Modified Halloysite Nanotubes: Nanoarchitectures for Enhancing the Capture of Oils from Vapor and Liquid Phases Giuseppe Cavallaro, Giuseppe Lazzara,* Stefana Milioto, Filippo Parisi, and Vincenzo Sanzillo Dipartimento di Fisica e Chimica, Universita ̀ degli Studi di Palermo, Viale delle Scienze, pad. 17, 90128 Palermo, Italy * S Supporting Information ABSTRACT: We prepared hybrid halloysite nanotubes (HNT/sodium alkanoates) in which the inner cavity of the nanoclay was selectively modified. Physicochemical studies evidenced the interactions between HNT and sodium alkanoates, ruled out clay exfoliation, quantified the amount of the loaded substance, and showed an increase of the total net negative charge, allowing us to obtain rather stable aqueous nanoclay dispersions. These dispersions were exploited as inorganic micelles to capture hydrocarbon and aromatic oils in the vapor and liquid states and were revealed to be nonfoaming but very efficient in encapsulating oils. Here, we have fabricated biocompatibile and low-cost inorganic micelles that can be exploited for industrial applications on a large scale. KEYWORDS: halloysite, gas absorption, inorganic micelles, hydrocarbon solubilization ■ INTRODUCTION For a long time, clay minerals, which are present in all types of sediments and sedimentary rocks, have been an object of scientific interest. Among them are the nanoclays, like imogolite and halloysite that both have a hollow tubular shape, which have a particular relevance for materials design. Imogolite, Al 2 SiO 3 (OH) 4, possesses an external diameter of ca. 25 Å and an internal diameter of ca. 10 Å. 1 In spite of its interesting structure, imogolite shows a certain toxicity, which may limit its applications. 2 Halloysite nanotubes (HNTs), Al 2 Si 2 O 5 (OH) 4 · 2H 2 O, possess a coil structure with an external diameter of ca. 50 nm and an internal diameter of ca. 15 nm. The different chemistry at each surface offers the opportunity to control the selective interaction with ionic molecules. 3,4 Recent studies proved that HNTs are ecological, biocompatible, and dispersible in water even at very high concentrations where they can form liquid crystalline phases. 5,6 HNTs are abundant all over the world and consequently are available at low cost. All of these features (i.e., chemical structure and sustainable cost and biocompatibility) make HNTs very appealing for several purposes. Two main application fields can be identified. The first one concerns the use of HNTs as an additive for polymers to increase the mechanical resistance and to confer an active response ability to external stimuli. 7-11 The second one deals with the suitability of the HNT cavity for catalysis 12 and drug release. 6 In this case, the modification of the HNT internal surface 13,14 was tuned to promote the affinity between nanotubes and target molecules. Very few studies are available on the enhanced solubilization of hydrophobic compounds exhibited by a modified HNT cavity. 14,15 In fact, the preparation of organic/inorganic hybrid materials based on HNTs may be of interest in terms not only because of their sustainability and eco-compatibility but also because they satisfy the demands of several applications. The modification of HNTs is an attractive and big challenge, with the goal being (1) to produce rather stable dispersions for applications where nanocontainers and nanocarriers are required to oppose the sedimentation of pristine HNTs and (2) to direct molecular species into the internal space to fabricate the desired nanostructures for the purposes of interest. In this work, we designed, prepared, and physicochemically characterized HNT/sodium alkanoates hybrids. Because their inner surface is positively charged and external surface is negatively charged over a wide pH range, 6 we chose anionic compounds to make a selective hydrophobic modification to the cavity. Among various anionic surfactants, we selected sodium alkanoates obtained from the neutralization of fatty acids, which are biocompatible. Moreover, these surfactants are available with different hydrophilic-hydrophobic balances. We employed sodium undecanoate, sodium dodecanoate, and sodium tetradecanoate to tune and manipulate the degree of the hydrophobicity of the nanotubes core. Thermogravimetry, SEM, and FTIR spectroscopy were used to define the composition and morphology of the as-prepared hybrid materials. Dynamic light scattering, ζ potential, and turbidim- etry allowed us to investigate the stability of their aqueous dispersions. These hybrids successfully entrap aliphatic and aromatic hydrocarbons, prototypes of hydrophobic contami- nants and solvents, from gas and liquid phases. The developed biocompatible inorganic micelles are appealing for industrial Received: October 23, 2013 Accepted: December 6, 2013 Published: December 6, 2013 Research Article www.acsami.org © 2013 American Chemical Society 606 dx.doi.org/10.1021/am404693r | ACS Appl. Mater. Interfaces 2014, 6, 606-612