The effect of sodium chloride, and pretreatment of cotton
with chitosan, on the adsorption of lac dye was also studied.
An increase in the sodium chloride concentration over the
range 4.31!103e6.84!101 M led to an increase in the
dye adsorbed onto the cotton and this remained constant in
the concentration range 10.3!101e17.1!101 M when
cotton fibres were dyed with lac dye solution without pH
control. It is believed that the sodium ion (NaC) can 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 onto
the cotton [3,16,17]. The adsorption isotherm for lac dye on
cotton in the presence of 0.5 M NaCl agreed well with both
the Langmuir and Freundlich isotherms with an enthalpy
(DHo) of 13.09 kJ. However, sodium chloride had no effect
on the adsorption of lac dye on cotton at a pH 2.5, 3.0 and
3.5, indicating that hydrogen ions (HC) play a more important
role than sodium ions (NaC). The pretreatment of cotton with
a 0.3% aqueous (v/v) solution of chitosan showed a significant
enhancement of dye uptake onto cotton and also a decrease in
the dye desorbed from cotton compared with the results in the
absence of chitosan or on lac dyeing in the presence of NaCl.The experimental data for lac dyeing on cotton pretreated with
chitosan fitted very well to the Langmuir isotherm with an
enthalpy change (DHo) of 17.43 kJ. Pretreatment of cotton
with chitosan is thus an alternative way to create cationic sites
on cotton and to subsequently increase the affinity between lac
dye and the fibre. The results from this study should help to
underpin future improved practical applications of lac dyeing
of cotton.