Description
Gregory N. Smith,†,‡ Sarah E. Rogers,¶ and Julian Eastoe†
† School of Chemistry, University of Bristol
Cantock’s Close, Bristol, BS8 1TS, UK
‡ Current address: Department of Chemistry, University of Sheffield
Dainton Building, Brook Hill, Sheffield, South Yorkshire, S3 7HF, UK
ISIS-STFC, Rutherford Appleton Laboratory
Chilton, Oxon, OX11 0QX, UK
g.n.smith@sheffield.ac.uk
The charge of poly(12-hydroxystearic acid) (PHSA) stabilized poly(methyl methacrylate) (PMMA) latexes dispersed in nonpolar solvents is influenced by many variables [1]. However, the effect of varying the volume fraction has not been studied for latexes charged by Aerosol OT (sodium dioctylsulfosuccinate or AOT) surfactant. For other types of charged PMMA latexes [2], the electrophoretic mobility depends on the particle volume fraction. This parameter is crucial for industrial uses of colloids in nonpolar solvents. Charging particles using surfactants is straightforward and versatile [3], so developing a fuller understanding of this process is of great interest.
Phase-analysis light scattering (PALS) measurements were used to study latex electrokinetics, and contrast-variation small-angle neutron scattering (CV-SANS) was used to characterize surfactant-latex interactions. PALS measurements were performed for dispersions of 76 nm diameter PMMA latexes in n-dodecane charged by AOT from volume fractions ranging 4×10–4 to 2×10–1. The electrophoretic mobility strongly depended on the volume fraction, decreasing by an order of magnitude from –4.2×10–10 m2 V–1 s–1 at a volume fraction of 4×10–4 to –0.34×10–10 m2 V–1 s–1 at a volume fraction of 2×10–1.
There are two possible explanations for this observation: one due to fundamental electrokinetics and one due to surfactant-polymer interactions. Ohshima devised an electrokinetic expression that shows how the electrophoretic mobility of charged particles with a constant potential is reduced at higher volume fractions [4]; this is due to the overlap of double layers. However, the reduction in charge could also be caused by there being insufficient free surfactant in solution to charge latexes at high volume fractions. CV-SANS measurements were performed on dispersions of PMMA latexes with deuterium-labeled AOT-d34 surfactant in latex contrast-matched n-dodecane, using an approach developed in previously published studies [5]. The total scattering intensity increases at higher latex volume fractions, but the scattering length density difference between the AOT-d34 in the latex core and the solvent is independent of volume fraction. This shows that the reduction in electrophoretic mobility is due to the overlap of double layers, as predicted from electrokinetics [4]. Without the CV-SANS experiments, it would not be possible to distinguish between the two possible explanations, so this study highlights the value of contrast-variation SANS in soft matter colloid science.
[1] Smith G N, Hallett J E, and Eastoe J 2015 Soft Matter 11 8029
[2] Vissers T et al. 2011 J. Colloid Interface Sci. 361 443; Gillespie D A J et al. 2014 Soft Matter 10 566
[3] Smith G N and Eastoe J 2013 Phys. Chem. Chem. Phys. 15 424
[4] Ohshima H 1997 J. Colloid Interface Sci. 188 481
[5] Smith G N, Heenan R K, Rogers S E, Eastoe J, et al. 2014 Langmuir 30 3422; Smith G N, Rogers S E, Eastoe J, et al. 2015 Langmuir 31 13690
Primary author
Gregory Smith
(University of Bristol)
