Separation of Bulk Effects and Bound Mass during Adsorption of Surfactants Probed by Quartz Crystal Microbalance with Dissipation: Insight into Data Interpretation

The assessment of adsorbed surfactant mass by quartz crystal microbalance with dissipation (QCM-D) monitoring is often complicated due to large bulk responses, particularly for surfactants with high critical micelle concentration (CMC). We present in this work means to interpret QCM-D data that enables the response from the bulk contribution to be separated from the response originating from adsorbed mass. Adsorption of two surfactants, Triton X100 and C12AspNa(2) with low and high CMCs, respectively, at the gold-liquid interface surface has been evaluated. Two different approaches to quantify the bulk response are compared. The first approach involves the use of a nonadsorbing surface (silica), yielding a calibration curve for the concentration dependent bulk response. The second method is based on the fact that the overtone-dependent QCM-D response that originates from changes in the bulk differs from that induced by the adsorbed layer of the surfactants. Under the reasonable assumption that the bulk solution and the adsorbed surfactants can be treated as a Newtonian liquid and an acoustically rigid film, it is demonstrated that the bulk contribution can be quantified without control measurements involving inert surfaces. An excellent agreement between the two methods is reported