How to determine CMC of mixed surfactants in a cleansing product?

Abdullah

These are the CMC of SLS & SLES. It means this amount of surfactant will be free in surface between air & water and above that would form micceles. Now if we have both SLES and SLS in formula, will there be 356 mg/L total free surfactant or any other amount? 

If 365 mg/L surfactant is free then when we have several surfactants, doesn't it make it more irritating to have a lot more free surfactants?

If it will have any different amount, how to determine and find that amount?

SLS 185 (mg/L)
SLES 171 (mg/L)

ketchito

@Abdullah CMC is determined using a tensiometer. For a mixture of surfactants, you'd need to make different blends, and use a tensiometer to evaluate the change on interfacial tension, same as what you'd do for a pure surfactant. It'd be wrong to just add the CMC of both surfactants since they'd find the way to form micelles to reduce the increasing interfacial tension.

Abdullah

@ketchito thanks 

My point was aslo just to know if the amount of free surfactant below CMC of each surfactant will interact and make micceles so total free surfactant is not the same as the sum of total free surfactant of each surfactant. 

ketchito

@Abdullah Indeed, total free surfactant below CMC wouldn't necessary be the sum of each free surfactant, measured separately.

Abdullah

ketchito said:

@Abdullah Indeed, total free surfactant below CMC wouldn't necessary be the sum of each free surfactant, measured separately.

That is good to know.
Thanks