Adsorption Studies of Molecules on the Halloysite Surfaces: A

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Adsorption Studies of Molecules on the Halloysite Surfaces: a Computational and Experimental Investigation Francesco Ferrante, Nerina Armata, Giuseppe Cavallaro, and Giuseppe Lazzara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12876 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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The Journal of Physical Chemistry

Adsorption Studies of Molecules on the Halloysite Surfaces: a Computational and Experimental Investigation Francesco Ferrante,∗ Nerina Armata, Giuseppe Cavallaro, and Giuseppe Lazzara Dipartimento di Fisica e Chimica - Università degli Studi di Palermo, Viale delle Scienze Ed. 17, 90128 Palermo, Italy E-mail: [email protected]

Phone: +39 091 23897979. Fax: +39 091 590015

Abstract We report the results of joint computational and experimental investigations on the adsorption capability of halloysite towards a set of common molecules (water, alcohols, halides and carboxylic acids). The halloysite system has been modelized by means of a cluster approach choosing a portion of a spiral nanotube; it has a slight curvature, with a convex aluminic layer. The adsorption geometries are described in terms of hydrogen bond network structures; calculated interaction energies invariably indicate that the inner aluminic surface is the place for preferential adsorption of polar molecules. The presence of substitutional defects on the outer or inner surface of the halloysite model causes sometimes slight variations in the adsorption properties. The calculated adsorption energy values confirm that the carboxylic group interacts with the substrate much more strongly than the alcoholic group, which in turn interacts stronger than the halides, a trend which is in agreement with the results obtained by means of thermogravimetric analysis on vapour phase adsorption. ∗

To whom correspondence should be addressed

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Introduction The study about the absorption of organic molecules on natural materials such as clay minerals allows to obtain information on the design of novel functionalized nanocomposites with industrial and technological relevance. Among clays materials, halloysite nanotubes have aroused great interest thanks to their non-toxicity 1,2 and economic availability. 3 Halloysite belongs to kaolinite group and naturally occurs with different morphologies: platy, spheroidal and hollow tubular shape. The dominant shape is the multiwalled spiral-like tubular one, 4,5 with sizes between 500 and 1000 nm in lenght and 15-100 nm in inner diameter depending on the deposit. 6 These special properties make halloysite nanotubes promising entrapment systems for the loading, storage and controlled realease of various species, 7–10 and as filler for bio-nanocomposites. 11–13 From a chemical point of view, halloysite can be described as a 1:1 aluminosilicate layer composed by two building blocks: SiO4 tetrahedra and AlO6 octahedra. The two sheets formed by these building blocks share a common plane of apical oxygen atoms. The ideal stoichiometric unit formula of halloysite is Al2 Si2 O5 (OH)4 ·nH2 O and, according to the classification scheme based on the hydration and the interlayer spacing, the dehydrated form (n=0) is named halloysite-7Å and the hydrated form (n=2) is named halloysite-10Å. The former presents the same interlayer distance observed in flat kaolinite while the increase of the tickness in halloysite-10Å is due to the presence of interlayer water molecules. The inner and outer surfaces of halloysite nanotubes have different chemical and physical properties; indeed, after the curving and rolling up process, whose comprehension is still under debate, 14,15 silico sheet forms the outer surface and alumino sheet forms the inner surface of the tubular system. The twofold nature of halloysite, which is used experimentally to perform selective adsorption, 16–18 can be explored by modeling, employing computational approaches, the adsorption of various organic molecules. Several theoretical investigations concerning the adsorption process of organic species, water molecules and ions on flat exfoliated kaolinite models were reported. 19–22 In this work, however, as far as we know, we report the first study on the adsorption of a water molecule, ethanol, ethyl halides and acetic acid on a model of halloysite which takes into account 2 ACS Paragon Plus Environment

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the curvature of the nanotube. The interest on the adsorption phenomenon of various compounds on the outer and inner surface of the nanotube of halloysite is obviously object of many experimental studies 16–18 . A technique that is widely employed in this field is the Thermogravimetric Analysis (TGA) 8,23 , which was used in this work to obtain information on the adsorption capability of halloysite nanotubes (HNT) toward some alcohols (ethanol, propanol and hexanol), chloralkanes (propyl-, butyl- and pentylchloride) and carboxylic acids with (propionic, butanoic and pentanoic) in vapor phase. This kind of investigation is devoted to assess the influence of the functional group and of the alkyl chains on the adsorption process.

