Clay films with variable metal ions and self-assembled silicate layer-void nanostructures

Abstract

The naturally occurring silicate clays with sodium counter-ions were exfoliated into nanoscale silicate platelets (NSP-ONa) and subsequently exchanged with various metal ions including lithium(I), potassium(I), magnesium(II), calcium(II), and aluminum(III) to afford silicate platelets with the corresponding metal counter-ions (NSP-OM, M = metal counter-ion). In solution, their ionic properties were characterized by measuring zeta potentials over pH to reveal the electrokinetic shifting from −50 mV to −5 mV. The gelling behavior was shown to be low at a concentration of 4 wt% for M = monovalent Na⁺, K⁺, and Li⁺; and 7 wt% for M = multivalent Mg²⁺, Ca²⁺, and Al³⁺. Under the process of solution coating and evaporating, the silicate platelets self-assembled into thin films of 20–100 μm in thickness with the exception of NSP-OAl. The regularity of the clay's self-assembled nanostructure increases as the valence of the metal counter-ion decreases. The nanostructures of the silicate films were further characterized by wide angle X-ray diffraction, density/void measurement, and scanning electron microscopy. These findings of NSP-OM and their self-assembled nanostructures formation were for the first time recorded and suggested for new film fabrications by bottom-up technology.

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1. Introduction

The naturally abundant silicate clays are academically and industrially interesting due to their attractive properties such as ionic character, 2D geometric shapes, high surface area, layered nanostructure, organic encapsulating ability, and high ionic exchange capacity. The commonly utilized phyllosilicate clay Na⁺–MMT is composed of multiple silicate layers with a fundamental 2 : 1 structure of two tetrahedral sheets sandwiched with an edge-shared octahedral sheet.^1–3 The fundamental stacks are generically composed of multiple silicate platelets of layered structure with crystalline defects and exchangeable metal ions. The presence of counter-ions such as Na⁺, Mg²⁺, and Ca²⁺ allows the organic incorporation and modification of the clay layered structure through ionic exchange reaction. For example, the conventional intercalation reaction on the clay is performed by the incorporation of organic alkyl quaternary ammonium salts to afford layered silicates/polymer hybrids.⁴ Owing to the ionic characters and possible organic modifications, the clay has many practical uses such as catalysts,⁵ adsorbents,⁶ metal chelating agents,⁷ hydrogels,⁸ and nanofillers in polymer nanocomposites.⁹

The homogeneous distribution of clay units in polymer matrix is often the main thrust for rendering the polymer composite properties such as gas barrier,^10,11 coefficient of thermal expansion,¹² mechanical strength,¹³ glass transition temperature,¹⁴ and anti-flame characteristic.¹⁵ The homogeneity of platelet distribution in the composites is hard to achieve due to the self-aggregating tendency of the silicate platelets. In literatures, the mechanism and the phenomena of clay self-assemblies or self-aggregation were reviewed.¹⁶ The self-assembly was affected by the layer-stacking tendency and also the swelling behavior in water due to the existence of interlayer cations in clay platelets,^17,18 pH environment,¹⁹ clay concentration,²⁰ and the process of preparation.²¹ The ionic charge interaction, geometric aspect-ratio, and layered stack structure are several key parameters for influencing their aggregating behaviors.

In viewing the differences between the layered structure and the exfoliated silicate platelet, we have made efforts on the process of exfoliating the layered Na⁺–MMT into individual silicate platelets. In order to overcome the strong mutual attraction between platelets of layered stack clays, the polymeric amines were synthesized by Mannich condensation and used as the exfoliation agents. New exfoliation mechanisms such as zigzag and phase-inversion were reported.^22–24 Ultimately, the layered Na⁺–MMT could be delaminated into individual silicate platelets which were isolatable through the toluene/water extraction procedure. The individual silicate platelets in the form of sodium counter-ion (NSP–ONa) existed in irregular polygonal shapes with extremely thin dimensions of 100 × 100 × 1 nm³, large surface areas (ca. 750 m² g⁻¹), and high ionic charges (ca. 18 000 ions/platelet). Distinctly different from the pristine clay, NSP-ONa have the total exposure of sodium counter-ions that are originally hidden in the multilayered stacks. The self-assembling properties through platelet-to-platelet alignment were also reported previously.^25,26

