Effect of long chain alcohols on micellar relaxation time and foaming properties of sodium dodecyl sulfate solutions

The effect of long chain alcohols (5 mol% CnOH for n = 8, 10, 12, 14, and 1 6) on the micellar stability of sodium dodecyl sulfate (SDS) solutions (SDS concentration ranging from 25 to 200 mM) was investigated and related to f oaming properties, such as foamability, dynamic and equilibrium surface ten sion, and surface viscosity. The slow micellar relaxation time tau(2), whic h 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 tau(2) and, hence, micella r stability, due to the strong ion-dipole interaction between the SDS and t he alkyl alcohol. However, above approximately 150 mM SDS, all alcohols exc ept C12OH decrease the micellar stability due to mismatching of the alkyl c hains. When the chain length of the alcohol and SDS are not equal, the exce ss hydrocarbon chain exhibits thermal motion, thereby increasing the area p er molecule in micelles as well as at the air/water interface. Foamability was determined by two methods: air blowing through a single capillary subme rged in the surfactant solution or vigorous hand shaking. When enough time is allowed for the interface to form (in case of single bubble foam generat ion), the dynamic surface tension approaches the equilibrium surface tensio n. Since the equilibrium surface tension for the SDS/C12OH mixture is signi ficantly lower (approximately 7 mN/m) than that for the pure SDS solution, more foam is generated with the mixed surfactant system. However, in very h igh shear rate processes (e.g., vigorous hand shaking), the break up time o f 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, tau(2)) than pure SDS micelles, higher dynamic sur face tensions are attained and thus less foam is generated with the surfact ant/alcohol mixture by the shaking method. I:n conclusion, we show that the foamability can exhibit opposite behavior depending upon the rate of foam generation (i.e., specific method used for foaming). (C) 1998 Academic Pres s.