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

Bojana Katana,Dóra Takács,Istvan Szilagyi,Tamás Szabó,Andrej Jamnik,Edit Csapó

doi:10.1021/acs.jpcb.0c07885

Bojana Katana, Dóra Takács + Show 4 more

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https://doi.org/10.1021/acs.jpcb.0c07885

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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.

Highlights

Halloysite nanotubes (HNTs) represent an interesting class of materials due to their advantageous properties such as biocompatibility, abundant surface functional groups, and availability in large scale, which are exploited in various applications.[1−3] HNTs are aluminosilicates of tubular structure with two different surface charge states.[4]
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 in the colloidal stability of the HNT particles in both inorganic salt and ionic liquid (IL) solutions
The validity of the Hofmeister series will be clarified for simple ions, and the series will be extended with IL cations

Summary

Introduction

Halloysite nanotubes (HNTs) represent an interesting class of materials due to their advantageous properties such as biocompatibility, abundant surface functional groups, and availability in large scale, which are exploited in various applications.[1−3] HNTs are aluminosilicates of tubular structure with two different surface charge states.[4]. Ions of dissolved salts are able to interact with surfaces, thereby modifying the charge density to different extents.[24−26] Such a variation in the surface charge affects the aggregation processes and, the colloidal stability of the samples. These phenomena prompted the establishment of the Hofmeister series of cations and anions to order their effect on particle aggregation processes. This theory was originally developed for the destabilization of protein solutions by inorganic salts,[27] but it has proven to be a powerful tool to predict the stability of particle dispersions on the basis of the type of co- and counterions.[19−21,28−32] The original Hofmeister series were established as follows: F− < Cl− < Br− < NO3− < I− < ClO4− < SCN−

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