Recently we have reported the fabrication of the NSP-ONa self-assembled films and their unique nanostructure.³ As a continuation of our work, we further found the effects of changing the NSP counter-ions on the solution properties and also film self-assemblages. By ionic exchange reaction, the exfoliated silicate platelets of various metal counter-ions, or NSP-OM (M = Li⁺, K⁺, Ca²⁺, Mg²⁺, and Al³⁺), have been synthesized. The electrokinetic properties of the ionic platelets in water were investigated by measuring zeta potential. The platelet suspensions were further allowed to water evaporation to afford clay films which were subsequently examined for the self-assembled nanostructure and the regularity of thin platelet self-alignment. Two-dimensional XRD and SEM were used to reveal the films' nanostructures with alternating arrangement of silicate platelet/air void composition. In addition, we have compared the difference in the volume percentage of voids in different NSP-OM films.

2. Experimental

2.1. Materials

Potassium chloride (KCl), lithium chloride (LiCl), calcium chloride (CaCl₂), magnesium chloride (MgCl₂), and aluminum chloride (AlCl₃) were obtained from Sigma-Aldrich Co. Na⁺–MMT with a CEC of 120 meq/100 g, was supplied from Nanocor Co., USA. The phyllosilicate clay is in the sodium form of layered silicates and aggregates from the primary stack units each consisting of approximately 8–10 silicate platelets. The individual NSP-ONa were obtained from the randomization of the clay by our exfoliation process reported previously.^22,25

2.2. General procedures for preparing aqueous dispersion of NSP-OM

The preparation of NSP with different species of metal counter-ions is described using CaCl₂ as the example for cationic exchange reaction. To a four-necked round-bottomed flask (2 L) equipped with a mechanic stirrer, a reflux condenser, and a thermometer, an aliquot of NSP-ONa dispersion (1.00 g, 1.00 wt%, 1.20 meq.) was charged into the glassware reactor. While the suspension was vigorously agitated and heated to 80 °C, CaCl₂ (0.133 g, 1.20 mmol) was added in one portion at 80 °C, and the stirring was maintained for one hour. The suspension was filtered and washed with de-ionized water repeatedly until no chloride was detected in the sample by using an EDX. The NSP-OCa sample was re-dispersible in de-ionized water up to 1.00 wt%. Other NSP-OM clays were prepared by using similar procedures with different metal halides including KCl, MgCl₂, and AlCl₃. In the preparation of NSP-OLi, 12.0 mmol of LiCl was used for this exchange reaction.

2.3. Preparation of self-assembled film by water evaporation

The films of NSP-OM were prepared by casting their dispersion (1.00 wt% in water) on a PET tray and water evaporation at 60 °C until complete dryness. Self-standing films with a smooth surface could be obtained for NSP-OM (M = Na⁺, Li⁺, K⁺, Ca²⁺, and Mg²⁺). In the case of NSP-OAl, only spherical and irregular pieces were formed.

