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Structure of Hybrid Materials Based on Halloysite Nanotubes Filled with Anionic Surfactants Giuseppe Cavallaro, Isabelle Grillo, Michael Gradzielski, and Giuseppe Lazzara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01282 • Publication Date (Web): 07 Jun 2016 Downloaded from http://pubs.acs.org on June 8, 2016

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The Journal of Physical Chemistry

Structure of Hybrid Materials Based on Halloysite Nanotubes Filled with Anionic Surfactants. Giuseppe Cavallaroa, Isabelle Grillob, Michael Gradzielskic, Giuseppe Lazzara*a a

Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Viale delle

Scienze pad 17, 90128 Palermo, Italy. [email protected] Tel: +39 09123897962 b

LSS Group, Institut Laue-Langevin, 6 rue Jules Horowitz BP 156, F-38042 Grenoble,

Cedex 9, France c

Stranski Laboratorium für Physikalische und Theoretische Chemie, Institut für Chemie,

Technische Universität Berlin, Straße des 17. Juni 124, Sekr. TC 7, 10623 Berlin, Germany

ACS Paragon Plus Environment


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Abstract The structure of pristine halloysite nanotubes (HNT) and ones functionalized by anionic surfactants (sodium dodecanoate and sodium dodecylsulfate) was investigated by Small Angle Neutron Scattering (SANS). These experiments evidenced the structural organization of the surfactants adsorbed onto the HNT cavity and the importance of the surfactant head group. Contrast matching experiments were employed in order to mask the dominant scattering effect of the clay hollow nanotubes and to focus on the surfactant organization within the lumen. Further investigation on the mesoscopic structure of the investigated materials was carried out by Electric Birefringence (EBR), which allowed to study the rotational mobility of both pristine and functionalized HNT. The gained structural insights were used to deduce some relevant properties of the hybrids, such as their surfactant loading, charge and solubilization ability towards hydrophobic compounds. For the latter, the HNT lumen hydrophobization was straightforwardly demonstrated by both Fluorescence Spectroscopy and Fluorescence Correlation Spectroscopy (FCS) using Nile Red as fluorescent probe. This paper correlates the structural properties of the hybrid material with micellar properties based on HNT and anionic surfactants, showing that for the dodecanoate a much more pronounced aggregation tendency within the HNT cavity prevails compared to the dodecylsulfate. The attained knowledge is crucial for designing innovative sustainable nanostructures that are based on ecofriendly halloysite and anionic surfactants that can be used for the solubilization and delivery of hydrophobic compounds from such hybrid materials.

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Introduction Over the last decades clay nanoparticles, mostly plate-like structured, have been investigated comprehensively because their properties may be tuned to obtain green nanomaterials appealing for several technological applications.1,2 Within this field, a recent review1 highlighted that the cylindrical halloysite (HNT), a nanotube, in the scientific realm has been a newly emerging clay, which possesses tuneable surface chemistry, huge specific area and biocompatibility. Due to these characteristics, HNT can be considered a proper filler for green polymer composites,3-5 an efficient catalytic support,6-8 and a suitable adsorbent for water decontamination,9,10 oil recovery11 or for delivery purposes. However, for adsorption and solubilisation it is important to modify the binding/solubilization properties of HNT, as it can be done by the addition of surfactants. HNT has a tubular hollow structure that makes it an effective nanocontainer for encapsulation and controlled release of antibacterial,12-14 anticorrosive,15,16 and selfhealing17 compounds. Chemically, HNT is very similar to the most common plate-like clay kaolin but its aluminosilicate sheets are rolled into tubes because of the presence of two water molecules between the multilayers creating a packing disorder between the neighboring alumina and silica layers.18 However, the reason bringing the flat kaolinite to be rolled into HNT is still unclear.19 As concerns the HNT sizes, the nanotubes are quite polydisperse with length and external diameter of about 1000 nm and 80 nm, respectively.1 It is interesting to note that the structural features of HNT were thoroughly investigated in the solid state by microscopy techniques20 as well as X-ray diffraction (XRD).19,21 Quasi-Elastic Neutron Scattering experiments revealed the peculiar diffusion behavior of the confined interlayer water on HNT.22 The influence of the surfaces’ 3



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modification on the HNT morphology was studied in order to maximize the loading ability toward target molecules.9,20,23 Transmission electron microscopy (TEM) evidenced that the treatment of HNT with sulfuric acid provides an efficient method for controllable enlargement of the lumen diameter, thereby e .g. enhancing the benzotriazole loading.20 On the other hand, very few studies have been reported on the structural analysis of HNT in aqueous solution, despite the fact that it would be important to know how these colloids are present in aqueous medium, e. g. as single nanotubes, aggregates thereof, etc.. Aqueous dispersion is important for processing of HNT but also for applications in solubilization/decontamination. Literature quotes only dynamic light scattering (DLS) experiments on aqueous dispersions of pristine and modified HNT.24 The obtained average translational diffusion coefficient of pristine HNT is 9.4 × 10-13 m2 s-1, corresponding to a hydrodynamic radius of 250 nm. Moreover, it was observed23 that the diffusion dynamics of the nanotubes in water is not altered by selective functionalization of the HNT lumen. However, DLS did not provide detailed information on the structure of the materials because of the limited spatial resolution. It is well known that small angle neutron scattering (SANS) is an appropriate technique to investigate the structural features of hybrid systems based on inorganic nanoparticles.25,26 As concerns the functionalized HNT, SANS studies can provide robust bulk average information on the structure of the hybrids and a straightforward description of the surfactant adsorption onto the HNT surfaces. Especially external contrast variation can be of great help by masking the scattering originating from certain domains of the hybrid structure. Interestingly, HNTs possess a positively charged lumen and a negatively charged outer surface in the pH range between 2 and 8,20 which influence the liquid crystalline phase behavior of aqueous HNT dispersions.27 This peculiarity is due to the 4


