Environmental Engineering Reference
In-Depth Information
in great contrast to the case measured in pure ethanol, indicating a soft silica
surface in CTAC ethanol solutions. Considering all the details in this force
profile, we can interpret the observed jump-in being the removal of a single layer
of micelles from the gap between the tip and sample under the applied force of
AFM tip. It is therefore reasonable to conclude that the silica surface is covered
by CTACmicelles, and the size (diameter) of micelles or thickness of single layer
micelles on silica is around 5.7 nm.
The AFM images of silica in ethanol and 5 mmol/L CTAC ethanol solution
are shown in Fig. 6.9a,b, respectively. The silica wafer in ethanol exhibits a
smooth, featureless topography (Fig. 6.9a). The image of silica wafer obtained
in CTAC ethanol solution (Fig. 6.9b), on the contrary, shows well-defined
features. It is interesting to note that the size of high spots is of 6 nm range, a
value very close to the jump-in distance observed on the force profile. It appears
that CTAC at 5 mmol/L concentration in ethanol forms pseudo-spherical
micelles. These micelles are assembled on silica surface with well-defined
voids of 10-nm diameter. For better view, a higher magnification image of the
marked area is shown on the left corner with circles to highlight the voids. By
filling in these voids with desired materials and removing the micelles, one can
obtain mesoporous surfaces of desired materials. Clearly these assembled
micelles can serve as templates for synthesis of mesoporous silica coatings on
silica-coated magnetic particles.
Structural properties of surfactants and micellar solutions are crucial when
preparing mesoporous structures. Surfactants in solution assemble into struc-
tures whose geometry can be described by the surfactant packing parameter [113,
114]. This packing parameter is defined as g=v/al, where n is the volume of the
surfactant tail, a is the effective head group area, and l is the length of the
extended surfactant tail. An increase in the packing parameter represents a
Fig. 6.9 Silica wafer in ethanol (a) and CTAC solutions (b)(circles represent voids to be filled
by silica using sol-gel method)
