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Issue 41, 2014
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The effect of antagonistic salt on a confined near-critical mixture

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We consider a near-critical binary mixture with addition of antagonistic salt (hydrophilic cations and hydrophobic anions) confined between weakly charged and selective surfaces. A mesoscopic functional for this system is developed from a microscopic description by a systematic coarse-graining procedure. The functional reduces to the Landau–Brazovskii functional for amphiphilic systems for a sufficiently large ratio between the correlation length in the critical binary mixture and the screening length. Our theoretical result agrees with the experimental observation [Sadakane et al., J. Chem. Phys., 2013, 139, 234905] that the antagonistic salt and the surfactant both lead to a similar mesoscopic structure. For very low salt concentration ρion the Casimir potential is the same as in the presence of inorganic salt. For larger ρion the Casimir potential takes a minimum followed by a maximum for separations of order of tens of nanometers, and exhibits an oscillatory decay very close to the critical point. For separations of tens of nanometers the potential between surfaces with a linear size of hundreds of nanometers can be of order of kBT. We have verified that in the experimentally studied samples [Sadakane et al., J. Chem. Phys., 2013, 139, 234905, Leys et al., Soft Matter, 2013, 9, 9326] the decay length is too small compared to the period of oscillations of the Casimir potential, but the oscillatory force could be observed closer to the critical point.

Graphical abstract: The effect of antagonistic salt on a confined near-critical mixture

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Publication details

The article was received on 12 Jun 2014, accepted on 27 Jul 2014 and first published on 29 Aug 2014

Article type: Paper
DOI: 10.1039/C4SM01264J
Citation: Soft Matter, 2014,10, 8188-8201
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    The effect of antagonistic salt on a confined near-critical mixture

    F. Pousaneh and A. Ciach, Soft Matter, 2014, 10, 8188
    DOI: 10.1039/C4SM01264J

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