solubility parameter, δ

Parameter used in predicting the solubility of non-electrolytes (including polymers) in a given solvent.

For a substance $B$ ,

$δ B = ( Δ vap E m , B V m , B ) 1/2$

where $Δ vap E m , B$ is the molar energy of vaporization at zero pressure and $V m , B$ is the molar volume.

Notes:

For a substance of low molecular weight, the value of the solubility parameter can be estimated most reliably from the enthalpy of vaporization and the molar volume.
The solubility of a substance B can be related to the square of the difference between the solubility parameters for supercooled liquid B and solvent at a given temperature, with appropriate allowances for entropy of mixing. Thus, a value can be estimated from the solubility of the solid in a series of solvents of known solubility parameter. For a polymer, it is usually taken to be the value of the solubility parameter of the solvent producing the solution with maximum intrinsic viscosity or maximum swelling of a network of the polymer. See J.H. Hildebrand, R.L.Scott, The Solubility of Nonelectrolytes, 3^rd ed., Reinhold Publishing (1950); Dover Publications (1964), Chap. VII, p.129; Chap. XXIII, for the original definition, theory, and extensive examples.
The SI units are $Pa 1/2 = J 1/2 m −3/2$ , but units used frequently are $µPa 1/2 = ( J cm −3 ) 1/2$ or $( cal cm −3 ) 1/2$ , where $1 ( J cm −3 ) 1/2 ≈ 2.045 ( cal cm −3 ) 1/2$ . The unit calorie is discouraged as obsolete.

Source:

PAC, 2008, 80, 233 (Glossary of terms related to solubility (IUPAC Recommendations 2008)) on page 264