  It is known that the presence of cations like Ca++ or Pb++ in the water
subphase alters the pressure-area isotherms for fatty acid monolayers. The
corresponding lattice constant changes have been studied using x-ray
diffraction. Reflection-absorption spectroscopy has been used to probe the
chemical composition of the film. We report on the first measurements of the
time evolution of the shear viscosity of arachidic acid monolayers in the
presence of Ca++ ions in the subphase. We find that the introduction of Ca++
ions to the water subphase results in an increase of the film's viscosity by at
least three orders of magnitude. This increase occurs in three distinct stages.
First, there is a rapid change in the viscosity of up to one order of
magnitude. This is followed by two periods, with very different time constants,
of a relatively slow increase in the viscosity over the next 10 or more hours.
The corresponding time constants for this rise decrease as either the subphase
pH or Ca++ concentration is increased.
