Abstract
Surfactants tend to adsorb on the surface/interface mostly in a directional manner. The alkyl chain orientation and conformation order for molecular monolayers of sodium dodecylbenzenesulfonate (SDBS) at low concentrations are studied by using the sum frequency generation vibrational spectroscopy (SFG-VS). The molecular arrangement of the surfactants adsorbed at the solid/liquid interface is further investigated. It is found that the arrangement of the SDBS at the interface becomes relatively ordered with increasing bulk concentration. Meanwhile, the orientation angle reduces gradually, and the molecules tend to be upright state. In addition, the effect of friction on the conformation order and orientation angle are also analyzed. The intensity of the SDBS vibrational contraction peak becomes lower after friction, which indicates that the anion has a reorientation process at the interface. The arrangement of molecules becomes more disordered due to friction. The orientation angle increases slightly, which indicates the monolayer has an inclined trend relative to the lateral direction on the interface. A modified adsorption model considering friction effect is proposed. This work may provide a reference for the further study of adsorption mechanism and application of surfactants.
Highlights
Surfactants are interfacially active compounds with amphiphilic molecules
3.1 Friction leads to the changes of conformation order and tilt angle
sum frequency generation (SFG) spectra of an sodium dodecylbenzenesulfonate (SDBS) monolayer on fused silica surface in the CH stretching region with various concentrations are plotted in Figs. 3(a) and 3(b)
Summary
Surfactants are interfacially active compounds with amphiphilic molecules. Surfactants consist of a charged headgroup and an extended hydrophobic alkyl chain [1]. One of the characteristics is that they tend to adsorb on the surface/interface in a directional manner. The adsorption behavior at the interface is determined by multiple forces including electrostatic force, hydrophobic interaction force, hydrogen bonding, van der Waals force, and other forces. The critical micelle concentration (CMC) of a surfactant is defined as the concentration of surfactant molecules that begin to self-aggregate to form micelles when the concentration reaches a certain level. Since micelles of surfactants can provide colloidal stability, corrosion inhibition, and boundary lubrication at solid/liquid interfaces, the surfactants are widely used in many fields
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