Adsorption of Laponite on a Cellulose Model Surface

Abstract

Summary: The adsorption behavior of laponite on cellulose model surfaces was studied by a quartz crystal microblance with dissipation (QCM-D). The influence of prior adsorption of a trihydroxysilane has also been investigated. XPS was used to determine the elemental composition of the surfaces.

Introduction

Laponite (Figure 1) is an artificial layer silicate with the empirical formula Na^+0.7[(Si₈ Mg_5.5 Li_0.3) O₂₀(OH)₄]^−0.7. Its idealized crystal structure is very similar to those of serpentin, a naturally ocurring layer silicate. Laponite is widely used in industry for coatings, as rheological modifyer and for pigment stabilization, just to name a few.1 Recent approaches are aiming at creating nano-composites using adsorption or embedding of clay platelets on or within the base material in order to improve specific properties like lowered flammability.2

Figure 1. Left: idealized crystal structure of laponite. For clarity, the sodium atoms have been omitted. Right: schematic presentation of an exfoliated laponite disc.

Inflammability is currently a hot topic in textile industry with respect to the manufacturing of fire resistant protection clothes.2 However, for the preparation of such a material, knowledge has to be gained on the adsorption behavior of laponite on cellulose. Especially cellulose model surfaces seem to be ideal to mimic the surface behavior of cellulosic materials. While the obvious strategy to use cellulose solutions is somewhat limited because of the limited number of commercially available solvents, the preparation of modified, better soluble cellulose derivatives that can be converted to cellulose after the coating procedure offer many possibilities for the preparation of cellulose model surfaces. Several derivatives (xanthogenates, cellulose acetates) have already been considered, however, from our point of view, the most straightforward approach to prepare cellulose model surfaces is the use of trimethylsilyl cellulose (TMSC). Several detailed studies on the preparation, characterization and utilization of thin cellulose films regenerated from TMSC are known in literature (Figure 2).3–5

Figure 2. Preparation of cellulose model surfaces starting from TMSC.

After coating on a flat substrate such as silicon wafers, the silyl groups can be removed very easily by simple exposure to an acidic atmosphere yielding cellulose model films, whose thickness can be tuned up to a certain degree by varying the concentration of the coating solution and the spin coating conditions. The formed byproduct, trimethylchlorosilane (TMSCl), is immediately hydrolyzed under these reaction conditions to yield the silanol that condenses in a subsequent reaction to give HMDSO (hexamethyldisiloxane).6

These model films can also be deposited on sensors used for quartz crystal microbalancing. The use of a quartz crystal microbalance with dissipation (QCM-D) enables the online monitoring of adsorption processes on the solid/liquid interface over a wide mass range (from 20 ng/cm² to some µg/cm²).7, 8 This is advantageous with respect to basic investigations on the adsorption behavior of laponite and tert-butylsilanetriol (TBS, Figure 3)9, 10 on a cellulose model surface. Results obtained from such studies as well as surface elemental analysis with X-ray photoelectron spectroscopy are presented in this contribution.

Figure 3. Crystal structure of ^tBuSi(OH)₃. Black: carbon, white: hydrogen, dark grey: oxygen, light grey: silicon.

Experimental Part

Materials

Laponite was provided by Rockwood Additives Ltd. (Widnes, UK). Aqueous laponite suspensions (1.0 wt.%) using 18 MΩ · cm Milli-Q water were prepared by rolling for 12 h. Gold coated sensors with a resonance frequency of 5 MHz (QSX301) for the QCM-D measurements were purchased from Lot-Oriel (Darmstadt, Germany). TBS was prepared according to a published literature procedure.10 The characterization of the laponite suspensions (dynamic light scattering and zeta potential measurements) was carried out with a Brookhaven ZetaPlus device (Worcestershire, UK) using a 30 mW laser.

Film Preparation

Cellulose model surfaces were prepared by depositing 50 µl of a TMSC solution in toluene (1.0 wt.%) on quartz sensors followed by spin coating (a = 2500 rpm/s, v = 4000 rpm, t = 60 s) according to published literature procedures.3, 4 For the regeneration of the TMSC films, 2 ml of hydrochloric acid (3 M) were placed adjacent to the coated QCM-D sensors in petri dishes, and the petri dishes were covered with a watchglass. The exposure time was set to 10 minutes.

