Leather, a textile based on collagen, usually requires the addition of sulfated oils that have been recently found to cause instability when heated in critical manufacturing processes. Here reactions between collagen and sodium dodecyl sulfate (SDS), sulfated castor oil, or a synthetic sulfated oil are studied calorimetrically. Sodium lauryl sulfate below its critical micelle concentration (cmc) displayed an immediate exotherm due to equilibrium binding of the reagents with stoichiometry n = 12.6 +/- 0.2, K = (2.02 +/- 0.8) x 10(7) M-1, and enthalpy Delta H = 62 +/- 2 Kcal/mol; and a delayed endotherm due to denaturation of collagen. The endotherms accompanying the reactions with sulfated oils with longer chains were smaller, with no apparent denaturation of collagen. The micellar nature of these surfactants was apparent from very large n for sulfated castor oil, 4082 +/- 11 and a very small value of Delta H, 0.77 +/- 0.01 cal/mol. The binding of sulfated castor oil at the polar bands of collagen crystallites, comprising extended molecules arranged side-by-side, was shown directly by electron microscopy.