Computational Details All calculations were performed within the density functional theory framework by using the Gaussian09 package. 24 In order to achieve a reliable description of dispersion interactions in the study of adsorption, the M06L exchange-correlation functional proposed by Thrular and coworkers 25 was used, joined with the split valence plus polarization (VZP) basis set by Ahlrichs and coworkers, 26 having contraction schemes: H (4s1p)/[2s1p]; C, O, N (7s4p1d)/[3s2p1d]; Mg(10s6p)/[4s2p] and Al, Si (10s7p1d)/[4s3p1d]. The M06L functional was preferred to the M06-2X one because it allows to take advantage of resolution of identity approximation, helpful when systems with more than 200 atoms are treated; the auxiliary functions corresponding to VZP basis set were used. 27,28 The halloysite model employed in this investigation is a portion of a nanotube spiral arm with stoichiometry H48 Al24 Si24 O126 . It was tailored from a model of a 5 nm diameter halloysite nanotube obtained by replicating along the tube axis a spiral supercell builded and relaxed as described elsewhere. 29 The experimental value of 5.2 Å was applied along the repeating direction. Hydrogen atoms were used to saturate the dangling bonds originated from the cutting and, in order to maintain the curvature own of the spiral arm, the position of the oxygen atoms placed in the Si-Al interlayer were kept frozen during the geometry optimization procedure. We are confident that this



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issue does not affect the general conclusions of the investigation, since adsorption/desorption of small molecules on inner or outer surfaces of the nanotube should not sensibly distort its whole geometry. Further, the width of a layer (i.e. a single spiral arm) of halloysite is large enough in the perpendicular direction to the nanotube axis to minimize possible polarization effects that other layers could have on the adsorption of molecules in the inner and outer surfaces. The model just described is depicted in Figure 1. [Figure 1 about here.] Interaction energies of the adsorbates with the halloysite surfaces were evaluated as difference between the energy of the whole system and the energies of its constituents. All reported interaction energies were corrected for basis set superposition error by means of counterpoise procedure. 30

Experimental Details Halloysite (HNT), ethanol (≥ 99.8%), 1-propanol (≥ 99.5%), 1-hexanol (≥ 99.9%), 1-chloropropane (98%), 1-chlorobutane (≥ 99.5%), 1-chloropentane (99%), butanoic acid (≥ 99%), pentanoic acid (≥ 99%) and hexanoic acid (≥ 99%) were purchased from Sigma Aldrich. All chemicals were used as received without further purification. The TGA experiments were performed by means of a Q5000 IR apparatus (TA Instruments) under the nitrogen flow of 25 cm3 min−1 for the sample and 10 cm3 min−1 for the balance. The weight of each sample was ca. 10 mg. The calibration was carried out by means of Curie temperature of standards as reported elsewhere. 31 The measurements were conducted by heating the sample at a rate of 10 ◦ C min−1 .

