Drainage and critical thickness of foam films from mixed solutions of bovine serum albumin and n-dodecyl-β-D-maltoside

Data for the kinetic stability of foam films from ``bovine serum albumin'' (BSA) solution, or from its mixed solutions with non-ionic surfactant ``n-dodecyl-β-D-maltoside'' (C12G2) are presented and discussed based on the specificity of interactions between the components analyzed by tensiometry and fluorescence spectroscopy.The thickness as a function of time in different areas of BSA or BSA/ C12G2 films formed at different experimental conditions (different molar ratios between protein and surfactant, film radius) is measured interferometrically from a video recording of the film evolution. The thinning velocity presented via drainage coefficient, α, in the areas where first, or other black spots appear, or in other film areas was determined from the lnh (t) dependences up to the critical state (first black spot formation in the film). It is found that BSA film has the slowest thinning and the most regular thickness. The thinning velocity of the both BSA and BSA/surfactant films in the area where first spot appears decreases considerably with increasing of the film radius. The thinning velocity values in other film areas are close to those obtained in areas with spots, but only in the initial period of film drainage. The thinning in these places almost stops around the critical state. In above mentioned different film areas the thicknesses, corresponding to the critical state are measured and compared. It is found that the values of critical thickness in areas with spots are noticeably smaller, than critical thickness in other areas, irrespectively of the film radius and molar ratio of the stabilizers. It means that the value of critical thickness for a protein film can not be obtained as an average value of the thickness values determined in different film areas just before the first black spot formation.