Adsorption at s/l interface

Enrichment in S/L interfaces is of great importance in numerous industrial purification processes (solvent purification, separation, water treatment, decoloriza-tion, flotation, oil recovery, detergency, and so on). The surface area of industrial adsorbents is also often derived from S/L adsorption isotherms. Adsorption at S/L interfaces can be divided into two types, namely adsorption from pure liquids and adsorption firom solutions. Interaction with pure liquids is often characterized by immersion calorimetry. 

Solid/liquid (S/L) interface, surfactant adsorption at, 24 138-144 Solid-liquid encapsulation process, 16 444 Solid-liquid equilibria (SLE), 22 302 strategic separation schemes and, 22 310-311t... 

The adsorption of surfactants at the S/L interface involves a number of complex interactions, such as hydrophobic,... 

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. 

The adsorption of surfactants at the solid/liquid (S/L) interface determines their efficiency in powder wetting and dispersion. A reduction of the S/L interfacial tension by surfactant adsorption leads to a reduction of the contact angle, which in turn ensures complete wetting of the powder by the Hquid. In addition, the... 

The free energy or tension of adhesion, r, for a solid-liquid system is defined as the difference between the interfacial free energies or tensions at the S/A and S/L interfaces on the contact line R between these two interfaces and the L/A interface. The variation of the adhesion tension with composition of the liquid phase therefore must follow the variations of the corresponding adsorption equilibria (and tensions) at the corresponding interfaces. 

The adsorption of molecules at an S/L interface creates a region of transition. of the order of molecular dimensions, in which the composition changes fmm the hulk solid pha.se to that of the bulk liquid. From a thermodynamic point of view, the adsorbed molecules modify the surface tension (surface free energy... 

Due to the relatively weak adsorption of homopolymers at the L/L interface, and in some cases at the S/L interface, homopolymers are seldom used as emulsifiers or dispersants. For this purpose, the molecule is modified to include some specific units that have strong adsorption to the surface. A good example is partially hydrolysed poly (vinyl acetate), which is commercially referred to as poly(vinyl alcohol) (PVA). The polymer contains 4-12% acetate groups (i.e. 96-88% hydrolysed) and these groups impart an amphipathic character to the chain. The polymer becomes surface-active at the L/L interface and hence it can be used as an emulsifier. In addition, on a hydrophobic surface such as polystyrene, the acetate groups become preferentially adsorbed on the surface of the particles, thus leaving the PVA units dangling in solution as loops and tails . The latter provide the required steric stabilization. 

In this overview, the first section will on general classification of polymeric surfactants. This is followed by a section on preparation of polymeric snrfactants, with particular reference to sugar-based molecules. This is followed by a discussion of their solntion properties. The next section will be devoted to the adsorption of polymeric snrfactants at the solid/liquid (S/L) interface, whereby a summary will be given to some of the theoretical treatments and the methods that... 

An alternative (and perhaps more efficient) polymeric surfactant is the amphipathic graft copolymer consisting of a polymeric backbone B (polystyrene or polymethylmethacrylate) and several A chains ( teeth ) such as polyethylene oxide. The graft copolymer is referred to as a comb stabilizer—the polymer forms a brush at the solid/liquid interface. The copolymer is usually prepared by grafting a macromonomer such as me-thoxy polyethylene oxide methacrylate with polymethyl methacrylate. In most cases, some polymethacrylic acid is incorporated with the polymethylmethacrylate backbone this leads to reduction of the glass transition of the backbone, which makes the chain more flexible for adsorption at the S/L interface. Typical commercially available graft copolymers are Atlox 4913 and Hypermer CG-6 (ICI). 

Schrodle, S., and Richmond, G. L. 2008. In situ non-linear spectroscopic approaches to understanding adsorption at mineral-water interfaces. J. Phys. D Appl. Phys. 41 033001. 

A-B, A-B-A block and BAn graft type polymeric surfactants are used to stabilise emulsions and suspensions [18]. B is the anchor chain that adsorbs very strongly at the 0/W or S/L interface, whereas the A chains are the stabilising chains that provide steric stabilisation. These polymeric surfactants exhibit surface activity at the 0/W or S/L interface. The adsorption and conformation of these polymeric surfactant at the interface has been described in detail in reference 18. 

The adsorption at the S/L interface between finely dispersed powders and aqueous solutions is measured by the same principle as described earlier, that is, by a decrease in the concentration c in solution, which can be most simply measured by the increase in the surface tension at the air-liquid interface. However, this method lacks accuracy, once the surfactant concentration exceeds the critical micellization concentration. For more accurate determination of surfactant concentrations, spectroscopic or chromatographic-mass spectrometric methods can be used. A review of the methods used to determine the concentration of different surfactants can be found in [5]. 

The adsorption at the S/L interface is different from the adsorption at the solid-gas interface in numerous aspects. Yet, the thermodynamics used to describe adsorption at the solid-gas interface are applicable to the S/L interface, and the polarity equalization rule remains valid. 

The most effective wetting agent is the one that gives a zero contact angle at the lowest concentration. For 0 = 0° or cos 0 = 1, Ysl and Ylv has to be as low as possible. This requires quick reduction of Ysl and Ylv under dynamic conditions during powder dispersion (this reduction should normally be achieved in less than 20 seconds). This requires fast adsorption of the surfactant molecules at both the L/V and S/L interfaces. 

Different types of adsorption excess isotherms (U- and S-shaped functions) naturally yield different free enthalpy functions A21G—/(xj), the course of which is characteristic of the minimum energy of wetting at the S/L interface, a parameter diagnostic of the stability of the disperse system. 

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