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Ion Specific Effects on the Stability of Layered Double Hydroxide Colloids

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Ion Specific Effects on the Stability of Layered Double Hydroxide Colloids Soft Matter View Article Online PAPER View Journal | View Issue Ion specific effects on the stability of layered double hydroxide colloids Cite this: Soft Matter, 2016, 12,4024 Marko Pavlovic,a Robin Huber,a Monika Adok-Sipiczki,a Corinne Nardinab and Istvan Szilagyi*a Positively charged layered double hydroxide particles composed of Mg2+ and Al3+ layer-forming cations À and NO3 charge compensating anions (MgAl–NO3-LDH) were synthesized and the colloidal stability of their aqueous suspensions was investigated in the presence of inorganic anions of different charges. The formation of the layered structure was confirmed by X-ray diffraction, while the charging and aggregation properties were explored by electrophoresis and light scattering. The monovalent anions adsorb on the oppositely charged surface to a different extent according to their hydration state leading to the ClÀ 4 À À À NO3 4 SCN 4 HCO3 order in surface charge densities. The ions on the right side of the series induce the aggregation of MgAl–NO3-LDH particles at lower concentrations, whereas in the presence of the left Creative Commons Attribution 3.0 Unported Licence. ones, the suspensions are stable even at higher salt levels. The adsorption of multivalent anions gave rise to charge neutralization and charge reversal at appropriate concentrations. For some di, tri and tetravalent ions, charge reversal resulted in restabilization of the suspensions in the intermediate salt concentration Received 14th December 2015, regime. Stable samples were also observed at low salt levels. Particle aggregation was fast near the charge Accepted 7th March 2016 neutralization point and at high concentrations. These results, which evidence the colloidal stability DOI: 10.1039/c5sm03023d of MgAl–NO3-LDH in the presence of various anions, are of prime fundamental interest. These are also critical for applications to develop stable suspensions of primary particles for water purification processes, www.rsc.org/softmatter with the aim of the removal of similar anions by ion exchange. This article is licensed under a Introduction later stage for their facile removal from the reaction mixture. The effect of anions on the colloidal stability thus needs to be Open Access Article. Published on 08 March 2016. Downloaded 9/29/2021 12:51:04 PM. Layered double hydroxides (LDHs) are anionic clays composed comprehensively understood to control particle aggregation. of mixed positively charged lamellar hydroxides of divalent The classical theory developed for aqueous suspensions of (e.g.,Mg2+,Zn2+ or Ca2+) and trivalent (e.g.,Al3+,Fe3+ or Cr3+) charged colloidal particles by Derjaguin, Landau, Verwey metal ions with charge compensating anions between the and Overbeek (DLVO) has proven to be suitable to predict layers.1 LDH particles and their composite materials are widely colloidal stability in the presence of electrolytes.14,15 It states used in energy storage,2,3 medical treatments,4–6 coating,7 that the acting interparticle force is the superposition of the catalysis8–10 and the removal of water pollutants.11–13 For the repulsive double layer and attractive van der Waals forces. latter application, their relatively high anion exchange capacity The first one originates from the overlap of electrical double À 2À is utilized to capture various types of anions (e.g.,NO3 ,SO4 , layers forming around the charged particles dispersed in a 2À 3À 3À SeO4 ,PO4 and AsO4 ) during water purification processes. salt solution. Its magnitude decreases with an increase in the Ion exchange can take place by intercalation between the layers ionic strength due to the screening effect of the counterions and also by adsorption on the particle surface. To achieve high on the surface charge. The attractive van der Waals forces are efficiencies of these procedures, stable LDH suspensions con- independent of the electrolyte concentration and only depend taining primary particles are required during the ion exchange on the size and composition of the particles. Accordingly, process. In addition, the particles have to be aggregated at a aqueous colloids are stable at low ionic strength and unstable at higher electrolyte concentrations, while these stability regimes are separated by the critical coagulation concentration a Department of Inorganic and Analytical Chemistry, University of Geneva, (CCC).16 30 Quai Ernest-Ansermet, CH-1205 Geneva, Switzerland. The presence of simple salts of different compositions can E-mail: [email protected]; Tel: +41 22 3796031 b Institut des Sciences Analytiques et de Physicochimie pour l’Environnement et les lead to different CCCs. This ion specificity on particle aggrega- Mate´riaux, Universite´ de Pau et des Pays de l’Adour, 2 Avenue du Pre´sident Angot, tion usually follows the tendency described by the Hofmeister 17–19 F-64053 Pau, France series for anions and cations as 4024 | Soft Matter, 2016, 12, 4024--4033 This journal is © The