WEI-LUN HSU

Silica/Electrolyte Interface

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Being the most abundant solid compound in the Earth’s crust with advantages over other materials such as low cost, material integrity and optical transparency, silica (SiO2) is one of the most commonly used materials for Lap-on-a-Chip devices. When exposed to an aqueous solution, surface SiO2 reacts with water molecules forming silanol functional groups SiOH. These silanol groups are amphoteric meaning they release/accept protons to/from the solution according to the local pH condition adjacent to the surface. When the surrounding pH is lower (high proton concentration) than its isoionic point, protons in the solution are prone to associabe with the oxygen atom of the silanol groups yielding a positively charged surface. Conversely, if the surrounding pH is higher (dilute proton concentration) than its isoionic point, protons on the silanol groups are unstable and tend to be released into the solution resulting in a negatively charged surface.

 

The results from NMR reveal that the silica surface contains multiple silanol group. An infrared study indicates that there are at least two types of silanol groups at the silica and water interface. It is argued that it is from the nonuniform surface density of silanol groups. The networks of hydrogen bonds between silanol groups yield a silica surface that is even more complicated in aqueous solution. If a silanol group is closely located near another silanol group, the proton on the silanol group interacts with the oxygen atom on the adjacent silanol group via hydrogen bonding. In consequence, the proton has less of a tendency to be released into the solution resulting in a higher equilibrium constant. These kinds of silanol groups are classified as vicinal silanol groups. On the other hand, if a silanol group located is far from other silanol groups, the proton on the oxygen atom has a stronger tendency to be released into the solution compared to those on the vicinal groups. These kinds of silanol groups have a lower equilibrium constant and are called isolated silanol groups. There is another kind of silanol group which has two hydroxyls on the Si atom which is called germinal silanol group.