Adsorption destabilisation

The potential for ion adsorption allows for two further coagulation mechanisms. The first may be described as adsorption destabilisation and ultimately has a similar effect to that described for indifferent electrolytes. Metal ions effectively act to neutralise particle charge, reduce the extent of double layer repulsive interactions and hence decrease the potential energy barrier to coagulation. The second, and perhaps more important, mechanism may be... 

Destabilisation of the alumina at pH 5 where the particles are positively charged shows quite similar trends, the principal difference being that the cationic polymer is completely ineffective. Presumably, the positive charges on both particles and polymer are sufficient to prevent adsorption of the relatively small, highly charged polyions. The behaviour of the non-ionic and anionic polymers is essentially the same as at pH 11. Destabilisation by the addition of... 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

At high surfactant concentrations the asymmetric films can be destabilised additionally as a result of the dissolution of the surfactant in the antifoam phase (for example, extraction of non-ionic surfactants such as oxyethyl alcohols, acids and aikylphenols) and its adsorption at the surface of the emulsion drops [66,67]. 

Thus, the destabilisation of asymmetric films in the presence of a dispersed antifoam will be as stronger as its solubility decreases and its affinity towards the aqueous phase increases. This is so because under these conditions the surfactant adsorption at the interface with the antifoam phase decreases. 

The effectiveness of heterogeneous defoaming is determined mainly by two factors the antifoam solubility and its ability to prevent adsorption of the surfactant at the aqueous film/antifoam interface, thus, destabilising the asymmetric aqueous films. The solubility of saturated alcohols in water and in aqueous surfactant solutions decreases with the increase in the molecular mass within the homologous series. The ability of alcohols to prevent adsorption change in the same direction. The difference between the interfacial tensions water/alcohol and aqueous surfactant solution/alcohol can serve as a quantitative measure for the change in the surfactant adsorption at the aqueous solution/alcohol interface... 

Many soHd particles with some degree of hydrophobicity have been shown to cause the destabilisation of foams, including hydrophobic sihca and poly(tetrafluoroethylene) (PTFE) particles. The PTFE particles exhibit a finite contact angle when adhering to the aqueous interface, and it has been suggested that many such hydrophobic particles can deplete the stabilising surfactant film by rapid adsorption, causing weak spots in the film. 

Note, that the bubble surface can be covered by an adsorption layer even without the special addition of surfactant due to contaminations in natural, tap, and industrial waters. Thus, the conclusion that wetting films are stable due to the stabilising effect of molecular forces is at least doubtful for natural waters containing traces of surface active compounds which can destabilise the wetting film and can provide contactless flotation. Collectorless microflotation has been observed for example by Goldman et al. (1974). However, they did not perform any colloid-chemical investigations. 

A quite different technique is necessary when a stabilising adsorption layer on the particle surface is formed. Apparently it is necessary to provide reversibility of surfactant adsorption so that a decrease of surfactant concentration in the bulk results in desorption. This decrease is used in water purification technology based on adsorption methods. Specifically, if a surfactant stabilises the adsorption layers on particles and also adsorbs at the water-air interface, a preliminary flotation of surfactant can decrease their adsorption and thus destabilise the particles. Then microflotation can be applied to extract destabilised particles. 

The important aspect of adsorption processes at a liquid interface is lateral mobility which can lead to lateral excess transport of adsorbed molecules. Lateral transport disturbs the equilibrium state of an adsorption layer. In many important systems, such as emulsions, foams, and bubbly liquids, the properties of a non-equilibrium adsorption layer can be essential. This has been demonstrated in the systematic work of the Russian and Bulgarian schools summarised in monographs like "Thin Liquid Films" by Ivanov, "Coagulation and Dynamics of Thin Films" by Dukhin, Rulyov and Dimitrov, and "Foams and Foam Films" by Krugljakov and Exerowa. These books pay most attention to thick film drainage and stabilisation/destabilisation of thin liquid films. This book is focused on other dynamic processes at liquid interfaces in general or connected with phenomena of emulsions and foams. 

Colloidal particles can adsorb chemical ions and radicals at their surface. If the adsorbed species carries a charge opposite to that of the colloid, the adsorption will lead to a reduction of the surface potential, causing the particle to become destabilised. 

In summary, there are two scenarios which cause the most severe flux decline. Firstly, poorly soluble organics at low pH (4,5) or in the presence of salt, which is often the case with surface waters, and secondly small colloids, that partially aggregate. Particles prepared with organics in the OPS order exhibit a low flux decline for the small colloids, due to a low rejection and an incomplete adsorption within the pores. The solution chemistr) is important, as pH influences the adsorption of organics and, thus, particle stabilisation. Additionally, calcium can destabilise the previously stable colloids. 

The role of alkali promoters has been subject to many studies, and it has been commonly accepted that the effect of alkali is to lower the activation barrier for the dissociation of chemisorbed N2. Recently, the role of potassium has been accounted for by density functional theory calculations indicating that on iron catalysts destabilisation of NHx species, creating more free sites for N2 adsorption, may be the main reason for the rate enhancement observed, whereas on Ru and Co-based catalysts promotion becomes effective through weakening of the N=N bond of the adsorbed species (38). 

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