Solvent-mediated force

In a good solvent, the segment - solvent interaction tends to pull a pair of segments apart, so that the solvent - mediated force F should be repulsive as is F. On the other hand, F should be attractive in poor solvents. Hence, as the solvent is made poorer by changing either solvent species or temperature, the situation should be reached in which the attractive F cancels or suppresses the repulsive F so that the net force F + F becomes zero or even negative. 

Nevertlieless the sketched picture provides the most basic view of a glass transition in colloidal suspensions, connecting it with the increase of the structural relaxation time T. Increased density or interactions cause a slowing down of particle rearrangements which leave the HI relatively unaffected, as these solvent mediated forces act on all time scales. Potential forces dominate the slowest particle rearrangements because vitrification corresponds to the limit where they actually prevent the final relaxation of the microstructure. The structural relaxation time T diverges at the glass transition, while stays finite. Thus close to arrest a time scale separation is possible, T T . 

Solvent-Mediated Forces. These break into two classes ... 

Because the interactions between different chains and between chains and solvent molecules are so complicated, it is normal to consider instead a model polymer in which all the microscopic details, such as bond angles, torsional potentials, solvent mediated forces between different monomers, etc, are coarse grained into a few parameters which are then generally fixed empirically or alternatively from computer simulation. 

The study of McMillan-Mayer level models, in which the solvent coordinates have been averaged over so that only solvent-mediated ion-ion forces need be treated, is relatively well developed. However the real forces at this level are even more poorly known than the forces at the Born-Oppenheim level referred to above. It is found that McMillan-Mayer level models can be brought into good agreement with solution thermodynamic data. 

Solvent-structure (mediated] forces enter when h is so small that overlap occurs between the liquid layers adjacent to the two boundaries. Close to these surfaces the ordering of the fluid molecules differs from that in the bulk, as discussed in sec. 11.2.2. If h is reduced, the overlap of two such layers requires work to be done by or on the system, that is, it leads to an additional contribution to G(h) and to /7(h). 

If the electrostatic interaction between two like-charged particles is attractive, we call the interaction inverted and if the interaction between oppositely charged particles is repulsive, we also call it inverted. Ionic force inversion is not a denial of Coulomb s law. If it exists, it must be a manifestation of many-body interactions. For example, if two like-charged polymers electrostatically attract each other, it can only be through the mediation of counterions and/or polarizable solvent molecules. Force inversion is a topic currently of widespread interest in polyelectrolyte physics (this volume, Chapter 5). We currently believe that inverted forces can arise through several distinct types of interactions, and it may not always be easy to pick... 

Theoretical and experimental studies of the role of solvent on polymorphic crystallization and phase transformations abound in the literature of the last few years and some pertinent examples are described here. For solvent-mediated transformations, the driving force is the difference in solubility between different polymorphs. An important earlier paper on the kinetics of such phase transformations [51 ] described a model featuring two kinetic processes in sohd to solid phase changes via a solution phase, namely dissolution of the metastable phase and growth of the stable one. 

The solvent mediated potential of mean force (PMF) between molecular solutes is defined as... 

The energy conversions that produce motion in living organisms consist of two distinct but interlinked physical processes of hydrophobic association and elastic force development, collectively referred to as consilient mechanisms in that they each provide a common groundwork of explanation. The association of oil-like domains, hydrophobic association, has been characterized in terms of the comprehensive hydrophobic effect (CHE), and elastic force development has been described in terms of the damping of internal chain dynamics on deformation, whether deformation occurs by extension, compression or solvent-mediated repulsion (see section E.4.1.2 and Figures E.3 and E.4, below). 

The main shortcoming of DLVO theory is that it treats the interactions between the ions and the colloidal particle as purely electrostatic. Ions close to the particle are also subject to ion-type specific, often solvent-mediated attractive forces. Their inclusion on the level of an extended Poisson-Boltzmann equation leads to a more complex scenario, in which the salt concentration also changes the effective surface charge of the colloid [9]. This can induce resolubilization at high salt concentrations when a potential barrier reappears due to overcharging. The inclusion of ion-colloid dispersion interactions into the Poisson-Boltzmann equation can also induce similar effects. 

There are no expressions of equivalent reliability and generality for the contributions to Ui,(r) from steric interactions. A full understanding of steric stabilization requires further progress in the theory of physical adsorption of macromolecules. The problem is essentially a molecular one, and will not be discussed in this Report which is concerned with statistical mechanics at the particulate level. For the same reason, we are not concerned here with short-range solvent-mediated structural forces, which can become important when particles are very close together. 

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