having little effect. Analyses of the numerical simulations revealed that the difference in
membrane surface potential between points directly below the tip and off to the side
where the tip had no effect was only 2% [1]. The association constant for cation binding
to the PS headgroup is K = 1 M^1 [84] and Co = 0.5 mM is the known bulk electrolyte
concentration. We ran the finite element simulation with this cation binding model. Here,
the simulation was run at a separation of 11 run to get the far values. Next, the tip was
brought closer, and the surface charge was adjusted based on the previous step's surface
potential, in this case it was adjusted according to the Langmuir Isotherm (see Section
3.2.3). The simulation was run with this new surface charge and the force was calculated.
However, as shown in Figure 3.8 below, cation binding based charge regulation was
insufficient to fit the experimental data.
Figure 3.8: Comparison of the AFM data with the van der Waals and stem layer inclusive
computational model. The AFM data and the electrostatic simulation shown here is the
same as in Figure 3.2.
37