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different chemical composition: the external surface is composed of Si−O−Si groups while the inner surface consists of a gibbsite-like array of Al-OH groups. Recent studies28 highlighted that selective modification of the HNT inner surface promotes the affinity between the nanotubes and hydrophobic target molecules. Our previous work9,23 demonstrated that functionalization of the HNT cavity with sodium alkanoates represents an easy strategy to obtain tubular inorganic micelles with efficient solubilization ability towards both aliphatic and aromatic hydrocarbons. For example, we calculated that the toluene removal from aqueous dispersions is increased by ca. 10 % because of the modification of HNT lumen with sodium dodecanoate (NaL).9 This result can be explained by HNT lumen hydrophobization. The Gibbs-Thomson effect observed for the n-decane adsorbed onto the NaL/HNT represents a further indirect prove of the formation of an hydrophobic pocket in the functionalized nanotubes.23 However, so far still missing is a mesoscopic structural picture of the surfactant modified HNTs. This work reports the first SANS study on both pristine and surfactant functionalized HNTs. In particular, we performed SANS experiments on pure clay nanotubes as well as surfactant/HNT hybrids. Sodium dodecyl sulfate (NaDS) and NaL were selected as surfactants in order to hydrophobize the HNT cavity, both being anionic C12-surfactants but differing with respect to their hydrophilic head group. SANS curves of NaL/HNT and NaDS/HNT were studied both in D2O and in a condition matching the HNT. The analysis of the SANS curves allowed us to identify in detail the structure of the surfactant adsorbed into the HNT cavity. These insights were supported by measurements of thermogravimetry, ζ-potential, and electric birefringence (EBR) to gain a comprehensive structural picture. Moreover, we studied the solubilization ability of the pristine and functionalized HNT towards Nile red, which is very sensitive to the 5



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hydrophobicity of the microenvironment as evidenced in oil-in-water emulsions29 and fluorescence correlation spectroscopy (FCS) allow to gain further structural information.

Materials and Methods Chemicals. Halloysite (HNT, purity ≥ 99.5%) was a gift from Applied Minerals. Water was of Millipore grade. D2O was purchased from Eurisotop in 99.9 % isotopic purity. Sodium dodecanoate (NaL, purity ≥ 99.0%), sodium dodecyl sulphate (NaDS, purity ≥ 98.0%) and Nile Red (purity ≥ 99.5%) were Aldrich products. All the chemicals were used without further treatment.

Preparation of surfactant/HNT hybrid materials. Aqueous surfactant solutions were prepared by dissolving 5 g of surfactant (NaDS or NaL) in 250 cm3 of water. Then, 5 g of HNT were added and the obtained dispersion was magnetically stirred for 48 h at 20 °C. Successively, the dispersion was centrifuged to recover the functionalized material. The precipitate then was washed with water several times until the washing water reached the surface tension value of pure water (72 mN m-1). This procedure ensures that eventually no more free surfactant is present and that the encapsulated surfactant is not released within one month at least as evidenced by surface tension measurements over time. Aqueous dispersions of the functionalized nanomaterials were prepared by magnetically stirring for 2 h at 20 °C.

Thermogravimetry (TG). Experiments were performed by using a Q5000 IR apparatus (TA Instruments) under nitrogen flow of 25 cm3 min-1 for the sample and 10 cm3 min-1 for the balance. The 6


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explored temperature interval ranged between 25 and 900 °C at a heating rate of 10 °C min-1. The surfactant loading in the hybrid materials was determined from TG curves by taking into account for the moisture loss and the residual matter at 600 °C as described in the literature.9 TG curves of pristine and modified HNT are reported in the Supporting Information.

ζ-potential. ζ-potential measurements were carried out by means of a Zetasizer NANO-ZS (Malvern Instruments) at 25.0 ± 0.1 °C.

Small angle neutron scattering (SANS). SANS experiments were performed at Institut Max von Laue-Paul Langevin (ILL), Grenoble (France) on the instrument D11. A wavelength of 5.9 Å (fwhm 9%) was selected. The sample to detector (and in parenthesis collimation) distances were chosen as 1.2 m (8 m), 8 m (8 m) and 34 m (34 m) in order to cover the 0.017 to 5.18 nm-1 range of the magnitude of the scattering vector (q). Data reduction was performed on 2D patterns; data were corrected for the detector efficiency using the scattering by a 1 mm H2O sample and the contribution from the empty cell was subtracted. Data reduction was performed with LAMP.30 Absolute scale was obtained using the intensity of water; all corrected patterns being isotropic were radially averaged. The incoherent scattering from the samples was not subtracted from the intensity curves. SANS curves were fitted by means of SASfit 0.94.2 software. All samples were prepared in D2O and in some cases in H2O/D2O mixtures that match the HNT scattering length density (ρ). The experimental ρ was determined by measuring the total scattered intensity of several 7



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HNT dispersions at variable D2O/H2O ratio (See Figure in Supporting information). It should be noted that the experimental ρ value for HNT is very similar to that calculated by considering the chemical formula and the density (δ) of halloysite (Table 1). As concerns the surfactants, critical micellar concentration (cmc) values are reported in Table 1.

