Eur. Phys. J. E (2012) 35: 55
https://doi.org/10.1140/epje/i2012-12055-4

Regular Article

Small-angle neutron scattering study of temperature vs. salt dependence of clouding in charged micellar system

¹ Solid State Physics Division, Bhabha Atomic Research Centre, 400 085, Mumbai, India
² Laboratory for Neutron Scattering, Paul Scherrer Institut, CH-5232, PSI Villigen, Switzerland

^* e-mail: vkaswal@barc.gov.in

Received: 25 January 2012
Revised: 4 April 2012
Accepted: 5 June 2012
Published online: 6 July 2012

Abstract

Clouding is studied by small-angle neutron scattering (SANS) on a charged micellar system of sodium dodecyl sulphate (SDS) and tetrabutylammonium bromide (TBAB) with varying temperature and salt NaCl. We show that the clouding occurs as a result of increase in the attractive potential between the micelles mediated by the dehydrated TBA⁺ counterions on increasing temperature and in the presence of salt. Both micelles and clusters coexist at cloud point temperature (CP) and beyond CP. The addition of salt can be used to obtain CP at room temperature (30 ^° C). The relative effect of different salts on clouding has been found in the order CaCl₂ > MgSO₄ > Na₂SO₄ > NaF > NaCl > KCl > CsCl > NaBr > NaNO₃ . This order is explained on the basis of two important roles played by salt ions: i) counterion condensation that increases the size of the micelles and ii) dehydration of TBA⁺ counterions by salt ions for bridging the micelles.