Entropy-driven disordering

Note that because diffusion is entropy-driven, it is one of the means whereby the system attempts to minimize the magnitude of the concentration gradient, i.e. to maximize the overall disorder that is present. Accordingly, diffusion will always be present even in the absence of both convection and migration. 

The mixing of two gaseous substances, or of two non-polar liquids, are further examples of entropy-driven processes. These involve negligible enthalpy changes (no strong chemical bonds are formed or broken) but the increased randomness and disorder in the system lead to a positive entropy change. 

It is apparent that the cell reaction is thermodynamically downhill (AG < 0) but that it is entropy driven (A5 > 0), the process being enthalphically unfavourable (A/T > 0). The positive E value reflects the increase in disorder in converting the solids Hg2Cl2 and Ag into solid AgCl and liquid Hg. 

Although the thermodynamic analysis of weak flocculation and colloidal phase separation, given above, illustrates the basic principles, some of the details are incorrect, in particular for more concentrated dispersions. One missing feature is the prediction of an order/disorder transition in hard sphere dispersions (for which Vmin is 0), where, at equilibrium, a colloidal crystal phase is predicted to coexist with a disordered phase over a narrow range of particle volume fractions (ip), that is, 0.50 < tp < 0.55 (Dickinson, 1983). In molecular hard-sphere fluids this is known as the Kirkwood-Alder transition , and is an entropy-driven effect. 

Figm 1 Two structural levels of the gel tensile-blob model. Shown are the tensile coil, and the extended tensile structure, L Adapted from Sukumaran, S. K. Beaucage, G. Europhys. Lett. 2002, 59, 714-720, Rgure 1 Tension on the chain overcomes entropy-driven chain disorder at large scales resulting in a linear stmcture that is random at scales smaller than the tensile-blob size,... 

Systems at constant temperature and pressure, which are common laboratory conditions, have a tendency toward lower enthalpy and higher entropy. A chemical reaction is driven toward the formation of products by a negative value of AH (heat given off) or a positive value of AS (more disorder) or both. When AH is negative and AS is positive, the reaction is clearly favored. When AH is positive and AS is negative, the reaction is clearly disfavored. 

Figure 1.12. From Order to Disorder. The spontaneous mixing of gases is driven by an increase in entropy.

The fluid that flows through the coils in the back of a refrigerator removes heat from the interior by the same process. In fact, the refrigerant used in freezers will absorb so much heat that it can freeze skin. The fluid is compressed by the compressor, then allowed to expand through the coils. The compression is driven by an electric motor, and the expansion is driven by entropy the gas phase is a more disordered state than the liquid phase. 

As indicated in Table 6, the —AGa values of the three SX samples obtained for all nonpolar probes in the alkane series followed the order SX-II > MSX = SX-I, suggesting that the adsorption of the alkane probes is energetically similar toward MSX and SX-I, but thermodynamically more favorable on SX-II. Analysis of the temperature dependence of AGa for the SX samples revealed that the MSX and SX-I had statistically equivalent AHa and ASa and hence comparable —AGa, whereas SX-II had a less negative AHa and a much less negative ASa than MSX and SX-I, the net result being a higher —AGa for SX-II (Table 6). In other words, the more favorable adsorption of the nonpolar probes on SX-II is driven by a eonsiderably reduced loss in surface entropy (disorder or molecular mobility). 

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