Regulation of the Stability of Titania Nanosheet Dispersions with

Article Cite This: Langmuir XXXX, XXX, XXX−XXX

pubs.acs.org/Langmuir

Regulation of the Stability of Titania Nanosheet Dispersions with Oppositely and Like-Charged Polyelectrolytes Szilaŕ d Saŕ inger,†,‡ Paul Rouster,§ and Istvań Szilaǵ yi*,†,‡ MTA-SZTE Lendület Biocolloids Research Group and ‡Interdisciplinary Excellence Center, Department of Physical Chemistry and Materials Science, University of Szeged, H-6720 Szeged, Hungary § Institute of Condensed Matter and NanosciencesBio and Soft Matter, Université Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium Downloaded via UNIV OF NEW ENGLAND on March 28, 2019 at 13:03:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

ABSTRACT: Charging and aggregation processes of titania nanosheets (TNS) were extensively studied in the presence of oppositely charged or like-charged polyelectrolytes in aqueous dispersions. The surface charge of the TNS was systematically varied by the pH; therefore, positive nanosheets were obtained at pH 4 and negative ones at pH 10. Strong adsorption of poly(styrene sulfonate) (PSS) of high negative line charge density on the TNS was observed at pH 4, leading to charge neutralization and reversal of the original sign of charge of the nanosheets. The adsorption of like-charged poly(diallyldimethylammonium chloride) (PDADMAC) was also feasible through a hydrophobic interaction. The predominating interparticle forces were mainly of the DLVO-type, but additional patch−charge attraction also took place in the case of PSS at low surface coverage. The TNS was found to be hydrophilic at pH 10 and no adsorption of like-charged PSS was possible because of strong electrostatic repulsion between the polyelectrolyte and the surface. The PDADMAC showed high affinity to the oppositely charged TNS surface in alkaline dispersions, giving rise to neutral and positively charged nanosheets at appropriate polyelectrolyte doses. Formation of a saturated PDADMAC layer on the TNS led to high resistance against salt-induced aggregation through the electrosteric stabilization mechanism. These results shed light on the importance of polyelectrolyte concentration, ionic strength, and charge balance on the colloidal stability of TNS, which is especially important in applications, where the nanosheets are dispersed in complex solution containing polymeric compounds and electrolytes. structures,23 however, they may be overruled in the future because of the potential development and applications of other titanium oxide morphologies, such as nanotubes,24 nanowires,25 and nanosheets.26 For example, titania nanosheets (TNS) possess well-defined layered or unilamellar structures associated with high surface area, good thermal stability, pHtunable properties, biocompatibility, and versatile surface modification possibilities, which make these two-dimensional materials, for instance, very advantageous in bio-related applications. It is obvious that the colloidal stability of the dispersed systems is a critical issue once the titania nanoparticles are applied in heterogeneous systems such as in blood, aqueous environmental samples, or industrial manufacturing processes.2 In these applications, stable dispersions of primary particles are required once they are used either as catalysts or delivery agents. Experimental conditions like ionic strength, temperature, and pH significantly influence the charging and the

1. INTRODUCTION Inorganic nanomaterials are widely used in numerous applications in chemistry, biology, and materials science because of their high reactivity, increased surface area, lower melting point, ductility, and possible biocompatibility.1−3 Among them, nanosized titanium oxides and their derivatives are applied as catalysts,4−6 food additives,7 sunscreen ingredients,8 parts of photovoltaic devices,9−11 environmental purification agents,12−16 and carriers in delivery processes.17−20 Despite the extensive use of titanium oxide nanomaterials in other fields, their history in medical, biological, or pharmaceutical science is relatively short.21 For instance, the photodynamic therapeutic properties of titanate particles were recently utilized against cancer cells.22 Another bio-related field, where titanium oxides are used in large extent, is drug delivery. Apart from biocompatibility, improved physicochemical properties and ease of processability allow relatively facile immobilization of various drugs or other biologically active molecules in their structures.17,18 Depending on the synthesis conditions, titanium oxides of various shapes and compositions have been prepared. Among them, spherical nanoparticles are the most commonly used © XXXX American Chemical Society

