Monomer exchange dynamics of self-assembled surfactant monolayers at the solid-liquid interface.

Self-assembly of surface-active agents at solid–liquid interfaces is critical for many processes in nature and technology, including the stability of suspension, detergency, wetting and mineral separation. Historically, a detailed understanding of nanometer-scale layers has emerged. However, surprisingly little is known about their dynamics—the key to non-equilibrium phenomena. Even in equilibrium, there is constant exchange of monomers between bulk and monolayer. Determination of the associated rate constants has been undertaken by evaluating adsorption and desorption kinetics. These methods are not practical near equilibrium adsorption as they rely upon a changing surface concentration. Hence, little can be learned about the true equilibrium state of a monolayer. The importance of dynamical properties at solution interfaces has long been realized and relaxation techniques, utilizing the system’s response to perturbation from equilibrium, have developed into powerful tools. Unfortunately, these methods do not adapt easily to the rather slow kinetics typically found at the solid–liquid interface. The nature of the solid–liquid boundary as a buried interface, together with the vanishingly small fraction of monolayer to bulk molecules poses a serious experimental challenge. Certain spectroscopic techniques are applicable, and in fact infrared attenuated reflection spectroscopy (IR-ATR) has been used to study exchange reactions of aggregates at interfaces. However, this linear IR technique probes an interfacial region extending to hundreds of nanometers—including molecules far beyond the monolayer. Higher interfacial specificity is obtained with nonlinear optical methods. Kinetic studies at the air–water interface by second harmonic generation have been reported. The related but more powerful vibrational sum-frequency spectroscopy (VSFS) exploits resonant effects of vibrational transitions, and has demonstrated its applicability for the study of monolayers at mineral–water interfaces. However, its current use for such systems has heretofore been limited to static investigations. Herein, we exploit a wide-band, rapidly tunable IR source within a VSFS system to investigate dynamic phenomena. A simplified, three step, monomer exchange process is described in Figure 1. The surfactant (sodium dodecanoate, C12) is first released from the monolayer (rate constant kD). Secondly monomer adsorption occurs from the bulk into a randomly oriented state (kA) on the substrate (fluorite CaF2). Finally an ordered monolayer is reformed (kO).