Читать книгу The Rheology Handbook - Thomas Mezger - Страница 94

Dispersions and gels are showing yield points due to intermolecular forces (van-der-Waals forces). This includes dipole-dipole interactions between particles, and between particles and the surrounding dispersion agent. There are different kinds of interaction forces (with specification of the typical bonding energy per mol): Electrostatic interactions between permanent dipoles (Keesom forces; < 30 kJ/mol), interactions due to induction between permanent and induced dipoles (Debye forces; < 2 kJ/mol), and dispersion forces between mutually induced dipoles (London forces; < 40 kJ/mol) [3.22] [3.27] [3.64] [3.82]. They are all based on physical-chemical bonds (secondary bonds) between the molecules and have a considerably lower bonding energy, usually below 20 kJ/mol, compared to the chemical primary valency bonds which are acting within the molecules. These primary bonds are covalent electron-pair bonds, ionic or metallic “electron gas” bonds which are usually showing energy values of 50 to 400 kJ/mol, and maximum values of 1000 kJ/mol [3.20] [3.28]. Bonds via intermolecular hydrogen bridges (< 50 kJ/mol) are an exceptional type of physical-chemical secondary bonds. In large numbers, however, they can have a great effect on rheological behavior.

Interactions may build up a three-dimensional network of forces. In the low-deformation range, this network occurs as a stable and solid-like structure resulting in elastic behavior (gel-like character, or gel-like state, see also Chapter 8.3.2a).