We measured the frequency dependence of the longitudinal viscosity and the layer compression modulus of homeotropically aligned multilamellar film of hydrated dimyristoyl phosphatidylcholine (DMPC) at low frequencies from 10 Hz to 1 kHz, varying temperature (T) and relative humidity (RH) systematically. We have, for the first time, demonstrated unambiguously that in the La phase (the lyotropic smectic-A phase) the longitudinal viscosity diverges as 1 /co when the circular frequency goes to zero. In addition, we were able to construct the T-RH phase diagram of the La- Lp. transition of hydrated DMPC, based upon the T and RH dependences of the layer compression modulus. The phase boundary thus determined agreed well with that obtained by X-ray scattering by Smith et al. (J. Chem. Phys. 92 (1990) 4519).
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