ADSORPTION BEHAVIOUR OF 5-METHYLRESORCINOL AND ITS MIXTURES WITH LYSOZYME AT AIR-WATER INTERFACE - Biochemistry and Biotechnology: Research and Development - page 98

molecules due to the intermolecular forces which is measured by the surface tension. In order
to avoid the unfavorable energetic interaction of its nonpolar part with water, a surfactant
molecule prefers to partition to the air-water interface, resulting in the reduction of the
surface tension. A dynamic equilibrium always exists among the surfactant monomers at the
interface with the species in the bulk solution below CMC. Beyond CMC, surfactant
monomers at the interface equilibrate with micelles in the bulk medium [Dash, Misra, 2011].
In figure 1. dynamic curves of MR adsorption at air/water interface from 0.05M PBS (pH 7.4)
solutions of varying concentration (0.16-128 mM) are presented. The data relates to time of
storage of MR solutions 2 and 24 hours at 25 O C in darkness (Figure 1a and Figure 1b,
respectively). One can see that MR in aqueous media exhibits behavior similar to that of
surfactants which reduce the surface tension of water at the interface with air.

a b

Figure 1. Dynamic surface tension of MR concentrations 0.16-128 M after 2 h (a) and 24 h (b)
incubation and storage t 25 O C in darkness.

The surface activity of MR is a function of concentration and previous storage time of
solutions. The form of dynamic curves changes with increasing storage time of solutions from
poorly reproducible multistep (Figure 1a) to more simple, well reproducible (Figure 1b).
Possibly this change manifestoes the change of the MR structure in water solution.The
existence of lag- or induction period which is typical for all dynamic curves of MR adsorption
means that adsorption process is diffusion-limited [Fainerman et al., 1994].
The value of induction period drops with increasing of MR concentration up to 0.0165 М,
and then does not change (Figure 2). Early on the base of ITC data [Martirosova, 2011] we
proposed that this point corresponds to transition of MR from molecular dispersed state to
associative one. The surface tension isotherm of MR adsorption obtained at 2 and 24 hours of
time storage is shown in Figure 3. In agreement with expectation isotherm has a form typical
for surfactant.