ABSTRACT
The more interesting experiments involve COOH and OH groups in both nonpolar and hydrogen-bonding solvents. The acid-base components of surface free energy of these SAMs are not readily available from contact angle measurements, because most test liquids will completely wet such surfaces. The CFS in these systems can potentially provide, along with a dispersion component, the values of y + and y~, which together will completely characterize adhesion between these SAMs and other organic surfaces. For example, STC treatment of WOH/HD/OH and W0H/HD/CH3 values (HD is hexadecane) listed in Table 3 using equation (22) and assuming y ^ % ^ 19.3 mJ/m2 and taking KCH3/H2O % 51 mJ/m2, yields ^OH ^ 1-1-1-5 mJ/m2. On the other hand, if for the same monolayers we used work of adhesion values found with CFS in water (Table 3), we would obtain ^OH ^ 24.6 mJ/m2. Warszynski et al. argued [57] that the discrepancy can be resolved if one assumes that in combining relationship (equation (21)) one needs to take the values for surface free energy of solids saturated with respective liquids. Possible rearrangement of surface groups in response to different environments is another plausible explanation. In addition, the STC model has been shown to have internal inconsistencies and CFS could be a powerful tool to explore them. Clearly, the yAB values for high surface energy groups are not available by other means (e.g., contact-angle measurements); in these situations, CFS can be used as an independent method to construct the respective acid-base scales. It also can provide insights in the behavior of surface groups in contact with varying liquid medium. A systematic study that takes advantage of CFS to derive STC values in a self-consistent way, however, is lacking in the present literature.
