Effect of sodium dodecylsulfate monomers and micelles on the stability of aqueous dispersions of titanium dioxide pigment nanoparticles against agglomeration and sedimentationby Yung-Jih Yang, Aniruddha V. Kelkar, Xilan Zhu, Guanrong Bai, Hou T. Ng, David S. Corti, Elias I. Franses

Journal of Colloid and Interface Science

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Accepted Manuscript

Effect of sodium dodecylsulfate monomers and micelles on the stability of aqueous dispersions of TiO2 pigment nanoparticles against agglomeration and sedimentation

Yung-Jih Yang, Aniruddha V. Kelkar, Xilan Zhu, Guanrong Bai, Hou T. Ng,

David S. Corti, Elias I. Franses

PII: S0021-9797(15)00226-X

DOI: http://dx.doi.org/10.1016/j.jcis.2015.02.051

Reference: YJCIS 20286

To appear in: Journal of Colloid and Interface Science

Received Date: 27 January 2015

Accepted Date: 20 February 2015

Please cite this article as: Y-J. Yang, A.V. Kelkar, X. Zhu, G. Bai, H.T. Ng, D.S. Corti, E.I. Franses, Effect of sodium dodecylsulfate monomers and micelles on the stability of aqueous dispersions of TiO2 pigment nanoparticles against agglomeration and sedimentation, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/ j.jcis.2015.02.051

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Effect of sodium dodecylsulfate monomers and micelles on the stability of aqueous dispersions of TiO2 pigment nanoparticles against agglomeration and sedimentation

Yung-Jih Yang,a Aniruddha V. Kelkar,a Xilan Zhu,a Guanrong Bai,a Hou T. Ng,b David S.

Corti,a Elias I. Fransesa,* a School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA b

Printing and Content Delivery Lab, HP Labs, Hewlett-Packard Co., 1501 Page Mill Road, Palo Alto, CA 94304, USA * Corresponding author; franses@ecn.purdue.edu; 765-494-4078; fax 765-494-0805

Highlights  Above the cmc of SDS, TiO2 nanoparticles of size over 200 nm settle without agglomeration.  Above 60 mM in water, SDS micelles produce strong depletion forces among TiO2 nanoparticles.  Depletion forces cause fast flocculation, with no coagulation, and fast sedimentation.  New model of how a fast flocculation rate affects cluster size and sedimentation rate.

Abstract

Hypothesis

As more sodium dodecylsulfate (SDS) monomers adsorb at the water/TiO2 nanoparticles interface, the particles become more stable against agglomeration and sediment more slowly.

SDS micelles are not expected to adsorb on the particles and affect the stability against agglomeration or sedimentation. Since micelles are smaller than the 300 nm TiO2 nanoparticles studied, they may introduce depletion forces which may affect the dispersion stability.

Experiments and Models

Sedimentation times were measured in water and in 100 mM NaCl for SDS concentrations from 0.1 to 200 mM. Adsorption densities of SDS and zeta potentials of particles were measured.

Dynamic light scattering was used to measure average diameters of particles or particle agglomerates. Modeling of sedimentation/diffusion was done to predict sedimentation times of 2 particles. Modeling of agglomeration rates was done to help predict sedimentation rates of clusters.

Findings

At SDS concentrations close to or above the cmc, up to 60 mM in water or 115 mM in 100 mM

NaCl, the nanoparticles sediment most slowly without any agglomeration. At higher micelle concentration, SDS micelle depletion forces are very strong, causing fast flocculation, without coagulation. Then sedimentation occurs much faster. The effective micelle depletant size includes about 4 Debye lengths of the charged micelles or particles.

Keywords: Titanium dioxide, Sodium dodecylsulfate, agglomeration, sedimentation, depletion potential, stability 1. Introduction

Titanium dioxide (TiO2) particles are widely used as a white pigment in the ink industry providing ink layers with good whiteness and opacity (hiding power) because of their high refractive index, 2.7. Over four million tons of TiO2 are used as paint or ink pigments worldwide, with annual sales of $16 billion [1]. Dispersions of these high density (4.2 g/cm3) particles are quite susceptible to settling. In one example (see Section 4.2), TiO2 particles with diameters of 300 nm are predicted to sediment by 1 cm within 15 hours in water if there is no agglomeration, and even faster when there is agglomeration. This property is detrimental for potential ink applications, since effective inks should remain stable for weeks with no stirring needed.

Although reducing the particle size may alleviate this problem, inks of smaller particles can have inferior optical properties, such as color strength, opacity, and transparency [2]. Increasing the ink dispersion viscosity might help slow down the sedimentation rate, but may result in worse inkjet performance. Particle agglomeration can increase the sedimentation rate and affect the 3 performance of the print heads because of channel or nozzle clogging [3]. Producing dispersions that are stable against both agglomeration and sedimentation with particle sizes from 100 to 500 nm is crucial for formulating good quality white inks [4-5].

Most of the literature studies have focused on ways to stabilize TiO2 particles against agglomeration [6-16] by using surfactants or polymers as dispersants. When a dispersant adsorbs on the particles surface, it affects the electrostatic interactions and often introduces additional long-range steric interactions. The DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory and the Fuchs-Smoluchowski model are widely used to predict the stability of an aqueous dispersion of charged particles [17]. Few studies have addressed the particle settling behavior.

Liu et al. investigated the effects of the pH and the ionic strength on the agglomerate size, zeta potential, and sedimentation rate [16]. They showed that different compositions can result in different aggregation and sedimentation behavior. Other authors have shown that particle crystallinity and morphology have no effect on the dispersion stability. The particle chemical composition was found to affect the stability by influencing the particle surface charges [18].