Conformation at the solid/liquid interface

FIGURE 7.32 Schematic representation of the variation of polymer conformation at the solid-liquid interface. (From Tjipangandjara, K.F. et ah, Colloids Surf., 44, 229, 1990. With permission.)... 

Fig. 4.23. Schematic representation of the variation of polymer conformation at the solid-liquid interface under shifted pH conditions.

The key to understanding how surfactants and polymers (to be referred to as polymeric surfactants) function as stabilisers is to know their adsorption and conformation at the solid/liquid interface. This is the objective of the present chapter, which is a survey of the general trends observed and some of the theoretical treatments. 

All three investigations require several sophisticated techniques, such as zeta potential measurements, surfactant and polymer adsorption and their conformation at the solid/liquid interface, measurement of the rate of flocculation and crystal growth, and several rheological measurements. 

For the full assessment of the properties of suspension concentrates, three main types of investigations are needed (i) the fundamental investigation of the system at a molecular level (ii) investigations into the state of the suspension on standing (iii) bulk properties of the suspension. All these investigations require a number of sophisticated techniques such as zeta potential measurements, surfactant and polymer adsorption and their conformation at the solid/liquid interface, measurement of the rate of flocculation and crystal growth, and several rheological measurements. Apart... 

The interfacial properties of chain-like molecules in many polymeric and colloidal systems are dependent on the conformation of the chains adsorbed at the interface (.1). Chains adsorbed at the solid-liquid interface may be produced by anchoring diblock copolymers to particles in a polymer dispersion. Such dispersions are conveniently prepared by polymerizing in the presence of a preformed AB diblock copolymer a monomer dissolved in a diluent which is a precipitant for the polymer. The A block which is... 

The system described in this investigation is polystyrene-14C adsorbed on Graphon carbon black (graphitized Spheron 6) from six solvents comprising a wide spectrum from good to poor solvent power. Well-characterized materials were selected to elucidate the conformation of polymer molecules at the solid/liquid interface. So far two models have been postulated to describe the conformation of the adsorbed polymer molecules at the solid/liquid interface (9, 13, 14, 18, 19, 21, 27). In the first model the polymer assumes a loop or coil structure in which only a fraction of the polymer segments are attached directly at the interface, and in the second model the polymer forms a relatively flat and compressed interfacial layer with many segments attached to the solid substrate. 

There has been considerable interest in recent years in the formation of condensed films of purine and pyrimidine bases at the solid-liquid interface. It is well recognised that non-covalent affinities between base pairs play a prevalent role in determining nucleic acid conformation and functionality. Likewise, there has been interest in the role of substrate and non-covalent intermolecular interactions in the configuration of ordered monolayers of purine and pyrimidine bases. There is also more general interest in the interaction of bases with metal surfaces and metal complexes. In the latter case it is noted that the biological role of nucleic acids and certain nucleotides are dependent on metal ions, particularly Mg, Ca, Zn, Mn, Cu and Ni. " Also certain metal complexes, notably of platinum, have the anti-tumour activity, which is linked to their ability to bind to bases on DNA. On a different note, the possibility that purine-pyrimidine arrays assembled on naturally occurring mineral surfaces might act as possible templates for biomolecular assembly has been discussed by Sowerby et al. 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

To understand the role of polymers in dispersion stability, it is essential to consider the adsorption and conformation of the macromolecule at the solid/liquid interface [1]. 

This section will deal with the above interfacial aspects starting with the equilibrium aspects of surfactant adsorption at the air/water and oil/water interfaces. Due to the equilibrium aspects of adsorption (rate of adsorption is equal to the rate of desorption) one can apply the second law of thermodynamics as analyzed by Gibbs (see below). This is followed by a section on dynamic aspects of surfactant adsorption, particularly the concept of dynamic surface tension and the techniques that can be applied in its measurement. The adsorption of surfactants both on hydrophobic surfaces (which represent the case of most agrochemical solids) as well as on hydrophilic surfaces (such as oxides) will be analyzed using the Langmuir adsorption isotherms. The structure of surfactant layers on solid surfaces will be described. The subject of polymeric surfactant adsorption will be dealt with separately due to its complex nature, namely irreversibility of adsorption and conformation of the polymer at the solid/liquid interface. 