Table 1. Critical micellar concentration of the surfactants and properties (molar mass, density and scattering length density) used in the SANS data analysis.

HNT

a

cmc / g dm-3 Molar mass / g mol-1 δ / g cm-3 ρ / 1010 cm-2 294.19a 2.5316b 2.17c; 2.54d

H2 O

18.02

0.9970

-0.558

D2 O

20.03

1.1099

6.37

NaDS

2.31e

288.38

1.1966f

0.589

NaL

6.24e

222.30

1.1005f

0.267

For the unitary cell. bFrom ref [24]. cexperimental. dcalculated. eFrom

ref [31]. fFrom ref [32].

Electric birefringence (EBR). The electric birefringence measurements were performed using a Cober high power pulse generator, Model 606, to generate rectangular pulses of the electric field (1.08 ·105 V m-1). The quartz cuvettes were illuminated with a He/Ne laser (633 nm), the signal detected by a photomultiplier and recorded on a Datalab transient recorder, DL 920. Measurements were done on aqueous dispersions of both pristine and functionalized HNT with a concentration of 0.1 g dm-3. A distribution analysis of EBR data was performed by means of Xpfit. All dispersions were thermostated at 25 °C.

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Fluorescence spectroscopy. The experiments were carried out by using Nile Red as a fluorescent probe which is sensitive to the polarity of its microenvironment.33 The steady-state Nile red fluorescence spectra of the air equilibrated samples were registered with a Fluoromax 4 (Jobin-Yvon) spectrofluorometer (right angle geometry, 1 cm х 1 cm quartz cell) at 25.0 ± 0.1 °C. The excitation wavelength was of 558 nm and the emission spectra were recorded from 590 to 640 nm. The widths of slits were set at 5 and 3 nm for excitation and emission, respectively. The mixtures for the measurements were prepared as described in the following. Known aliquots of a Nile red solution in ethanol were carefully added into dark flasks by a Hamilton microsyringe. After solvent evaporation, the water as well as the aqueous dispersions of the investigated materials were added and equilibrated at room temperature for approximately 3 days under vigorous stirring. For all of the systems, the final concentration of Nile red was 10-8 mol dm-3.

Fluorescence correlation spectroscopy (FCS). FCS measurements were performed with a Leica TCS SMD FCS system with hardware and software for FCS from PicoQuant (Berlin, Germany) integrated into a high-end confocal system Leica TCS SP5 II instrument. The confocal volume (0.135 fL) was calibrated by a measuring the characteristic time of Rhodamine 6G (5 × 10-9 mol dm-3) in water with a known diﬀusion coeﬃcient of 4.0 × 10-10 m2 s-1.34 Excitation of Nile red

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was performed using an Ar ion laser at 514 nm. The obtained correlation functions were fitted with the following expression:35

()

 t G(t) = G(0) · 1 + τc 

γ

−1

()

  1 t  · 1 + 2   S τc  

γ

   

−0.5

(1)

where G(0) and τc are the intercept and the decay time and S accounts for the anisotropy of the confocal volume and is defined as the ratio of its vertical and lateral extension. The exponent γ is 1 for pure diffusion. The diffusion coefficient was calculated from the characteristic τc as:

D = ω02 / 4τc

(2)

with ω0 being the lateral extension of the confocal volume, which was 590 nm.

Results and Discussion The surfactant/HNT hybrids were prepared as described elsewere9,24 with the aim to selectively modify the halloysite cavity and, consequently, to develop tubular inorganic hybrid materials. The HNT lumen hydrophobization in the hybrids was directly proved by fluorescence spectroscopy as well as FCS using Nile red as fluorescent probe. First SANS experiments on aqueous dispersions of pristine HNT and HNT functionalized with anionic surfactants (NaL and NaDS) were performed in order to study their structure in a detailed fashion. Complementary insights on the structural properties of the investigated materials were then obtained also by EBR. Both the structure and hydrophobic character of the lumen of the modified HNT were correlated with the functionalization degree, which depends on the polar head group of

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the surfactant. In particular, the surfactant loading values estimated by TG data (see SI) were 4.5 wt% and 1.2 wt% for NaL/HNT and NaDS/HNT, respectively.

Surfactant/HNT hybrid: lumen hydrophobization Figure 1 shows the emission fluorescence spectra of Nile red aqueous solutions at the concentration of 10-8 mol dm-3.

