are negatively charged at a pH higher than 4 [3,4], and dyeing
occurs by the transfer of dye from the external to the internal
solution. Upon addition of sodium chloride (as an electrolyte)
to the dye solution, the sodium ions (NaC) distribute between
the external solution and internal solution so that the negative
charge on the cellulose surface is neutralized or shielded, allowing
the lac dye molecules to be adsorbed on the cotton
[3,23,26]. However, sodium chloride had no effect on the adsorption
of lac dye on cotton at pH 2.5, 3.0 and 3.5 (Fig. 4)
since the carboxylic acid and hydroxyl groups would not be
ionized at these pH values. This indicated that the hydrogen
ion (HC) plays a more important role in the adsorption process
than sodium ion (NaC); the adsorption isotherm of lac dye on
cotton in the presence of 0.5M sodium chloride is reported in
the next section.