Results and Discussion The trend of adsorption energies obtained from the computational investigation will be compared with that of adsorption Gibbs free energies measured by vapour phase TGA experiments. In par4 ACS Paragon Plus Environment

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ticular, the information on the molecules studied in this work will be achieved by extrapolation of measurements performed on sequences of n-alcohols, n-chloroalkanes and n-carboxylic acids with increasing carbon chain lengths. This procedure allowed to discriminate the contribution of adsoprtion from the one arising from lateral interactions between coadsorbed molecules; this, for a proper comparison with computational results. In order to investigate the adsorption of molecules on the halloysite surfaces by a computational approach, the adsorbates were initially located at the center of the halloysite model, either on silicic outer surface or on aluminic inner surface. If compared to the nanotube portion, the investigated molecules are sufficiently small to consider negligible border effects on the adsorption geometry. As a matter of fact, after geometry optimization the adsorbed species remain still far from the edges of the model and interact mainly with its central hexagonal arrangement. In the following, hydrogen bonds will be described by means of the A···B distance, where A is the donor (acceptor) atom of the adsorbed species and B is the acceptor (donor) atom of the halˆ angle formed between the OH moiety of the donor and the O atom loysitic system, and the OHO of the acceptor, being the adsorbed species always on the left of the symbol. Hydrogen bonds with ˆ angle outside the 140-180◦ range were considered as secA···B distance larger than 3 Å and OHO ondary interactions and, even though they are on the whole relevant in determining the adsorption geometry and to some extent its energetic, they will not be described in details.

Adsorption of water Since the adsorption of water molecules on kaolinite-type systems has been experimentally and computationally widely debated, a large amount of information helpful to verify the quality of the model and method used in this work already exists. Particular interest is always addressed to the characterization of the interlayer water of halloysite-10Å, which led to the identification of two kinds of water molecules: hole water, forming hydrogen bonds with basal oxygens of the SiO4 sheet, with a molecular HH vector parallel to the surface, and associated water, which interact with both the hole water and the hydroxyl groups of the octahedral sheet, and have a 5 ACS Paragon Plus Environment


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molecular HH vector almost perpendicular to the surface normal. 32,33 Hu and Michaelides, 34 in a theoretical investigation on the formation of ice on kaolinite by means of the PBE functional and a periodic approach, studied the adsorption of water monomer, water clusters and chains only on the hydrophilic hydroxylated (001) surface. They classified various sites of adsorption: four of them are threefold hollow sites, two are twofold bridge sites and three are onefold top sites. After the full relaxation of water and kaolinite geometries, they proved that the threefold hollow sites are the most stable. From a geometrical point of view, the most stable sites would be those where the water molecule is positionated upright in the plane of the surface normal, donating one hydrogen bond (lenght 1.73 Å) and accepting two hydrogen bonds (ca. 2 Å). The authors reported that the adsorption energy of the water molecule at this site is -56 kJ mol−1 . Tunega et al. studied the interaction of a water molecule with a dickite model, by means of a ONIOM B3LYP:PM3 cluster approach. 35 They found that, when the water molecule is located above the center of the octhahedral cavity, it forms hydrogen bonds with three surface hydroxyl groups. Two of these OH groups are proton donors to the water oxygen atom with distances of about 1.9 Å and they are tilted up with respect to the cavity. The oxygen atom of the third hydroxyl group acts as acceptor with respect one proton of the water molecule. Therefore, the hydrogen atom of this hydroxyl group is tilted out with a distance of 1.628 Å. The tetrahedral SiO4 sheet is able to give only weak interactions with the water molecules, which are oriented with hydrogen atoms directed toward two basal oxygen atoms, with the H-H vector parallel to the surface plane. The distances between the water molecule hydrogen atoms and the basal oxygen atoms are 2.142 and 2.515 Å, respectively. The authors found that the BSSE-corrected adsorption energy of water on the SiO4 sheet side of kaolinite is about -17.2 kJ mol−1 , while the adsorption energies on the AlO6 side is -34.7 kJ mol−1 . According to our calculations, on the silicic surface the water molecule acts as donor of two hydrogen bonds formed with the oxygen atoms of two adjacent SiO4 tetrahedron, with H···O distances of 2.223 and 2.334 Å and OH···O angles of 164◦ and 156◦ , as depicted in Figure 2a. The molecule lies in a plane tilted of about 20◦ with respect to the surface normal. Despite the formation of two hydrogen bonds, the interaction energy is only -11.9 kJ mol−1 , witnessing a not


