The molar concentrations of two individual surfactants required to yicld a surface tension value
of 36 dynes/cm in aqueous solution are 2.6 x 10-3 and 1.15 x 10-), respectively. The total molar
surfactant concentration required to yield a surface tension of 36 dynes/cm is 6.2 x 10 for a
mixture of the two surfactants in which the mole fraction of the first surfactant (on a surfactant-
only basis) is 0.41. Calculate the value of Xi, the mole fraction of surfactant 1 in the total
surfactant at the aqucous solution air interface for this mixturc.
Solution
The stronger the intermolecular forces of a liquid, the stronger its surface tension.
Normally, the concentration of whatever substances H2O contains has nothing to do with its
surface tension. An exception is the concentration of surfactants. The most commonly known
surfactant is soap, i.e. sodium stearate. The more sodium stearate molecules present in a certain
amount of H2O, the lower the surface tension.
For other liquids such as some organic solvents, there are no “oil surfactants―, and
whatever substance the solvent holds probably won’t affect the strength of intermolecular
forces between solvent particles, so the is no relation between surface tension of a glass of oil
and the concentration of a certain substance in it