Polymer adsorbed layers surface pressure

Figure 2. Effect of ageing and compression of adsorbed layers of unmodified and hydrophobe modified cationic celluloslc polymers on their surface pressure. (Reprinted with permission from ref. 21. Copyright 1985 Allured Publishing.)...

Another kind of wall-effect was proposed by El perin (1967). He suggested that an adsorbed layer of polymer molecules could exist at the pipe wall during flow and this could lower the viscosity, create a slip, dampen turbulence pulsations, and prevent any initiation of vortices at the wall. Later work (Little 1969), however, with a transparent pipe and dyed polymer, showed that the adsorption could in be fact an experimental artifact (a quantity of polymer solution, trapped in pressure gage piping, slowly diffused back into the solvent flow). Although polymer molecules do more or less adhere to clean surfaces in thin films, there is no interaction with the bulk of the solution which could alter the flow properties (Gyr, 1974). Thus, it is evident that adsorption of the additives on surfaces is not the reason for the drag reducing effect. 

As our first case study, dealing with pol)miers, we consider Langmuir mono-layers of poly(methacrylic ester), PMA, at the water-air interface. Data for these layers can be used to illustrate some trends and principles, laid down in sec. 3.41. In that section we discussed how the surface pressure of physisorbed polymers depends on surface concentration. In a dilute monolayer of pancakes, the surface pressure was found to be given by the ideal term plus an excluded-area contribution. We rewrite [3.4.56] in terms of the adsorbed amount r = n°/A = N°/ N A) in moles of chains per unit area... 

The interfacial shear viscosity, is the ratio between the shear stress, a, and shear rate, y, in the plane of the interface, i.e. it is a two-dimensional viscosity. The unit for surface viscosity is, therefore, N s (surface Pa s). A liquid/liquid (or liquid/ vapour) interface with no adsorbed surfactant or polymer shows only a negligible interfacial shear viscosity. However, in the presence of an adsorbed surfactant or polymer layer, an appreciable interfadal shear viscosity is obtained (which can be orders of magnitude higher than the bulk viscosity of the film). This appreciable shear viscosity can be accounted for in terms of the orientation of the surfactant or polymer molecules at the interface. For example, surfactant molecules at the 0/W interface usually form a monolayer of vertically oriented molecules with the hydrophobic portion pointing to (or dissolved in) the oil, leaving the polar head groups pointing in the aqueous phase. A two-dimensional surface pressure, n, may be defined, i.e. 

Because of restrictions on the number of possible configurations, non-adsorbing polymers tend to stay out of a region near the surfaces of the particles, known as the depletion layer. As two particles approach, the polymers in the solution are repelled from the gap between the surfaces of the particles. In effect the polymer concentration in the gap is decreased and is increased in the solution. As a result, an osmotic pressure difference is created which tends to push the particles together. The resulting attractive force is the reason for depletion flocculation. In contrast to this, depletion stabilisation has been mentioned above. 

Two basic approaches are possible one Involves compression of a dispersion of (monodlsperse) particles canylng an adsorbed polymer layer and monitoring the pressure as a function of the volume fraction. In the second approach the force between two macroscopic surfaces with adsorbed pol5rmer layers is measured as a function of the surface separation. 

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