Adsorption from polymer solutions

A recent design of the maximum bubble pressure instrument for measurement of dynamic surface tension allows resolution in the millisecond time frame [119, 120]. This was accomplished by increasing the system volume relative to that of the bubble and by using electric and acoustic sensors to track the bubble formation frequency. Miller and co-workers also assessed the hydrodynamic effects arising at short bubble formation times with experiments on very viscous liquids [121]. They proposed a correction procedure to improve reliability at short times. This technique is applicable to the study of surfactant and polymer adsorption from solution [101, 120]. 

There are numerous references in the literature to irreversible adsorption from solution. Irreversible adsorption is defined as the lack of desotption from an adsoibed layer equilibrated with pure solvent. Often there is no evidence of strong surface-adsorbate bond formation, either in terms of the chemistry of the system or from direct calorimetric measurements of the heat of adsorption. It is also typical that if a better solvent is used, or a strongly competitive adsorbate, then desorption is rapid and complete. Adsorption irreversibility occurs quite frequently in polymers [4] and proteins [121-123] but has also been observed in small molecules and surfactants [124-128]. Each of these cases has a different explanation and discussion. 

Fleer, G. J., and J. Lyklema (1983), "Adsorption of Polymers", in G. D. Parfitt and C. H. Rochester, Eds., Adsorption from Solution at the Solid/Liquid Interface, Chapter 4, Academic Press, London. 

No attempt has been made to discuss the voluminous literature on molecular sieves or supported metals and the reader is referred to papers by Nacchace and Uytterhoeven (31) and Sheppard (32) respectively. IR studies on adsorption from solution are particularly relevant to polymer adsorption and have been reviewed by Rochester (33). 

A wide variety of methods exist for the immobilisation of enzymes on a sensor surface. Screen-printed carbon electrodes are often the favourite base for these sensors due to their inexpensiveness and ease of mass production. Methods used for the construction of AChE-containing electrodes include simple adsorption from solution [22], entrapment within a photo-crosslinkable polymer [20,23], adsorption from solution onto microporous carbon and incorporation into a hydroxyethyl cellulose membrane [24], binding to a carbon electrode via Concanavalin A affinity [25,26] and entrapment within conducting electrodeposited polymers [27]. 

The adsorption from solution of polymers has been studied extensively. The amount of polymer adsorbed usually reaches a limiting value as the concentration of polymer in solution is increased, but this value is usually well in excess of that which would be expected for a monomolecular layer of polymer adsorbed flat on the solid surface. This suggests that the adsorbed polymer is anchored to the surface only at a few points, with the remainder of the polymer in the form of loops and ends moving more or less freely in the liquid phase179. 

Polymer adsorption from solution is a very large subject and it is difficult to provide an exhaustive treatment. We will try to describe the scaling and self- consistent field descriptions of homopolymer adsorption, together with experimental data selected to illustrate the important aspects. 

For our purposes, adsorption from solution is of more direct relevance than gas adsorption. Most, if not all, topics in the five volumes of FICS Involve one or more elements of it. In the present chapter, the basic elements will be introduced, restricting ourselves to low molecular weight, uncharged adsorbates and solid surfaces. Adsorption of charged species leads to the formation of electrical double layers, which will be treated in chapter 3. Adsorption at fluld/fluid Interfaces follows in Volume III. Adsorption of macromolecules will be Introduced in chapter 5. Between monomers, short oligomers, longer oligomers and polymers there is no sharp transition in the present chapter we shall go as far as non-ionic surfactants, but omit most of the association and micelle formation features, which will be addressed in a later Volume. There will be some emphasis on aqueous systems. 

Another feature of adsorption from solution is the variety and complexity of molecules that may be involved in the processes. Indeed one can be interested either by a simple organic molecule, like benzene and its derivatives, or by much larger molecules like proteins, surfactants, or polymers, which bear many different chemical functions and may adopt a large number of conformations at the interface. For such molecules, a good knowledge of both the surface chemistry and the accessibility of porous materials are crucial to understand the adsorption phenomenon. 

Fig. 2 The construction of a polymer-cushioned lipid bilayer membrane. (A) Architecture constructed in a sequential way first, onto the functionalized substrate a polymer layer (cushion) is deposited by adsorption from solution and covalent binding, followed by the (partial) covalent attachment of a lipid monolayer containing some anchor lipids as reactive elements (B) able to couple the whole monolayer to the polymer cushion. (C) Alternatively, a lipopolymer monolayer, organized, e.g., at the water-air interface can be co-spread with regular low-mass amphiphiles and then transferred as a mixed monolayer onto a solid support, prefunctionalized with reactive groups, able to bind covalently to the polymer chains of the lipopolymer molecules, (B). (D) By a fusion step (or a Langmuir Schaefer transfer) the distal lipid monolayer completes the polymer-tethered membrane architecture...