Received: January 25, 2019 Revised: March 18, 2019 Published: March 19, 2019 A

DOI: 10.1021/acs.langmuir.9b00242 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

For the determination of the mobilities, 5 mL solutions were prepared. In brief, 0.5 mL dispersion of the uncoated or coated TNS of 10 mg/L concentration was added to 4.5 mL solution composed of the polyelectrolyte and/or NaCl at appropriate concentrations. The samples were allowed to rest 2 h at room temperature before measuring the electrophoretic mobilities after 1 min equilibration time in the device. The reported values are the average of five individual measurements. The experiments were performed in 350 μL volume omega-shaped plastic cuvettes (Anton Paar). Time-resolved DLS measurements were carried out to determine the apparent aggregation rate coefficients (kapp) of particle dimer formation as42

aggregation processes and hence, the colloidal stability of the titanium oxide compounds.27 Although numerous articles can be found in the literature, which deal with functionalization of titania nanostructures by various compounds, only a limited number of studies was concerned with the effect of such a functionalization on the colloidal stability of the particles.28−32 Polyelectrolytes have proved to be efficient aggregation or stabilization agents for nanoparticles dispersed in an aqueous medium.33 Furthermore, charging and aggregation processes of titanium oxide particles of various shapes and compositions were studied in the presence of polyelectrolytes.27 In these studies, the particles were usually negatively charged because of the relatively low point of zero charge (PZC) of the titanium oxide compounds.34−36 In this way, oppositely (positively) charged polyelectrolytes such as poly(diallyldimethylammonium chloride) (PDADMAC)26 or poly(amido amine) dendrimers31 were applied to tune the colloidal stability of the dispersions. Besides, the effect of like-charged (negative) polyelectrolytes [e.g., poly(acrylic acid)37 or humic acid38 on the charging and aggregation of titanium oxide nanoobjects was also investigated but in a much less extent than the oppositely charged systems. Nevertheless, no systematic studies were performed with samples, where the same polyelectrolytes were used as oppositely charged and likecharged by changing the sign of the charge of titanium oxide particles by the pH. Accordingly, positively charged particles can be obtained at low pH and negative ones are present at higher pH, below and above the PZC, respectively.39 On the other hand, the possible protonation of the polyelectrolytes has to be precisely known because variation in the pH often leads to significant changes in the line charge densities.40,41 By considering these issues, the charge balance can be tuned by varying the surface charge of the titanium oxide material from positive to negative by the pH, while keeping the charge of the polyelectrolyte the same, that is, no changes in the charge density occur in that pH range. Therefore, in the present study, systematic electrophoretic and dynamic light scattering (DLS) measurements were performed to assess the surface charge properties and aggregation processes of the TNS in the presence of PDADMAC and poly(styrene sulfonate) (PSS) polyelectrolytes. The measurements were performed at pH values below or above the PZC. In this way, both polyelectrolytes were probed as like-charged or oppositely charged substances. The results shed light on the adsorption mechanism of the PDADMAC or PSS on TNS and on the origin of the major interparticle forces induced by polyelectrolyte adsorption.

kapp =

1 dR h(t ) R h(0) dt

(1)

where Rh is the hydrodynamic radius, t is the time of the experiment and Rh(0) is the Rh of TNS determined in stable dispersion. The measurements were run for 40−120 min depending on the speed of aggregation. The same particle concentration (1 mg/L) was used in all of the time-resolved measurements. In order to compare the tendencies, the sample preparation for the DLS was done in a similar manner as the one described above for electrophoresis. The only difference in the sample preparation was that the total volume was 2 mL for DLS and that after adding the particles to the solutions of polyelectrolyte and/or NaCl, the samples were stirred with a Vortex (VWR) for 25 s. Furthermore, the samples were equilibrated for 30 s in the instrument before starting the time-resolved measurements. The colloidal stability was expressed in terms of stability ratio (W), which was calculated from the kapp values as follows42−45 W=

fast kapp

kapp

(2)

where the fast condition corresponds to the diffusion-controlled aggregation of the particles achieved in 1 M NaCl solution. One can realize that W = 1 is associated to unstable dispersions, where all the particle collisions result in dimer formation.

