Measurement of Dilational Modulus of an Adsorbed BSA Film Using Pendant Bubble Tensiometry: From a Clean Interface to Saturation

Abstract

Two open issues on the measurement of the dilational modulus (E) for an adsorbed protein film during the adsorption process have been unacknowledged: how E varies during the adsorption and the length of time needed to attain a stable E value. A new approach for detecting the E variation from a clean air–water interface to saturated film and estimating the time needed to reach a saturated state was proposed. A pendant bubble tensiometer was utilized for measuring the relaxations of surface tension (ST) and surface area (SA), and the E was evaluated from the relaxation data of minute distinct perturbances. The data showed a clear variation in E during the BSA adsorption: E sharply decreased to a minimum at the early stage of BSA adsorption; then, it rose from this minimum and oscillated for a while before reaching an E corresponding to a saturated BSA film after a significant duration. The adsorbed BSA film took ~35 h to reach its saturated state, which was much longer than the reported lifetime of the adsorbed film in the literature. A rapid surface perturbation (forced bubble expansion/compression) could change the E, causing a significant drop in E followed by a slow increase to the original stable value.