ABSTRACT
In physical chemistry of biological surfaces, one of the most sought after properties is surface (interfacial) tension (see preceding chapters). Surface tension is not only an intrinsic thermodynamic quantity but also closely related to surface adsorption (1-4). Different techniques have been employed to measure the surface tension of biomolecules at air-water and oil-water interfaces, such as the Wilhelmy plate (5,6), the du Noiiy ring tensiometer (7), and those based. on the volume (5,8), weight, or shape o(a pendant drop (9,10). In the ring method, the force required to pull a ring from the surface of a liquid is determined. This method has the disadvantage of enlarging the surface area during the measurement process, which leads to alteration of the adsorption state of biomolecules. Viscoelastic effects in addition to surface tension effects may also come into play. The Wilhelmy plate technique requires the establishment of a zero contact angle, which is difficult to guarantee with systems involving biomolecular solutions because of adsorption onto the plate. Moreover, this is even more difficult in liquid-liquid systems, which are relevant to many biological processes. The ring method also suffers further complications in liquid-liquid systems. The calculation of interfacial tension with the du Noiiy tensiometer requires a correction factor for the weight of the column of liquid while the ring is removed. In liquid-liquid systems, consideration of the density difference across the interface is required for an accurate correction. The drop volume technique relies on the volume of a liquid drop detaching from a capillary tube to determine the interfacial tension.
