Thermodynamically favourable interactions

Let us turn now to consider systems with thermodynamically favourable interaction (A24 < 0) (i.e., mutual attraction) between protein and polysaccharide. Here there is little measurable effect on the protein loading (see Table 3.1) (Semenova et al., 1999). However, an important con-... 

Figure 7.5 Effect of the character of the interactions between dextran and legunhn on the time-dependent interfacial pressure jc of the adsorbed layer of legumin at the planar o-decane-watcr interface (o) 0.001 wt% legumin alone, and ( ) 0.001 wt% legumin + 2 wt% dextran. (a) Thennodynanhcally unfavourable interaction pH = 7.0, ionic strength = 0.01 M (dextran A/w = 48 kDa). (b) Thermodynamically favourable interaction pH = 7.8, ionic strength = 0.01 M (dextran A/w = 270 kDa).

Thermodynamically Favourable Interactions between Biopolymers in the Bulk... 

Finally, we note here that the dehydration of biopolymers associated with their attractive non-ionic interactions (dipole-dipole, hydrophobic) can increase the contribution of the mixing entropy. This in turn can lead to a tendency towards enhancement of the thermodynamically favourable interactions between them (Appelqvist and Debet, 1997 Cai and Arnt-field, 1997 Semenova a/., 1991b). 

In considering the impact of thermodynamically favourable interactions between biopolymers on the formation and stabilization of food colloids, a number of regular trends can be identified. One of the most important aspects is the effect of complexation on interfacial properties, including rates of adsorption and surface rheological behaviour. 

The presence of a thermodynamically favourable interaction between protein and polysaccharide is commonly associated with a marked decrease in protein surface activity at the air-water or oil-water interface (see Figures 7.5b and 7.15). There is a slower decay in the surface tension for complexes in comparison with the pure protein, and also higher values of the tension in the steady state. Data establishing these trends have been reported for the following biopolymer pairs in aqueous media legumin + dextran and legumin + maltodextrin (Antipova and Semenova,... 

Figure 7.15 Effect of thermodynamically favourable interactions between biopolymers on protein surface activity at the planar oil-water or air-water interface. The surface pressure n reached after 6 hours is plotted against the polysaccharide concentration ( ), legumin (0.001 wt%) + dextran (Mw = 270 kDa) at / -decane-water surface at pH = 7.8 and ionic strength = 0.01 M, (Ay = -0.2 x 105 cm3 mol1) (Pavlovskaya et ah, 1993) ( ), legumin (0.001 wt%) + maltodextrin (MD6, Mw = 102 kDa) at air-water surface at pH = 7.2 and ionic strength = 0.05 M (Ay = - 0.02 x 105 cm3 mol-1) (Belyakova et ah, 1999) (A), legumin (0.001 wt%) + maltodextrin (MD10, Mw = 45 kDa) at air-water surface at pH = 7.2 and ionic strength = 0.05 M (.1 / = - 0.08 x 105 cm3 mol-1) (Belyakova et ah, 1999).

On the basis of available data, it would appear that there are several possible reasons that may account for the observed decrease in surface activity of proteins, depending on the strengths of their thermodynamically favourable interactions with different polysaccharides. In the case of a rather weak interaction, which does not lead to the formation of a stable complex between protein and polysaccharide, the decrease in the surface activity of protein is evidently determined by the corresponding... 

Adsorbed layers of mixed biopolymers are potentially non-equilibrium systems in terms of their structure and composition. Therefore one has to be aware that the impact of thermodynamical favourable interactions between biopolymers on the formation and stabilization of food colloids is dependent, not only on the total system composition, but also on the experimental procedure whereby the two interacting biopolymers are brought to the interface (McClements, 2004 Jourdain et aL 2008, 2009 Dickinson, 2008a). 

The existence of thermodynamically favourable interactions between two biopolymers influences their gel-forming ability in aqueous media. As an example, let us refer here to the effect of low-methoxyl amidated (LMA) pectin (DE = 30 %) on the gelation ability of sodium caseinate (Matia-Merino, 2003 Matia-Merino et al., 2004 Dickinson, 2008a,b). 

The solubility parameter is calculated at 20 MPa and therefore the polymer is swollen by liquids of similar cohesive forces. Since crystallisation is thermodynamically favoured even in the presence of liquids of similar solubility parameter and since there is little scope of specific interaction between polymer and liquid there are no effective solvents at room temperature for the homopolymer. 

In such cases slow rearrangements to the thermodynamically favoured form occurred. With kinetically labile complexes, the bridging mode adopted by the dithiooxalate ligand was a reflection of the thermodynamic stability of M—S2C2O2 vs. (R3P)2—Ag—S2C2O2 interactions. 

