Interaction between lipid bilayers

Nearby lipid bilayers experience attractive van der Waals forces, screened electrostatic repulsion, and fluctuation-induced steric repulsion.

The Hamaker constant sets the strength of pairwise-summed van der Waals attraction between macroscopic bodies across a medium.

Debye–Hückel screening length ( $ξ$ )

 0  0

The Debye–Hückel screening length is the distance over which mobile ions suppress an electrostatic potential in the linearized electrolyte theory.

Multivalent-electrolyte screening-length ratio ( $λ_{D, 2} / λ_{D, 1} = I_{1} / I_{2}$ )

 0  0

For fully dissociated electrolytes at the same temperature and solvent permittivity, linear screening theory gives

λ_{D}^{- 2} \propto \sum_{i} c_{i} z_{i}^{2} = 2 I

. A divalent symmetric salt of formula concentration

c

has

I = 4 c

, not

c

2 c

. ion association and nonideal concentrated-solution effects limit this comparison.

Helfrich repulsion

 0  0

Helfrich repulsion is the entropic interaction produced when neighboring membranes suppress each other's thermal undulations. Its free energy per area scales as

(k_{B} T)^{2} / (k_{c} d^{2})

A membrane unbinding transition sends the equilibrium layer spacing to infinity. In a dilute lamellar stack, competition between a quadratic virial attraction and cubic Helfrich repulsion gives a continuous transition with spacing exponent one.