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Peptizing Mechanism at the Molecular Level of Laponite Nanoclay Gels Philip Kensbock, Dan Eugen Demco, Smriti Singh, Khosrow Rahimi, Radu Fechete, Andreas Walther, Annette M. Schmidt, and Martin Moeller Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03592 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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Langmuir

Peptizing mechanism at the molecular level of Laponite nanoclay gels Philip Kensbock1, Dan Eugen Demco*1,2, Smriti Singh1, Khosrow Rahimi1, Radu Fechete3, Andreas Walther1, Annette Monika Schmidt2, Martin Möller*1 1

DWI-Leibniz-Institute for Interactive Materials, e.V., RWTH-Aachen University,

Forckenbeckstraße 50, D-52074 Aachen, Germany 2

University of Köln, Institute of Physical Chemistry, Luxemburger Str. 116, D-50939

Köln, Germany 3

Technical University of Cluj-Napoca, Department of Physics and Chemistry, 25 G.

Baritiu Str., RO-400027, Cluj-Napoca, Romania

*Corresponding authors: Fax: +49-241-233-01 E-mail addresses: [email protected] and [email protected]

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ABSTRACT: In the presence of additives like etidronic acid (1-hydroxy ethane-1,1-diphosphonic acid, HEDP) a process of peptizing of Laponite clay gels takes place. The peptizing process at molecular level was directly revealed by the

31

P and 1H high-resolution magic-angle sample

spinning (HRMAS) NMR spectroscopy. Two NMR spectral components were detected and assigned to free etidronic acid and bound to the Laponite disk edges. Furthermore, with increase of temperature the ratio of bound-to-free etidronic acid increases. This thermal activation process could be explained by the increase in electrical polarization of the hydroxyl group at the edges and by the exfoliation of the tactoids that leads to more access to the additive molecules to the electrical charges of platelet edges. Phosphorous-31 HRNMR spectroscopy on sodium-fluorohectorite with aspect ratio of ~750 shows a reduction of the bound etidronic acid molecules. Transmission electron microscopy (TEM), field-emission scanning microscopy (FESEM), UV-Vis spectrophotometry, dynamic light scattering (DLS) and zeta potential results support the proposed peptizing mechanisms.

Keywords: Nanoclay; Laponite; Peptizing; Etidronic acid;

31

P and 1H NMR spectroscopy;

TEM; FESEM; Dynamic light scattering; Zeta potential.


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I. INTRODUCTION Clays are natural or synthetic lamellar inorganic crystals that give rise to colloidal suspension.1-3 These particles having nanosizes within a defined range of concentration form gels out from water dispersions.4 These dispersions have found a broad range of applications in oil drilling, as paints and ceramic additives, in agriculture, cosmetic, pharmaceutical, paper and polymer film industry.5-7 Furthermore, it has also been used for scavenging amino acid from soil by the processes of amino acid adsorption at the clay/water interfaces.8 Nanoclays have also been explored as carriers for organic molecules like the delivery agent of chemotherapeutics to cells9 and as fluorescent reporters.10 A well-defined model for the clay family is represented by the Laponite RD which is a synthetic

version

of

hectorite

clay

with

the

empirical

formula

(Na0.7+[Si8Mg5.5Li0.3O20(OH)4]0.7-).11 The Laponite RD clay has on average a disk-like shape with an average diameter of 25 nm and a height of about 1 nm. The nanocrystal consists of an octahedral layer of magnesium cations coordinated by six oxygen surrounded by two tetrahedral layers of silicon oxide. Negative charges are present on the Laponite faces that are induced by the isomorphic substitution of Mg2+ with Li+, which is counterbalanced by the corresponding number of cations, typically Na+ located at the silicon dioxide layers. The cation exchange capacity of Laponite is 0.55 mequiv per gram. At the edge of the Laponite disks are located reactive hydroxyl groups in the form of Si-OH groups, and within the interior of the clay sheet in the form of Mg-OH and Li-OH groups. The charges on the edge is pH dependent12 and covers less than 10% of the overall cation exchange capacity of the clay.13 The Laponite clay at low concentration ( < 1 wt % )

4

can be dispersed in water

containing a small amount of salts, in the form of individual platelets and tactoids composed of two to four superimposed platelets.14 Using static and dynamic light scattering (DLS) it could be shown that dispersion of Laponite contains high polydispersity in size with ACS Paragon Plus Environment

