Monolayer desorption, cohesive energies

This study presents an experimental approach for measuring the cohesive energies in condensed lipid monolayers which avoids the difficulty of measuring extremely low values of tt. The approach depends on evaluating the free energy of desorption (or adsorption) of condensed films and comparing this value with the free energy of desorption of the film in the ideal-gas state. 

Cohesive Energies from Monolayer Desorption Studies... 

The cohesive energies in the monolayers will be obtained with Equation 11. For the cohesive energy in the hydrocarbon region of the mono-layer, Wc, the rates of desorption of homologous compounds will be compared at constant pH. For the forces in the polar region of the film, the variation of k8 with pH, for a given compound (constant m), will be examined. 

Measurement of the free energies of monolayer desorption from the rates of desorption depends on whether equilibrium exists between the monolayer and a thin region of solution immediately beneath the film. The relation which tests this condition (Equation 8) must correctly predict the dependence of the rate constant for desorption, k8, on 7r. For the sulfate, phosphonate, and carboxyl films in this study Equation 8 is obeyed within the range of experimental error (2 to 5%). Therefore, it is reasonable to assume that the necessary equilibrium condition does exist. The cohesive forces in the monolayer follow directly from the evaluation of the free energies of desorption. 

Implicit in the determination of Wr, the cohesive energy in the hydrocarbon region of the long-chain monolayer, is the assumption that W0, the free energy of desorption for one —CH2— group, obtained with relatively short hydrocarbon chains, is independent of the length of the hydrocarbon moiety. The validity of this assumption can be checked for sulfate films where W(. may be obtained from desorption studies as... 

To evaluate the free energy of cohesion in the hydrocarbon region of the monolayer, Wc, it is necessary only to obtain the value of W0, the free energy of desorption of the hydrocarbon in the absence of interac-... 

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