Royal Society of Chemistry 2016 View Article Online Paper Soft Matter 2À 2À À À À À À SO4 o HPO4 o HCO3 o Cl o Br o NO3 o I For LDH particles, interactions with multivalent anions (e.g., 2À 2À 2À 3À 3À À À CO3 ,SO4 , CrO4 ,PO4 and AsO4 ) have been extensively o ClO4 o SCN investigated to determine the adsorbed amount and to clarify 45–51 + + + + + + + 2+ 2+ the adsorption mechanism. In spite of the large number of N(CH3)4 o NH4 o Cs o Rb o K o Na o Li o Mg o Ca such studies, no comprehensive investigation on the effect of For the correct interpretation of this order, one has to consider anion adsorption on aggregation kinetics and related colloidal the charge and the hydrophobicity of the particle surface. stability has been published yet. Accordingly, ions located on the right side of the series (e.g., In the present research, we aimed at investigating the surface SCNÀ and Ca2+) stabilize negatively charged, hydrophobic charge properties and aggregation of LDH particles composed of 2+ 3+ À particles in aqueous suspensions leading to a higher CCC, Mg and Al metal ions and NO3 interlayer anions (denoted as 2À + + while the ions on the left side (e.g.,SO4 and N(CH3)4 ) MgAl–NO3-LDH) in the presence of the K salts of various mono- À aggregate the particles at lower concentrations (direct Hofme- and multivalent anions. The NO3 intercalated LDHs are widely ister series). The order is reversed (indirect Hofmeister series) applied, since other anions can be easily intercalated between for negatively charged, hydrophilic particles. These tendencies the layers by ion exchange. Electrophoretic and dynamic light were established by aggregation studies with negatively charged scattering experiments were performed in aqueous suspensions latex,20,21 clay,22,23 silver iodide24 and titania.25 For positively to explore possible ion specific effects on the colloidal stability of charged, hydrophobic particles, the indirect Hofmeister series the samples over a wide range of electrolyte concentrations. On has to be considered, i.e., ions on the right side of the series the basis of the measured CCCs, we have clarified the Hofmeister induce aggregation at low concentrations, whereas the ones on series and the SchulzeÀHardy rule for LDH particles. the left side are able to destabilize suspensions only at high CCC. The order is reversed and the direct Hofmeister series is valid for positively charged, hydrophilic particles. The inter- Experimental section 20,26,27 28 29 Creative Commons Attribution 3.0 Unported Licence. action of latex, peptide and gold particles of positive Materials charge with ions from the Hofmeister series followed the rule. Analytical grade KCl, KSCN, K2HPO4 (Acros Organics), KHCO3 The effects of sodium salts of different anions (SCNÀ,BrÀ, (Alfa Aesar), KNO3,K2SO4,K2HAsO4,K3Fe(CN)6 and K4Fe(CN)6Á3H2O ClÀ,SO 2À, HPO 2À and CO 2À) within the Hofmeister series on 4 4 3 (Sigma-Aldrich) were used. The salt solutions were prepared by the aggregation of weakly charged LDHs have been also inves- dissolving the calculated amount of solid in ultrapure water tigated.30 The reported CCC values showed good agreement (Millipore) and the pH was adjusted to 9 with 0.1 M KOH with the tendency predicted by the indirect Hofmeister series (Sigma-Aldrich) solution. This pH was also maintained in all for positively charged, hydrophobic particles. However, only a stock suspensions and water was used for sample preparation weak dependence on the type of salt was discovered for mono- throughout the experiments. The protonation constants (pK)of This article is licensed under a valent ions. The presence of multivalent anions decreased the inorganic anions together with their actual charge at pH 9 are CCCs significantly. This fact indicates the importance of the presented in Table 1. All stock solutions were filtered using a counterion valence on the colloidal stability of the particles, 0.1 mm pore-size filter (Millipore) prior to sample preparation Open Access Article. Published on 08 March 2016. Downloaded 9/29/2021 12:51:04 PM. which has to be treated differently from the monovalent case. and the measurements were carried out at 25 1C. Although the DLVO theory takes the valence of ions into The MgAl–NO3-LDH particles were prepared using the account through the ionic strength, the type of salt (i.e., ion 52–55 coprecipitation method. The solutions of Mg(NO3)2 and specificity) is not considered. Accordingly, electrical double Al(NO3)3 (Sigma-Aldrich) were added dropwise under vigorous layer forces decrease with the valence of counterions at the stirring to N2-blanketed 1.0 M NaOH (Acros Organics) until the same salt concentrations and such a decrease leads to a final pH reached 10. The resulting suspension was subjected to dependence of the CCCs on the valence (z)as 1 Table 1 Protonation constants and calculated actual charges of the CCC / n (1) z inorganic anions used in the present study 31,32 a b b b c This is the so-called SchulzeÀHardy rule and n can Anions pK1 pK2 pK3 Valence at pH 9 vary between 2 and 6 depending on the surface charge density ClÀ À0.71 — — 1.00 33–35 35–39 À of the particles.
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