Figure 1. Emission fluorescence spectra of Nile red aqueous solution at the concentration of 10-8 mol dm-3 in the presence of HNT, NaL/HNT, NaDS/HNT and the pristine surfactants. The concentration of pristine and functionalized HNT was 0.35 g dm-3. The concentrations of NaL and SDS were 0.016 g dm-3 and 0.0043 g dm-3, respectively. For details see the experimental section. 11



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The presence of HNT does not influence the Nile Red fluorescence intensity (Figure 1a) in agreement with the hydrophilic nature of the pristine nanotubes, which are not able to solubilize hydrophobic compounds. On the other hand, we observe a strong increase of Nile red fluorescence intensity (Figure 1a) for both NaL/HNT and NaDS/HNT. Literature reports that the Nile Red fluorescence lifetime markedly decreases with the enhancement in the hydrogen-bonding capability of the medium.36 Therefore, the rise in fluorescence intensity is a consequence of the Nile Red encapsulation within the hydrophobic cavity of the functionalized HNT, where the confinement effect within the may contribute to the enhancement of the fluorescence intensity by protecting the Nile Red more from the aqueous medium. It should be noted that the data reported in Figure 1 refer to aqueous dispersions of both pristine and modified HNT with a concentration of 0.35 g dm-3 (no larger concentrations were investigated because of the very high turbidity of more concentrated dispersions). As concerns the hybrid materials, we estimated that the concentrations of NaL and NaDS were 0.016 g dm-3 and 0.0043 g dm3

, respectively. These values, which were calculated by considering the surfactant

loadings into the HNT, are far below the cmc values. For a comparison fluorescence experiments were performed in the presence of the pristine surfactants at the same concentrations reported above (Figures 1b and 1c), which showed no variation of the Nile red fluorescence intensity. As expected, the Nile red is not solubilized by the surfactant because of the absence of micelles in the solution. The Nile red fluorescence intensity in the NaDS-water system only increases for surfactant concentrations above 2.31 g dm-3, which corresponds to the NaDS cmc.33 Furthermore, we investigated the effect of the functionalized HNT concentration (cfHNT) on the Nile red emission fluorescence signal (FS), which was determined by the 12


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integration of the fluorescence intensity peak. Figure 2 shows that FS monotonically increases with cfHNT approaching a constant value, which indicates the saturation of the hydrophobic lumen.

Figure 2. Integrated fluorescence signal for Nile red (10-8 mol dm-3) as function of the functionalized HNT concentration. Red circles and green rhombi refer to NaL/HNT and NaDS/HNT, respectively. Lines are best fit according to the equations 3 and 4. Inset shows the percent of Nile Red adsorbed by the functionalized HNT as a function of the surfactant concentration (red solid line and green dotted line for NaL and NaDS, respectively). A quantitative evaluation of the affinity of Nile red to the micellar aggregates of the hybrids of NaL-HNT and NaDS-HNT can be provided by the distribution constant of Nile red between the water and the hydrophobic lumen (KN) using the following approach37

FS = FSw · χN,w + FSl · (1-χN,w)

(3)

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where χN,w is the fraction of Nile red in the aqueous phase, while FSw and FSl are the Nile red emission fluorescence signal in water and in the hydrophobic lumen, respectively. KN is related to χN,w as

χN,w =

1 (1 + KN ⋅ cfHNT)

(4)

where cfHNT is the concentration of the HNT in unit of mass(HNT)/mass(water). Equations 3 and 4 were successfully applied to the FS vs cfHNT trends (Figure 2) yielding KN values of 40 ± 12 dm3 g-1 and 4.2 ± 1.8 dm3 g-1 for NaL/HNT and NaDS/HNT, respectively. These results clearly indicate that the affinity of Nile Red to the micellar domains of the HNT hybrid depends strongly on the type of surfactant employed and the free enthalpy of transfer of Nile Red is higher by 2.25 kT (~ 5.6 kJ/mol) for NaL compared to NaDS. Namely, NaL/HNT straightforwardly exhibits a more pronounced Nile Red solubilization ability being that the KN value is one order of magnitude larger than that of NaDS/HNT; the latter is consistent with the stronger hydrophobic character of the NaL/HNT lumen due to larger surfactant loading. It should be noted that encapsulation of Nile Red into the modified cavity is driven by the hydrophobic interactions between the CH2 groups of the surfactants tails and the aromatic ring of the dye molecules. Therefore, the formation of hydrophobic surfactant pockets induced by HNT already well below the cmc is demonstrated. It is interesting to note that here the NaL is hydrophobically more active than the NaDS, despite having one CH2-group less. This then is an indication that the reason for this is most likely to found in a different supramolecular organisation.

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On the basis of the fit parameters of the fluorescence data we determined the adsorption isotherms of Nile Red related to the surfactants (see inset in Figure 2) by taking into account their loading into the functionalized HNT. The differing affinity of NaL and NaDS towards Nile Red could be affected by the peculiar organization of the surfactant adsorbed onto the HNT inner surface. Namely, the surfactant structure induced by HNT could influence the hydrophobicity of the hybrid systems and, consequently, the strength and the amount of Nile Red that can be solubilized into the modified lumen. The Nile Red encapsulation into the hydrophobic cavity of NaL/HNT was further confirmed by FCS experiments (Figure 3).

