Page 114 - Polymer-based Nanocomposites for Energy and Environmental Applications

90 Polymer-based Nanocomposites for Energy and Environmental Applications

are generally found in the place of origin because they are produced as a result of
weathering of rock or shale [69]. The weathering environment frequently presents
as a chemical decomposition of granite containing silica and alumina or as the solution
of limestone rock or finally as the disintegration and solution of shale. The second type
named the sedimentary nanoclay or transported nanoclays are taken off from the orig-
inal deposit through erosion of silicate minerals at the surface of the earth and depos-
ited to a distant place [70]. Accordingly, the nanoclay is usually associated with
various impurities like quartz, anatase, rutile, pyrite, siderite, feldspar, and iron rocks
[71] depending on the nanoclay origin (mother rock) and depositional environment.
While mean, their presence nonetheless of small quantities may be sufficient enough
to affect negatively the thermal and optical properties of nanoclays. For this reason,
there are yet as several physical and chemical treatments to remove or decrease the
amount of these impurities in the nanoclays; physical like as sieving, magnetic sep-
aration, selective flocculation, and froth flotation [72] and the chemical process as
leaching with various chemical component like oxalic, H 2 SO 4 , and other organic
acids, in the presence of a fermented medium, bioleaching, etc. [73], the first stage
devoted to the purification, based on the wet sieving of rock to separate the fine frac-
tions represent nanoclay from the slurry [74]. As already mentioned, the purification
has a large impact on the thermal and optical properties of nanoclays, besides improv-
ing the mechanical properties using the chemical modification to produce high
nanoclay quality. Hence, Johnston (1996) develops the fundamental concept of
“active sites” appear in nanoclay that underlie their mechanism of chemical modifi-
cation and contribute to nanoclay interaction with other substances, as in PCM [75].
The interaction between the chemical molecules and nanoclay takes place by the six
different active sites that generally are (1)“broken edge” that are the sites and exposed
surface silanol and aluminol groups of the nanoclay platelets, (2) isomorphic substi-
tutions, (3) hydrophobic silanol surfaces, (4) exchangeable cations, (5) hydrophobic
sites on adsorbed molecules, and (6) hydration shell of exchangeable cations. Among
the active sites, the first four are more important in clarifying nanoclays to be devel-
oped as supporting materials in PCMs [76]. So, the first site called broken edge cor-
responds to the hydroxyl edges of the nanoclay particles such as Si-OH and A1-OH (or
Mg-OH); these edge sites have a very important contribution in the cation-exchange
capacity (CEC). A number of surface hydroxyls protonated or deprotonated as a func-
tion of the pH and the ionic strength of the electrolyte represent a negligible amount
5%–10% of the total surface area of the nanoclay particles, which generally equivalent
to the amount adsorbed of organic molecules [77]. The second named isomorphic
substitutions or charge-imbalanced substitutions in the crystalline network. For the
substitutions in the tetrahedral and/or octahedral sheets creating a negative charge
3+
+
such as Al 3+ substituting Si 4+ in the tetrahedral sheets or Li ,Fe , and Mg 2+
replacing Al 3+ in the octahedral sheets, such charges are counterbalanced by loosely
+
+
2+
held surface cations mostly sodium (Na ), potassium (K ), calcium (Ca ), and mag-
2+
nesium (Mg ) cations in the nanoclay lattice of the 2:1 layer silicates. Thirdly,
hydrophobic silanol surfaces are also extremely important as active sites. Sorption
of organic molecules on surfaces of the particles can impart a hydrophobic nature