Surface tension semi-static methods

As compared to static methods, the semi-static methods for surface tension measurement are based on achieving a metastable equilibrium, and focused mainly on investigating the conditions under which the system loses that equilibrium. 

The threshold of the equilibrium state can generally be reached slowly, and thus the surface tension values obtained by semi-static methods closely resemble those obtained by static ones. The rate of approaching the equilibrium state should be optimized in each system, in order to avoid lengthy measurements and to obtain surface tension values as close to the equilibrium ones as possible. Among the most common semi-static methods are the method of maximum pressure, the du Noiiy ring method and the drop-weight method. 

The low rate at which equilibrium between the adsorption layer and the bulk is established is typical for high molecular weight surface active substances for which the surface tension gradually decreases with time. The measurement of the surface tension by static and semi-static methods (see Chapter I, 4) as a function of time during the formation of adsorption layers allows one to retrieve information on the kinetics of adsorption phenomena [11,12]. 

The specific surface free energy and the surface tension, a, numerically equal to it can be accurately determined at the gas-liquid and liquid-liquid interfaces. In this section we describe the general principles on which various methods for the determination of surface tension are based. In general, these methods can be classified as static, semi-static and dynamic. This classification, however, may be further extended with the dynamic versions of some methods listed below as static or semi-static exist. Extensive review of the methods used to determine the surface tension is given in [6,16-18]. 

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