Ion Specific Effects on the Stability of Halloysite Nanotube Colloids—Inorganic Salts versus Ionic Liquids

Abstract

Charging and aggregation processes were studied in aqueous dispersions of halloysite nanotubes (HNTs) in the presence of monovalent inorganic electrolytes and ionic liquid (IL) constituents. The same type of co-ion (same sign of charge as HNT) was used in all systems, while the type of counterions (opposite sign of charge as HNT) was systematically varied. The affinity of the inorganic cations to the HNT surface influenced their destabilizing power leading to an increase in the critical coagulation concentration (CCC) of HNT dispersions in the Cs+ < K+ < Na+ order. This trend agrees with the classical Hofmeister series for negatively charged hydrophobic surfaces. For the IL cations, the CCCs increased in the order BMPY+ < BMPIP+ < BMPYR+ < BMIM+. An unexpectedly strong adsorption of BMPY+ cations on the HNT surface was observed giving rise to charge neutralization and reversal of the oppositely charged outer surface of HNT. The direct Hofmeister series was extended with these IL cations. The main aggregation mechanism was rationalized within the classical theory developed by Derjaguin, Landau, Verwey, and Overbeek, while ion specific effects resulted in remarkable variation in the CCC values. The results unambiguously proved that the hydration level of the surface and the counterions plays a crucial role in the formation of the ionic composition at the solid–liquid interface and consequently, in the colloidal stability of the HNT particles in both inorganic salt and IL solutions.

Influence of adsorption of ionic liquid constituents on the stability of layered double hydroxide colloids

“…Previous studies [30][31][32][33][34] on other particle-IL systems pointed out that ion specific effects, similar to inorganic electrolytes, [35][36][37] are important in the presence of IL constituents since they govern the adsorption of ions from solutions. Moreover, IL cations and anions can be arranged in sequences based on their adsorption affinity to a given surface.…”

“…Moreover, IL cations and anions can be arranged in sequences based on their adsorption affinity to a given surface. 31,32,34 For standard inorganic salts, 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.…”

“…45 In addition, surface adsorption of cations followed the above order and results of calculations revealed that the hydration of both the surface and the ions played a major role in the adsorption mechanism. 46 Particle aggregation studies 31,[34][35][36][37]47,48 shed light on the fact that critical coagulation concentration (CCC) data determined with the same particles in different electrolyte solutions can be ordered in the Hofmeister series too. However, the DLVO (Derjaguin, Landau, Vewey and Overbeek) theory, 49 which describes the surface forces in the presence of salt constituents, failed to predict the dependence of such forces on the type of salts present and consequently, the trends in CCC values measured in various salt solutions.…”

“…31,47,[50][51][52] Besides, the CCC of particles in IL solutions were also found to be sensitive to the type of ions. 31,32,34 Therefore, the traditional Hofmeister series of ions developed for the destabilization effect of ionic species in particle dispersions can be extended with IL constituents. This issue can be adequately addressed in aqueous IL solutions by investigating the charging characteristics and aggregation properties of the colloidal particles.…”

“…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 Nevertheless, no similar investigations have been performed with LDH particles yet.…”