山东大学：《物理化学》课程教学资源（讲义资料）8.4 Surface adsorption of solution

88.4 Surface adsorption of solution 8.4.2 Gibbs adsorption isotherm d=-S++al+∑om Ad+∑ndA=0 Let n, be the excess amount of the solute(2) At constant T and p in the surface layer compared to that in a solution of uniform composition. Then the dgo=odA+>u dn lowering of free energy due to the adsorption of solute at the interface is n, du, Integration gives This lowering of free energy in the surface G=a4+∑n is equivalent to -Ado, hence do Further differentiation Ado au2 dG=ad4+Aa+∑om+∑

i i i dG SdT Vdp dA dn   = − + + +    At constant T and p = + i dG dA i dni     = +   G A i ni i i i i dG dA Ad dn n d    = + + +       Integration gives 0 Ad n di i    + =  Let n2 be the excess amount of the solute (2) in the surface layer compared to that in a solution of uniform composition. Then the lowering of free energy due to the adsorption of solute at the interface is n2 d2 . 8.4.2 Gibbs adsorption isotherm Further differentiation n2 d2 = −Ad This lowering of free energy in the surface is equivalent to - Ad, hence: 2 2 T       = −    §8.4 Surface adsorption of solution

88.4 Surface adsorption of solution 8.4.2 Gibbs adsorption isotherm and the surface excess of solute per unit area Is O 00 rr\T Gibbs adsorption isotherm u2=u2+tINc The sign of T2 is determined by(ao/ac), RT while the value of l is determined by both(ao/ac)and c

and the surface excess of solute per unit area is: 2 2 2   = + RT c ln 2 2 2 RT d dc c  = 2 2 2 T c RT c      = −     Gibbs adsorption isotherm The sign of 2 is determined by (/c), while the value of 2 is determined by both (/c) and c. 8.4.2 Gibbs adsorption isotherm 2 2 T       = −    §8.4 Surface adsorption of solution

88.4 Surface adsorption of solution 8.4.3 The types of surface adsorption Three tv f surface adsorption Type I: salts, non-volatile acids and bases, 70 sucrose etc 30 10 C Spring water contains solvable salts

8.4.3 The types of surface adsorption Three types of surface adsorption. Type I: salts, non-volatile acids and bases, sucrose, etc. Spring water contains solvable salts. 10 30 50 70 I II III c  §8.4 Surface adsorption of solution