Shear response of Langmuir monolayers of heneicosanoic (C-21) acid studied using a torsion pendulum

We have studied the shear relaxation of heneicosanoic (C-21) acid monolayers on the surface of water at different temperatures and pressures, in the CS, L(2) '', L(2)', and S phases. A torsion pendulum was displaced from its equilibrium position by an angle theta(0) and the relaxation theta(t) was measured. Only the CS phase sustains a static shear (similar to 160 dyn/cm) at the time scales studied (approximate to 10(3) s). The data fit well to a stretched exponential form theta(t) = theta(0) + {theta(0) - theta(infinity)}exp{-(t/tau)(n)}. We find that k tau(n)/n is independent of the torsion constant k; this implies that the viscosity is time dependent. k tau(n)/n is a strong function of pressure and temperature. At phase boundaries there is a discontinuity in either k tau(n)/n or its slope.