The interfacial tension r of the test solutions (3 wt% WPI) was
measured with a K12 tensiometer (Krüss GmbH, Germany) using
the Wilhelmy’s plate method at 20 C during 3 h. With a constant
WPI concentration, the interfacial tension is 43 ± 1 mN m1 at long
time (equilibrium value) for all the solutions. Neither xanthan gum
nor sodium chloride changes the interfacial tension. The composition
and properties of the solutions are gathered in Table 2