1. Introduction
Cotton is the most important natural textile fiber. Its structure consists of cellulose, a polymer constructed of monomeric glucose units. Since cellulose is susceptible to hydrolysis when heated in acidic medium, dyeing of cotton must be achieved at pH values above 4 (Broadbent, 2001). The pendant hydroxymethyl groups in the cellulose chains are partially oxidized to carboxyl groups during plant growth, and subsequent processing. These carboxyl groups confer a negative charge to the surface of cotton uponimmersion during the dyeing process. At higher pH values (>8),side chain hydroxyl groups can be ionized, thus increasing the negative surface charge significantly (Rattee, 1995). Therefore, cotton generally shows poor affinity toward anionic dyes due to charge repulsion, leading to low uptake of such dyes (Janhom et al., 2006;