Salt Effects in Free Nonionic Films

value for the enthalpy of adsorption of both CO and NO. A plausible explanation of the correlation observed is as follows. The standard entropy of adsorption is ascribable to the stretch of the cation-carbon (or cation-nitrogen) bond and to two bending modes of CO (or NO). The frequencies involved are usually (see, e.g., ref 19) assumed to parallel the strength of adsorption AaH. As a matter of fact, the Ni-N (or similar) motion can be represented by a Morse function, whose depth is the adsorption energy: hence the relationship between the cation-carbon (or nitrogen) stretching frequency and the strength of adsorption. As to the bending vibrations (deformations), there is little direct evidence. In a set of systems, however, not too far from that under study (namely H-bonded complexes of the type FH-B, where B is a base molecule), the libration modes of HF have been observed to parallel the strength of adsorption.20 The entropy associated with a harmonic vibration is a decreasing function of the frequency of vibrat i~n. '~ In conclusion, the greater the strength of adsorption, the smaller the entropy associated with the residual vibrations of the adsorbed molecules, as observed.