Adsorption energy per segment

In most situations the experimental system is more complicated than one (homodisperse) polymer adsorbing from a single solvent. In multicomponent systems preferential adsorption always plays a role. A common example is the adsorption of a polydisperse polymer, where usually long chains adsorb preferentially over short ones, even if the adsorption energy per segment is the same. 

Figure 6.2 Variation of adsorption amount F with adsorption energy per segment...

Because many segments of the polymer can be in contact with the surface, a low bonding energy per segment may suffice to render the affinity of several segments together so high that their adsorption is virtually irreversible. 

Perhaps the simplest type of a polymeric surfactant is a homopolymer, that is formed from the same repeating units, such as PEO or poly(vinyl pyrrolidone). These homopolymers have minimal surface activity at the O/W interface, as the homopolymer segments (e.g., ethylene oxide or vinylpyrroUdone) are highly water-soluble and have little affinity to the interface. However, such homopolymers may adsorb significantly at the solid/liquid (S/L) interface. Even if the adsorption energy per monomer segment to the surface is small (fraction of kT, where k is the Boltzmann constant and T is absolute temperature), the total adsorption energy per molecule may be sufficient to overcome the unfavourable entropy loss of the molecule at the S/L interface. 

State (h) represents the case whereby the particles are not completely covered by the polymer chains. In this situation, the simultaneous adsorption of one polymer chain onto more than one particle occurs, leading to bridging flocculation. If the polymer adsorption is weak (low adsorption energy per polymer segment), the flocculation may be weak and reversible, but if the adsorption of the polymer is strong then tough floes will be produced and the flocculation will be irreversible. The latter phenomenon is used for soUd/Uquid separation, for example in water and effluent treatment... 

In a number of ways the adsorption of polymers, including proteins, differs from that of low molecular weight substances. A polymer molecule attaches to the sorbent surface via several segments. Even if the adsorption free energy per segment is low. 

Very weak adsorption type isotherms are very unusual for polymers because the adsorption energy per macromolecule is exponentially increasing with the number of segments per molecule actually adsorbed in trains. 

However, under our experimental conditions, the adsorption energy per adsorbed segment is expected to be very small due to the electrostatic repulsions between anionic polymer molecules and highly negatively charged sand surface. 

A MC study of adsorption of living polymers [28] at hard walls has been carried out in a grand canonical ensemble for semiflexible o- 0 polymer chains and adsorbing interaction e < 0 at the walls of a box of size C. A number of thermodynamic quantities, such as internal energy (per lattice site) U, bulk density (f), surface coverage (the fraction of the wall that is directly covered with segments) 9, specific heat C = C /[k T ]) U ) — U) ), bulk isothermal compressibility... 

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