Ionic bridges

Scheme 3.3 Structure of hydroxyl groups on metal oxides I covalent, terminal II ionic, terminal. III ionic, bridging IV ionic, triply bridging.

No individual cation can hold the chains together permanently, because its lifetime in the polymer domain is short, and this mechanism is therefore quite different from the simple ionic bridge or simple chelation model. This distinction is insisted on, because polyelectrolyte theory can lead to important predictions concerning, for example, the effect of a change of distribution of anionic groups... 

Hydrogen bonding (Section 6.2) is another weak interaction that helps maintain the proper three-dimensional structure of a protein. The positively and negatively charged amino acids within a protein can interact with one another to form ionic bridges. This is yet another attractive force that helps to keep the protein chain folded in a precise way. 

The forces that hold the quaternary structure of a protein are the same as those that hold the tertiary structure. These include hydrogen bonds between polar amino acids, ionic bridges between oppositely charged amino acids, van der Waals forces between nonpolar amino acids, and disulfide bridges. 

Globular proteins contain varying amounts of a-helix and P-pleated sheet folded into higher levels of structure called the tertiary structure. The tertiary structure of a protein is maintained by attractive forces between the R groups of amino acids. These forces include hydrophobic interactions, hydrogen bonds, ionic bridges, and disulfide bonds. 

The formation of an alginate junction zone between two buckled guluronic acid chains with divalent calcium as the ionic bridge. 

The clay and the calcium superimposed show that counterions are located at the interface between the polymer and lamellae. It was possible to elaborate an adhesion mechanism in which the cations form ionic bridges between the negative surface of the clay and the latex after water evaporation. Furthermore, it shows how the interface adhesion is important to the mechanical properties of the nanocomposites and how it is possible to formulate materials with the same polymer and clay, but with differentiated properties depending only on the counterions. The results of microscopy and mechanical tests lead to the conclusion that the lamellar separation and the interface adhesion of these lamellae at the polymer matrix play a crucial role in the properties of these systems. 

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