Effect of Micellar Lifetime on the Wetting Time of Cotton in Sodium Dodecyl Sulfate Solutions

Textile wetting involves the displacement of air by surfactant solutions in the fiber matrix. The kinetics of surfactant adsorption is more important than the equilibrium properties of surfactant solutions for the textile wetting process because equilibrium conditions are rarely attained during the time allowed for wetting in textile processing.' Fowkes observed that the log of the wetting time decreases linearly with the log of the bulk phase concentration of the surfactant below the critical micelle concentration.2 Electrolytes that decrease the surface tension of surfactant solutions, such as Na2S04, NaC1, and KCl, increase the wetting power of ionic surfactant solution^.^^^ As the temperature of the solution is increased, the chain length for optimum wetting by ionic surfactants generally increases, probably because of the increased solubility of the surfactant a t higher temperatures and its decreased tendency to adsorb to the surface.5 Fowkes also found that the rate of wetting was determined not by the bulk phase concentration of the surfactant but by the rate of diffusion of the surfactant to the wetting front when adsorption of the surfactant onto the surface is strong.2 The surfactants of short alkyl chain length and branched hydrophobic groups are more efficient than the surfactants of long and straight alkyl chains as wetting agents due to their higher diffusion coefficient^.^^^ This is presumably due to the less efficient molecular packing in the micelles of branched chain surfactants which results in relatively unstable micelles in the solution. The surfactants which cannot be effectively packed in the micelles have strong wetting power due to the higher rate of mass transfer of surfactant monomers to the interface. The addition of polyoxyethylenated nonionic surfactant increases the wetting efficiency of anionic surfactant solutions but decreases that of cationic surfactant solutions. This is due to the increase in the mobility of anionic surfactant monomers in solution and decrease in the mobility of cationic surfactant monomers by the addition of nonionic surfactants? resulting in rapid diffusion of anionic and slow diffusion of cationic surfactants. When a piece of a fabric is placed on the surface of a surfactant solution, the solution penetrates into the interfiber spaces of the fabric (Figure 1). These interfiber spaces can be considered as the cylindrical capillaries of radius r and length 1. For the case of thin fabric, where the gravity effect of the liquid inside the capillary can be neglected, the rate of penetration of a liquid into a capillary