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sufficient satisfaction of their optimal length. The water molecule can act also as an acceptor, involving the interaction with the stoichiometric hydrogen atom located in the layer underlying the exposed silicic surface (intercalated water, see Figure 2b). Indeed, this hydrogen bond, joined with two secondary interactions with surface O atoms, is sensibly stronger that the one described above, being -18.5 kJ mol−1 the calculated interaction energy. Its geometric parameters are H···O = 1.905 Å and O···HO = 169◦ . A much larger interaction energy, -54.4 kJ mol−1 , has been calculated on the other hand when the water molecule is adsorbed on the aluminic surface. Here, H2 O forms a strong hydrogen bond as donor (H···O distance: 1.782 Å, OH···O angle: 149◦ ) and two hydrogen bonds as acceptor (O···H: 1.938 and 1.969 Å; O···HO 155 and 159◦ ); it places almost symmetrically at the center of the Al6 O6 hexagon, with a hydrogen atom pointing outward, as in Figure 2c. This adsorption geometry of water recalls the one described by Tunega et al., 35 but this time the water molecule shows a larger adsorption energy. [Figure 2 about here.] The results obtained on the adsorption of a water molecule on our halloysite model are quite in agreement with the other computational studies reported in literature, so the model used should give reliable outcomes also for the adsorption of the other kind of molecules investigated in the present work.

Adsorption of ethanol and ethylhalides The optimized adsorption geometries of ethanol on the two halloysitic surfaces are reported on Figure 3. The ethanol molecule can interact with the silicic surface by the formation of one hydrogen bond, with geometry: H···O = 2.106 Å and OH···O = 152◦ . The interaction energy, equal to -11.7 kJ mol−1 , is essentially the same of that of water having the corresponding adsorption geometry; presumably due to the steric hindrance of the ethyl group, ethanol does not show intercalation. In effect, the unique hydrogen bond is sensibly shorter than that of the water case. 7 ACS Paragon Plus Environment


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Also when adsorbed on the inner aluminic surface the ethanol molecule locates in a way that closely resembles that of the water molecule, though less symmetrically: the hydroxyl group acts once as donor, with H···O distance of 1.793 Å and OH···O angle of 146◦ , and twice as acceptor of hydrogen bonds (O···H: 1.903 and 2.002 Å; O···HO 169 and 154◦ ); the ethyl moiety points outward the surface. Also in this case the adsorption energy, -55.5 kJ mol−1 , is much larger than the one calculated for the silicic surface, and is essentially the same as the interaction energy evaluated for water. This would mean that ethanol could compete with water with respect to the adsorption on the halloysitic surfaces, but it must be taken into account that the former, at variance with the latter, can not originate new efficient hydrogen bonds with other molecules of the same type, if a solvent wetted surface is to be considered. [Figure 3 about here.] From the data reported in Table 1 it can be noticed that all ethylhalides, EtX (X = F, Cl, Br), adsorb very weakly on the silicic surface, being the calculated interaction energies comprised in the 5.7-10.3 kJ mol−1 range; it increases with the halogen atomic number. EtCl and EtBr show the same adsorption geometry: they locate with the halogen atom pointing toward the center of the SiO hexagonal arrangement, with distances from the six oxygen atoms comprised in the ranges 3.22-3.75 and 3.38-3.79 Å, respectively. The ethyl moiety is close to the surface and can form two weak CH–O hydrogen bonds, two oxygen atoms bonded to the Si acting as acceptor, with H···O distances equal to 2.633 and 2.805 Å in the case of EtCl, and more symmetric H···O distances of 1.786 and 2.788 Å in the case of EtBr. A different interaction geometry is adopted by EtF: the fluorine atom is too scarcely polarizable to interact with the surface oxygen atoms and it is small enough to intercalate the SiO hexagon and form a very weak hydrogen bond with the intralayer H atom, with F···H of 2.277 Å. The adsorption energies on the aluminic surface are sensibly larger, with EtF showing the stronger interaction. As regards the formation of two not symmetrical hydrogen bonds, where the halogen atom acts as acceptor, all EtX molecules adsorb in a similar manner to EtOH; not having hydrogen bond donor capability, the molecule can rotate and the ethyl group can come 8 ACS Paragon Plus Environment