3. RESULTS AND DISCUSSION The detailed structural characterization of the bare TNS can be found elsewhere.26 In brief, transmission electron microscopy analysis yielded 94 nm in lateral dimension with a polydispersity of 0.3, while the average height of the TNS was found to be 9 nm by atomic force microscopy. These data result in an aspect ratio of 10.4 for the nanosheets. The PZC of the TNS was reported to be at pH 5.2 in our earlier study.26 Therefore, electrophoretic and DLS measurements were carried out at pH 4 and 10 to investigate the effect of oppositely charged or like-charged polyelectrolytes on the positively and negatively charged nanosheets, respectively. 3.1. Charging and Aggregation of Positively Charged TNS. Electrophoretic mobilities and stability ratios of TNS were determined at pH 4 in the presence of PSS and PDADMAC. In this situation, the former polyelectrolyte is oppositely charged, while the latter one possesses the same sign of charge as TNS. The nanosheets were of positive charge at low polyelectrolyte doses as indicated by the positive mobility values under these experimental conditions. However, the trend in the electrophoretic mobilities was different once PSS or PDADMAC was applied (Figure 1a). PSS adsorption was indicated by the decrease of the mobilities by increasing the PSS concentration. Such an adsorption process led to charge neutralization at the isoelectric point (IEP) and charge reversal at higher doses. The main governing forces responsible for the latter phenomenon are the entropy gain due to the release of the

2. EXPERIMENTAL SECTION 2.1. Materials. The synthesis of TNS together with the structural characterization is detailed elsewhere.26 Ionic strength and pH were adjusted by analytical grade NaCl, HCl, and NaOH, which were bought from VWR. Positively charged PDADMAC, (20 wt % aqueous solution with an average molecular mass of 275 kg/mol) and negatively charged PSS, (sodium salt, 10.6 kg/mol molar mass) were purchased from Sigma-Aldrich. All the measurements were carried out at 25 °C. High-purity water (VWR Purity TU+) was used for all the sample preparations. Water as well as the NaCl, HCl, and NaOH solutions were filtered with a 0.1 μm syringe filter (Millex). 2.2. Methods. Electrophoretic mobility and DLS measurements were performed with a Litesizer 500 instrument (Anton Paar) equipped with a 40 mW semiconductor laser (658 nm wavelength) operating in the backscattering mode at a scattering angle of 175°. B


Article

Langmuir

different by increasing the polyelectrolyte dose (Figure 2). Dispersions possessing a limited stability were observed at low

Figure 2. Hydrodynamic radii of TNS as a function of the experiment time at different PSS doses indicated in the inset. The measurements were carried out at 1 mM ionic strength, 1 mg/L TNS concentration and at pH 4.

