Implementation of  Gaines Thomas ion exchange model.

@phase(gt_surf, ads, 0.45)

@entity(X, gt_surf, 0)
@primary_entity(X, 1e-6, gt_surf, 1)

@entity(X-Na, gt_surf, 1)
@logKreaction(X-Na, 0.7, 1, X, 1,  Na+)

@entity(X2-Ca, gt_surf, 0.5)
@logKreaction(X2-Ca, 0.8,  2, X, 1, Ca+2)