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Flocculated Laponite-PEG/PEO dispersions with monovalent salt, a SAXS and simulation study.

Thuresson, Axel LU ; Segad, Mo; Turesson, Martin LU and Skepö, Marie LU (2016) In Journal of Colloid and Interface Science 466. p.330-342
Abstract
It is well-known that clay can form lamellar structures i.e. tactoids, and recently it has been shown that the tactoid formation is dependent on the platelet diameter. To the authors knowledge, no tactoid formation has been observed for montmorillonite platelets with a diameter less than 60nm. In this study, small angle X-ray scattering in combination with coarse-grained modeling and molecular dynamics simulations have been utilized to study the sediment of Laponite-polyethylene glycol/polyethylene oxide (PEG/PEO) at elevated salt concentrations (150mM-1M). Laponite consists of platelets with a diameter of 25nm and it is known to have a relatively monodisperse size-distribution. At pH 10, the face of the platelets has a strong negative... (More)
It is well-known that clay can form lamellar structures i.e. tactoids, and recently it has been shown that the tactoid formation is dependent on the platelet diameter. To the authors knowledge, no tactoid formation has been observed for montmorillonite platelets with a diameter less than 60nm. In this study, small angle X-ray scattering in combination with coarse-grained modeling and molecular dynamics simulations have been utilized to study the sediment of Laponite-polyethylene glycol/polyethylene oxide (PEG/PEO) at elevated salt concentrations (150mM-1M). Laponite consists of platelets with a diameter of 25nm and it is known to have a relatively monodisperse size-distribution. At pH 10, the face of the platelets has a strong negative charge, whereas the rim is slightly positive. Here we show that it is possible to induce tactoids for Laponite if two constraints are fulfilled: (1) addition of high amount of salt such as NaCl, and (2) addition of a neutral polymer such as PEG. The role of the salt is to screen the repulsive interactions between the platelets and the role of the polymer is to bridge the platelets together: hence the loss in configurational entropy of the polymer is counteracted by the gain in attractive polymer-platelet interaction. As the concentration of NaCl and/or PEG increases, the Bragg peak becomes sharper, which is an indication of that larger tactoids are formed. Comparison between Laponite and montmorillonite shows that the interlayer distance between the platelets increases linearly with an increased Debye screening length for both type of clays, whereas the structure peaks of Laponite are broader compared to the montmorillonite. We argue that the main reason to the latter is due to the size of the platelets: (i) smaller platelets are less rotationally restricted and (ii) the effect of positive edge charges is larger when the platelets are smaller, which results in more irregular aggregates. In absence of the polymer, montmorillonite form tactoids above ∼0.3MNaCl whereas Laponite does not. Even though the model used is simple, we find qualitative agreement between experiments and simulations, which verifies that the underlying physics for tactoid formation is captured. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Journal of Colloid and Interface Science
volume
466
pages
330 - 342
publisher
Elsevier
external identifiers
  • pmid:26748065
  • wos:000369119100038
  • scopus:84962312108
ISSN
1095-7103
DOI
10.1016/j.jcis.2015.12.033
language
English
LU publication?
yes
id
60d07aba-5dd2-44a5-8e46-9e005dd47fd4 (old id 8592522)
date added to LUP
2016-02-10 14:04:51
date last changed
2017-06-11 03:15:12
@article{60d07aba-5dd2-44a5-8e46-9e005dd47fd4,
  abstract     = {It is well-known that clay can form lamellar structures i.e. tactoids, and recently it has been shown that the tactoid formation is dependent on the platelet diameter. To the authors knowledge, no tactoid formation has been observed for montmorillonite platelets with a diameter less than 60nm. In this study, small angle X-ray scattering in combination with coarse-grained modeling and molecular dynamics simulations have been utilized to study the sediment of Laponite-polyethylene glycol/polyethylene oxide (PEG/PEO) at elevated salt concentrations (150mM-1M). Laponite consists of platelets with a diameter of 25nm and it is known to have a relatively monodisperse size-distribution. At pH 10, the face of the platelets has a strong negative charge, whereas the rim is slightly positive. Here we show that it is possible to induce tactoids for Laponite if two constraints are fulfilled: (1) addition of high amount of salt such as NaCl, and (2) addition of a neutral polymer such as PEG. The role of the salt is to screen the repulsive interactions between the platelets and the role of the polymer is to bridge the platelets together: hence the loss in configurational entropy of the polymer is counteracted by the gain in attractive polymer-platelet interaction. As the concentration of NaCl and/or PEG increases, the Bragg peak becomes sharper, which is an indication of that larger tactoids are formed. Comparison between Laponite and montmorillonite shows that the interlayer distance between the platelets increases linearly with an increased Debye screening length for both type of clays, whereas the structure peaks of Laponite are broader compared to the montmorillonite. We argue that the main reason to the latter is due to the size of the platelets: (i) smaller platelets are less rotationally restricted and (ii) the effect of positive edge charges is larger when the platelets are smaller, which results in more irregular aggregates. In absence of the polymer, montmorillonite form tactoids above ∼0.3MNaCl whereas Laponite does not. Even though the model used is simple, we find qualitative agreement between experiments and simulations, which verifies that the underlying physics for tactoid formation is captured.},
  author       = {Thuresson, Axel and Segad, Mo and Turesson, Martin and Skepö, Marie},
  issn         = {1095-7103},
  language     = {eng},
  pages        = {330--342},
  publisher    = {Elsevier},
  series       = {Journal of Colloid and Interface Science},
  title        = {Flocculated Laponite-PEG/PEO dispersions with monovalent salt, a SAXS and simulation study.},
  url          = {http://dx.doi.org/10.1016/j.jcis.2015.12.033},
  volume       = {466},
  year         = {2016},
}