The name is absent



prominent over the fluid phase as opposed to gel phase. When the tip is further than 7 nm
from the surface, we see an overlap of the two force curves. This is due to the equality in
surface charge of the two phases when the tip is far away and not perturbing the surface.
Figure 3.18 also shows the gel and fluid data compared with a theoretical force curve
obtained using the model in Chapter 2. The gel phase force data follows the same general
shape as the theoretical model due to immobility of the lipids and weak counterion
binding. The fluid phase shows the short range deviation from the model when the tip is
closer than 7 nm from the surface. As shown in this study, this is due to tip induced
mobile lipid charge regulation.

3.4 Discussion

We have used a combination of finite element computer simulations and experimental
AFM data to show that mobile charge regulation accounts for the short range (< 1 Debye
length) electrostatic force over anionic lipids measured by AFM in a low salt solution.
We have accounted for van der Waal interactions and cation binding based charge
regulation in our calculations and found the mobility of the lipid to be the dominant
factor in characterizing the short range AFM electrostatic force data over lipids. Control
experiments on silicon nitride surfaces, whose surface charges are immobile, showed that
the short ranged AFM force data could be adequately accounted for by the formation of a
stem layer due to cation binding. In contrast, a stem layer was insufficient to account for
the short-range force seen over fluid phase lipids. The strongest evidence for tip-induced

47



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