Interfacial relaxation of phospholipid layers at a liquid–liquid interface R. Miller a,b, *, J.B. Li a,b , M. Bree a , G. Loglio c , A.W. Neumann d , H. Mo¨hwald a a Max Planck Institute of Colloids and Interfaces, Rudower Chaussee 5, D-12489, Berlin-Adlershof, Germany b International Joint Lab between Institute of Photographic Chemistry and Max Planck Institute of Colloids and Interfaces, Chinese Academy of Sciences, De Wai, Bei Sha Tan 100101, Beijing, People’s Republic of China c University of Florence, Institute of Organic Chemistry, Via G. Capponi 9, 50121 Florence, Italy d Department of Mechanical Engineering, University of Toronto, 5 King’s College Road, Toronto, M5S 1A4, Canada Abstract The phospholipid, l-a-dipalmitoylphosphatidylcholine (DPPC), forms a monomolecular film onto a pendent drop by adsorption at the water–chloroform interface. Transient relaxations of the adsorption layer after an area change of the drop surface induced by a definite drop volume increase are studied by axisymmetric drop shape analysis (ADSA). The dynamic interfacial tension response to small-amplitude disturbances of the adsorption equilibrium are recorded and interpreted in terms of a diffusion controlled relaxation theory. The experi- ments provide information on the interfacial dilational elasticity of the phospholipid layer at the water–chloroform interface.  1998 Elsevier Science S.A. All rights reserved Keywords: Interfacial relaxation; Diffusional exchange of matter; Phospholipid adsorption layers; Liquid–liquid interface; Axisymmetric drop shape analysis (ADSA) 1. Introduction Interfacial rheology is regarded as the mechanical and flow properties of interfacial layers [1]. The study on sur- face rheology is to build up a relationship between the strain on a surface element, which involves the surface deforma- tion, and the forces acting in the surface. In practice, it reflects the surface tension changes due to changes of the surface area during surface compression or expansion. There are two types of surface deformation – dilational and shear deformations. Under dilational deformation the surface area increases or decreases while the shape of the liquid interface remains the same. Under shear deformation the interfacial area is constant while its shape is changing [2]. In the processes of foam or emulsion formation, the interfacial area between the two immiscible fluids is chan- ged dramatically [1] so that dilational deformations become extremely important. At present, many studies on surface dilational rheology of low-molecular-weight surfactants at the air–water interface are published while investigations of adsorption layers at oil–water interfaces are very rare. This situation is due to experimental difficulties in performing compressions and expansions of adsorption layers at liquid–liquid. On the other hand, phospholipids are being widely employed in chemical engineering and food products as emulsion stabi- lizers [3]. Thus, there is need of knowledge of dynamic and mechanical properties of lipid adsorption layers at liquid– liquid interfaces. The axisymmetric drop shape analysis (ADSA) has been developed recently by Neumann and his group to measure dynamic interfacial tensions [4,5]. The central idea of this technique is to analyze the shape of a pendent drop on the basis of the Gauss–Laplace equation. This technique is par- ticularly advantageous for the determination of dynamic interfacial tensions of surfactant solution–liquid interfaces. In the present study, a pendent drop of DPPC in chloroform was immersed into a water phase to produce a chloroform– water interface. The aim of the present paper is to demonstrate the appli- cation of the ADSA methodology for studies of the inter- facial dilational rheology of surfactant layers at liquid– liquid interfaces. After a certain period of time, phospholi- Thin Solid Films 327–329 (1998) 224–227 0040-6090/98/$ - see front matter  1998 Elsevier Science S.A. All rights reserved PII S0040-6090(98)00633-6 * Corresponding author. Tel.: +49 3063923135; fax: +49 3063923102; e-mail: miller@mpikg.fta-berlin.de