polyelectrolyte concentrations as indicated by the fairly low stability ratio values determined in this regime. By increasing the amount of PSS, the stability ratio values decreased and reached a minimum followed by an increase at high PSS doses, where stable samples were obtained. Similar destabilization− restabilization mechanism has already been reported for titanium-oxide compounds in the presence of oppositely charged polyelectrolytes,26,27,30,31,47 also with the same charge balance using nanowires and PSS.29 Comparing the trend in the mobilities and stability ratios, one can easily realize that the minimum in the latter values is located near the IEP. Therefore, the aggregation features of TNS at different PSS doses resembles to the one predicted by the classical theory developed by Derjaguin, Landau, Verwey, and Overbeek (DLVO).50,51 Indeed, the nanosheets rapidly aggregate once the surface charges are neutralized at the IEP, where the repulsive electrical double layer interaction is absent and the attractive van der Waals force predominates. More stable dispersions were observed at low and high PSS doses, where the double layer forces overcome the attractive ones due to the sufficient charge of the nanosheets. On the other hand, the difference in the slopes in the stability ratios in the slow aggregation regimes at low and high doses indicates the presence of additional (non-DLVO) attractive forces. This difference originates from the so-called patch-charge effect,52,53 which is often induced by adsorption of strong polyelectrolytes on oppositely charged particles.54−56 In the present situation, PSS tends to form islands (patch) on the surface upon adsorption, while empty positive places (charge) are still available on the TNS surface at low polyelectrolyte coverage. The patches are electrostatically attracted by the charges giving rise to the evolution of an attractive interaction in addition to the already existing van der Waals attractive forces. The patch−charge interaction leads to faster aggregation of the nanosheets and thus, to lower stability ratios and to a smaller slope in the slow aggregation regime at low surface coverage. Nevertheless, such an additional attraction is not pronounced at higher doses above the IEP, where the PSS coverage on the TNS surface is high. This is because of the insufficient number of positive charges on the surface, which prevents the electrostatic attraction with the

Figure 1. Electrophoretic mobility (a) and stability ratio (b) of TNS as a function of the polyelectrolyte dose at pH 4. The ionic strength and the TNS concentration were set to 1 mM and 1 mg/L, respectively. The PDADMAC@TNS dispersions were stable in the entire polyelectrolyte range, that is, no stability ratios could be measured. The unit in the x-axis indicates mg of polyelectrolyte per 1 g of TNS. The lines serve to guide the eyes.

solvent molecules upon polyelectrolyte adsorption46 and the electrostatic attraction between the PSS and the empty places on the oppositely charged TNS surface. The adsorption continued until the mobilities reached a plateau at high PSS concentration. The onset of this plateau (ASP) corresponds to the polyelectrolyte dose, at which a saturated PSS layer forms on the surface of the oppositely charged TNS. Similar charging behavior has been previously reported with titanium oxide particles in the presence of oppositely charged polyelectrolytes.27,28,47,48 In the case of PDADMAC, however, the mobilities remained constant up to a dose of about 10 mg/g and then slightly increased at higher concentrations. This result indicates that adsorption of like-charged PDADMAC may occur, even on the positively charged TNS, at high polyelectrolyte loading. Similar adsorption process of like-charged polyelectrolytes was observed for negatively charged titanium oxides above the PZC,37,38,49 but to the best of our knowledge, this is the first time, when adsorption of positively charged polyelectrolyte is reported on titanium oxide compounds of the same sign of charge. Stability ratios were determined under the identical experimental conditions as in the mobility study (Figure 1b). By varying the PSS dose, significant changes in the colloidal stability of the nanosheets were observed. This was clearly demonstrated by the results of the time-resolved DLS measurements, where the increase in Rh was remarkably C


Article

Langmuir PSS patches. Although, bridging interaction between PSSfunctionalized particles was reported earlier,33,54 no evidence was found for this phenomenon in the present system most likely because of the low molecular mass of the PSS and the low ionic strength applied in the experiments. Besides, no stability ratios could be determined for positively charged TNS in the presence of PDADMAC. As indicated by the results of the mobility measurements, PDADMAC adsorbs on the like-charged surface giving rise to an increase of the surface charge and hence, to stronger electrical double layer repulsion. Due to the fact that the bare particles were stable (see details later) at pH 4 and 1 mM ionic strength and that the presence of PDADMAC further enhanced the stabilizing electrostatic forces, the nanosheets formed stable dispersions in the entire PDADMAC concentration regime investigated. Therefore, stability ratio measurements were not possible under these experimental conditions. 3.2. Charging and Aggregation of Negatively Charged TNS. Electrophoretic and time-resolved DLS experiments were also carried out at pH 10 above the PZC of the TNS in a similar fashion than with the positively charged particles. Because the charge of the PSS and PDADMAC is pH-independent, the former one is of the same sign of charge as the nanosheets, while the latter one is oppositely charged. All other experimental conditions were the same as in the measurements discussed in the previous section. The electrophoretic mobility values are shown in Figure 3a.