Figure 3. Normalized FCS decay curves for Nile Red (10-8 mmol dm-3) in water (top) and in aqueous dispersion of NaL/HNT (0.25 g dm-3). 15



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Table 2 reports the diffusion coefficient of Nile Red calculated from the fitting of FCS data. Based on the Stokes-Einstein equation the apparent hydrodynamic radius (Rh) was determined (Table 2).

Table 2. Fitting parameters obtained from the description of the fluorescence correlation functions. G(0)

τc / ms

D / µm2 s-1

Rh / nm

Water

0.194 ± 0.002

0.32 ± 0.08

271 ± 68

0.9 ± 0.2

NaL/HNT

1.216 ± 0.004

17 ± 4

5.1 ± 1.2

48 ± 11

According to literature,38,39 the diffusion decay-time of Nile Red in water is around 0.3 ms for the size of the confocal volume employed by us, while the dynamic behavior of Nile red adsorbed onto NaL/HNT is much slower because of the reduction of the fluorophores’ mobility. Namely, the larger τc value (Table 2) agrees with the hindered diffusion of Nile Red within the HNT cavity. On this basis, both D and Rh of NaL/HNT (Table 2) refer to the dynamics of the fluorescent probe confirming its confinement in the hydrophobically modified lumen. It should be noted that the mobility of the nanotubes was not monitored by FCS experiments because the HNT size is comparable with the confocal volume. Moreover, the G(0) increase observed in the presence of NaL/HNT (Table 2) can be attributed to the encapsulation of Nile Red into the hydrophobic cavity of the modified nanotubes, as that reduces correspondingly the number of independent fluorescent sites in the solution. It should also be noted the actual diffusion process of the Nile Red here will be rather complicated as it is expected

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to occur rapidly along the axis of the HNT, but this process then is overlaid by the much slower translational and rotational diffusion of the HNT.

Structural properties SANS data Figure 4 reports the scattering curve in full contrast (D2O as a solvent) for pristine HNT.

Figure 4. SANS intensity as function of q, the magnitude of the scattering vector, for a dispersion of HNT (concentration of 0.46 g dm-3) in D2O. The solid line represents the best fit according to a hollow cylinder (schematically presented in the Figure; see also eq. 5). Inset shows the distribution of the HNT external radius of a population of such objects estimated from both the SANS data (line) and from the SEM micrographs (circles). The absence of oscillations indicates a relatively large polydispersity of the radii9 that smears out the scattering features expected for a hollow cylinder, as they are for instance seen for well-defined self-assembled nanotubes.40 The SANS curve is well described (Figure 4) by a model of hollow cylinders with a uniform ρ. For such a model the scattering intensity is given by 17



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1

I(q)= N×P(q, Ri, ∆R, σ, L, ρHNT, ρs) + Ibck

Page 18 of 37

(5)

1

where N is the number density of particles determined by the concentration and HNT geometry taking into account for the polydispersity. The form factor of the halloysite is P(q, Ri, ∆R, σ, L, ρHNT, ρs) (see Supporting Information), Ri and L being the inner radius and the nanotube length, ρHNT, and ρs are the scattering length densities (Table 1) of HNT and D2O, respectively. Namely, the hollow cylinder presents ρHNT and ρs as scattering length densities for the shell and the core, respectively, while the fitting parameters are Ri, ∆R, σ, L. For the shell thickness (∆R) a polydispersity, defined as σ = (/2)-1, was taken into account using the Schulz-Zimm distribution. Details on the SANS model and the residuals of the fitting are given in Supporting Information. A structure factor was not considered as we are far below the overlapping concentration (63 g dm-3).24 The obtained L and Ri values were 400 nm and 18 nm, respectively, while the external radius distribution (calculated as Ri+∆R) is centered at ca. 78 nm (Figure 4). It should be noted that the length of 400 nm is at the limit of detection for the given q-range and, therefore, its value does not influence significantly the scattering curves (see Supporting Information). For the latter, it is interesting to note that the results obtained from the SANS data analysis are rather close to those calculated from the statistics based on SEM micrographs (Figure 4), which highlighted the hollow cylinder mesostructure of HNT (some examples are reported in Figure 5).

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Figure 5. SEM images of pristine HNT [3] (a). Distribution of the external radius (b) and length (c) of a population of nanotubes estimated from a statistical analysis of SEM images. Based on geometric considerations, the HNT cavity is ca. 10 vol % of the nanotube.18 This value can be compared with the loading for NaL/HNT (4.5 wt%). Within this, it should be noted that the adsorbed surfactant does not generate a monolayer onto HNT cavity. In such case, the NaL loading should be 1.2 wt % by considering the average specific area of the HNT inner surface (6.9 m2 g-1)20 and the occupied area of the surfactant with a carboxylate head group at the water/alumina interface (0.41 nm2 molecule-1).41 Since the NaL amount contained is much larger, we can conclude that NaL generates organized structures induced by the HNT. In contrast, the amount of NaDS entrapped into the HNT cavity is much lower (1.2 wt%) and in good agreement with surfactant monolayer adsorption onto HNT inner surface, as for considering the head surface area of NaDS (0.52 nm2 molecule-1)42 a theoretical loading of 1.5 wt% is predicted. Based on these data we calculated the ratio (R) between the HNT 19



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functionalization and the surfactant loading corresponding to the formation of one full layer onto the nanoclay inner surface. R values (3.75 and 0.8 for NaL/HNT and NaDS,/HNT, respectively) highlighted that the structural organization of the surfactant loaded into the cavity is affected by the polar head group. Particularly, the carboxylate group induced the formation of complex structures, while monolayer adsorption was estimated for sulphate alkanoate surfactant. Figure 6 shows that the HNT functionalization with both surfactants does not alter the tubular morphology. In addition, the sizes of modified HNT are comparable with those of the pristine nanotubes.

