The cleansing action of soap

My textbook says that:



> 
> The formation of micelles takes place only above a particular
> temperature called **Kraft temperature (Tk )** and above a particular
> concentration called **critical micelle concentration (CMC)**. On
> dilution, these colloids revert back to individual ions. Surface active
> agents such as soaps and synthetic detergents belong to this class.
> **For soaps, the CMC is $10^{–4}$ to $\pu{10^{–3} mol L^–1}$**.
> 
> 
> 


Would someone please tell me:


* How this apply practically to real life situations when we use
soaps or detergents to wash laundry?
* Is the Kraft temperature for soaps so less that it is attained by
friction and heat generated while scrubbing the clothes during
washing?
* And since the critical micelle ***concentration*** seems to be low,
does lathering of soap two or three times satisfy the requirements
for attaining micelle formation?




First, let's get the spelling correct: it's the Krafft temperature.


Second, let's imagine that there are two kinds of solubility curves of surfactants in water: one type of behavior is that solubility increases as temperature rises (this type of surfactant has a Krafft temperature), but there is another type of surfactant whose solubility *decreases* as the temperature rises, especially at certain temperatures.


The second type of surfactant is generally composed of a hydrophobic portion like an alkylated phenol connected to a long chain of ethylene oxide units. Such surfactants have *cloud points*: temperatures above which they become more insoluble in water, leading to formation of larger and larger micelles, and eventually a liquid layer. Micelles are tiny agglomerations of surfactant molecules, not individual molecules in solution. These surfactants are useful in dishwashing machines, where hot water is beneficial to removing fats by melting them, but foaming is bad because it clogs the spraying action, so the insolubilization of the surfactant provides a liquid layer with defoaming action in addition to providing micelles to emulsify the fats.


My experience has been mostly with this second type of surfactant, but it suggests that the opposite type of behavior might also occur.


And when it does, a given *solution* of surfactant might be cooled enough to become cloudy. A warmer solution can dissolve more of this kind of surfactant, so the temperature at which the surfactant precipitates out will vary.


Now there is an important concentration which differs for each surfactant: the *critical micelle concentration* (CMC). Each surfactant equilibrates between solubility (complete dispersion on a molecular level) and micelle formation (aggregation into micelles, which are composed of several/many molecules, forming tiny aggregates which disperse well in water, and do not totally agglomerate into one phase).


The Krafft temperature is the temperature at which the solubility of the surfactant does not exceed the CMC, so lowering the temperature of this solution will not enable micelles to form. Only above the Krafft temperature will there be enough surfactant *in solution* so that if the temperature is reduced, surfactant will come out of solution into micelles.


The three questions are good ones:


Laundering with cold water suggests that enough surfactant should be added to generate micelles, but the manufacturer should be sure to use a surfactant that has enough solubility to be an effective cleanser.


Friction and heat would raise the temperature, so would minimize micelle formation. Whether this would reduce washing ability is debatable, because emulsification can occur even from soluble molecules.


Lathering - generating foam or bubbles - suggests that air bubbles are coated with a continuous film, not really individual molecules and not micelles. Foam is helpful in removing dirt, but it is not clear whether this friction/rubbing is dependent on micellization more than just plain solubility. If you have a concentrated solution, even without micelles, friction and rubbing could solubilize a lot of fats and oils.


Also see <https://en.wikipedia.org/wiki/Krafft_temperature>.


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