Stabilisers adsorption

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 performance of stabilisers in respect of their physical persistency can also be improved by physical adsorption on surfaces of reinforcing fillers, e.g. of CB or amorphous microground silica [589]. Mineral fillers are well known to adsorb polymer additives, especially stabilisers necessary for processing and... 

The most spectacular results with temperature-programmed LC have been obtained for some notoriously difficult polymeric additives. Characterisation of the oligomeric HALS stabiliser poly [[6-[(l,l,3,3-te-tramethylbutyl) amino]-l,3,5-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidyl)imino]-l,6-hexanediyl [(2,2,6,6-tetramethyl-4-piperidyl)imino]] (I) (Figure 4.12) is difficult for several reasons it has a broad MWD, may contain isomers, and has several amino groups that promote almost irreversible adsorption to silica based column packings in LC. 

It is usually difficult to discuss unambiguously on the role of the formation of sulphate, which may explain the deactivation. Their formation can equally occur on the support and on the noble metals. The poisoning effect of S02 has been reported by Qi el al. on Pd/Ti02/Al203 [112], However, in the presence of water, the stabilisation of hydroxyl groups could inhibit the adsorption of S02 [113], Burch also suggested a possible redispersion of palladium oxide promoted by the formation of hydroxyl species [114], Such tentative interpretations could correctly explain the tendencies that we observed irrespective to the nature of the supports, which indicate an improvement in the conversion of NO into N2 at high temperature. Nevertheless, the accentuation of those tendencies particularly on prereduced perovskite-based catalysts could be in connection with structural modifications associated with the reconstruction of the rhombohedral structure of... 

Chlorinated hydrocarbons can be purchased without stabiliser, or the stabiliser can be removed by adsorption onto alumina, or by extraction with water, followed by drying. IJnstabilised di- or trichloromethane will slowly decompose, producing HC1, which corrodes stainless steel. The rate of decomposition may be accelerated by the presence of other solvents. 

Ethers contain additives to stabilise them against peroxide formation. For instance, tetrahydrofuran is commonly stabilised by the addition of small amounts of hydroquinone. This absorbs uv radiation strongly and so interferes with uv absorbance detection. It can be removed by distilling the solvent from KOH pellets. If you use inhibitor-free tetrahydrofuran, it should be stored in a dark bottle and flushed with nitrogen after each use. Any peroxides that form should be periodically removed by adsorption onto alumina. 

The unstable explosive azide may be stabilised by adsorption for reprographic... 

In many practical cases, stabilisation by polymers involves a combination of steric and charge interactions. Unlike simple electrolytes, multiple adsorption effects permit polyelectrolytes to continue to adsorb well beyond the point where the adsorbed layer charge exceeds that of the particle surface. In this way, the effective charge on particles can be increased substantially at relatively low surface coverage by the polymer. 

In the detergency process, fatty materials (i.e. dirt, often from human skin) are removed from surfaces, such as cloth fibres, and dispersed in water. It is the surfactants in a detergent which produce this effect. Adsorption of the surfactant both on the fibre (or surface) and on the grease itself increases the contact angle of the latter as illustrated in Figure 4.7. The grease or oil droplet is then easily detached by mechanical action and the surfactant adsorbed around the surface of the droplet stabilises it in solution. 

Figure 8.4 Surfactant adsorption at the bubble surface stabilises the films which create a foam.

Mineral fillers can produce both pro-degradant and stabilising effects in thermoplastic polymers. The reasons for this are varied and complex with factors such as adsorption of stabilisers and the presence of detrimental trace impurities such as certain metals playing a part. Until recently this was a little explored area,but several useful reviews have recently appeared [31,32]. 

Ford and coworker [104] have studied HIPEs of water-in-xylenes, stabilised by a variety of surfactants, and postulated three properties which an emulsifier should possess in order to form stable w/o HIPEs of high volume fraction a) a lowering of the interfacial tension between water and oil phases, b) the formation of a rigid interfacial film and c) rapid adsorption at the interface. 

A stabilising effect in the presence of salt was also noted by Aronson and Petko [90]. Addition of various electrolytes was shown to lower the interfacial tension of the system. Thus, there was increased adsorption of emulsifier at oil/water interface and an increased resistance to coalescence. Salt addition also increased HIPE stability during freeze-thaw cycles. Film rupture, due to expansion of the water droplets on freezing, did not occur when aqueous solutions of various electrolytes were used. The salt reduced the rate of ice formation and caused a small amount of aqueous solution to remain unfrozen. The dispersed phase droplets could therefore deform gradually, allowing expansion of the oil films to avoid rupture [114]. 

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