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close to the surface to give rise to CH–O interactions, with H···O distance comprised between 2.3 and 2.5 Å. EtF is again an exception, since its interaction geometry is forced by more directional hydrogen bonds. For comparison, the hydrogen bond interaction of ethylhalides with water in vacuo has been investigated by using the same theoretical methods employed for the adsorption on halloysite. It resulted that at least three possible configurations exist. In the first of them, whose geometric parameters and energetics are reported in the last rows of Table 1, the water molecule forms hydrogen bond as donor and accepts a weak interaction with a CH2 hydrogen atom; this has been calculated as to be the most stable structure in all three cases. In a slightly less stable configuration, the water molecule locates as on the first geometry but with a different orientation, which results in longer X···H and CH···O distances. Finally, a very small interaction energy has been calculated for a configuration where the water molecule interacts only with the methyl group of the ethyl chain; this geometry does not exist in the case of EtF. It is worth to note that the interaction energy between EtX and the aluminic inner surface of halloysite is almost twice the one calculated for the EtX–H2 O system, indicating that in this surface the ethylhalide molecules are subjected to substantially optimal interactions. [Table 1 about here.]

Adsorption of acetic acid In Figure 4 the optimized adsorption geometries of acetic acid over the halloysitic surfaces are reported. AcOH interacts with the oxygen of the silicic exposed surface as H2 O in the first adsorption mode and EtOH, but with a slightly larger interaction geometry, equal to 16.7 kJ mol−1 . The geometric parameter of the hydrogen bond are H···O = 2.161 Å and OH···O = 142◦ . On the other hand, when adsorbed on the aluminic surface, the AcOH molecule can adopt at least two geometries, both with interactions sensibly stronger than the H2 O and EtOH counterparts, being 78.7 and 68.0 kJ mol−1 the corresponding interaction energies. In the first of them, the acetic acid molecule lies almost symmetrically along a diagonal of the (AlO)6 hexagonal arrangement, and perpendicular to the surface. The carbonyl oxygen atom acts as acceptor (O···H = 1.951 and 2.048 9 ACS Paragon Plus Environment


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Å; H···OH = 165 and 166◦ ) while the carboxylic group as donor (H···O = 1.600 Å; OH···O = 174◦ ) of two and one hydrogen bonds, respectively; the methyl moiety is normal to the surface. In the second, less stable, adsorption geometry AcOH forms instead only two hydrogen bonds, one, presumably very strong, as acceptor (O···H = 1.874 Å; H···OH = 164◦ ) and one as donor to an oxygen atom located outside the central hexagon (H···O = 1.658 Å; OH···O = 160◦ ). [Figure 4 about here.]

Effects of the substitutional Al/Si and Mg/Al defects Kaolinite mineral has a heterogenous surface charge attribuited to the isomorphous substitution of Si4+ by Al3+ on the basal surface, and to the charge on the edges, due to pH depending protonation/deprotonation processes. 36 It has been suggested that in the halloysite species Si4+ /Al3+ substitution could be considered a cause of the spiralization process. 37 Moreover, the isomorphous substitution of Al3+ by Mg2+ on kaolinite has been experimentally proved. 38 According to these aspects and to what reported by Scott et al., 39 in their theoretical work on the adsorption of water and formamide on kaolinite surface, in our model the substitution of a tetrahedral Si4+ by Al3+ and of an octahedral Al3+ by Mg2+ has been carried out. In order to investigate the effects of these substitutional defects on the hydrogen bond strenght, and therefore on the adsorption geometries and energies, three different systems, having a net negative charge, for the H2 O/halloysite, EtOH/halloysite and EtCl/halloysite species have been considered. In the first of them, in the silicic outer surface a Si atom linking the O atom acting as acceptor of hydrogen bond has been substituted by Al; in the second and third system, in the aluminic inner surface the substitution with Mg involved, one at the time, the Al atom which is bonded the O atom acting as acceptor (Ala ) and the other one bonded to the O atom acting as donor (Ald ). The second of these systems has been considered also for the case of EtCl/halloysite, even if the oxygen atom does not act as acceptor. The atomic centers subjected to substitution are highlighted in Figure 3, with the EtOH/halloysite system taken as example. In the H2 O/halloysite


