Entropic processes

Passive diffusion is the simplest transport process. In passive diffusion, the transported species moves across the membrane in the thermodynamically favored direction without the help of any specific transport system/molecule. For an uncharged molecule, passive diffusion is an entropic process, in which movement of molecules across the membrane proceeds until the concentration of the substance on both sides of the membrane is the same. For an uncharged molecule, the free energy difference between side 1 and side 2 of a membrane (Figure 10.1) is given by... 

Furthermore, a real machine designed for adiabatic compression does not reach the ideal point of reversible iso-entropic process, because of unavoidable irreversible transformations. The reversible work associated to the pressure increase inside a fluid can be always calculated as vdp, while the net enthalpy variation is directly related to mechanical energy consumption, which increases with irreversibilities. 

The formation of hydrophobic bonds is similar to the reverse of the processes shown in Table 5.6. It has a negative AG (i.e., is spontaneous), a positive AH, and a positive AS. Thus, the tendency for hydrophobic bonds to be formed results not because water can form more hydrogen bonds. On the contrary, there is a decrease in hydrogen bonding and a corresponding increase in entropy, which allows the process to be spontaneous. Hydrophobic bonding thus provides another example of an entropic process. 

The structural- (or a-) relaxation time ts may be somewhat arbitrarily defined by the time when the ratio between the contribution of the glassy-relaxation process (G) to the relaxation modulus G(t) and the total contributed by all the entropic processes (R), G/R reaches 3. Physically, this means that G/R has decayed nearly by factor of e (2.72) from 10, which is the G/R ratio at the relaxation time t = t ) of the highest Rouse-Mooney mode when the temperature is at the glass transition point. At the same time, the contribution from the glassy component in such a state is still significant. The structmal-relaxation time defined this way is basically equivalent physically to that defined by ... 

It should be recognized that it is hard to know whether a column is operating exclusively in the SEC mode. The studies outlined previously can help one ascertain whether interactions are occurring. In addition, Becker and Unger [45] noted that SEC is a temperature-independent entropic process, whereas surface interactions are temperature-dependent enthalpic processes. Therefore, changes in retention that result from changes in temperature would indicate nonsize exclusion, partitioning mechanisms. 

Enthalpic (heat) effects arising from molecular interactions are reflected in Ah. SEC is a strictly entropic process that is, Ah = 0 and temperature has no significant effect on the elution volume. 

Relaxation of the nuclei perpendicular to the static field direction is characterized by spin-spin or transverse relaxation time T2. This is an entropic process, where the coherence of spins is gradually lost without any energy exchange with the lattice. The main significance of this relaxation lies in its relationship with the homogeneous linewidths of the resulting spectral bands. 

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