3.1. Adsorption of naphthalene in the dark
According to the Langmuir model, the coverage θ at equilibrium
varies as
θ = Qads
Qmax
= KLCeq
(1 + KLCeq)
(1)
where Qads is the number of adsorbed naphthalene molecules,
Qmax the maximum number of molecules that can be adsorbed
on the surface, Ceq the concentration at adsorption equilibrium
and KL the Langmuir adsorption constant.
The linear plot in Fig. 1 can be identified with:
1
Qads
= 1
Qmax
+ 1
(QmaxKLCeq)
(2)
The intercept at the origin gives Qmax and the slope gives
KL via the product KL Qmax. Table 2 gives the adsorption
constants measured. We found a value of 4100 L mol−1 for
KL and 24mol g−1 for Qmax. Coverage ranges from 0.06
to 0.38 when naphthalene concentration ranges from 15.9 to
175mol L−1. At concentrations below 40 mol L−1, adsorption
equilibrium can be approximated by a linear law, where
Qads =KL ×Qmax ×Ceq.
Another value of the adsorption constant KL was given by
Barrios et al. [18] (KL = 1152 L mol−1) in phosphate buffer
and Triton X-100 surfactant. This value is lower because of
the competitive adsorption of phosphate ions and surfactant
molecules.