The last of the important concepts that we will consider is self-assembly. Most chemists have, at some time in their careers, wondered why molecules cannot just make themselves. The process by which molecules build themselves is termed self-assembly and is a feature of many supramolecular systems. If the molecular components possess the correct complementary molecular recognition features and their interaction is thermodynamically favourable then simply mixing them could result in the specific and spontaneous self-assembly of the desired aggregate. This assumes that there is no significant kinetic barrier to the assembly process. The recognition features within the components may be any of the intermolecular bonding processes mentioned above, but we will be concerned with interactions between transition metal ions and polydentate ligands. 

Thallium (Tl), which appears to exhibit conservative behaviour in seawater, has two potential oxidation states. As Tl1, thallium is very weakly complexed in solution. In contrast, Tl111 should be strongly hydrolysed in solution ([T13+]/[T13+]t — 10 20 5) with Tl(OH)3 as the dominant species over a very wide range of pH. The calculation of Turner et at. (1981) indicated thatTl111 is the thermodynamically favoured oxidation state at pH 8.2. Lower pH and p()2 would be favourable to Tl1 formation. Within the water column, pH can be considerably less than 8.2 and /)( )2 lower than 0.20 atm. In view of these factors, and the observation that redox disequilibrium in seawater is not uncommon, the oxidation state of Tl in seawater is somewhat uncertain. The existence of Tl in solution as Tl+, a very weakly interactive ion, would reasonably explain the conservative behaviour of Tl in seawater. However, the extremely strong solution complexation of Tl3+ suggests that Tl3+ may be substantially less particle reactive than other Group 13 elements (with the exception of boron). 

Fluorine exists as F and MgF+ in seawater in approximately equal concentrations. The free ion fractions of Cl and Br- are essentially 100%. Iv in the form IO3 is the thermodynamically favoured (stable) form of iodine in oxygenated seawater. In the upper ocean, where thermodynamic disequilibria are common, a significant fraction of total iodine is present as iodide (I-). IO3, like I , is weakly interactive. 

Immobilization of the aminosilane molecule changes its interaction characteristics. Because the surface silanols are more acidic than silane silanols, the interaction with the surface silanols is thermodynamically favoured over intramolecular interaction. Kelly and Leyden10 measured the enthalpy of adsorption of the aminosilane molecules. Their results indicate that interaction with the surface involves more proton transfer than in the closed form dissolved molecules. 

Unmodified silica was found to have a low adsorptive capacity with respect to vitamin E. In the case of modified silica the quantity of immobilized biomolecules is significantly increased (Figure 2). The adsorption of vitamin E does not prevent interaction of silica with vitamin C (Figure 3). It was found that the adsorption of vitamin C from ethanol solution, on the surface of modified silica with preadsorbed vitamin E, is thermodynamically favourable (AGads = -31 kJ/mol). 

If apolar hydration is characterized by the conditions that AG° > 0, TAS < 0 and AH < 0, then a process which minimizes exposure of apolar groups to water should be a thermodynamically favoured process. Then if two apolar groups of either the same or different molecules come together in water, AS for this process will be positive because some of the structured water is released into the bulk solvent. Such association is called hydrophobic, hydrophobic bonding or hydrophobic interaction (Kauzmann, 1959). The term bond is probably inappropriate because the association is due to entropy rather than to enthalpy effects, a consequence of the disruption of the clathrate structure around the apolar solute (Jolicoeur and Friedman, 1974). Despite the general acceptance of the concept of hydrophobic association, there are different approaches to the problem of understanding this phenomenon. 

It has been shown that the alumina—free zeolites are hydrophobic and hence interact unfavorably with water. This renders them thermodynamically unstable in aqueous solution with respect to dense phases. Only when organic molecules are occluded does water penetration decrease and the favourable interaction of occluded molecules with the zeolitic silica micropore wall become dominant. This is a means whereby the alumina-free zeolites may become thermodynamically stable. 

A redox reaction between Fe(III) and As(III) has not been observed, despite its thermodynamic favourability. In a heterogeneous study. As(III) was reacted with a slurry of silica gel impregnated with ferric hydroxide. Polarographic analysis showed that while As(III) was removed from solution, Fe(III) was released into solution however, As(V) and Fe(II) were not produced. A redox reaction was also not observed when solutions of Fe(III) (or freshly precipitated Fe(0H)3) were mixed with a solution of As(III) at pH 4 or pH 9. However at pH 4 interactions between Fe(III) and As (III) were observed that were significantly different from those between Fe(III) and As(V). 

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