Effect of Long Chain Alcohols on Micellar Relaxation Time and Foaming Properties of Sodium Dodecyl Sulfate Solutions

Abstract

The effect of long chain alcohols (5 mol% CnOH forn= 8, 10, 12, 14, and 16) on the micellar stability of sodium dodecyl sulfate (SDS) solutions (SDS concentration ranging from 25 to 200 mM) was investigated and related to foaming properties, such as foamability, dynamic and equilibrium surface tension, and surface viscosity. The slow micellar relaxation time τ2, which is directly related to micellar stability, was determined by the pressure-jump technique. It was found that below 150 mM all the long chain alcohols investigated in this study cause an increase in τ2and, hence, micellar stability, due to the strong ion–dipole interaction between the SDS and the alkyl alcohol. However, above approximately 150 mM SDS, all alcohols except C12OH decrease the micellar stability due to mismatching of the alkyl chains. When the chain length of the alcohol and SDS are not equal, the excess hydrocarbon chain exhibits thermal motion, thereby increasing the area per molecule in micelles as well as at the air/water interface. Foamability was determined by two methods: air blowing through a single capillary submerged in the surfactant solution or vigorous hand shaking. When enough time is allowed for the interface to form (in case of single bubble foam generation), the dynamic surface tension approaches the equilibrium surface tension. Since the equilibrium surface tension for the SDS/C12OH mixture is significantly lower (approximately 7 mN/m) than that for the pure SDS solution, more foam is generated with the mixed surfactant system. However, in very high shear rate processes (e.g., vigorous hand shaking), the break up time of micelles determines the flux of surfactant molecules to the interface and hence the foamability. Since the mixed SDS/C12OH micelles are more stable (longer relaxation time, τ2) than pure SDS micelles, higher dynamic surface tensions are attained and thus less foam is generated with the surfactant/alcohol mixture by the shaking method. In conclusion, we show that the foamability can exhibit opposite behavior depending upon the rate of foam generation (i.e., specific method used for foaming).