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dispersion consisting of platelets, dimmers, trimers, and even oligomers, though the effect of temperature on the size polydispersity (exfoliation behaviour) was not reported.15 The ultrasound attenuation spectra was used o show that in Laponite suspensions in the concentration range 1.5%-4% w/v a substantial fraction of aggregates are actually tactoids.16 Laponite gels in aqueous solutions are formed in the concentration range of 1-3 wt%.4 These gels form three-dimensional aggregates (“house of cards”) based on a T-shape morphology resulting from electrostatic attraction between the positive charges at the platelets edge and the faces of platelets.3 Actually, the situation could be more complex as shown by the gelation of clay suspensions studied by Monte Carlo simulations of clay disks carrying a point quadrupolar charge.17 The clay suspension undergoes a transition from a low concentration phase where platelets assemble into elongated T-clusters to a “house of cards” gel architecture at higher concentration.17 For many applications it is important to have the dispersion or peptizing Laponite gels into an aqueous solution. The transformation of gel to sol is induced by addition of certain compounds, e.g., condensed phosphates like tetrasodium pyrophosphate, glycols, or some non-ionic

surfactants

(http://www.byk.com/technical_brochures/BYK_B-

RI21_Laponite_EN.pdf).18 The basic feature of this process is related firstly to the formation of anions, like (P2O7)4-, that become associated with the positively charged edges of Laponite and secondly to the contribution of hydrated Na+ ions which form a double-layer at the clay surfaces that cause mutual electrostatic repulsions between individual platelets. Hence, the electrostatically stabilized “house of cards” architecture is not favourable. Nevertheless, details of the degelation mechanism at the molecular level and its dependence on clay aspect ratio, temperature, additive concentrations were not reported in literature. The main aim of this work is to investigate the NMR spectroscopic signature of peptizing effect on Laponite RD clay aqueous suspensions, in the medium range of Laponite concentrations were gels are formed using as dispersant etidronic acid (1-hydroxy ethane-1,1ACS Paragon Plus Environment

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diphosphonic acid, HEDP) that has a structure analogue of pyrophosphate. The microscopic information is obtained using

31

P and 1H high-resolution magic-angle sample spinning

(HRMAS) NMR spectroscopy. The evidence for free and bound etidronic acid molecules to Laponite edge as well as the temperature dependence of these fractions is provided by the NMR method. The temperature dependence can partially be explained by the tactoids-toplatelets conversion as well as cluster dissolution which is investigated by FESEM, UV-Vis spectrophotometry, and DLS. Furthermore, the evolution of the disks net charges with temperature is measured by the zeta potential.

II. MATERIALS AND METHODS A. Synthesis of Laponite RD Dispersed Gels by Etidronic Acid (HEDP). Laponite of RD grade (Rockwood Additives Ltd) was dried at 130 °C for 5 hours, which resultd in loss of up to 10 wt% of its weight due to absorbed water. On dispersion of Laponite in water, release of the Na+ ions leads to a negative charge on the faces, while protonation of the OHgroups, localized at the single crystal rim, forms a positive charge. The Laponite RD dispersions have been prepared by the procedure described in Reference (4). In short Laponite RD was added to water with electrical conductivity of 0.15 mS/cm, slowly under vigorous stirring and afterwards the dispersions were stirred for 30 min to produce a homogeneous solution. All the stirring had been made at 500 rpm. The final dispersion product was stored immediately after stirring under nitrogen to avoid acidification by CO2. Laponite disks show a chemical dissociation in acid solutions, which could be the result of the contact with atmospheric CO2.19 This aging process is characterized by leaching of Mg2+ ions.

B.

