olielektrolitlerin, önemli bir endüstriyel hammadde olan kaolen mineralinin elektrokinetik özellikleri üzerindeki etkisi ve. bu mineral üzerindeki adsorplanma mekanizmasının bilinmesi, başta seramik ...ve kâğıt yapımı olmak üzere, bu mineralin kullanıldığı birçok endüstriyel alan için büyük önem arz etmektedir. Bu çerçevede, çalışmalarda kullanılmak üzere 3 'ü yerli ve 3 'ü de ithal olmak üzere toplamda 6 farklı bölgeden değişik özellikte kaolen numuneleri temin edilmiş ve bunlar üzerinde anyonik karakterli 3 farklı polielektrolit (anyonik polilektrolit) kullanılarak elektrokinetik ve adsorpsiyon çalışmaları gerçekleştirilmiştir. Elektrokinetik çalışmalarda, kaolenlerin pH'ya, polielektrolit .türü ve konsantrasyonuna bağlı olarak zeta potansiyel ölçümleri gerçekleştirilmiştir. Yapılan ölçümler sonucunda hiçbir kaolen numunesinde, gerek pH'ya ve gerekse de polielektrolit türü ve konsantrasyonuna bağlı olarak sıfır yük noktası tespit edilememiştir. Numunelerin zeta potansiyeli, ölçüm yapılan tüm pH değerlerinde ve polielektrolit konsantrasyonlarında sıfırdan küçük olup, ortam pH'sı ve ortamdaki polielektrolit konsantrasyonu arttıkça zeta potansiyel değerlerinin de mutlak değerce artış gösterdiği saptanmıştır. Adsorpsiyon çalışmalarında ise polilektrolitlerin kaolen alümina sitelerine sterik olarak adsorplandığı ancak adsorplanma miktarlarının kaolen pH'sı, özgül yüzey alanı ve boyut özelliklerine göre farklılıklar gösterdiği tespit edilmiştir. En yüksek polilektrolit adsorpsiyonu yoğunluğu, özgül yüzey alanı en büyük ve pH'sı en düşük olan kaolende, en düşük polielektrolit adsorpsiyonu yoğunluğu ise özgül yüzey alanı en küçük ve pH'sı en büyük olan kaolende gerçekleşmiştir.
Kaolin, a clay mineral and one of the important industrial raw materials, has a broad variety of applications in industry. For example, it is often used in the ceramics, paper coatings, water-based paints and inks and as an additive in polymers. Kaolin crystals consist of alternating layers of silica tetra-hedra and alumina octrahedra, the unit cell of the kaolin lattice has the composition Si2Al205(0H)4. Each kaolin particle consists of a stack of about 50 sheets of twin-layers, held together with hydrogen bonds. The particles are plate-like and the aspect ratio (particle diameter/particle thickness) is about 5-15. There is a significant difference in the chemical composition of the edges and the basal planes of the kaolin particles. There are often crystal imperfections in the kaolin crystals, with ionic substitutions of AI for Si or Mg for A I, which result in an overall deficit of positive charge. This substitution thereby gives rise to a net anionic charge of the basal surfaces of the particles. However, it has been shown that the negative charge of the basal surfaces varies slightly with pH, the origin of this pH-dependent charge could be due to the presence of some amphoteric silanol groups on the basal surfaces. At the edges of the kaolin particles, the octahedral alumina and tetrahedral silica sheets are broken, exposing aluminol and silanol groups which can yield positive charges in acidic solution and negative charges in alkaline solution. The point of zero charge of the edges is around pH 7.
Interparticle interactions between the edges and the basal planes of the kaolin particles promote aggregation of the particles into edge-to-edge, edge-to-face or face-to-face structures in concentrated aqueous dispersions. However, adsorption of polyelectrolytes or surfactants on the particle surfaces will change the interactions and thereby alter the colloidal stability and rheological properties of kaolin dispersions. Thus, knowing the effect of polyelectrolytes onto electrokinetic properties of kaolin and their adsorption mechanism is very important especially for production of ceramics and paper.
For this purpose, totally six kaolin samples from different deposits, (which are coded as K-730 (Bali-kesir-Düvertepe region kaolin), Eti600 (-38m)
(Istanbul-Şile region), Ömerli (İstanbul-Şile-Ömerli region), K-2 (a Bulgarian kaolin), CC-31 (an English kaolin) and ESK-410 (an Ukranian kaolin)) and three different polyelectrolytes in anionic character (anionic polyelectrolyte) were supplied for this study.
The results of studies can be explained as follows: In experimental studies point of zero charge could not be obtained either by pH values or polyelectrolyte type and concentration for all kaolin samples. The zeta potential of samples for all pH values and polyelectrolyte concentrations was found to be negative, thereby it was observed that increasing the pH value and polyelectrolyte concentration at medium also increased the absolute value of zeta potential. It was also determined that, the surface charge was found to be negative, while the kaolin samples pH is between 4-10 and polyelectrolyte concentration was found to be 10-50 mg/L.
The adsorption of anionic polyelectrolytes onto the kaolin surfaces was obtained by steric holding of free anionic poles of polymer chains onto the positive charged kaolin sites and also the pH of medium has an important role for adsorption quantity. Adsorption was found to be decreasing by increasing pH. That was the result of the deduced number of free anionic sites of polyelectrolyte on to the decreasing charged sites on kaolin surfaces and thereby forming lesser electrostatic chain. And also the adsorption of anionic poles of polyelectrolyte onto the basal sites of kaolin (-) was electrostatically impossible. Thus, the highest adsorption density was obtained for Ömerli kaolin at pH value of 4. It was observed that, for kaolin sample coded by CC-31 for which the pH value is about 4, the adsorption quantity of polyelectrolyte was approximately quarter of Ömerli kaolin. This situation originated by the different pH value for each kaolin, and also, the specific surface area and dgo size which were directly according to these parameters. The specific surface areas of Ömerli kaolin and kaolin coded CC-31 were found to be 27.99 m2/g and 12.30 m2/g respectively. Consequently, it was found that the positive sites of Ömerli kaolins were approximately four times of kaolin sample coded by CC-31 according to pH changes in addition to these parameters.
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