Fig. 4 (a) The reactive-ester analogue of a carboxy-terminated monochloro-silane derivative self-assembles onto a glass substrate to result in a reactive monolayer, (b) Onto this, an ethyleneimine-containing polymer coil, obtained by the partial conversion of a polyoxazoline precursor polymer binds covalently after adsorption from solution to give a stable polymer cushion for the binding of a monolayer of a reactive amphiphile, a reactive ester derivative of a fatty acid in the example given in (c)... 

Fig. 5 Surface-plasmon resonance curves, i.e., reflectivity-vs-incident angle scans of the bare substrate, a Ag coated glass slide with a thin SiC>2 layer evaporated on top (A), after the self-assembly of a reactive monochlorosilane derivative (cf. Fig. 4a) (B), after the adsorption (from solution), covalent binding, and soxhlet extraction of the polymer cushion (C), and after the deposition of a model lipid monolayer (a layer of reactive ester derivatives of a fatty acid) (D)...

When we introduce an insoluble solid into a solution, a change in composition of the solution usually occurs. This is as a result of preferential adsorption of one of the components on the adsorbent solid. Adsorption from solution is a broad subject including detergent, dye, ion, polymer and biological material adsorption on solids, and a huge amount of literature has been published in this field, since it is important to many industries. In this section, an introduction to the subject will be presented, but excluding ionic adsorption. 

Adsorption from solutions onto solid surfaces is important in many industrial practices, such as dye or organic contaminant removal, edible oil clarification by activated carbon, and ion exchange, where the adsorption of ions from electrolyte solutions is carried out. Adsorption from solution is also used in analytical chemistry in various chromatography applications. On the other hand, surfactant, polymer and biological material adsorption on solids, to modify the surface of solid particles in stabilizing dispersions, are also very important industrial fields. 

The study of ultrathin polymer layers on metals is relevant in understanding the behaviour of polymers on surfaces, as well as in the areas of adhesion and corrosion. Gold and copper surfaces can be covered with monolayers of polymers by adsorption from solution [227. 228, 229, 230, 231, 232, 233, 234 and 235]. 

Surface and Interfacial Aspects of Biomedical Polymers, vol. 2, Plenum Press New York, 1985 (b) Landau, M.A. Molecular Mechanism of Action of Physiologically Active Compounds, Nauka Moscow, 1981 (c) Protein at Interfaces Physicochemical and Biochemical Studies, Brash, JL. Horbett, T.A., Eds., ASC Symp. Ser., vol. 342, Amer. Chem. Soc. Washington, DC, 1987 (d) Parfitt, G.D. Rochester, C.H., Eds. Adsorption from Solution at the Solid/Liquid Interface, Academic Press London, 1983 (e) Sato, T. Ruch,... 

A relatively new approach is the consecutive adsorption of oppositely charged polymers or colloids onto solid supports, which yields films that possess internal structure on the nm scale and these films can also be homogenous over large areas. The control over the layer architecture is as straightforward as in the case of LB-films, but there seems to be limits with respect to control of molecular order and orientation. On the other hand, the process can easily be automated since it simply consists of multiple adsorption from solution. The resulting films have in some cases been observed to be stable for seven days at temperatures around 200 °C and at least for more than one year at ambient conditions. 

The simplest method for fabricating PMEs from preformed polymers is based on adsorption from solution by dipping the electrode in a polymer solution. The reproducibility of this method is questionable however because of the need to shake or wipe excess solution from the surface before the solvent evaporates. More reproducible films can be prepared by dropping a measured aliquot of a polymer solution onto the electrode surface, followed by the slow evaporation, preferably in a solvent-saturated chamber, of the solvent. For thin polymer films, the technique of spin coating can produce more homogeneous and reproducible layers. This technique involves the drop coating and solvent evaporation of the polymer solution on a rapidly rotating electrode. 

The second contribution influencing polymer adsorption from solution is the Flory-Huggins interaction parameter between poljnner and solvent. Such an enthalpy of mixing term adds a contribution of X (t)2 z) — 4> ) to the interaction energy contribution, where 4) is the bulk solution concentration of the polymer (2) and, when approaches thermodynamically poor values, then the adsorbed amoimt of pol3mier increases significantly. Figure 5.13 shows experi-... 

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