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case, the Mg/Al has been considered also when water is adsorbed on the silicic surface but interacts with the intralayer hydrogen atom. As a matter of fact, the Mg/Ald substitution has a large influence on the adsorption energy of water acting as acceptor of hydrogen bond. Indeed, after the Al/Si substitution in the silicic surface the interation energy of the H-bond donor water molecule increases from -11.9 to -18.5 kJ mol−1 , while after the Mg/Ala substitution on the aluminic surface the magnitude of the same interaction slightly increases from from -54.4 to -57.8 kJ mol−1 . Conversely, when water accepts H-bond, i.e. in the two cases of intercalated water in the silicic surface and of the H2 O···OAld contribution to the interaction with the aluminic surface, the adsorption energy decreases from -18.5 to -1.0 kJ mol−1 and from -54.4 to -38.1 kJ mol−1 , respectively, after the Mg/Ald substitution. As a consequence it seems that this substitutional defect ruled out the presence of intercalated water on the silicic surface; for comparison with the pristine halloysite, in this system the HO···HOMg Hbond length increases to 2.066 Å. In the aluminic surface, instead, as shown in Figure 5, a drastic rearrangement occurs on the optimized geometry: in this surface there are many choiches for the H-bond formation and water does prefer to shift and donate H-bond to a O site far from the one where substitution took place. In this configuration, therefore, water acts as acceptor of one Hbond and as donor of two H-bonds, which is pretty distant from the optimal interaction with the Al surface; so the adsorption energy experiences a decreasing. Hydrogen bond lengths are indeed: O···H = 2.004 Å, H···O = 1.957 and 1.992 Å, while H-bond angles are all around 145◦ . [Figure 5 about here.] When EtOH adsorbs on the substituted silicic surface, the Al/Si substitution has only a small effect on the interaction energy, which becomes -18.3 kJ mol−1 , even if the hydrogen bond lenght is shorter than in the pristine surface, being now 1.913 Å. When adsorption on the aluminic surface occurs, the hydrogen bond strength is slightly larger after the substitution Mg/Ala (-59.3 kJ mol−1 ) and slightly smaller when the substitution Mg/Ald takes place (-50.4 kJ mol−1 ). As a matter of fact, in the first case the EtOH···OMg hydrogen bond lenght is 1.659 Å, sensibly shorter than in the case where the substitution is absent, with a OH···O angle of 153◦ . This shortening is reflected also 11 ACS Paragon Plus Environment