2.4. Characterization

Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was performed on an ICAP 9000 from Jarrell-Ash. The instruments were used to determine the interlayer counter-ion contents of NSP-OM samples. The X-ray powder diffraction (XRD) was performed on a Schimadzu SD-D1 diffractometer with a Cu target (λ = 1.5405 Å) at a generator voltage of 35 kV, a generator current of 30 mA, and a scanning rate of 2° min⁻¹. The d-spacing (n = 1) was assigned on the basis of the Bragg's equation (nλ = 2d sin θ). Two-dimensional X-ray powder diffraction (2D-XRD) was performed on a Bruker D8 Discover with a Vantec 2000 area detector. Scanning electron microscopy (SEM) was performed on a JEOL JSM-5600 SEM system and operated at 15 kV. The samples were coated with Pt before measurements. Energy dispersive X-ray spectrometry (EDX) measurements with a resolution of 0.01 wt% were conducted on the above-mentioned SEM equipped with an Oxford X-act EDX micro analyzer. A particle size and zeta potential was performed on a 90Plus/BI-MAS (Brookhaven Instrument Corp., NJ). The zeta potential of NSP-OM suspensions at 0.01 wt% was measured in the range of pH 2 to 11. Aqueous hydrochloric acid and sodium hydroxide solutions were used to adjust the pH of clay suspensions. The reliability of the zeta potential measurements was controlled with standard deviation of reading less than 2. The particle size distribution of NSP-OM in aqueous dispersion was measured at 0.01 wt% concentration. In the measurement of the bulk density (ρ₁), the mass (M₁) of the sample was determined on a Mettler Toledo analytical balance with a resolution of 0.0001 g. The thickness (T₁) of the sample was determined by averaging three measurements on different areas of the sample by using a micrometer (Fisher) with a resolution of 0.001 mm. The length (L₁) and width (W₁) of the sample were determined by averaging three measurements on different areas of the sample for each side using a digital caliper (Fisher) with a resolution of 0.001 mm. The bulk density was calculated according to eqn (1): ρ₁ = M₁/V₁, where V₁ = T₁ × L₁ × W₁. The overall resolution for the bulk density measurement was 0.0001 g cm⁻³.

In the measurement of the apparent density (ρ₂), the mass (M₂) of the sample was determined on a Mettler Toledo analytical balance with a resolution of 0.0001 g. The volume (V₂) of the sample was measured in a 50.0 cm³ burette (tolerance = 0.1 cm³) pre-filled with toluene by immersing the pre-weighed sample into the solvent and comparing the difference between the starting and final volume. The apparent density was calculated according to eqn (2): ρ₂ = M₂/V₂. With the above parameters ρ₁ and ρ₂, we estimated the total void volume percentage according to eqn (3): TV = (1/ρ₁ – 1/2.6)/V₁, where 2.6 is the density of bulk MMT.³ The volume percentages of micro voids (Mi) and macro voids (Ma) could be calculated by eqn (4): Mi = (1/ρ₂ – 1/2.6)/V₁ and eqn (5): Ma = (1/ρ₁ – 1/ρ₂)/V₁, respectively.

3. Results and discussion

3.1. Ionic exchange reaction of NSP-ONa with various metal chlorides

We performed the ionic exchange reactions of the sodium counter-ions on the NSP surface with various metal chlorides at one equivalent to the CEC of clay, including LiCl, KCl, MgCl₂, CaCl₂, and AlCl₃, as the representative metal ions. A schematic drawing for the ionic exchange of NSP-OM is illustrated in Scheme 1. The overall process scheme demonstrates the conceptual description of the exfoliation, purification and isolation of NSP-ONa, and the ionic exchange reaction with various metal ions. The NSP-ONa source from Na⁺–MMT is illustrated by the chemical interaction through ≡SiO^–Na⁺ functionalities with the form of multiple ions on layered or single platelets. The transformation was performed in water at low clay concentration in avoiding the gel phenomenon.

Scheme 1 Process scheme illustrating the exfoliation/isolation of NSP-ONa and the followed metal ion exchange to NSP-OM.