Figure 6. SEM images of NaL/HNT (a) and NaDS/HNT (b)

In order to obtain more detailed structural information regarding the effect of surfactant incorporation onto the halloysite lumen SANS curves (Figure 7) of the functionalized HNT were obtained at different dispersion concentration (c).

20


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Figure 7. SANS intensity normalized for the concentration of the scattering objects as function of q, the magnitude of the scattering vector, for dispersions of functionalized HNT in D2O. Inset shows the details at q range between 0.9 nm-1 and 5 nm-1 in linear intensity scale. The concentration of the scattering objects is systematically changed, while the surfactant/HNT weight ratios are 0.047 and 0.012 for NaL and NaDS, respectively. Figure 7 shows that a peak appears at q = 1.79 nm-1 for the hybrid NaL/HNT system in full contrast. The corresponding characteristic length calculated by the Bragg law is 3.5 21



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nm. Based on the reported length of one NaL molecule (1.7 nm), one may asses that this reflection is compatible with either the formation of layered structures or cylindrical packing of NaL within the HNT lumen. The peak position and its concentration normalized intensity are not affected by the amount of dispersed hybrid material. Based on these findings and keeping in mind that in all cases the NaL concentration is far below its cmc, which is 44.5 mM,43 one can state that these organized structures are present within the lumen and their formation does not depend on the concentration in the studied concentration regime. It is interesting to note that NaDS/HNTs did not provide any peak in the scattering function (Figure 7) in agreement with the lower loaded amount. Besides the peak presence, the scattering curves for NaL/HNT hybrids are very similar to that of HNT once the background is substracted and the intensity is normalized by concentration (see Figure in Supporting Information). SANS curves of modified HNT were successfully fitted by using the hollow cylinder with uniform ρ as model (Figure 8). Supporting Information reports the residuals for the fitting as well as some examples of SANS curves after the background subtraction. Similarly to the SANS data analysis of pristine HNT, the scattering intensity is given by

1

I(q)= N×P(q, Ri, ∆R, σ, L, ρsurf-HNT, ρs) + Ibck

(6)

where ρsurf-HNT is the scattering length density of the cylinder shell, which is composed by HNT and surfactant. Namely, this model assumes that the surfactant is included into the shell of the hollow cylinder, while the core is based on pure D2O. The values of ρsurfHNT

were calculated (2.133×1010 cm-2 and 1.987×1010 cm-2 for NaDS/HNT and

NaL/HNT, respectively) by taking into account the chemical formula and density of 22


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each component (Table 1) as well as the surfactant loading estimated from TG data. Based on the SANS data analysis of pristine halloysite, L and ∆R were fixed at 400 and 60 nm, respectively, while Ri was considered as fitting parameter. The obtained Ri values (Table 3) endowed to estimate the number of surfactant layers (SL) confined into the HNT lumen through the following equation:

SL = (Ri(HNT) – Ri(fHNT))/Ls

(7)

where Ls is the length of the surfactant molecule, while Ri(HNT) and Ri(fHNT) are the inner radii of pristine and functionalized HNT, respectively. We calculated that NaL forms multilayered (ca. 5 layers) structures within the HNT cavity. On the contrary, NaDS/HNT presents 2 layers into the lumen. Figure 8b-c reports a scheme of the surfactants organization within the HNT. The formation of surfactant self-assembled structures onto the HNT is driven by hydrophobic interactions between the alkyl chains and electrostatic interaction between the surfactant head group and the alumina surface. Literature reports that the NaL binding to alumina flat surface is much stronger with respect to that observed for NaDS.44 That the NaL forms a multilayer structure may be attributed to the fact that the COO- group is much better able to interact synergistically with another COO- group than it is the case for the sulfate head groups.

23



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Figure 8. SANS intensity as function of q, the magnitude of the scattering vector, for the dispersions in D2O of modified HNT at various concentrations. Full lines represent the best fits according to a hollow cylinder model (a). Scheme for modified HNT (b). Detail on surfactants organization within the HNT (c).