In the case of PSS, the mobilities remained negative and constant within the experimental error in the concentration regime investigated. This fact indicates negligible adsorption on the like-charged surface. This result is somewhat surprising because like-charged PDADMAC adsorbed on the TNS at low pH and adsorption of other like-charged polyelectrolytes has already been reported for negatively charged titanium oxide materials.27,37,38 One can assume that the negatively charged TNS is more hydrophilic than the positive one and hence, the evolvement of hydrophobic interaction is not feasible with the PSS, which remains dissolved in the solution. Besides, such a hydrophobic character is rather typical for PSS of significantly higher molar masses than the one used in the present study. Another possible explanation for the negligible adsorption is that because of the lower molar mass of the PSS, the release of solvent molecules upon adsorption and subsequent entropy gain46 could not overcome the electrostatic repulsion between the polyelectrolyte and the surface. In the other system, PDADMAC adsorbed strongly on the oppositely charged nanosheets and charge neutralization occurred at the IEP. The adsorption continued after the IEP, leading to charge reversal and to the formation of a saturated polyelectrolyte layer on the TNS at the ASP. The trend in the magnitude of the electrophoretic mobilities resembled to the one discussed in the PSS@TNS system at pH 4 in the previous chapter because the main interactions between the polyelectrolytes and the surfaces are the same irrespective of the charge balance in the oppositely charged systems. No stability ratios could be determined in the samples containing PSS and negatively charged nanosheets because of the sufficiently high negative charge and the strong electrical double layer forces, which stabilize the dispersions under the experimental conditions applied. The tendency in the stability ratios in the PDADMAC system resembles a U-shape curve, which is typical for charged particles in the presence of oppositely charged polyelectrolytes.27,33,56 Similar trend was found with the PSS systems at pH 4. As discussed earlier, such a destabilization−restabilization mechanism can be qualitatively described by the DLVO theory,51 which states that particles of a neutralized surface charge at the IEP rapidly aggregate because of the absence of double layer repulsion and the presence of van der Waals attraction. On the other hand, nanosheets of sufficiently high charge below or above the IEP are stabilized by electrical double layer repulsion. Comparing the behavior of the negatively and positively charged TNS discussed so far, the following conclusions can be taken. First, like-charged polyelectrolyte was adsorbed only on the positive nanosheets at low pH. This is most likely because of the different hydrophobicities of the nanosheets under acidic and alkaline conditions as well as the different molar masses of polyelectrolytes. Second, patch−charge attraction was detected at pH 4 in the PSS system at low surface coverage, nevertheless, this type of interaction was absent in the PDADMAC samples at pH 10 because very similar slopes were measured in the stability ratios in the slow aggregation regimes at low and high polyelectrolyte doses in the latter case. This result shed light on the fact that PDADMAC adsorbs in a more extended conformation on the TNS and hence, no polyelectrolyte patches are formed on the surface. 3.3. Resistance Against Salt-Induced Aggregation of the Bare and Polyelectrolyte-Functionalized TNS. The nature of the interparticle forces was further explored by systematically changing the ionic strength in the dispersions

Figure 3. Electrophoretic mobility (a) and stability ratio (b) of TNS as a function of the polyelectrolyte dose at pH 10. The ionic strength and the TNS concentration were set to 1 mM and 1 mg/L, respectively. The PSS@TNS dispersions were stable in the entire polyelectrolyte range, that is, no stability ratios could be measured. D


Article

Langmuir

Figure 4. Electrophoretic mobility (top row) and stability ratio (bottom row) of TNS and its polyelectrolyte-functionalized derivatives as a function of the ionic strength at pH 4 (a) and pH 10 (b). The solid lines presented with the mobilities are just to guide the eyes, while the ones with the stability ratios are the results of calculations using eq 5.