31

P and 1H High-Resolution Magic Angle Sample Spinning (HRMAS) NMR

Spectroscopy. For proton high-resolution magic-angle sample spinning (HRMAS) NMR spectra of Laponate RD dispersion with a concentration of 3 wt% in deuterated oxid (D2O) in the presence of HEDP monohydrate with concentration of 5.35 mM, were measured at ACS Paragon Plus Environment

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various temperatures in the range of 15°C-55°C. For this purpose a wide-bore AV700 Bruker NMR spectrometer operating at the proton frequency of 700.2378 MHz was used. A crosspolarization MAS probe with a 3.2 mm rotor was employed at the rotor frequency of 10 kHz. The temperature was maintained within ± 0.5 ºK using a Bruker temperature control unit. All the 1H HRMAS spectra were externally referenced to TMS. For all measurements the recycle delay was 7s, the radio-frequency pulse length was 1.9 µs, while the dwell time was 10 µs, and the number of scans was 512. The time domain data were 4k and the zero filling was done with 16k. Proton decoupled

31

P high-resolution magic-angle sample spinning (HRMAS) NMR

spectra of the same Laponite RD dispersion as above were measured at various temperatures in the range of 15°C to 55°C. The spinial64 pulse sequence was applied to proton frequency for

31

P-1H heteronuclear decoupling and NMR spectrometer frequency for

MHz. All the

31

31

P was 283.367

P HRMAS spectra were externally referenced to ammonium phosphate

monobasic that has the

31

P resonance at +1 ppm relative to phosphorous acid 85% in H2O

used as zero ppm reference. The rotor frequency was 10 kHz, the recycle delay was 7s, the radio-frequency pulse length was 4 µs (75 W), while the dwell time was 4 µs, and the number of scans was 2016. The time domain data were 4k and the zero filling was done with 16k. The relative spectral integral intensities of the 1H and

31

P HRMAS spectra were measured using

TopSpin 3.2 Bruker software. The spectral decomposition was made using DMFIT program.

C. Transmission Electron Microscopy (TEM). TEM micrographs were taken on a Carl Zeiss LibraTM 120 Microscope (Oberkochen, Germany) equipped with an in-column filter. The electron beam accelerating voltage was set at 120 kV. Microtomed ultrathin sections were obtained using a LEICA 125 Ultracut microtome. The samples were embedded into epoxy resin prior cutting. For grey value analysis of the TEM images the software “Image J” (version 1.46τ) was used.


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D. Field Emission Scanning Electron Microscopy (FESEM). Electron micrographs of Laponite RD dispersion with HEDP were obtained using field emission scanning electron microscopy (HITACHI SU9000). For the measurements aqueous solutions of Laponite RD with a concentration of 3 wt% were used. The secondary electron images have the resolution of 1.0 nm using 20 kV accelerating electron beam. A droplet of dispersion was put on a silicon wafer, was fixed on a holder, and it was then transferred to the high-vacuum, and further inserted into the observation electron microscope chamber.

E. UV-Vis spectrophotometer. UV-visible spectra were measured using a Varian Cary 100 Bio-UV-Visible split-beam spectrophotometer running with Cary WinUV scan application. Laponite RD dispersions were scanned in the wavelength range of 200 nm-800 nm with spectral resolution of 1 nm. Before sample analysis, a water baseline measurement was recorded for use as a sample blank. A high-intensity Xe flash lamp was used as a source for UV light, which permits taking 80 data per second.

F. Dynamic light scattering measurements. The DLS measurements were performed on Laponire RD water dispersion with concentration of 0.01 wt%. Before the measurement the clay aqueous solution was passed through a 4 µm poly(tetrafluoroethylene) membrane filter. The intensity correlation function was directly obtained as

g 2 (τ ) =

I (q,τ )I (q,0 ) I (q,0 )

2

,

(1)

where I (q,τ ) is the intensity of the laser beam measured for the correlator time τ, and the symbol

(...)

, denotes the statistical average for an ergodic system. The modulus of the

scattering vector is defined as q = (4πn / λ ) sin (θ / 2 ) , where n is the refractive index of the solvent, λ is the wavelength of the electromagnetic radiation and θ is the scattering angle. Electric field autocorrelation functions defined as g1 (τ ) = ( g 2 (τ ) − 1)1 / 2 , relaxation rates Γ, and distributions were determined by photon correlation spectroscopy performed at the


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scattering angle of θ=90° with a set-up consisting of an ALV-SP2 goniometer, an ALV-SIPC photomultiplier, a multiple τ digital real-time AVL-7004 correlator. As a radiation source a solid-state laser (Koheras) with a wavelength λ=473 nm. The data were recorded in the temperature range of 20°C-70°C, using a toluene bath with the temperature controlled to a precision of ± 0.1°C. For individual dynamic light scattering measurements in the heating mode, individual data were collected for 180 s, with the temperature steps of 3 °C. At each temperature an equilibration time of 10 minutes was allowed. The dispersions were sufficiently dilute to diminish multiple scattering and at each temperature five data were taken and finally the average from these measurements was presented.

