SHORT COMMUNICATION Colloid Polym Sci 274:1183-1187 (1996) 9 Steinkopff Verlag 1996 J. Kr/igel J.B. Li R. Miller M. Bree G. Kretzschmar H. M6hwald Surface viscoelasticity of phospholipid monolayers at the air/water interface Received: 29 April 1996 Accepted: 19 June 1996 J. Kr/igel Universit~it Potsdam Institut fiir Festk6rperphysik 14415 Potsdam, FRG J.B. Li. Dr. R. Miller (12N)" M. Bree G. Kretzschmar 9 H. M6hwald MPI ffir Kolloid- und Grenzfliichenforschung Rudower Chaussee 5 12489 Berlin, FRG J.B. Li Institute of Photographic Chemistry Chinese Academy of Sciences 100101 Beijing, China Abstraet The surface viscoelastic properties of monolayers of two phospholipids DPPC (L-cc-dipalmi- toylphosphatidylcholine) and DMPE (L-c~-dimyristoylphosphatidyl- ethanolamin), at the air-water inter- face have been investigated. Two techniques were used for the inves- tigation. One involved use of an interracial shear rheometer (torsion pendulum apparatus ISR1), to provide measurements of the shear viscosity t/s as a function of surface pressure, and the second, a modified LB trough with an oscillating barrier to generate periodic dilation and compression so as to measure the dilational elastic modulus E as a function of surface area. Results indicate a strong depen- dence of t/s and E upon monolayer phases. This suggests that the viscoelastic relaxation of monolayers can be understood as molecular rearrangements, domain exchange and molecular reorientations between different monolayer states. Key words Phospholipid - Langmuir monolayers - surface rheology - surface dilational elasticity - surface shear viscosity Introduction Phospholipid monolayers are well recognised as model systems of biomembranes anchored perfectly at fluid inter- faces [1-5], and in technology as precursors of multilayer structures (LB films) I-6]. Hence much attention has been paid to the measurements of the physico-chemical proper- ties of the various phases of these monolayers and also their surface mechanical properties. These mechanical properties can be characterised by surface shear and dila- tional viscoelasticity and provide direct information on the stability of these films [7, 8]. Unfortunately, mechanical properties are rather difficult to measure as most mechan- ical devices used for measurement of these properties are based on the excitation of small amplitude surface waves with typical frequencies of about 100 Hz and cause a seri- ous disturbance of the monolayer state: The technique of Surface Light Scattering is established as an ideal probe for studying surface viscoelasticity without direct mechan- ical contact. However the difficulty of the technique itself and the strict requirements on the film preparation and manipulation still bring about some restrictions on experiments. In the present paper results of the surface mechanical properties of phospholipid monolayers at the air/water interface are given. Recently developed apparatuses are used in this study: the surface shear rheometer ISR1 and a modified Langmuir-Blodgett trough equipped with an oscillating barrier [9, 10]. Both instruments have an op- erational frequency of less than 0.1 Hz. The former instru- ment allows the determination of the shear viscosity, r/s, and the latter determines the dilational elasticity, E, of the monolayers [11] both parameters measured at varying surface pressures. ?