Effect of subphase Ca++ ions on the viscoelastic properties of Langmuir monolayers

It is known that the presence of cations like Ca++ or Pb++ in the water sub phase alters the pressure-area isotherms for fatty acid monolayers. The cor responding lattice constant changes have been studied using x-ray diffracti on. Reflection-absorption spectroscopy has been used to probe the chemical composition of the film. We report on the first measurements of the time ev olution 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 t o the water subphase results in an increase of the film's viscosity by at l east three orders of magnitude. This increase occurs in three distinct stag es. First, there is a rapid change in the viscosity of up to one order of m agnitude. This is followed by two periods, with very different time constan ts, 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 t he subphase pH or Ca++ concentration is increased. (C) 1999 American Instit ute of Physics. [S0021-9606(99)70432-0].