Intermolecular and interparticle forces

Similarities and Differences Between Intermolecular and Interparticle Forces... 

Thermodynamic aspects are very important in coUoid and smf ace science. They have been reviewed in several published articles, e.g. in Current Opinion in Colloid Interface Science (Aveyard, 2001 Texter, 2000 Lynch, 2001). This chapter offers a brief introduction to the most important concepts, especially those related to intermolecular and interparticle forces. 

Problem 2.1 Multiple choice questions on intermolecular and interparticle forces Fill in Table 2.4. Only one answer is correct for each question. 

For macromolecules and, in general, for particles/ droplets in the colloid domain, the vdW forces are much longer range than the forces between molecules. But both intermolecular and interparticle or interfacial forces depend on the intervening medium, which can be quantified via the relative permittivities or the Hamaker constants. The vdW forces are typically attractive, but because of the presence of an intervening medium they can be repulsive in some systems that consist of at least two different types of molecules or particles. 

In a solution of a solute in a solvent there can exist noncovalent intermolecular interactions of solvent-solvent, solvent-solute, and solute—solute pairs. The noncovalent attractive forces are of three types, namely, electrostatic, induction, and dispersion forces. We speak of forces, but physical theories make use of intermolecular energies. Let V(r) be the potential energy of interaction of two particles and F(r) be the force of interaction, where r is the interparticle distance of separation. Then these quantities are related by... 

On the other hand, solids are characterized by a very ordered structure in which each ion or molecule is surrounded by a fixed number of neighbors whose nature and orientation are determined by the interparticle forces in the crystal. These may be chiefly ion-ion interactions, as in an ionic crystal, or intermolecular forces, as in a molecular crystal. Because of the high state of order in crystals it is a reasonably straightforward problem to calculate their thermodynamic properties on the basis of quite simple statistical mechanical models. 

Liquids and solids are called condensed phases (or condensed states) because their particles are extremely close together. Electrostatic forces among the particles, called interparticle forces or, more commonly, intermolecular forces, combine with the particles kinetic energy to create the properties of each phase as well as phase changes, the changes from one phase to another. 

Flow or deformation involves the relative motion of adjacent elements of the material. As a consequence such processes are sensitive to interatomic or intermolecular forces. In the case of liquids containing dispersed particles, interparticle forces are also important. Because the rheological properties of colloidal suspensions exhibit such a rich variety of phenomena, rheological studies not only provide information on medium-particle and particle-particle interactions but also arc of immense technological importance. 

In Sections 4.2 and 4.3 we consider effects related to the surface tension of surfactant solution and capillarity. In Section 4.4 we present a review on the surface forces due to intermolecular interactions. In Section 4.5 we describe the hydrodynamic interparticle forces originating from the effects of bulk and surface viscosity and related to surfactant diffusion. Finally, Section 4.6 is devoted to the kinetics of coagulation in dispersions. 

With the exception of highly polar materials, London dispersion forces account for nearly all of the van der Waals attraction which is operative. The London attractive energy between two molecules is very short-range, varying inversely with the sixth power of the intermolecular distance. For an assembly of molecules, dispersion forces are, to a first approximation, additive and the van der Waals interaction energy between two particles can be computed by summing the attractions between all interparticle molecule pairs. 

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