The performance of nonionic surfactants
is mediated by the interfacial
interactions at the solid–liquid interface. Here we applied
sum frequency generation (SFG) vibrational spectroscopy to probe the
molecular structure of the silica–nonionic surfactant solution
interface in situ, supplemented by quartz crystal microbalance with
dissipation monitoring (QCM-D) and molecular dynamics (MD) simulations.
The combined studies elucidated the effects of nonionic surfactant
solution concentration, surfactant composition, and rinsing on the
silica–surfactant solution interfacial structure. The nonionic
surfactants studied include ethylene-oxide (EO) and butylene oxide
(BO) components with different ratios. It was found that the CH groups
of the surfactants at the silica–surfactant solution interfaces
are disordered, but the interfacial water molecules are ordered, generating
strong SFG OH signals. Solutions with higher concentrations of surfactant
lead to a slightly higher amount of adsorbed surfactant at the silica
interface, resulting in more water molecules being ordered at the
interface, or a higher ordering of water molecules at the interface,
or both. MD simulation results indicated that the nonionic surface
molecules preferentially adsorb onto silanol sites on silica. A surfactant
with a higher EO/BO ratio leads to more water molecules being ordered
and a higher degree of ordering of water molecules at the silica–surfactant
solution interface, exhibiting stronger SFG OH signal, although less
material is adsorbed according to the QCM-D data. A thin layer of
surfactants remained on the silica surface after multiple water rinses.
To the best of our knowledge, this is the first time the combined
approaches of SFG, QCM-D and MD simulation techniques have been applied
to study nonionic surfactants at the silica–solution interface,
which enhances our understanding on the interfacial interactions between
nonionic surfactants, water and silica. The knowledge obtained from
this study can be helpful to design the optimal surfactant concentration
and composition for future applications.