Adsorption of dianionic surfactants based on amino acids at different surfaces studied by QCM-D and SPR

The adsorption of three dicarboxylic amino acid-based surfactants, disodium N-lauroylaminomalonate, disodium N-lauroylaspartate, and disodium N-lauroylglutamate, has been studied by surface plasmon resonance (SPR) and the quartz crystal microbalance with dissipation monitoring (QCM-D). These surfactants have high cmc values, which means that the unimer concentration is high at the plateau value of adsorption. This gives rise to a considerable "bulk effect", which must be deducted from the observed value in order to obtain the true value of the adsorbed amount. In this article, we show how this can be done for the QCM-D technique. Adsorption is studied on silica, gold, gold hydrophobized by a self-assembled layer of an alkane thiol, and hydroxyapatite. Adsorption on hydroxyapatite differs very much among the three surfactants, with the aspartate derivative giving the strongest and the glutamate giving the weakest adsorption. This difference is explained as the difference in ability of the dicarboxylic amphiphiles to chelate calcium in the crystal lattice.