Surface dilational behaviour of spread dipalmitoyl phosphatidyl glycerol monolayers

The stress relaxation process in spread dipalmitoyl phosphatidyl glycerol (DPPG) monolayers was investigated by using the pendant drop technique as a microfilm balance in combination with the axisymmetric drop shape analysis. The stress relaxation caused by a transient drop volume change was analysed by a new model, which takes into account material inhomogeneity by means of a distribution of relaxation γ. The surface pressure decay in the DPPG monolayers at 35 and 40 °C is governed by 2 main relaxation γ. One is of the order of 20 s, the other one is in the range of 200-600 s. The pi/A isotherms, the surface dilational modulus, and the higher relaxation γ are influenced by temperature and by the presence of electrolyte in the subphase; 0.15 mol dm-3 NaCl yields a smaller area demand per molecule in the monolayer. Harmonic oscillation experiments were carried out and the dilational elasticities and viscosities were determined for different frequencies in the range between the two main relaxation γ. Both, elasticities and viscosities, depend on frequency. The surface rheological behaviour is structural viscous, i.e. high viscosities at small frequencies, which decrease at faster deformations. The results are related to the surface dilational rheological behaviour of DPPC monolayers