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Table 3. Inner cylinder radius and surfactant layers into the HNT lumen obtained from fitting the scattering patterns in D2O for modified nanotubes at different concentrations c.a c / g dm-3 Ri(fHNT) / nm SL NaL/HNT

9.8

9.9

4.8

NaL/HNT

19.7

9.8

4.8

NaL/HNT

29.5

9.8

4.8

NaL/HNT

47.8

9.5

5.0

NaDS/HNT

20.0

14.7

1.9

The advantage of the SANS technique is the possibility to change the contrast between the solvent and the scattering objects by exchanging hydrogen with deuterium. In particular, we performed experiments in D2O/H2O mixtures that match the clay nanoparticle ρ. This experiment can straightforwardly highlight the presence of surfactant organized structures in the hybrid material. On this basis, dispersions of HNT, NaL/HNT and NaDS/HNT in D2O/H2O mixed solvent (41 v% D2O, ρ=2.28×1010 cm-2) close to the HNT matching point (ca. 39.4 v% D2O, ρ=2.17×1010 cm-2) were prepared and investigated. Although the HNT scattering intensity was not completely matched, the experimental curve for HNT (see Supporting Information) showed a significantly smaller intensity than the dispersions of modified HNT (Figure 9), which is still influenced by the presence of HNT. The analysis of SANS curves (Figure 9) in contrast variation conditions was carried out by using the same model previously described for the data in full contrast. In detail, SANS curves were successfully simulated by means of a hollow cylinder with uniform ρ. The scattering intensity was expressed as 25



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1

I(q)= N×P(q, Ri, ∆R, σ, L, ρsurf-HNT, ρD20/H2O) + Ibck

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(8)

where ρD20/H2O represents the scattering length density of the prepared D2O/H2O mixture, which is confined within the core of the cylinder. The geometrical parameters (Ri, ∆R, L) and the scattering length density of the cylinder shell (ρsurf-HNT) were fixed to the values determined by the experiments in D2O. The very good agreement between simulated and experimental curves (considering that no adjustable parameters were used) supports the validity of the proposed model, which is sensitive to the amount and organization of surfactant into the HNT. In contrast, simulations assuming different surfactant assembly (5 layers of NaDS and 1 layer of NaL) did not show a good agreement with experimental data (Figure 9). SANS curves after the background subtraction are reported in Supporting Information.

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Figure 9. SANS intensity as function of q, the magnitude of the scattering vector, for the dispersions in D2O/H2O mixed solvent (41 v% D2O) of modified HNT. Full lines represent the simulations according to a hollow cylinder model with parameters determined by the experiments in D2O (2 layers of NaDS and 5 layers of NaL). Broken lines represent the simulations assuming 5 layers of NaDS and 1 layer of NaL.

Electric birefringence. Further insights on the structure of the pristine and functionalized HNT were obtained by EBR experiments. Figure 10 shows the time response of the birefringence to an electric field (1.08 ·105 V m-1) applied in the form of a rectangular voltage pulse at aqueous dispersions of HNT and NaL/HNT. The conductivity values were 3.94 µS cm-1, 4.09 µS cm-1 and 8.91 µS cm-1 for NaL/HNT, NaDS/HNT and pristine HNT, respectively. Because of the nanotubes’ anisometry the applied electric field leads to a partial alignment of the nanotubes, which are free to reorient in suspensions. The 27



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nanotubes’ alignment with the electric field caused an induced transient birefringence. Figure 8 shows that the magnitude of the birefringence (∆n) is much larger for the functionalized nanotubes with respect to the pristine ones. Similarly, the modified HNT showed an increase of charge as evidenced from the ζ-potential values (-32 mV, -41 mV, and -19 mV for NaL/HNT, NaDS/HNT, and HNT, respectively).

Figure 10. Transient electric-field-induced birefringence signals for aqueous dispersion of NaL/HNT and HNT (0.1 g dm-3). The applied electric field is 1.08 ·105 V m-1 and the pulse length is 1.7 ms. Upon termination of the voltage pulse, ∆n exponentially decays with a characteristic relaxation time (τ). The ∆n field free-decay over time (t) was successfully fitted by using the following equation

∆n = ∆n0 exp(-t/τ)

(9)

where ∆n0 is the birefringence magnitude at the instant of electric field termination. Fitting of EBR data for NaL/HNT is reported in Supporting Information. Table 4 reports the τ values for both functionalized and pristine HNT calculated by the equation 9. These data highlight that the presence of the surfactant in the HNT cavity increases

28


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the rotational mobility of the nanotubes. Such effect can be attributed to the increase of the ζ-potential reducing the tendency for HNT clustering.

Table 4. The Relaxation time τ, rotational diffusion coefficient Drot, and the length L for HNT, NaL/HNT and NaDS/HNT. τ / ms

Drot / s-1

L / nm

HNT

5.53 ± 0.02

30.1 ± 0.1

520 ± 2

NaL/HNT

3.454 ± 0.007

48.24 ± 0.09

400 ± 2

NaDS/HNT

2.998 ± 0.007

55.5 ± 0.1

365 ± 2

As reported for single-walled carbon nanotubes45 as well as for rodlike polymers,46 τ might be identified as the rotational relaxation time by the equation

τ = (6Drot)-1

(10)

where Drot is the rotational diffusion coefficient. Equation 10 is valid for dilute systems with free rotation of the particles. Once obtained Drot, the lengths of the investigated nanotubes (Table 4) were calculated by using the diffusivity equation of thin rigid rods of length (L) expressed as

Drot =

 3kBT   L   ln  − 0.8  3  πηL   d  

(11)

where d is the diameter of the rods, η is the viscosity of the solvent, T is the temperature and kB is the Boltzmann constant. Based on SEM results, d was fixed at 178 nm. As general result, L values for both pristine and modified HNT are lower with respect to that estimated from the analysis of SEM images. These results can be explained by 29