Experimentally, the ICP-AES was used to analyze the metal element distribution as shown in Table 1. It was revealed that the sodium counter-ions were only partially replaced by the cations in the exchange reaction with respect to the original clay CEC equivalence. The non-exchangeable sodium might be caused by an equilibrium between sodium or multivalent cations and the existence of less exchangeable sites. Furthermore, the exchange reaction is following the equilibrium between two metal exchange potentials. For example, K⁺, Mg²⁺, Ca²⁺, and Al³⁺ had a greater exchange potential than Na⁺ for possible exchange, and the trend was reported as Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ = NH₄⁺ > Na⁺ > Li⁺.²⁷ In Table 1, the analyses of metal distribution in relative amounts indicated the consistency with the trend of exchange potentials. It is noteworthy that the use of equal molar ratio of Li cations to the NSP exchange capacity failed to replace the sodium counter-ions due to the equilibrium limitation. Hence, 10 equivalent excess amount of LiCl was used to force the completion by overwhelming the equilibrium of Na⁺/Li⁺ in the exchange reaction to obtain NSP-OLi.

Table 1 Relative composition of metallic elements in NSP-OM

NSP-OM	Relative composition^a
	Na	Mg	Al	M^b
a Measured by using ICP-AES and normalized by setting Si element to 100.b Exchanged metal ions.
NSP-ONa	11	8.8	40	—
NSP-OLi	1.2	6.8	39	10
NSP-OK	1.2	6.2	36	9.9
NSP-OMg	0.30	8.8	38	6.5
NSP-OCa	0.34	7.2	36	6.9
NSP-OAl	0.26	8.0	38	6.5

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3.2. Electrokinetic properties of NSP-OM

The zeta potential is an indication of repulsion force among the charged particles in dispersion. A higher zeta potential (in absolute value) indicates a weaker tendency for the particles to aggregate. Thus, we measured the zeta potential of the silicate clay suspensions at 0.01 wt% concentration against pH for evaluating their electrokinetic properties (Fig. 1). In general, the zeta potentials (absolute value) of NSP-OM with multivalent Mg²⁺, Ca²⁺, and Al³⁺ are lower than those with monovalent Na⁺, Li⁺, and K⁺. The valence-dependent trend could be explained by the Gouy–Chapman theory, 1/k = [ε_oεkT/2n_oe²v²]^0.5, where 1/k, ε_o, ε, k, T, n_o, e, and v stand for the thickness of diffuse double layer, dielectric permittivity of vacuum, the dielectric constant of water, Boltzmann constant, absolute temperature, ion concentration, electrical charge, and cation valence, respectively. The equation indicates that an increase in cation valence would lower the thickness of the diffuse double layer comprised of a layer of surface charge and a layer of loosely attracted ions, thus resulting in a decrease in the zeta potential.²⁸ In other words, the higher valence of metal ion may weaken the repulsive force among the silicate platelets or strengthen their face-to-face attraction. Further, NSP-OLi has a different zeta potential expression from NSP-ONa and NSP-OK. Similarly, NSP-OMg is slightly different from NSP-OCa under most pH conditions. In view of the relative size of the metal ions, the observation could be explained by following the trend of the hydrated radius of cations (Li⁺ = 3.8 Å, Na⁺ = 3.6 Å, K⁺ = 3.3 Å, Mg²⁺ = 4.4 Å, and Ca²⁺ = 4.2 Å). The hydrated radius of these ionic charges plays an important role in controlling the thickness of the diffuse double layer and subsequent the expression of zeta potentials.²

Fig. 1 Zeta potentials of NSP-OM (M = Na⁺, Li⁺, K⁺, Ca²⁺, Mg²⁺, and Al³⁺) at varied pH environment.

In correlation with the trend of zeta potentials, the particle sizes were measured for NSP-OM (Fig. 2). The size distribution for NSP-OM with mono-, di-, and trivalent ions appeared at 120 nm and 900 nm (major); 430 nm and 1850 nm (major); and 440 nm and 4200 nm (major, broad peak), respectively.

Fig. 2 Particle size distributions of NSP-OM (M = Na⁺, Li⁺, K⁺, Ca²⁺, Mg²⁺, and Al³⁺).

The trend of the measured particle size differences is in good correlation with zeta potentials.

