Molecular basis of thermodynamics

As we have discovered, thermodynamics gives an account of the interrelation of macroscopic properties. It does not directly invoke the molecular nature of matter and its conclusions appear to be independent even of the existence of molecules. However, we can relate the conclusions of thermodynamics to the way in which molecules behave and the establishment of this link, statistical thermodynamics, is the subject of this chapter. 

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Since the First Edition the most significant change to the book has been the addition of a new chapter which provides a simple treatment of the molecular basis of thermodynamics. Though this chapter has been placed at the end of the book it has been written in such a way that it could be employed with advantage at an earlier stage of a first course in chemical thermodynamics. Indeed, a prompt introduction to the elements of statistical thermodynamics can be very helpful in reinforcing the fundamental concepts of classical thermodynamics. 

In an effort to go beyond regular solution theory and better understand the molecular basis of mixed surfactant system behavior, several molecular-thermodynamic theories for surfactant mixtures have been developed. The molecular-thermodynamic theory will be brieLy introduced and discussed below. Readers are referred to a recent summary (Shiloach and Blankschtein, 1998) o can obtain more detailed description and discussion in literature (Nagarajan, 1985, 1986 Puwada and Blankschtein, 1990,1992 Nargarajan and Ruckenstein, 1991 Bergstroem and Eriksson, 1992 Sarmoria et al., 1992 Zoeller and Blankschtein, 1995, 1998 Almgren et al., 1996 Barzykin and Almgren, 1996 Bergstroem, 1996 Zoeller et al., 1996 Blankschtein et al., 1997 Shiloach and Blankschtein, 1997,1998 Thomas et al., 1997 Shiloach et al., 1998). 

Free energy perturbation calculations use Monte Carlo or molecular-dynamics approaches to calculate relative free energies (e.g., of solvation or host-guest binding) on the basis of thermodynamic cycles involving the binding processes of two related systems and the artificial mutation of one system into the other. 

During the past few years, a large number of crystal and NMR structures (Table I), structure-based mutations (Table II) and thermodynamic data (Table III) have become available. These studies have led to a much more advanced understanding towards the molecular basis of signaling transduction of the TNFR superfamily. 

As mentioned above, solution chemistry was bom at the end of the 19th century and developed on the basis of thermodynamics and statistical thermodynamics. Main fields supporting the solution chemistry were physical chemistry and coordination chemistry, which also absorbed some other parts of chemistry in order to establish an interdisciplinary field of chemistry in 1950-1960. Even in this period, however, solvent, e.g, water, was recognized as a continuum in most studies except for some works on ionic hydration, and thus, molecular picture of water was not clearly recognized. In modem solution chemistry, ion-solvent and ion-ion interactions should be depicted more clearly at the molecular level. 

These systems also provide an understanding of the molecular basis of interfaces, since the amphiphile molecules consist of alkyl chains and hydrophilic groups. Thermodynamic analyses on surface adsorption and micelle formation of a anionic surfactants in water were described by surface tension (drop volume) measurements. These data are analyzed in Table 3.19. These data show that at 20°C (Table 3.20) the magnitude of surface tension changes nonlinearly (varying from 1.7 to 0.7 mN/m per CH2) with alkyl chain length. 

Fletcher (50, 51 see 42 also) concluded on the basis of thermodynamic arguments, that an ice surface can not have a surface orientation because of the large loss of configurational entropy that this entails. The surface energy may, however, be lowered if the surface is covered by a thin water layer or quasiliquid film in which a transition from molecular dipole orientation to the normal random ice surface can take place. Fletcher showed that the dipole charge on this layer should be negative. These investigations apply to the static ice surface. 

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