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Lamellar miscibility gap in a binary catanionic surfactant-water system

Silva, Bruno F. B.; Marques, Eduardo F. and Olsson, Ulf LU (2007) In The Journal of Physical Chemistry Part B 111(48). p.13520-13526
Abstract
The coexistence of two lamellar liquid-crystalline phases in equilibrium for binary surfactant-water systems is a rare and still puzzling phenomenon. In the few binary systems where it has been demonstrated experimentally, the surfactant is invariably ionic and the miscibility gap is thought to stem from a subtle balance between attractive and repulsive interbilayer forces. In this paper, we report for the first time a miscibility gap for a catanionic lamellar phase formed by the surfactant hexadecyltrimethylammonium octylsulfonate (TASo) in water. Synchrotron small-angle X-ray scattering, polarizing light microscopy, and H-2 NMR unequivocally show the coexistence of a dilute (or swollen) lamellar phase, L-alpha', and a concentrated (or... (More)
The coexistence of two lamellar liquid-crystalline phases in equilibrium for binary surfactant-water systems is a rare and still puzzling phenomenon. In the few binary systems where it has been demonstrated experimentally, the surfactant is invariably ionic and the miscibility gap is thought to stem from a subtle balance between attractive and repulsive interbilayer forces. In this paper, we report for the first time a miscibility gap for a catanionic lamellar phase formed by the surfactant hexadecyltrimethylammonium octylsulfonate (TASo) in water. Synchrotron small-angle X-ray scattering, polarizing light microscopy, and H-2 NMR unequivocally show the coexistence of a dilute (or swollen) lamellar phase, L-alpha', and a concentrated (or collapsed) lamellar phase, L alpha ''. Furthermore, linear swelling is observed for each of the phases, with the immiscibility region occurring for 15-54 wt % surfactant. In the dilute region, the swollen lamellar phase is in equilibrium with an isotropic micellar region. Vesicles can be observed in this two-phase region as a dispersion of L alpha' in the solution phase. A theoretical cell model based on combined DLVO and short-range repulsive potentials is presented in order to provide physical insight into the miscibility gap. The surfactant TASo is net uncharged, but it undergoes partial dissociation owing to the higher aqueous solubility of the short octylsulfonate chain. Thus, a residual positive charge in the bilayer is originated and, consequently, an electrostatic repulsive force, whose magnitude is dependent on surfactant concentration. For physically reasonable values of the solubility of the octyl chain, assumed to be constant with surfactant volume fraction, a fairly good agreement is observed between the experimental miscibility gap and the theoretical one. (Less)
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published
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keywords
L-ALPHA, RICH SIDE, AQUEOUS MIXTURES, VESICLE FORMATION, CRITICAL MICELLE CONCENTRATIONS, LYOTROPIC LIQUID-CRYSTALS, GENERAL PHASE-BEHAVIOR, EQUATION-OF-STATE, SDS-DDAB-WATER, ANGLE X-RAY
in
The Journal of Physical Chemistry Part B
volume
111
issue
48
pages
13520 - 13526
publisher
The American Chemical Society
external identifiers
  • wos:000251291600008
  • scopus:37349023324
ISSN
1520-5207
DOI
10.1021/jp076321f
language
English
LU publication?
yes
id
10899278-9692-4618-9283-01c48ae05791 (old id 968835)
date added to LUP
2008-01-30 08:40:29
date last changed
2017-10-22 04:49:32
@article{10899278-9692-4618-9283-01c48ae05791,
  abstract     = {The coexistence of two lamellar liquid-crystalline phases in equilibrium for binary surfactant-water systems is a rare and still puzzling phenomenon. In the few binary systems where it has been demonstrated experimentally, the surfactant is invariably ionic and the miscibility gap is thought to stem from a subtle balance between attractive and repulsive interbilayer forces. In this paper, we report for the first time a miscibility gap for a catanionic lamellar phase formed by the surfactant hexadecyltrimethylammonium octylsulfonate (TASo) in water. Synchrotron small-angle X-ray scattering, polarizing light microscopy, and H-2 NMR unequivocally show the coexistence of a dilute (or swollen) lamellar phase, L-alpha', and a concentrated (or collapsed) lamellar phase, L alpha ''. Furthermore, linear swelling is observed for each of the phases, with the immiscibility region occurring for 15-54 wt % surfactant. In the dilute region, the swollen lamellar phase is in equilibrium with an isotropic micellar region. Vesicles can be observed in this two-phase region as a dispersion of L alpha' in the solution phase. A theoretical cell model based on combined DLVO and short-range repulsive potentials is presented in order to provide physical insight into the miscibility gap. The surfactant TASo is net uncharged, but it undergoes partial dissociation owing to the higher aqueous solubility of the short octylsulfonate chain. Thus, a residual positive charge in the bilayer is originated and, consequently, an electrostatic repulsive force, whose magnitude is dependent on surfactant concentration. For physically reasonable values of the solubility of the octyl chain, assumed to be constant with surfactant volume fraction, a fairly good agreement is observed between the experimental miscibility gap and the theoretical one.},
  author       = {Silva, Bruno F. B. and Marques, Eduardo F. and Olsson, Ulf},
  issn         = {1520-5207},
  keyword      = {L-ALPHA,RICH SIDE,AQUEOUS MIXTURES,VESICLE FORMATION,CRITICAL MICELLE CONCENTRATIONS,LYOTROPIC LIQUID-CRYSTALS,GENERAL PHASE-BEHAVIOR,EQUATION-OF-STATE,SDS-DDAB-WATER,ANGLE X-RAY},
  language     = {eng},
  number       = {48},
  pages        = {13520--13526},
  publisher    = {The American Chemical Society},
  series       = {The Journal of Physical Chemistry Part B},
  title        = {Lamellar miscibility gap in a binary catanionic surfactant-water system},
  url          = {http://dx.doi.org/10.1021/jp076321f},
  volume       = {111},
  year         = {2007},
}