Surface dilatational properties of mixed sodium dodecyl sulfate/dodecanol solutions

The surface dilatational properties of aqueous solutions of sodium dodecyl sulfate (SDS) and n-dodecanol are investigated in the frequency range 1≤f≤500 Hz using the oscillating bubble method. The results demonstrate that a pure dodecanol solution has an elastic surface without viscous effect whereas the surface of a SDS solution without added dodecanol exhibits a strong viscoelastic behavior. Mixtures show graduated properties. The time behavior of their surface dilatational moduli demonstrates that dodecanol molecules drive the SDS molecules slowly out of the surface. Therefore, the known one-component model describing the surface dilatational modulus can be used also for these mixtures. A simple theoretical consideration explains this effect.