containing bare TNS, TNS and PSS, and TNS and PDADMAC at both pH 4 and 10. The doses of the polyelectrolytes were set at the ASP determined earlier in the oppositely charged systems. Electrophoretic mobilities and stability ratios are presented in Figure 4. In general, the tendencies in the absolute mobilities were very similar independent of the charge balance. Although the values were system specific, the magnitude of the mobilities decreased by increasing the ionic strength because of the screening effect of the dissolved salt constituents on the surface charges.33 Concerning the stability ratios, slow and fast aggregation regimes were separated by the critical coagulation concentration (CCC) values in each cases. This behavior is in line with the DLVO theory. At low-salt level, accordingly, the electrical double layers are strong and stabilize the dispersions, while these repulsive forces are weak or even vanish at higher ionic strengths due to charge screening by the salt constituent ions. Similar DLVO-type interparticle forces were also reported for titanium oxide particles of various shapes in different ionic environments.27,57,58 Although the tendencies were similar in both mobility and stability curves obtained at different pHs by changing the ionic strength, the values showed remarkable system specificities. Looking at the results obtained at pH 4, the magnitude of the mobilities followed the TNS < PDADMAC@TNS ≈ PSS@TNS order at low doses (Figure 4a and Table 1). To assess the charge density (σ) of the bare and functionalized nanosheets, electrophoretic mobilities (μ) were converted to zeta potentials (ζ) with the Smoluchowski model as59

Table 1. Characteristic Composition, Size, Polydispersity, Charging and Aggregation Data for TNS in the Bare and in the Polyelectrolyte-Functionalized Forms bare TNS μ (m /Vs) σ (mC/m2)b CCC (mM)c ζCCC (mV)d 2

a

PDADMAC@ TNS

PSS@TNS

pH 4

pH 10

pH 4

pH 10

pH 4

pH 10

2.0 16 17 24.6

−3.1 −18 100 −28.6

2.5 16 45 25.0

1.6 19 400 17.7

−2.5 −18 100 −27.0

−3.1 −17 80 −27.0

a

Measured at 1 mM ionic strength. bCalculated with eq 4. Determined from the stability ratio versus ionic strength plots using eq 5. dZeta potentials at the CCC. c

ζ=

ημ ε0ε

(3)

where η is the viscosity of the medium (8.9 × 10−4 Pa s), ε is the dielectric constant of water (78.5), and ε0 is the dielectric permittivity of vacuum (8.9 × 10−12 F/m). Thereafter, the surface charge densities were calculated with the Grahame equation as50 ÄÅ ÉÑ 2kBTε0 εκ ÅÅÅ qζ ÑÑÑ ÑÑ σ= sinhÅÅÅ ÅÅÇ 2kBT ÑÑÑÖ q (4) where kB is the Boltzmann constant, T is the temperature, q is the elementary charge, and κ is the inverse Debye length, which represents the contribution of all the ionic species present in the dispersion to the quantitative description of the electrical double layer.59 One can conclude from the σ values E


Article

Langmuir

where e is the base of the natural logarithm, LB is the Bjerrum length, and H is the Hamaker constant. The values applied in the calculations were 2.72, 0.71 nm, and 1.60 × 10−20 J. The ζCCC is the zeta potential at the CCC (Table 1) in this case. The calculated and measured data are shown in Figure 5.

