Structure of Micelles Calcium Didodecyl Sulfate: A SAXS Study

Priyadarshi Mahapatra

doi:10.4172/2157-7439.1000307

Abstract

This paper reports the structure of micelles of Calcium Didodecyl Sulfate (CDS), Ca(DS)2 , as studied using Small Angle X-ray Scattering (SAXS). CDS is a dianionic surfactant consisting of two DS- tails attached to Ca++ divalent ion. There is considerable interest in understanding the structure (especially the outer shell of counter-ions) of micelles of CDS as unlike conventional surfactants (e.g. CTAB, SDS) where counter-ion is monovalent, CDS has a divalent counter-ion. SAXS is an ideal technique for obtaining information about the outer shell of the Ca(DS)2 micelle, as the constituents (S, Ca, O etc.) of the shell are strong X-ray scatterers. The SAXS measurements have been made on salt-free aqueous solutions of calcium didodecyl sulfate for surfactant concentrations of 0.5, 1.0, 2.5, and 20 weight % (or 8.8, 17.7, 44.92 and 438.01 mM/dm3 ) respectively. Single step indirect Fourier transformation method has been utilized to generate particle distance distribution function. It is found that micelles are prolate ellipsoidal in shape. The size parameters of the ellipsoidal micelles have been determined. Other relevant parameters like mean aggregation number and effective fractional charge has been determined by fitting an ellipsoidal shaped core-shell model to the Fourier transformed scattering data. It is seen that increase in surfactant concentration results in lowering of aggregation number, increase of shell thickness and lowering of total charge and probably lowering of water association.

Highlights

Surfactant molecules (e.g., CTAB, SDS etc.) in dilute aqueous solutions self assemble to form variety of supra-molecular structures such as micelles, vesicles and liquid crystalline structures [1,2,3]
This paper reports the sizes and shapes of Ca(DS)2 micelles as obtained from Small Angle X-ray Scattering (SAXS) studies
The Pair Distance Distribution Function (PDDF) obtained by inverse Fourier Transformation is similar to what one expects from a prolate ellipsoidal micelle with a core shell structure [17]

Summary

Introduction

Surfactant molecules (e.g., CTAB, SDS etc.) in dilute aqueous solutions self assemble to form variety of supra-molecular structures such as micelles, vesicles and liquid crystalline structures [1,2,3]. This study deals with aggregation behavior of calcium didodecyl sulfate, Ca(DS), which constitutes of a cylindrical monomer [7,8] Anionic surfactant molecules such as Sodium Dodecyl Sulfate, NaDS, ionize in aqueous solution and the corresponding micelles are aggregates of DS- ions [2,9]. The intensity A(q) of X-rays scattered from a micellar solution is expressed in terms of the elementary scattering amplitude A(q) of the micelles and of the structure factor S(q) as given in various sources [19,22] as. The volumes and hydration numbers used were taken from various sources [27,28,29,30] Another important free-parameter ξ =∆ρc ∆ρsh was introduced to account for scattering lengths in the core and shell regions. In the above equation Ii,app and Ii,model denote the approximated and fitted intensities, respectively, I i,app is the mean value of the approximated intensities

Results and Discussion

Background

Conclusions