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considering that the experimental conditions induced the orientation of only the shorter nanotubes, which preferentially are aligned for small pulse duration. It should be noted that no longer pulses were applied because of the high conductivity of the dispersions (as such longer pulses would lead to a substantial heating). Drot of the investigated nanotubes was compared to the theoretical Drot value of rigid rods with the characteristic sizes of HNT determined by SEM (Figure 5). According to Broersma theory47, the theoretical Drot of rigid rods can be calculated as

Drot =

 3kBT   L   ln  + δ  3  πηL   d  

(12)

−1

L L with δ = -0.662 + 0.917   - 0.050   d d

−2

(13)

L Equation 12 can be used for rods with 2 ≤   ≤ 30. Thus, the Broersma theory is d

appropriate to study the rotational mobility of cylinders with the average HNT sizes (L = 770 nm and d = 178 nm) obtained from the analysis of the SEM images (Figure 5). The corresponding average Drot value is 11.3 s-1, which is lower with respect to the experimental data (Table 4). Moreover, the polydispersity distibutions of τ (Figure 10a) were determined through the distribution analysis (inverse Laplace transformation) of the experimental ∆n vs t trends. Based on the equation 10, we estimated the corresponding experimental Drot distributions (Figure 10b) that were compared with the theoretical one obtained by means of the Broersma theory (equations 12 and 13) taking into account the HNT length polydispersity given by the SEM micrographs (Figure 5). As expected, the experimental Drot distributions are centered at lower values confirming that the ∆n vs t trends reflect the relaxation of only the shorter nanotubes.

30


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Figure 11. Polydispersity distributions of the relaxation time (a) determined by the fitting of EBR data and the corresponding distribution functions of the rotational diffusion coefficient (b) calculated by the equation 10 (green, blue, red curves for NaL/HNT, NaDS/HNT and HNT, respectively). According to Broersma theory (equations 12 and 13), Figure 11b reports also the theoretical distribution function (green curve) of the rotational diffusion coefficient for rigid cylinders with the HNT sizes calculated from SEM (outer radius of 89 nm and length polydispersity reported in Figure 5).

Conclusions In this work, a structural investigation of composite materials based on halloysite nanotubes (HNT) and anionic surfactants (sodium dodecanoate and sodium dodecyl sulfate) is presented. We proved that these mixtures represent a new class of inorganic hybrid material containing micellar aggregates within the HNT cavity. The hydrophobization of the HNT lumen induced by the selective adsorption of the 31



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surfactants onto the halloysite inner surface has been demonstrated by using Nile Red as fluorescent probe. Within this, FCS measurements highlighted the reduction of Nile Red mobility because of its confinement into the HNT cavity. As evidenced by SANS experiments, the head group affects the surfactant organization and, consequently, the hydrophobic character of the HNT lumen. In particular, only the carboxylate group promotes the formation of organized structures, such as densely packed multilayers of sodium dodecanoate within the halloysite cavity, as seem from a correlation peak in SANS. This is presumably due to the better possible favorable interaction of the COO- head groups compared to the situation of the sulfate head groups. In contrast, for the dodecyl sulfate much lower loading and no micelle formation within the cavity is observed. Corroborating our structural model for all composite materials (both in full contrast and in HNT contrast matching conditions) we found that the scattering intensity was well described by a hollow cylinder with uniform scattering density as model. Interestingly, the inner radius of these cylinders is reduced in hybrid materials. The amount of surfactant present in these cylinders is much higher for the dodecanoate than for the dodecyl sulfate. Furthermore, EBR data evidenced the rotational mobility of the nanotubes, where the birefringence signal is much larger for the surfactant filled HNTs. In conclusion, this paper reports the first SANS data on both pristine and modified HNT. We proved that SANS studies can provide a straightforward description of the surfactant adsorption onto the HNT surfaces, which is crucial for designing innovative sustainable nanostructures perspective for the solubilization and delivery of hydrophobic molecules, as the formation of micellar entities within the HNT cavities allows to tune their solubilization properties largely.

Acknowledgement. The work was financially supported by the University of Palermo, PRIN 2010-2011 (prot. 2010329WPF), FIRB 2012 (prot. RBFR12ETL5) and PON Ricerca e 32


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competitività 2007–2013, asse 1. ILL is thanked for granting SANS beamtime. We are grateful to Ms. Carolin Ganas for performing some FCS measurements.

Supporting Information. TG curves of pristine and modified HNT; TG parameters of HNT, NaL/HNT, NaDs/HNT and pristine surfactants; SANS experiments for the determination of the HNT scattering length density; SANS curve of pristine HNT performed in its contrast matching conditions; SANS curves of modified HNT in D2O; SANS curves of HNT and NaL/HNT after background substraction and normalized by concentration in D2O; SANS curves of modified HNT after background substraction in D2O and HNT contrast matching conditions; Residuals for the fit of SANS curves; Fitting of EBR data for NaL/HNT; Model of Hollow cylinder for SANS fitting of pristine and modified HNT.

33



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References (1) (2) (3)

(4)

(5)

(6)

(7)

(8)

(9)

(10) (11)

(12)

(13)

(14)

(15)

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