We further correlated the self-aggregation behavior of different NSP-OM suspensions at 0.1 vol% concentration by simply observing their kinetic profile of solution sedimentation (Fig. 3). The dilute concentration was chosen to minimize particle interference.²⁹ It was observed that NSP-ONa, NSP-OLi, and NSP-OK suspensions had only little sedimentation after standing for 1 h and completely settled after 72 h (Fig. 3a–c, respectively). In the case of trivalent NSP-OAl, the sedimentation was apparently formed at the bottom of the vial after only a few minutes of standing. By comparison, the sedimentation phenomenon of NSP-OMg and NSP-OCa was moderate as shown in Fig. 3d and e. The sedimentation of NSP in water was caused by the ionic-charge induced cross-face interaction among platelets. The silicate platelets tended to aggregate in forming the settled sedimentation. In summary, the tendency for their self-aggregation in water is in the order of NSP-OAl > NSP-OCa > NSP-OMg > NSP-OK = NSP-ONa = NSP-OLi. We have observed that the suspensions of NSP-OM with multivalent counter-ions gelled at 7 wt% and lost flowability, where as that of NSP-OLi, NSP-ONa, and NSP-OK gelled at 4 wt%. The difference in the gelling behavior is mainly attributed to the swellability and the water hydration ability.¹⁸ Hence, the NSP of monovalent counter-ions are swelled more easily than those of multivalent counter-ions.

Fig. 3 Photographs showing the relative degree of sedimentation in silicate suspensions: (a) NSP-ONa, (b) NSP-OLi, (c) NSP-OK, (d) NSP-OMg, (e) NSP-OCa, and (f) NSP-OAl after standing for 1 h.

3.3. XRD Analysis on the d-spacing of the self-assembled NSP-OM films

The nanostructure was generated via self-stacking of the NSP-OM platelets under the process of solution casting and water evaporation at 60 °C. It was found that all NSP-OM species, with the exception of Al³⁺, had good film formability in generating free-standing and flexible clay films as shown in Fig. 4.

Fig. 4 Self-standing clay films of (a) NSP-ONa, (b) NSP-OLi, (c) NSP-OK, (d) NSP-OMg, (e) NSP-OCa, and (f) NSP-OAl under the process of solution coating and water evaporation.

These silicate films were characterized for their self-aligned nanostructures by XRD for basal d-spacing. As shown in Fig. 5, the basal spacing increased as the valence of the metal counter-ion increased, particularly in the change from monovalent to divalent metal ions. Specifically, the d-spacing of the film was shown to be 14.4 Å for NSP-ONa, 14.8 Å for NSP-OLi, 13.3 Å for NSP-OK, 15.0 Å for NSP-OMg, 15.3 Å for NSP-OCa, and 15.5 Å for NSP-OAl. These values are in agreement with the size of metal ion hydrated radius.¹⁸

Fig. 5 X-ray diffraction profiles of NSP-OM films.

3.4. Two-dimensional X-ray analysis on the alignment of NSP-OM films

The orientation and regularity of silicate platelet alignment by two transmission modes, one along z-axis, i.e. perpendicular to film surface; and the other along x–y plane, i.e. parallel to film surface, were examined. The experimental setup is shown in Fig. 6a. In Fig. 6b, the 2D diffraction patterns along z-axis was illustrated for NSP-OM films (M = Na⁺, Li⁺, K⁺, Mg²⁺, and Ca²⁺). The ring-like diffraction patterns by the 2D images indicated irregular orientations on the x–y plane. However, the arc patterns, confirmed the preferential stacking of platelets along the z-axis (Fig. 6c). In particular, the arc reflection of NSP-ONa and NSP-OLi films were sharp in relative to that of NSP-OK film, suggesting a high degree of ordered orientation in the platelet stacking. For comparison, less oriented nanostructures were observed for NSP-OMg and NSP-OCa films by showing more ring-like images with an expanded radius. It is mentioned that the NSP-OAl had poor film formability and was unable to be measured.