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in the hydrogen bond lengths of the donor part: 1.957 and 1.924 Å, with angles of 158 and 175◦ . When the Mg/Ald substitution is considered, on the other hand, a major distortion of the adsorption geometry occurs: the HO···HOMg hydrogen bond is almost broken (2.581 Å) and the ethanol molecule is tilted toward the zone far from the defect to form one hydrogen bond as acceptor (1.839 Å, 151◦ ) and one as donor (1.885 Å, 164◦ ). Also in this case therefore a rearrangement of the adsorption configuration takes place but the drastic decrease of the interaction energy after the Mg/Ald substitution on the aluminic surface found for water is not observed. The chloroethane molecule adsorbed on the silicic surface with Al/Si substitution is subjected to sensible effects on the interaction geometry and a negligible effect on the interaction energy (-7.3 kJ mol−1 ). Now the molecule is shifted from the center of the SiO hexagon, with distances from five oxygen atoms comprised between 3.32 and 3.69 Å, while the distance from the O atom bonded to the Al substitution is 4.023 Å. In the inner surface, the Mg/Ala substitution lead to a shift of the adsorbed molecule, since in the donor zone one hydrogen bond is stretched and bended (2.454 Å, 159◦ ) while the other is merely shortened (2.586 Å, 148◦ ). The interaction energy is sensibly smaller, -8.7 kJ mol−1 , almost one third of that calculated in absence of substitution. When the Mg/Ald defect is present, however, hydrogen bonds become more symmetric (Cl···HOMg: 2.504 Å, 152◦ ; Cl···HOAl: 2.579 Å, 168◦ ) but the interaction energy, -24.3 kJ mol−1 , is essentially unchanged with respect to the unsubstituted case. In conclusion, the variations of interaction energy values sometimes occurring in the presence of substitutional defects are determined by both the changes in the hydrogen bond networks existing between the adsorbed species and the substrate, and the changes of the intra-layer interactions of the substrate. For example, the Mg/Ala replacement determines the distortion of the geometrical parameters of the hydrogen atom of an OH group bonded to the Mg that leads to the formation of an “intra-ring” interaction between this hydrogen and OH group bonded to an aluminum atom in the opposite location. This interaction is weakened in the case of adsorption of the molecule of water and ethanol while it is reinforced in the case of chloroethane.


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Adsorption experiments in vapor phase The halloysite nanotubes adsorption capacity towards several small organic molecules (n-alcohols, n-chloroalkanes and n-carboxylic acids) in vapor phase was investigated as follows. The HNT powders (ca. 0.2 g) were equilibrated in a desiccator saturated with the selected vapor at 298.1 ± 0.1 K. The amount of organic molecules adsorbed onto HNT surfaces was determined through gravimetric method and vapour release was verified by TGA. Then, we estimated the gas-to-surface adsorption constant as Kads = θ /V P, where V P is the vapor pressure of the adsorbed compounds at 298.1 K and θ is the molar surface coverage, defined by the equation

θ=

Qe MMOM SSAHNT

where Qe is the adsorption efficiency, while MMOM and SSAHNT are the molecular weight of the adsorbed organic molecule and the HNT specific area, respectively. The investigated molecules and the results obtained are reported in Table 2. [Table 2 about here.] Both the chain length and the functional group of the adsorbed compounds were systematically changed with the aim to explore their influence on the adsorption process. The Kads values per area unit of adsorbent were calculated by taking into account the HNT specific surface, which is 65 m2 g−1 . 6 As general result, Kads increases with the chain length of the adsorbed molecule. The affinity of chloroalkanes to the HNT surfaces is lower with respect to the other investigated compounds as evidenced by the smaller Kads values. The energetics of the adsorption process was highlighted by the standard free energy (∆G0ads ), calculated as -RTln(Kads ). The negative ∆G0ads values evidenced that the HNT/organic molecules interactions are spontaneous as expected for the gas-to-surface adsorption of volatile organic compounds onto clay surfaces. As showed in Figure 6, ∆G0ads linearly decreases with the carbon atoms number of the alkyl chain. The slope of the ∆G0ads vs n line is correlated to the influence of the CH2 groups on the adsorption process, while the 13 ACS Paragon Plus Environment


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intercept provides the contribution of the functional groups. In the lower part of Table 2 the linear fitting parameters of ∆G0ads vs n are reported. The intercept data highlight that the chloride/HNT interactions are not favored, while the carboxylate group significantly enhances the adsorption of the molecule onto the nanoclay surfaces. Accordingly, the larger slope for carboxylic acids could be due to a more packed structure of the adsorbed organic compounds ad consequence of the stronger interactions between COOH groups and HNT surfaces. On the other hand, the similar slope values observed for n-chloroalkanes and n-alcohols indicate that the specific interactions between the functional groups (OH and Cl) and the HNT surfaces slightly affect the packing of the alkyl chains and, therefore, their lateral interactions. [Figure 6 about here.]

