Simple diffusion is the movement of substances from high concentration to low concentration across the lipid bilayer without the help of intermediate.The driving force for simple diffusion is the concentration gradient, and membrane potential gradient.
In order for molecules to diffuse either in/out of the cell, they have to pass through the hydrophobic core of the lipid bilayer. This is the rate-limiting step for simple diffusion, the rate is determined by the hydrophobicity of the substrate.
During equilibrium there will be no net movement of solutes across the membrane.
To measure the flux of a certain substrate:
J= P X (C2-C1)
J= flux; P= permeability; (C2-C1)= concentration gradient.
For charged solute such as ions, etc.. they diffuse down the lipid bilayer very poorly. The charge that they possess, either +ve/-ve causes them to be repelled from similar charges. Their charge also causes them to bind to water molecules thus becoming quite large.For charge solute, we have to also take account that when they move down their concentration gradient, they create membrane potential.
Equilibrium state for a charged solute is when the membrane potential exactly balances the concentration gradient.
To measure the membrane potential required to reach equilibrium:
E= (RT/ZF) ln ( (C in)/(C out) )
E= membrane potential; R= Gas constant; T= Absolute temp; F= Faraday constant