Fig. 6 (a) Illustration of orientations, (b) 2D diffraction patterns along z-axis, and (c) 2D diffraction patterns along x–y plane.

3.5. SEM Observation on the morphology of NSP-OM films

The nanostructure and surface morphology of the silicate films were further analyzed by using SEM. As revealed by SEM on the film's cross section, the NSP-ONa were shown the most regular alignment along the substrate surface among the six NSP-OM films (Fig. 7).

Fig. 7 SEM image on the cross section of: (a) NSP-ONa, (b) NSP-OLi, (c) NSP-OK, (d) NSP-OMg, (e) NSP-OCa, and (f) NSP-OAl films.

The film structure became less orderly as the valence of the metal ion increases. In particular, the silicate multilayers appeared finer and more parallel in alignment for NSP-OM with monovalent Na⁺, Li⁺, and K⁺ than those with divalent Mg²⁺ and Ca²⁺. In contrast, the formation of orderly aligned lamellae disappeared in NSP-OAl films, of which the aggregates appeared spherical or irregular in shape. The self-assembly by the NSP of trivalent aluminum counter-ions seems to undergo a cross-face interaction among the thin platelets rather than an ordered stacking process. A fine dispersion of the individual NSP units is necessary for self-stacking into an ordered nanostructure during water removal. Secondly, the regularity of platelet stacking is depended on the valence of the metal counter-ion species; for example, the multivalent charges may behave as pseudo-cross-linking agents which interfere with the platelet self-alignment. In solution and in bulk, the NSP interaction is reflected by the ionic force of metal ions in their water hydration sizes and charge interaction.

Overall, the characterizations of film morphology and NSP stacking orientation demonstrated the consistency between the SEM observation for alignment regularity and 2D X-ray analysis for self-stacking nanostructure with respect to the trend of metal counter-ion effect. The correlation can also be made to the properties of NSP-OM in the solution behaviors.

3.6. Multilayered nanostructure of alternating layers of platelets and voids

For the NSP-ONa film formation, the multilayered nanostructures from the platelet self-assembling to generate an alternating regularity of parallel NSP alignment and entrapped void space was reported previously.³ Here we compare the difference of NSP-OM besides M = Na⁺ in forming the corresponding films. The self-assembled films with different metal counter-ions were compared for their volume percentage of macro- and micro-void. During water evaporation and the NSP units self-assembling, air-void is entrapped in the same direction along the neighboring platelets in the multilayered structures. The self-assembly into layer arrangement and nanostructure largely depended on the thin platelet-to-platelet affinity. The density of the films is an indication of regularity and tightness in considering the simultaneous platelet alignment and air trapping. The volume percentage of voids was measured in two different means, the bulk density (in air) and the apparent density (in toluene penetration). Two types of air voids were determined in the nanostructures of stacking NSP-OM platelets. The toluene penetrable void is defined as a macro void, which is subtracted out from the bulk density to be micro void. Hence, the total volume, the sum of macro and micro voids, can be determined from the bulk density in air and apparent density in toluene. Overall, these NSP films have their bulk densities in the range of 1.1 and 2.1 g cm⁻³, significantly lower than the reported 2.6 g cm⁻³ for MMT clay in bulk (Table 2).³ The observation of the generally low density is attributed to the presence of entrapped air voids in films. Since the film thickness affects density, the regularity of NSP stacking is assumed to be different during the film's increase in its thickness from the bottom subtract surface. For the film below 50 μm, the apparent density of NSP films with monovalent counter-ions is generally higher than those with divalent analogues. The tightness of NSP self-alignment, rendering low macro voids, is consistent with the measurement of low d-spacing or inter-platelet distance in X-ray diffraction for the monovalent over the divalent metal NSP films. As the thickness increased from 20 to 100 μm, the bulk density increased or the total void volume decreased.

