Effect of tannic acid on the surface free energy of cotton fabric dyed with a cationic dye

The effect of tannic acid and/or Rhodamine B on the surface free energy components of bleached cotton was studied. The zeta potential of cotton fabric was also measured in 10(-6)-10(-2) M tannic acid solutions and in the presence of 10(-3) M NaCl. It was found that the zeta potential of cotton is negative and that its absolute value decreases with increasing concentration of tannic acid. In the presence of NaCl the potential is more negative than in the tannic acid solutions alone. One may conclude that both H+ and Cl- are potential-determining ions for the cotton surface. The surface is practically monopolar Lewis basic with an electron donor, gamma s(-), interaction of 50.1 mJ/m(2), while its electron acceptor interaction, gamma s(+), equals only 0.92 mJ/m(2). This polar character of the surface results from the presence of carboxyl end-groups. The apolar Lifshitz-van der Waals interaction, gamma(s)(LW), amounts 45.0 mJ/m(2). The cotton surface with adsorbed tannic acid from 10(-3) or 10(-2) M solutions, or Rhodamine B from 10(-3) M solutions, as well the surface dyed with Rhodamine after adsorption of tannic acid, shows different values of the surface free energy components. The changes are within ca. 14 mJ/m(2) for apolar y(s)(LW) component and within ca. 13 mJ/m(2) for polar gamma(s) component, It is believed they result from the presence of the above mentioned polar end-groups of bleached cotton, phenolic hydroxy groups in the tannic acid molecule, and two amine groups and one carboxyl group in the Rhodamine B molecule. The adsorption processes occurring in the systems are due to both electrostatic interaction and hydrogen bonding between the polar groups. It appeared that the thin-layer wicking technique adapted to the fabric materials is very useful for the surface free energy study, especially because the contact angle method cannot be applied for such materials. (C) 1998 Academic Press.