Importance of micellar relaxation time on detergent properties

As we enter the new millennium, manufacturers of laundry detergents would l ike to provide new products for the twenty-first century. With the goal of achieving new and better performance characteristics, design strategies for research and development should be defined. This paper highlights the impo rtance of micellar relaxation kinetics in processes involved in detergency. Earlier Shah and coworkers showed that the stability of sodium dodecyl sul fate (SDS) mice[les plays an important role in various technological proces ses. The slow relaxation time (tau(2)) Of SDS micelles, as measured by the pressure-jump technique, was in the range of 10(-4) to 10(1) s, depending o n the surfactant concentration. A maximal relaxation time and thus a maxima l micellar stability were found at 200 mM SDS (5 s), corresponding to the l east-foaming, largest bubble size, longest wetting time of textile, largest emulsion droplet size, and the most rapid solubilization of oil. These res ults are explained in terms of the flux of surfactant monomers from the bul k to the interface, which determines the dynamic surface tension. More stab le micelles lead to less monomer flux and hence to a higher dynamic surface tension. The relaxation time for nonionic surfactants las measured by the stopped-flow technique) was much longer than for ionic surfactants because of the absence of ionic repulsion between the head groups. The tau(2) was r elated to dynamic surface-tension experiments. Stability of SDS micelles ca n be greatly enhanced by the addition of long-chain alcohols or cationic su rfactants. In summary, relaxation time data of surfactant solutions enable us to predict the performance of a given surfactant solution. Moreover, res ults suggest that one can design appropriate mice[les with specific stabili ty, or tau(2), by controlling surfactant structure, concentration, and phys icochemical conditions, as well as by mixing anionic/cationic or ionic/noni onic surfactants for a desired technological application, e.g., detergency.