Film Characterization

Mass increase as a function of laponite and TBS adsorption, respectively, was measured using the QCM-D method (BiolinScientific/Q-Sense, Västra Frölunda, Sweden). The temperature of the QCM-D measurement cell was kept at 293.15 K during the whole measurement. Surface elemental analysis was performed by X-ray photoelectron spectroscopy in a UHV surface analysis system (Omicron Nanotechnology, Traunstein, Germany) using monochromatic Al-K_α (1486.6 eV) radiation.

Results and Discussion

Characterization of the Laponite Suspensions and Preparation of the Cellulose Model Surfaces

In a first step, the laponite exfoliation behavior of a 1.0 wt.% aqueous solution is investigated. For this purpose, the laponite suspensions are subjected to dynamic light scattering and zeta potential measurements. The particle size determination exhibits an effective diameter of 25 nm which correlates well with the data from literature.1 The zeta potential of the laponite particles is −35 mV, which is in the expected range indicating a stable colloidal suspension according to the DLVO - theory.11 In a subsequent step, the cellulose model surfaces are prepared by depositing a toluene solution of TMSC (1.0 wt.%, DS_Si: 2.55) on QCM-crystals followed by spin coating. Subsequent regeneration in hydrochloric acid atmosphere for 10 minutes according to a modified published literature procedure yielded pure cellulose films with a layer thickness of 25 nm.3, 4

Adsorption of Laponite and TBS onto Cellulose Model Surfaces

The adsorption studies are carried out with 1.0 wt.% aqueous laponite suspensions and additionally in combination with 1.0 wt.% aqueous solutions of a stable, water soluble silanetriol, tert-butylsilanetriol (Figure 3). It is stable at neutral pH towards self condensation reactions but in alkaline/acidic media condensation occurs.12 The natural pH (ca. 10) of the laponite suspensions should favor such a condensation with TBS.

For this purpose, two different adsorption processes are monitored by QCM-D: one where laponite is adsorbed prior to TBS onto a cellulose surface and a second where TBS is adsorbed onto a cellulose surface followed by adsorption of laponite. After a final rinsing step, the amount of adsorbed material can be easily calculated by using the Sauerbrey equation (1), which gives the correlation between the frequency of a crystal in dependence of the adsorbed mass.

• ((1))

Δf gives the change in frequency, Δm is the change in mass, n is the overtone of the oscillation (1, 3, 5, 7, 9, considered automatically by the software) and C is the Sauerbrey constant (17.7 ng/cm²).

As depicted in Figure 4, TBS is not irreversibly adsorbed on the surface and desorbs upon rinsing with water while laponite remains on the surface (ca. 620 ng/cm²) after a washing step. However, if the cellulose surface was rinsed with the laponite suspension directly after TBS-treatment without an intermediate water rinsing step, the TBS remained on the surface after subsequent washing with water. In contrast the adsorbed amount of laponite is quite similar compared to the adsorption without TBS. It seems that condensation between the OH functions of the laponite with the silanol functionalities of the silanetriol occurs. To further corroborate this assumption, we also performed X-ray photoelectron spectroscopy of the surfaces (Figure 5). While for a laponite treated cellulose surface a Si:Mg ratio of 1.7:3.3 (theoretical 8:5) is found, for the silanetriol/laponite surfaces the Si:Mg ratio is significantly higher (4.4:3.4) indicating the presence of another silicon containing species. A possible explanation for these results is that TBS has undergone condensation with laponite.

Figure 4. QCM-D adsorption measurements of laponite and TBS on cellulose. The dashed lines are showing the dissipation in each case. Left: Laponite is firstly adsorbed on cellulose. Right: Prior to laponite adsorption, TBS was adsorbed on cellulose.

Figure 5. XPS of different modified cellulose surfaces.

Conclusion

The QCM-D data (Figure 4) exhibit irreversible adsorption of laponite onto the cellulose surface under aqueous conditions, whereas TBS was completely removed from the surface by rinsing with water. However, if the cellulose surface was rinsed with laponite suspension directly after TBS-treatment without an intermediate water rinsing step, the TBS remained on the surface after subsequent washing with water as confirmed by XPS measurements. It is assumed that the higher silicon content in these films originates from condensation reactions between the hydroxyl groups of TBS and laponite.

On the basis of these results it is intended to prepare hybrid materials with improved properties in the future.

Acknowledgements

The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 214653. Alexander Fian from the Joanneum Research Institute for Surface Technology and Photonics is highly acknowledged for the XPS measurements. Prof. Thomas Heinze from the Friedrich Schiller University Jena is acknowledged for providing the TMSC.