G. Zeta potential and conductivity measurements. A key indicator of the colloidal dispersions stability can be inferred from the zeta potential (ζ). This quantity was measured using Zetasizer Nano M3-PALS, Malvern Instruments Ltd. UK. The instrument reports a ζpotential mean value, a distribution and the width of the distribution. A Laponite RD dispersion in water was used having pH=9 and concentration of 0.01 wt%. Measurements were performed in the temperature range of 20 °C- 60 °C in steps of 3 °C. The temperature stabilization time was 180 s. At each temperature eight measurements were made and the zeta potential

was

calculated

using

Henry´s

equation.

Laponite

RD

dispersion

microelectrophoretic and electrical conductivity measurements were made using disposable polystyrene cuvette.

III. RESULTS AND DISCUSSION A. Temperature dependence of the etidronic acid fractions in aqueous solution of peptized Laponite RD by 31P and 1H HRMAS NMR spectroscopy. High-resolution magic angle sample spinning (HRMAS) NMR spectroscopy of

31

P and 1H nuclei can offer a direct

proof of the binding of additive compounds like HEDP onto the edges of the clay. This process leads to the peptization of the clay gel by reducing the positive charge of the edge. ACS Paragon Plus Environment

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Figure 1 shows the 31P HRMAS NMR spectra of HEDP with the concentration of 5.35 mM in a peptized sample of Laponite RD with concentration of 3 wt% in tap water at two different temperatures. Composite spectra were detected that have being decomposed in two components using DMFIT program. The narrow spectrum with chemical shift at ~+19 ppm and the full linewidth at half height of ∆ν 1 / 2 ≈ 120 Hz, corresponds to the free HEDP molecules in the aqueous solution. This assignment is in good agreement with the 31P spectra of HEDP 60 wt% in aqueous solutions (Figure S1a, see Supporting Information) and

31

P

HRMAS spectrum of etidronic acid monohydrate (Figure S1b, see Supporting Information). The assignments of 31P HRMAS spectra were confirmed by the spectra simulations made by MestRecC program (Figure S2a, see Supporting Information). The broad spectral component is downfield shifted and the full linewidth at half height is ∆ν 1 / 2 ≈ 1.5 kHz (Figure 1). This heterogeneous broadening is originating from chemical shift distribution induced by defects of the Laponite single crystal at the edge and the magnetic susceptibility mismatch between solvent and Laponite platelets and tactoids. Phosphorous-31 spectrum simulation of a modified HEDP that mimics the bounding to Laponite edge is shown in Figure S2b (see Supporting Information) having a downfield shift of 1 ppm relative to the free HEDP molecule. The temperature increase from 15 °C to 60 °C will reduce the intensity of the 31P free HEDP spectral component as it is shown in Figure 1a and Figure 1b.


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a)

15 °C

30 28 26 24 22 20 18 16 14 12 10

ppm

b)

60 °C

30 28 26 24 22 20 18 16 14 12 10

ppm Figure 1. Phosphorus-31 HRMAS NMR spectra (continuous lines) of etidronic acid (cHEDP= 5.35 mM) used as peptizing additive for Laponite RD (c=3 wt%) with aspect ratio of 25 in tap water (σ=0.15 mS/cm) at temperatures of 15 °C (a), and 60 °C (b). The spectra were decomposed into two spectral components a narrow (point line) and a broad (dash line) one assigned to free and bound etidronic acid molecules, respectively. The simulated spectrum is shown with dash-point line.


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Recently, the binding of phosphonic acid to silica nanoparticles was investigated by 31

P and 1H solid-state 1D and 2D NMR spectrometry and a clear signature of the physisorbed,

monodental, bidental, and tridental binding was detected.20 In our case the

31

P broad

resonance (Figure 1) does not show any evidence of spectral asymmetry that could be interpreted as asymmetric binding of HEDP molecules to the edge surface via Si-OH groups. Hence, most probably the HEDP molecules are lying on the rim surface having a bidental binding. The existence of bound HEDP molecules to the Laponite edge can be also proved by 31

