The right-hand side is a function of the number of moles n 1 and volume fraction ϕ ϕ 1 of solvent (component 1 ), the number of moles n 2 and volume fraction ϕ ϕ 2 of polymer (component 2 ), with the introduction of a parameter χ χ to take account of the energy of interdispersing polymer and solvent molecules. R is the gas constant and T is the absolute temperature. The volume fraction is analogous to the mole fraction, but is weighted to take account of the relative sizes of the molecules. For a small solute, the mole fractions would appear instead, and this modification is the innovation due to Flory and Huggins. In the most general case the mixing parameter, χ χ , is a free energy parameter, thus including an entropic component.