Entropies of compression

Entropies of Compression of Charged Monolayers at Aqueous Interfaces... 

Entropies of compression (ASC) can now be calculated from the usual thermodynamic relation... 

By means of the Gouy-Chapman assumptions (21, 22) the entropy of compression of a smeared-out head group charge, and its associated... 

Anionic at the Air/Water Interface. In Figure 2 we show II—1/A isotherms obtained by one of us (38) for Ci8 sulfate on several salt solutions at 20°C. We also show a few points where the A values overlap with the results on 0.01 M NaCl reported in our paper on entropies of compression (33). The favorable agreement between both sets of data at lower A is discussed in Ref. 33. The most likely explanation of the slight discrepancies is trace residual alcohol in the older experiments. No other data are available for comparison at these large A values. 

We can now examine the influence of unsaturation on alkyl chain behavior. This significant influence is seen by comparing the total enthalpy and entropy of compression of oleic acid with those for stearic acid (14). The values at 25°C are —6,537 cal/mole and —29 e.u. for oleic... 

For condensed phases (solids, liquids away from the critical point), both / and Vare small. Accordingly, the contribution of the terms V (i pT)dP and pvdP to the enthalpy and entropy is generally negligible when compared to the contribution of the temperature term. This is the reason that we often take the enthalpy and entropy of compressed liquids to be independent of pressure. This accuracy of this approximation is tested in the example below. 

The conclusion is that pressure has a very weak effect on the enthalpy and entropy of compressed liquid, and may be neglected, unless the pressure change is very large. The effect of temperature is much stronger. In fact, at the conditions of this example, one degree kelvin is equivalent to a few hundred bar ... 

Yet another model for obtaining Astmc S values from Ahydr-S ones was suggested by Marcus and Loewenschuss (1985) and Marcus (1986). An irrelevant entropy of compression of Acomp5 = - 26.7 J mol is included in the absolute Ahydr val-... 

A somewhat different approach by Marcus [50] specified the irrelevant entropy of compression as = -26.7 J K moL, using the standard state of 0.1 MPa for the ideal gaseous ions and IM for the ions in solution. The same expression. Equation 5.5, for the electrostatic entropy of the solvent beyond the first solvation shell of the ion I was used for A5 j, but for the solvent molecules within this shell that were translationally immobilized, the deduction of their entropy from A,, i 5,°° was ... 

The standard molar entropy of dissolution, Asoi S° pertains to the transfer from a 1 atm gas state to a 1 mol L solution and hence includes compression of the gas phase from 1 mol contained in 24.61 L (at 300 K) to 1 mol present in 1 L. Since theoretical calculations disregard volume contributions, it is proper to exclude the entropy of compression equal to -R In 24.61 = -26.63 J K" mol, and instead to deal with Thus,... 

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