Davies-Guastalla equation

Dashed lines are the Davies-Guastalla equation for Cm TAB with A0 = 20... 

Dashed lines 1, 2, and 3 represent the Davies-Guastalla equation for 0.1, 0.01, and 0.00IM NaCl. A0 = 20 Az/molecule. 

Using surface pressure (u)-area (A) isotherms for sodium octadecyl sulfate (C18 sulfate) and octadecyl trimethylam-monium bromide (C18 TAB) monolayers spread at the air/water (A/W) and n-heptane/water (O/W) interfaces, we show that the Davies-Guastalla and Davies surface equations of state are not followed over a wide range of temperature and salt concentration. We attribute the discrepancies to Davies incorrect estimate of the chain-cohesion term at the A/W interface, the inadequacy of the electrical contribution to n at all but the highest A studied, and a possible chain-cohesion term at the O/W interface. 

Guastalla attributed Us to dipole interactions, but Davies (16) showed that the data of Saraga (17), Phillips (18), and Guastalla (15) supported a chain length dependence of the Guastalla coefficient and more reasonably explained the cohesion in terms of hydrocarbon chain interaction. Davies claimed a linear dependence of ns on n, the number of methylene groups in the monolayer chain, and proposed the equation of state... 

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