Abstract
The influence of ionic liquid (IL) anions and cations on the charging and aggregation properties of layered double hydroxide (LDH) nanoparticles was systematically studied. Surface charge characteristics were explored using zeta potential measurements, while aggregation processes were followed in dynamic light scattering experiments in aqueous IL solutions. The results revealed that the aggregation rates of LDHs were sensitive to the composition of ILs leading to IL-dependent critical coagulation concentration (CCC) values being obtained. The origin of the interparticle forces was found to be electrostatic, in line with the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, as the experimental aggregation kinetics were in good agreement with the predicted data. The ion specific adsorption of IL anions led to different surface charge densities for LDHs, which decreased in the order Cl- > Br- > DCA- > SCN- > NO3- for counterions and BMIM+ > BMPYR+ > BMPY+ > BMPIP+ in the case of coions resulting in weaker electrical double layer repulsion in these sequences. Since van der Waals forces are always present and their strength does not depend significantly on the ionic strength, the CCC values decreased in the above order. The present results shed light on the importance of the interfacial arrangement of the IL constituent ions on the colloidal stability of particle dispersions and provide important information on the design of stable or unstable particle-ionic liquid systems.
Highlights
Numerous layered double hydroxide (LDH) applications based on the intercalation or exchange of specific guest molecules have been comprehensively studied in the past decades
Antiinflammatory, anti-cardiovascular and anti-cancer drugs were successfully immobilized in LDH supports and the obtained hybrids were used for biomedical therapies.[7,8]
The surface charge properties and colloidal stability of positively charged LDH particles were investigated in the presence of aqueous ionic liquid (IL) solutions by electrophoresis and dynamic light scattering (DLS), respectively
Summary
Numerous LDH applications based on the intercalation or exchange of specific guest molecules have been comprehensively studied in the past decades. The extent of ion specific adsorption and its effect on the stability can be ordered in the Hofmeister series of anions and cations This theory was originally developed to describe the influence of salt constituents on the stability of protein solutions as follows.[38]. The CCC of particles in IL solutions were found to be sensitive to the type of ion.[31,32,34] the traditional Hofmeister series of ions developed for the destabilization effect of ionic species in particle dispersions can be extended by using IL constituents This issue can be adequately addressed in aqueous IL solutions by investigating the charging characteristics and aggregation properties of the colloidal particles.[31,32,34] no similar investigations have been performed with LDH particles yet.
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