P HRMAS spectroscopy taking into account the effect of the increase in the aspect ratio of

the platelets.21 Let us first rationalize the geometrical constrains imposed by the aspect ratio using the simple model for disk-shaped nanoclay with the radius R and the height h. In order to proof the dependence of the number of bound molecule onto the platelet rims to the platelet aspect ratio we consider in the following two disks with the radius R1 and R2 and the same thickness h. The aspect ratio for the disk i (i=1,2) is defined by (2) and we assume that R2 > R1. Moreover, the total mass of the clay i (i=1,2) in the dispersion solution can be written as ,

(3)

where ρ is the disk density and we assume that ρ1 ≈ ρ2. The number of patelets of i specie is denoted by Ni. Furthermore, we assume that the mass of the clay in the solution is the same for the two species, i.e. m1 = m2. In this case from Eqs. (2) and (3) we get (4) Hence, the number of platelets with larger aspect ratio AR2>AR1 will decrease, i.e. N2>1, at high ionic strength. The opposite temperature trend of µ E compared to the ζ potential can be related to the reduction of the electrical double layer thickness with increase in temperature. The temperature dependence of the electrical conductivity σ of the same Laponite RD aqueous dispersion is shown in Figure S9b (see Supporting Information). The increase in the effective electrical charge with the temperature leads to the increase of the electrical conductivity in the heating process. Enhanced conductivity is related to dissociation of Na+ ions from the Laponite disks and is associated with the decrease of the Debye screening length.30 Our freshly prepared Laponite aqueous dispersion with concentration of 0.01 wt% has a conductivity about two-orders of magnitude smaller compared to the Laponite dispersion with concentration of 3.5 wt% and 0.1 mM NaCl, aged after 13 days.31 In all the measurements of ζ potential, electrophoretic mobility, and electrical conductivity a hysteresis effect is detected (Figure S8 and Figure S9, see Supporting Information) that shows that the reverse process of ions migration and the reformation of tactoids are less efficient after were produced by the heating process.

IV. CONCLUSION The peptizing effect at the molecular level and its temperature dependence in the case of etidronic acid additive was investigated by

31

P and 1H HRMAS NMR spectroscopy. The

NMR spectroscopy is non-invasive and therefore is not affecting the dispersion ACS Paragon Plus Environment

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microstructure. Moreover, can prove directly the increase in the number of etidronic acid molecules bound to the Laponite edge due to the temperature increase or other factors and can be understood by considering the increase in the positive charge of the edge. There are different mechanisms that lead to this effect: (i) the thermal activation of the reactive hydroxyl groups in the form of Si-OH and Mg-OH groups; (ii) the temperature increase in the negative charge of the disk faces due to the substitution of a fraction of Mg2+ ions by Li+ and hence the increase of the edge positive charge induced by the broken bonds; (iii) the thermal activation of the exfoliation process that leads to the transformation of tactoids (dimers, trimers, and multimers) or clusters into platelets. The FESEM micrographs statistics show that the ratio between the number of platelets and tactoids increases with the increase of temperature. Furthermore, the UV-Vis spectra reveal that the Laponite RD dispersions prepared at higher temperature have more platelets shown by the increase in Rayleigh scattering. Exfoliation of tactoids to platelets was proved by DLS measurements using temperature dependence of the normalized probability distributions of the relaxation rates associated with the decay of electrical field autocorrelation functions. Hence, DLS data of peptised dispersion of Laponite RD with HEDP confirm the presence of tactoids that decreases in number with the increase of temperature. The Laponite colloidal solution becomes more instable at elevated temperatures due to the decreases of the ζ potential. The heating and cooling response of Laponite dispersion in aqueous solutions shows hysteresis on the time scale used in DSL and ζ potential measurements. The process of tactoids formation from exfoliated platelets is strongly hindered by the electrostatic interactions.

ACKNOWLEDGMENTS We thank Dr. Xiaomin Zhu for useful discussions. We express our gratitude to BYK-Chemie GmbH for material support. This work was performed in part at the Center for Chemical


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Polymer Technology (CPT), which was supported by the EU and the federal state of North Rhine-Westphalia (grant EFRE 30 00883 02).


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REFERENCES [1] Weaver, C.E.; Pollard, L.D. The Chemistry of Clay Minerals; Elsevier, Amsterdam, 1973. [2] Swartzen-Allen, S.L.; Matijevic, E. Surface and Colloid Chemistry of Clays. Chem. Rev.

