Polymers solvation forces

At separations of a few molecular diameters, the solvation force due to the specific structure of the confined liquid, can be substantial. Polymers at surfaces can be used to stabilize disperions by steric interaction. 

Unperturbed dimensions n. The dimension of a polymer coil in dilute solution at the theta temperature. Under these conditions, the long-range interactions between segments of the polymer chains, causing the chain to contract, are just balance by solvation forces. 

Theoretical approaches to determine solution structures of neutral polymers act on the assumption of the pseudo ideal state. Here the solvation forces from the solvent and the aggregation forces of the chain segments are in an equilibrium where the coil appears to be unaffected by forces. This pseudo ideal state is called the theta state. Theta conditions exist when the exponent a of the [q]-M-relationship has the value a=0.5 and at the same time the value of the second virial coefficient is A2=0 (see the chapter "The [q]-M-relationship in this monograph). Accordingly, a pseudo ideal solvent is called theta solvent and the corresponding temperature is called theta temperature. As Flory showed in his Nobel price lecture [1], the theta state can be described mathematically exactly from the chemical structure of the chain. 

At theta-conditions, the intramolecular interactions of the polymer chain are compensated by the solvation force of the solvent molecules and the polymer coil resumes its unperturbed dimensions ( Dimensions of a real polymer coil in Chap. 8). These theta-solvents correspond to thermodynamically poor solvents. The temperature at which the theta-conditions occur (theta-temperature) is normally close to the precipitation point of the polymer-solvent system. 

There are a number of interaction forces, which can exist between particles—attractive van der Waals, repulsive electrostatic, structural and solvation forces, forces due to adsorbed and non-adsorbing polymer etc. These forces can be manipulated by a variety of different means—by adding salt to screen the charge on the particles, to index match the solvent and particles to screen van der Waals interactions and changing the solvency to affect the state of the adsorbed polymer to name a few. The chemical sensitivity of these interactions becomes particularly important when dealing with sub-100 nm particles or for situations where particles are held at separations on the order of a few nanometers. In both cases, the granularity of the continuous phase and chemical details of the interactions of the species in solution with the solid surface begin to dominate interactions. If conditions where the particles are held at small separations are of interest, these interactions must be characterized, as they cannot at present, be predicted. 

Hydrophilic colloids, e.g. polymers and proteins, are stabilized by a combination of electrostatic and solvation forces. Such colloidal systems are unaffected by small electrolyte concentrations but at very high electrolyte concentrations they may precipitate (salting-out effect). For example, gelatin has strong affinity for water even at the lEP, but casein has a weaker hydrophilic behaviom and precipitates from water close to the lEP. 

For a given K>Iymer/solvent system, the temperature below which the solvent behave as a non-solvent and above which it behaves as a poor solvent is called Flory Theta (0) Temperature. At this temperature, the association forces (cohesive forces between the polymer segments) arc equal to the solvation forces (solvent dispersion forces) and the polymer... 

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