Table 2 Measurement of bulk/apparent density, macro-/micro-void, and total void fractions in film compositions

NSP-OM film	Thickness (μm)	Apparent density (g cm^−3)	Bulk density (g cm^−3)	Total void (%)	Micro void (%)	Macro void (%)	Micro/macro ratio
a Ref. 3.
Silicate bulk^a	—	2.6	—	—	—	—	—
MMT-ONa^a	20	1.5	2.2	42	10	32	0.31
	55	1.8	2.2	31	13	18	0.72
	87	2.0	2.2	23	14	9.0	1.6
NSP-ONa^a	19	1.5	2.1	42	14	28	0.50
	55	1.7	2.0	35	20	15	1.3
	86	1.8	1.9	31	26	5.0	5.2
NSP-OLi	28	1.3	2.1	50	12	38	0.32
	51	1.6	1.9	38	23	15	1.5
	82	1.7	1.9	35	24	11	2.2
NSP-OK	27	1.3	2.1	50	12	38	0.32
	62	1.5	2.0	42	17	25	0.68
	98	1.7	2.0	35	20	15	1.3
NSP-OCa	29	1.2	1.9	54	17	37	0.46
	55	1.5	1.9	42	21	21	1.0
	80	1.6	1.9	35	24	11	2.2
NSP-OMg	28	1.1	2.0	58	13	45	0.29
	58	1.4	1.9	46	20	26	0.77
	88	1.5	1.9	42	21	21	1.0

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The bulk density of NSP-OM films with monovalent counter-ions is in the order of NSP-ONa > NSP-OLi > NSP-OK. It was observed that NSP-ONa and NSP-OLi films had only minor differences in the bulk density. Although the three monovalent counter-ions had approximately the same hydrated radius, the NSP-OK film had a much lower density than the Na analogue due to the relatively irregular nanostructure of NSP-OK film. Thus, the contribution from macro voids becomes the main factor for a low bulk density.

In the case of NSP-OMg and NSP-OCa, the divalent counter-ions rendered the NSP films even lower bulk densities than those of NSP-ONa, NSP-OLi, and NSP-OK. The larger hydrated radius of divalent counter-ions may also behave to be pseudo-crosslinking interaction among the platelets and render the silicates unable to self-stack in a regular manner, thus resulting in the formation of even greater volume percentage of macro voids.

The volume ratio of micro to macro voids could serve as an index for estimating the degree of orderly nanostructure, the increase of micro/macro-void ratio the more regularity in self-assembling. It was noted that the volume percentage of macro-void could be correlated to the occurrence of the wavy layer morphology in the cross-section of films. The overall trend for the NSP-OM films is consistent in all aspects of density (void fraction), SEM morphology, and XRD regularity.

4. Conclusion

The previously developed clay nanoplatelets, NSP-ONa, were investigated for the transformation through ionic exchange reaction to NSP-OM with different metal ion species including monovalent Li⁺ and K⁺, divalent Ca²⁺ and Mg²⁺, and trivalent Al³⁺. The resulted NSP-OM with varied metal counter-ions demonstrated different electrokinetic properties in gelling and charge interactions in water solution. The valence-depended zeta potentials (absolute value) showed a decreasing trend from NSP-OM of monovalent Li⁺, Na⁺, and K⁺ to trivalent Al³⁺. The trend was in good agreement with the calculation using Gouy–Chapman theory. A lower zeta potential (absolute value) implied a stronger platelet aggregation force in solution. The electrokinetic property was pH dependent in solution and correlated to the ability of forming highly regular self-assemblages or nanostructure in films. The nanostructures of these platelet-aligning films were dependent of the counter-ion species. A decrease in the valence of metal counter-ions resulted in an increase in the regularity of the nanostructure and a slight decrease in the volume amount of nanoscale voids of the clay films.

Acknowledgements

We acknowledge financial supports from the Ministry of Economic Affairs and National Science Council (NSC) of Taiwan. We thank Professor Tzong-Ming Wu at the Department of Materials Science and Engineering, National Chung Hsing University, for XRD analysis.

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