1974, 74, 385-400. [3] van Olphen, H. An Introduction to Clay Colloidal Chemistry; 2nd ed. John Wiley and Sons, New York, 1977. [4] Ruzicka, B.; Zaccarelli, E. A Fresh Look at Laponite Phase Diagram. Soft Matter 2011, 7, 1268-1286, and references therein. [5] Neumann, B.S. Behaviour of Synthetic Clay in Pigment Dispersions. Rheo. Acta 1965, 4, 250-255. [6] Bippus, L.; Jaber, M.; Lebeau, B. Laponite and Hybrid Surfactant/Laponite Particles Processed as Spheres by Spray-Drying. New J. Chem. 2009, 33, 1116-1126. [7] Heath, D.; Tadros, Th.F. Influence of pH, Electrolyte and Poly(vinyl alcohol) Addition on the Rheological Characteristics of Aqueous Dispersions of Sodium Montmorillonite. J. Colloid Interface Sci. 1983, 93, 307-319. [8] Yang, Y.; Wang, S.; Liu, J.; Xu, Y.; Zhou, X. Adsorption of Lyosine on NaMontmorillonite and Competition with Ca2+: A Combined XRD and ATR-FTIR Study. Langmuir 2016, 32, 4746-4754. [9] Lezhnina, M.M.; Grewe, T.; Stoehr, H.; Kynast, U. Laponite Blue: Dissolving the Insoluble. Angew. Chem. Int. Ed. 2012, 51, 10652-10655. [10] Grabolle, M.; Starke, M.; Resch-Genger, U. Highly Fluorescent Dye-Nanoclay Hybrid Materials Made from Different Dye Classes. Langmuir 2016, 32, 3506-3512. [11] Bergaya, F., Lagaly, G. Handbook of Clay Science; Vol. 5, 2nd ed., Elsevier, Amsterdam, 2013.


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[12] Daniel, L.M.; Frost, R.L.; Zhu, H.Y. Edge-Modification of Laponite with Dimethyloctylmethoxisilane. J. Colloid Interface Sci. 2008, 321, 302-309. [13] Bourgeat-Lami, E.; Herrera, H.H.; Putaux, J.-L.; Reculusa, S.; Perro, A.; Ravaine, S.; Mingotaud, C.; Duguet, E. Surface Assisted Nucleation and Growth of Polymer Latexes on Organically-Modified Inorganic Particles. Macom. Symp. 2005, 229, 32-46. [14] Rosta, M.; von Gunten, H.R. Light Scattering Characterization of Laponite Sols. J. Colloid Interface Sci. 1990, 134, 397-406. [15] Nicolai, T.; Cocard, S. Light Scattering Study of the Dispersion of Laponite. Langmuir

2000, 16, 8189-8193. (16) Ali, S.; Bandyopadhyay, R. Use of Ultrasound Attenuation Spectroscopy to Determine the Size Distribution of Clay Tactiods in Aqueous Suspensions. Langmuir 2013, 29, 1266312669. (17) Dijkstra, M.; Hansen, J.P.; Madden, P.A. Gelation of Clay Colloid Suspension. Phys. Rev. Lett. 1995, 75, 2236-2239. (18) Au, P.-I.; Hassan, S.; Liu, J.; Leong, Y.-K. Behaviour of LAPONITE® Gels: Rheology, Ageing, pH Effect and Phase State in the Presence of Dispersant. Chem. Eng. Res. Design,

2015, 101, 65-73. (19) Mourchid, A.; Levitz, P. Long-Terms Gelation of Laponite Aqueous Dispersions. Phys. Rev. E 1998, 54, R4887-R4890. (20) Davidowski, S.K.; Holland, G.P. Solid-State NMR Characterization of Mixed Phosphonic Acid Ligand Binding and Organization of Silica Nanoparticles. Langmuir 2016, 32, 3253-3261. (21) Das, P.; Malho, J.-M. Rahimi, K.; Schacher, F.H.; Wang, B.; Demco, D.E.; Walther, A. Nacre-Mimetics with Synthetic Nanoclays up to Ultrahigh Aspect Ratios. Nature Commun.

2015, 6, 5967-5981.


