fl-Casein Adsorption at the Air/Water Interface JAMES R. HUNTER, PETER K. KILPATRICK, AND RUBEN G. CARBONELL ~ Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905 Received April 10, 1990; accepted September 3, 1990 The adsorption of reductively methylated bovine/3-casein to the air/water interface has been measured using the radiotracer technique. This method allows determination of both the surface excess concentration at the air/water interface and the amount of protein adsorbed on the Teflon sample container. The present results differ from previous B-casein isotherms reported in the literature due to improvements in the calibration and radiolabeling methods and accounting for protein adsorption on the container surface. The isotherm shows that increasing the bulk concentration increases the surface concentration except in the ranges 2 × 10 -5 to 8 × 10 -5 wt% and 2 × 10 -2 to 0.1 wt%, where constant surface coverages are observed. This behavior can be interpreted as being due to molecular reorientation of adsorbed/3- casein molecules. A kinetic model which adequately characterizes this adsorption behavior is presented. Below 10 .4 wt% the directly measured air/water isotherm and the Teflon/water isotherm inferred from the radiotracer measurements are very similar. Adsorption experiments performed by sequential addition of protein to the bulk solution show that t-casein molecules adsorbed at the air/water and Teflon/water interfaces exchange freely with/3-casein molecules in the bulk solution. © 1991Academic Press, Inc. INTRODUCTION The important role played by the adsorption of proteins to solid-liquid and gas-liquid in- terfaces in many different areas of technology has been well documented ( 1-5 ). Numerous reviews have been written on the subject, summarizing the results obtained with the wide array of techniques that have been em- ployed to measure the qualitative and quan- titative features of protein adsorption to sur- faces (6-8). In spite of these efforts, there are many important questions that remain to be answered to complete our understanding of protein adsorption phenomena (7). Because of the diversity of protein structures, and the complexity of the hydrophobic and hydro- philic domains on their surfaces, it is not yet clear which molecular parameters are mainly responsible for the observed levels of adsorp- tion of different proteins to a given interface. It is also not clear to what extent adsorbed proteins are able to exchange with macro- l To whom correspondence should be addressed. Journal of Colloid and Interface Science, Vol. 142, No. 2, March i5, 1991 molecules in the bulk fluid, or if certain types of protein structures are more likely than oth- ers to exchange easily with their surroundings. The air/water interface serves as an excel- lent model hydrophobic surface for studying protein adsorption because it is homogeneous and easily reproduced. In addition, the nature of the adsorption of some proteins to the air/ water interface bears strong similarities to their adsorption to solid surfaces. The radiotracer technique (9) is well suited for measuring the equilibrium values and the kinetics of adsorp- tion at the air/water interface because it allows a direct measurement of the adsorbed protein concentration without disturbing the sample. In a previous study (10), the adsorption isotherm of chicken egg white lysozyme at the air/water interface was measured using the radiotracer technique. Improvements in ex- perimental protocols led to differences be- tween these results and previously published data for lysozyme adsorption ( 11, 12). It was found that lysozyme adsorbs with its major axis parallel to the interface at low bulk con- centrations until a saturated monolayer is 429 0021-9797/91 $3.00 Copyright © 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.