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I am looking to measure the effect of dishwashing soap on the surface tension of water. To select appropriate values for the concentration of dishwashing soap is to find the saturation point first, but I can't find any values for it and don't know how to measure it. How can I select appropriate values for the concentration and if finding the saturation point is a way of doing that, how do I find the saturation point of dishwashing soap in water?

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    $\begingroup$ Dishwashing soap is a generic term. Different brands contain different mixtures of multiple surfactants and other additives in various concentrations and ratios. You would have to restrict the question to a single compound or a products, as the value is generally different for each. // You may want to search for surface tension measurement, e.g. Tensiometer $\endgroup$
    – Poutnik
    Commented Feb 15, 2022 at 9:20

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Critical Micelle Concnetration (CMC) is a commonly used parameter to measure when a solution is saturated with surfactant (soap). CMC is the concentration where the surfactant starts to form small droplets (micelles) instead of spreading out onto the water/air surface.

One way to determine CMC is by measuring the surface tension. When the surfactant concentration is (very) low, the surface tension more or less linearly decreases with increasing concentration, but at some point the surface tension reaches a constant level, as Poutnik mentions. This concentration is called the CMC. Kruss tensiometers: CMC and surfactant concentration

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I want to point out that saturation usually refers to a situation where chemical precipitates out of solution, but it seems you are interested in soap's effect. That's very different thing.

Soap's effect is very dependent on a situation - amount of evaporation and even type of soap. But in general even few parts per million can greatly change parameters. And then even a few percents won't change much further. While actual saturation is even further, at equal parts or so.

Something like this:

1 part per million - bubbles are normal size

100 parts per million - bigger bubbles

10 000 parts per million - bubbles are large

100 000 parts per million - bubbles are as large, but you can keep dissolving soap as much as you want

Why is it so? Unlike most materials like salt and conductivity, soap's effect doesn't rely on how much soap is in the bulk of water. Soap acts only on the water's surface. As you make a bubble, soap accumulates on bubbles' surface. And because molecules are small, you have a lot of volume against small amount of surface area. So, even few parts per million in water's volume can mean almost complete fill of soap bubble's surface. And if soap bubble's surface is filled with soap molecules, nothing will happen if you keep adding soap in water's volume.

What it can affect is how much evaporation is needed for soap's film to form fully, or how thin can you extend a soap film.

If you want to make bubbles that are more than 10cm, you will need to use more tricks than just soap. Water itself evaporates too quickly at that point. You can fix it by adding glycerine, it evaporates slower. Adding just more soap won't help make bubbles bigger after this point.

Anyway, soap is very different than all other chemicals, because it acts on a surface, rather than in volume. And it is better to ask more specific question, than trying to use a term that doesn't help much with it.

If you meant what you asked, you can mix about equal amounts of liquid soap and water. But for most uses it is too much.

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    $\begingroup$ I suppose the OP may mean saturation in mathematical meaning, where value of measured quantity reaches a plateau on the curve of its dependence on other quantity. Like surface tension not decreasing with another soap addition any more. $\endgroup$
    – Poutnik
    Commented Feb 15, 2022 at 9:41

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