergent) than the old one, yet cleans clothes just fine. $\endgroup$
    – Jon Custer
    Commented Dec 2, 2022 at 15:58
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    $\begingroup$ I’m voting to close this question because it more properly belongs on Chemistry. $\endgroup$ Commented Dec 2, 2022 at 21:04

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The washing power (or liquid) works due to the surfactant it contains. The surfactant has to do two things:

  1. detach the soil¹ from the fabric

  2. suspend the soil in solution so it doesn't redeposit on the fabric

The ability of the surfactant to do this is roughly related to its chemical potential. For very low concentrations the chemical potential is proportional to the concentration, but surfactants are unusual in undergoing a phase change to a micellar form at quite a low concentration called the critical micelle concentration (CMC). Above this concentration the chemical potential plateaus, and as concentration increases the chemical potential increase much more slowly.

So to achieve the best wash you need to keep the surfactant concentration at or above the CMC. In practice you want it to be above the CMC as in washing machines the kinetics of surfactant adsorption and soil removal are important and increasing the concentration does speed up the rates even above the CMC.

Adding extra water doesn't make that much difference during the wash phase as once the soil is solubilised it's off the fabric and will be removed in the rinse stage of the wash. There is a limit, but as long as there is enough water to get the soil off the fabric then adding extra water won't make much difference. In fact extra water might dilute the surfactant concentration to below the CMC and harm the cleaning performance.

Several decades ago I worked as a colloid scientists for the multinational company Unilever, which makes washing powder (and ice-cream). I didn't personally work on washing powders though I did study some related background science. The dosing instructions on your washing powder have been carefully selected to give an optimal tradeoff between performance and cost. Unless you are in a position to measure the surfactant concentration yourself it's best to just follow the dosing instructions.


¹ for washing powder scientists (yes, such people exist!) the term soil means anything that dirties clothes i.e. grease as well as dust or clay.

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More water means less cleaning, unless you add an equivalent/proportional amount of soap/detergent. The one that will do the most cleansing is the one with the best ratio of water to soap/detergent.

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    $\begingroup$ There does not seem to be any physics in this answer. [Maybe there is physics underlying the assertion, but the bold assertion without any explanation doesn't count.] $\endgroup$
    – WillO
    Commented Dec 2, 2022 at 16:43
  • $\begingroup$ @WillO Yes there is. It talks about a proportional chemical mixture, which is related to physics $\endgroup$ Commented Dec 2, 2022 at 16:47
  • $\begingroup$ @WillO maybe the question this answers isn't a physics question either. $\endgroup$ Commented Dec 2, 2022 at 16:48

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