The stem layer formed by cation binding is included in the simulation using the
following. The charge density for a mixed PC∕PS membrane, σg, is given by Xps, the mole
fraction of PS, and A, the membrane area per lipid.
Electrolyte cations bind to form a Stem layer on the PS head groups resulting in a
reduced surface charge density. Assuming that this follows a Langmuir isotherm, the
charge density due to the remaining charged lipids is,
σo
σ'(Γ77cj <3∙3>
where C is the molar concentration of the cation, and K is its association constant to the
lipid headgroup. Since the surface potential alters the surface cation concentration
according to the Boltzmann relation,
1 + KC0 exp
ey∕
kJ.
(3.4)
where Co is the bulk molar concentration of the electrolyte and ψ is the surface potential.
Since the Chapter 2 model does not apply charge regulation due to cation binding onto
the lipids, the charge density calculated from that models long-range fit is σo∙ Based on
Equation 3.4, we use the following iterative formula for the charge density in each step of
our calculation
1 + KC0 exp
eΨn
kJ
(3.5)
Here, ψn is the potential on the surface of the lipid, vertically below the apex of the tip.
We take the potential just below the tip because the force signal is largely determined by
the membrane region directly below the tip with variations in the potential elsewhere
36