pHThe measurement of pH in the micellar environment has not been technically feasible,
but the idea that a variation from the pH of the initial buffer solution occurs is widely
spread. This accounts for the shift registered in the pH profile (1–2 units) of
microencapsulated enzymes, in comparison with aqueous solutions (Menger and
Yamada, 1979). Regardless of the pH measurement, the ionogenic groups of the enzyme
will be affected by the microenvironment and their ionisation state severely influences
the interaction with substrate(s) and the inhibition by products (Petersen et al., 1998). In
the case of AOT reversed micelles, the acidic impurities are, at least partially, responsible
for the alkaline shift in the pH profile (Luisi and Magid, 1986).
Several methods for monitoring the pH of micellar solutions have been proposed.
Luisi and Magid (1986) and Khmel’nitskii et al. (1984) suggested the use of
(hydrophilic) indicators to monitor the pH.^31 P NMR was also used with the same
objective, using the^31 P nucleus presence in the phosphate ion. Karpe and Ruckenstein
(1990) made calculations of the pH variation with W 0 in the first hydration shell of the
reversed micelle (pHRM) and in the center of the micelle (pH 0 ). In the two models
presented (for phase transfer method and for injection method) the variations were more
significant, up to W 0 15–20, being the pH 0 comparable to the pH of the stock solution and
always higher than the pHRM. This acidification may affect enzymes, which interact with
the micellar interface.
The pH value for optimum activity greatly depends on the exposed residues of protein
especially those close to the active site. The pH may also affect the microencapsulation of
protein and the pH values below the isoelectric point and low ionic strength usually
improve the uptake of protein.
Buffer molarityWhen solubilising biopolymers, buffer solutions are often used to constitute the aqueous
phase forming the water pools. The presence of electrolytes in the water pools will after
the maximum Wo value, usually decreasing it (Luisi and Magid, 1986). This may be
attributed to the increase of repulsion charges among the surfactant head groups. Another
explanation for the decrease in droplet size and interdroplet attractive interaction with an
increase in salinity was given by Hou et al. (1988). An increase in salinity decreases the
interfacial area per mole head of surfactant molecule and makes the interface more rigid
and less penetrable. This causes the decrease in strength of attractive interactions
favouring a greater curvature of the interface (with consequent expulsion of water).
García-Río et al. (1994) noticed a reduction in the overall micellar viscosity caused by
salts and also attributed this fact to a decrease in the attractive interactions among
droplets by salt addition. All the electrolytes they used increased the temperature
percolation threshold.
The effect of added salts may be understood within the more general framework of the
effect of salts on the properties of surfactants in solution. The increase of salt
concentration lessens the size of the polar effective area of the surfactants thereby
increasing the curvature parameter of the surfactant. Hence, this explains the lower
capacity of AOT to incorporate water molecules, since water solubilisation implies an
increase of the micelles shortening the negative curvature of the interface. Another effect
Reversed micellar bioreaction systems 199