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(22) Naqvi, K.R.; Marsh, J.; Chechik, V. Formation of Self-Inhibiting Copper (II) Nanoparticles in an Autocatalytic Fenton-Like Reaction. Dalton Trans. 2014, 43, 4745-4751. (23) Bailey, G.W.; Karickhoff, S.W. UV-Vis Spectroscopy in the Characterization of Clay Mineral Surfaces. Anal. Lett. 2016, 6, 43-49. (24) Kumar, A.; Saxena, A.; De, A.; Shankar, R., Mozumdar, S. Facile Synthesis of SizeTunable Copper and Copper Oxide Nanoparticles Using Reverse Microemulsions. RSC Adv.

2013, 3, 5015-5021. (25) Bhattacharjee, J.; Banik, S.; Hussain, S.A.; Bhattacharjee, D. A study of the Interactions of Cationic Porphyrin with Nano Clay Platelets in Layer-by Layer (LbL) Self Assembled Films. Chem. Phys. Lett. 2015, 633, 82-88. (26) Ruzicka, R.; Zulian, L.; Ruocco, G. More on the Phase Diagram of Laponite. Langmuir

2006, 22, 1106-1111. (27) L. Li, L. Harnau, S. Rosenfeldt, M. Ballauff, Phys. Rev. E, 2005, 72, 051504-1,10. (28) Pecora, R. Spectrum of Light Scattered from Optically Anisotropic Macromolecules. J. Chem. Phys. 1968, 49, 1036-1043. (29) Bolisetty, S.; Hoffmann, M.; Lekkala, S.; Hellweg, Th.; Ballauff, M.; Harnau, L. Coupling of Rotational Motion with Shape Fluctuation of Core-Shall Microgels Having Tunable Softness. Macromolecules 2009, 42, 1264-1269. (30) Schmitz, K.S. An Introduction to Dynamic Light Scattering by Macromolecules; Academic Press, Boston, 1990. (31) Shahin, A.; Joshi, Y.M. Physicochemical Effects in Aging Aqueous Laponite Suspensions. Langmuir 2012, 28, 15674-15686.


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Peptizing mechanism at the molecular level of Laponite nanoclay gels Philip Kensbock1, Dan Eugen Demco*1,2, Smriti Singh1, Khosrow Rahimi1, Radu Fechete3, Annette Monika Schmidt2, Andreas Walther1, Martin Möller*1 1 DWI-Leibniz-Institute for Interactive Materials, e.V., RWTH-Aachen University, Forckenbeckstraße 50, D-52074 Aachen, Germany 2 University of Köln, Institute of Physical Chemistry, Luxemburger Str. 116, D-50939 Köln, Germany 3 Technical University of Cluj-Napoca, Department of Physics and Chemistry, 25 G. Baritiu Str., RO-400027, Cluj-Napoca, Romania

Grafic for Tables of Contents (TOC)


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Langmuir

a)

15 °C

30 28 26 24 22 20 18 16 14 12 10

ppm

b)

60 °C

30 28 26 24 22 20 18 16 14 12 10

ppm

Figure 1


Langmuir

2

31P spectrum Na-fluor-hectorite 750 aspect ratio

25.0 ppm (t1)

19.044

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20.0

15.0

ppm

Figure 2


Page 33 of 38

a)

ratio of bound-to-free HEDP

3

4.0 3.5

heating

3.0 2.5 2.0 1.5 1.0 0.5 0.0 10

20

30

40

50

60

temperature [°C]

b)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

4.0 3.5

cooling

3.0 2.5 2.0 1.5 1.0 0.5 0.0 10

20

30

40

50

60

temperature [°C]

Figure 3


Langmuir

4

0.274

1.591

a)

4.928

6.0

5.0

4.0

3.0

1.00

0.47

1H spectrum Laponite RD 3 wt.% 10 mM HEDP in D2O 25 °C

2.0

1.0

0.0

ppm (t1)

b)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4.0 3.5

heating

3.0 2.5 2.0 1.5 1.0 0.5 0.0 10

20

30

40

50

60

temperature [°C]

Figure 4


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Langmuir

5

Figure 5


Langmuir

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6

c

Figure 6


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7

1.0 1/2

S 2e

g1()=(g2()-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

-2

0.8 0.6 0.4 0.2

S 1e

-1

0.0 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0.01

0.1

1

10

 [s]

Figure 7


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8

Figure 8


Peptizing Mechanism at the Molecular Level of ... - ACS Publications

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