Conclusion For the first time, in this work, the computational study on molecular adsorption processes was performed on a model that has a curvature which take into account the distortions present in a halloysite nanotube. The adsorption energies obtained are comparable to those found in other computational work on flat systems kaolinite-type but the slight differences highligheted could be attributed to the tetrahedral and octahedral distortions of the curved sistem. The reported calculations suggest that even uncharged polar molecular species preferably adsorb on the halloysite aluminic surface, where the presence of many OH groups allows the formation of a network of multiple hydrogen bonds. High absorption energies are especially recorded for water, ethanol and acetic acid. The ethyl halide, instead, can only behave as acceptors of hydrogen bonds. The presence of defects and the subsequent formation of a net negative charge was also assessed by the appropriate substitutions on the pristine halloysite model. Major interaction energy differences were found in the case of water when making the substitution Mg/Al, where a strong distortion of the hydrogen bonds formed with the surface is noticed when water acts as an acceptor, with a decrease of the adsorption energy. 14 ACS Paragon Plus Environment

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To support the computational results, experimental thermogravimetric investigations on the organic molecules studied in this work were also performed. Through the analysis of the results it was possible to achieve information on the adsorption capacity of the nanotube of halloysite against molecules that carry different functional groups. It was therefore possible to draw up a scale of interaction, namely carboxyl > hydroxyl > halide, a trend which coincides with the one obtained from the computational study. Besides modelistic approximations, the deviations between measured free energies and calculated interaction energies highlight the importance of entropic effects on the adsorption process.

Acknowledgement Funding is gratefully acknowledged by P.O.N. Ricerca e Competitività 2007-2013 - Project: “Nanotecnologie e nanomateriali per i beni culturali (TECLA)” (PON03PE_00214_1)

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Table 2: Measured molar surface coverages (mol cm−2 ), equilibrium constants (mol cm−2 MPa−1 ) and standard free energies (kJ mol−1 ) for the adsorption process of n-alcohols, n-chloroalkanes and n-carboxylic acids on HNT in vapor phase. Bottom of the table: linear fitting parameters of ∆G0ads vs n. n

θ

Kads

∆G0ads

2 3 6

0.9 2.1 1.4

CH3 (CH2 )n-1 OH (1.69 ± 0.08)·102 (4.7 ± 0.2)·102 (1.75 ± 0.08)·105

-12.4 ± 1.4 -15.3 ± 1.5 -24.2 ± 1.7

3 4 5

0.045 0.058 0.038

CH3 (CH2 )n-1 Cl 1.15 ± 0.05 4.9 ± 0.2 11.1 ± 0.5

-0.4 ± 1.0 -3.9 ± 1.1 -5.9 ± 1.2

3 4 5

25 69 233

CH3 (CH2 )n-1 COOH (1.73 ± 0.09)·105 (2.64 ± 0.13)·106 (4.0 ± 0.2)·107

-29.9 ± 1.8 -36 ± 2 -43 ± 2

n-alcohols n-chloroalkanes n-carboxylic acids

slope / kJ mol−1 n−1

intercept / kJ mol−1

-2.95 ± 0.03 -2.77 ± 0.04 -6.74 ± 0.02

-6.4 ± 0.1 7.7 ± 1.8 -9.64 ± 0.06




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Figure 6: Dependence of ∆G0ads on the number of carbon atoms of the alkyl chain for n-alcohols, n-chloroalkanes and n-carboxylic acids. Dashed lines are linear fits.




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Adsorption Studies of Molecules on the Halloysite Surfaces: A

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