DYNAMIC PROPERTIES OF A PHOSPHOLIPID THIN-FILM AT AN AIR WATER INTERFACE WITH A PERIODIC CHANGE IN SURFACE-AREA/

Dynamic surface pressure (pi)-surface area (A) characteristics were me asured using periodic changes in the surface area of an unsaturated ph ospholipid (1,2-dioleoyl-3-sn-phosphatidylcholine; DOPC) thin film for med at an air/water interface. The dynamic pi-A curve traced a single closed hysteresis loop after several cycles of compression and expansi on, and the shape of the loop depended on the frequency of the change in surface area, the subphase temperature, and the chemical species di ssolved in the aqueous subphase. To evaluate the loop shape in a quant itative manner, we carried out Fourier transformation of the time-depe ndent change in surface pressure. On the basis of the higher harmonic components in the Fourier spectra, it is clear that a DOPC thin film b ehaves as a nonlinear viscoelastic body during compression and expansi on. We established a useful kinetic model by taking into account the c ooperative aggregation/dissociation of DOPC domains at the air/water i nterface with a periodic change in surface area. Hysteresis loops were examined with the aid of a computer simulation which included the coo perative factor.