The rate of capillary wetting in fibrous assemblies was measured by a newly developed device. The change in specific electric conductivity, λ/λc, with time, t, was measured using a plug of cellulose filter paper in a cylindrical Teflon cell. The yield time, t0, and initial slope, S, in λ/λc versus t curves were used as parameters of the wetting rate. The aqueous ionic surfactant solutions were used as wetting liquids. The effect of the alkyl chain length of alkyltrimethyl ammoniumchloride (ATAC) on the wetting rate was investigated. The effect of the electrolyte (KCl) was also examined for sodium dodecyl sulfate (SDS). The wetting rate was found to decrease with increasing ATAC concentration, and it decreased with increasing alkyl chain length for any given concentration of ATAC. In the case of SDS, the rate increased with increasing SDS concentration below the critical micelle concentration (cmc). Above the cmc, the wetting rate decreased with increasing concentration of SDS. The resultant maximum rate was found in the region of low SDS concentration by the addition of KCl. The results obtained below the cmc were discussed in terms of the Washburn equation; the wetting rate was affected by change in the surface free energies of liquids and in the advancing dynamic contact angle by the force balance at the interface due to adsorption of surfactants.