Polymers are also essential for stabilization of nonaqueous dispersions, since in this case electrostatic stabilization is not possible (due to the low dielectric constant of the medium). To understand the role of polymers in dispersion stability, it is essential to consider the adsorption and conformation of the macromolecule at the solid/liquid interface which was discussed before. 

This chapter will start with a short account of the general classification and description of polymeric surfactants. This is followed by a summary on then-solutions properties. The adsorption and conformation of polymeric surfactants at the solid-liquid interface will be discussed at a fundamental level and some experimental results will be presented to illustrate the prediction of the theories. The interaction energies between particles or droplets containing adsorbed polymeric surfactants will be briefly described. The final section will give some applications of polymeric surfactants in suspensions, emulsions, and multiple emulsions. 

The structure of the adsorbed layer is described in terms of the segment density distribution, p(z). As an illustration, Fig. 5 shows some calculations by Scheutjens and Fleer [17] for loops and tails with r = 1000,4>. = 10 , and X = 0.5. In this example, 38% of the segments are in trains, 55.5% in loops, and 6.5% in tails. This theory demonstrates the importance of tails which dominate the total distribution in the outer region of the adsorbed layer. As we will discuss in the next section on experimental techniques for characterization of the adsorption and conformation of polymers at the solid liquid interface, determination of the segment density distribution is not easy and usually assigns a value for the adsorbed layer thickness 6. This increases with increase of the molecular weight of the polymer and increase of solvency of the medium for the chains. 

The advantage of using an SECM-based configuration is that the fast mass transfer achieved at the local scale avoids the mass transfer limitation on kinetic measurements commonly encountered in flow injection SPR systems. This work evidenced that SECM-SPR is amenable for studies of rapid functional and dynamic conformational changes of protein at the solid-liquid interface. 

It is well-known that the interfacial properties are significantly influenced by the conformation of polymer chains grafted at interfaces [1-4]. However, the in situ real-time characterization of the conformational change of grafted chains still remains a challenge [5, 6]. Generally, polymer chains at the solid/liquid interface... 

The proposed structure of the complex does not assume a static distribution of the sequences. The system is of course a dynamic one, but we study it at equilibrium. A given COOH group, involved in a complex at the moment t, may be free or in the carboxylate from at t + dt. However the average number of complexed sequences remains invariant with time for a fixed composition of the system. The situation can be compared with the behaviour of macromolecules adsorbed at a solid-liquid interface their mean conformation is stable even if locally an adsorption/desorption equilibrium occurs. 

Rudolph, W. T. "The Conformation of Synthetic Polymers Adsorbed at a Solid, Liquid Interface" M. S. Thesis, University of Minnesota, 1976. 

Distribution between trains and loops in molecules adsorbed at solid/Iiquid interfaces is also possible and has been shown to occur for flexible polymers. There are some indications that protein molecules at solid/liquid interfaces do not always undergo the drastic conformational changes that occur at fluid/fluid interfaces. At a solid/liquid interface, an adsorbing molecule cannot penetrate the solid phase. Furthermore, adsorption may be confined to sites and thus be localized. Using infrared difference spectroscopy, Morrissey and Stromberg (1974) found a bound fraction (number of carbonyl surface... 

Fundamental investigation of the system at the molecular level. This requires investigations of the structure of the solid/liquid interface, namely the structure of the electrical double layer (for charge-stabiUsed suspensions), adsorption of surfactants, polymers and polyelectrolytes and conformation of the adsorbed layers (e.g., the adsorbed layer thickness). It is important to know how each of these parameters changes with the conditions, such as temperature, solvency of the medium for the adsorbed layers, and the effect of addition of electrolytes. 

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