(Table 1) that the absolute charge determined at the slip plane (i.e., zeta potentials were used in eq 4) is higher for the PSS@ TNS system and about the same for the PDADMAC@TNS and bare TNS. A clear sequence of TNS < PDADMAC@TNS < PSS@TNS was observed in the CCC values at pH 4 (Table 1), which were calculated as follows60 ÅÄÅ ÑÉβ Å CCC ÑÑÑ ÑÑ W = 1 + ÅÅÅÅ ÅÅÇ c NaCl ÑÑÑÖ

(5)

where cNaCl is the NaCl molar concentration and β was obtained from the stability ratios in the slow aggregation regime before the CCC as β=

dlog 1/W dlog c NaCl

(6)

Due to the fact that the shapes of the stability ratio versus ionic strength plots are very similar and only the CCC changes in the individual systems at pH 4, one can conclude that the aggregation mechanism is the same irrespective of the type of polyelectrolytes applied. The differences in the CCC are due to the different magnitude of charge, which is the highest for PSS@TNS and lowest for the bare TNS. Therefore, the electrical double layer forces are the strongest for PSS@TNS, leading to the highest CCC because an elevated salt concentration is needed to screen the repulsive double layer interaction and to destabilize the dispersions. For the negatively charged TNS at pH 10, the electrophoretic mobilities, stability ratios, and the corresponding CCC were about the same for the bare TNS and PSS@TNS, indicating negligible adsorption of the like-charged polyelectrolyte on the nanosheets (Figure 4b). However, a large increase was observed in the CCC (Table 1), once the TNS surface was saturated with a self-assembled PDADMAC layer. This finding is a clear signal for the presence of a remarkable stabilizing effect of the polyelectrolyte layer. In addition, the slope in the slow aggregation regime was also different compared to the bare TNS or to the PSS@TNS systems. Due to the fact that the charge density of PDADMAC@TNS was only slightly higher than the one determined for the bare nanosheets (Table 1), the stabilization mechanism cannot be explained within the DLVO theory but is clearly related to the adsorbed PDADMAC chains. Similar shifts in the CCC have already been observed for negatively charged particles with a saturated PDADMAC layer on their surface.26,30,61 This is most likely due to the steric stabilization mechanism,62 which originates from the overlap of the adsorbed polyelectrolyte chains upon the approach of two particles. Once the chains overlap, an osmotic pressure evolves, which gives rise to repulsive interaction and to the formation of stable dispersions. Obviously, electrical double layer forces are also present and thus, the joint effect is the so-called electrosteric stabilization.27,62,63 To unambiguously confirm the origin of the interparticle forces in the dispersions of the bare and functionalized TNS, the theoretical CCC values were calculated within the DLVO model as50 ÄÅ ÉÑ2 2 72π ÅÅÅÅ ε0εζCCC ÑÑÑÑ CCC = 2 ÅÅ Ñ e L B ÅÅÅÇ H ÑÑÑÑÖ

Figure 5. CCC values of bare and polyelectrolyte-functionalized TNS as a function of the zeta potentials at the CCC. Values determined at pH 4 are indicated by squares, while the ones at pH 10 are presented with circles. The solid line indicates the results of calculations using the DLVO theory (eq 7).

The experimental data agree well with the results of the theoretical calculation in four systems indicating the presence of DLVO forces as the major interparticle interaction. Nevertheless, two systems behaved somehow differently and hence, the presence of non-DLVO forces can be assumed. Although the deviation is small, the CCC of the positively charged bare TNS is lower than the predicted one, which is a clear sign for the presence of additional attractive forces. Because the pH in these samples is close to the PZC, the hydroxyl groups can be in different protonated stages39 and they may interact through electrostatic attraction or via hydrogen bonding. These interaction leads to additional (non-van der Waals) attraction and to a lower CCC value. On the other hand, the CCC of the PDADMAC@TNS system at pH 10 was significantly higher than the one calculated with the DLVO theory. The higher CCC is a clear signal for the operation of strong repulsive interparticle forces of the non-DLVO origin. As discussed above, this repulsion originates from the steric interaction between the adsorbed polyelectrolyte chains. The joint effect of steric and electrical double layer repulsions results in the formation of highly stable PDADMAC@TNS dispersions owing to the electrosteric stabilization mechanism.62

4. CONCLUSIONS The charging behavior and colloidal stability of TNS was investigated at different pHs in the presence of polyelectrolytes. The interactions between the polyelectrolytes and titania surface involved electrostatic, entropy, and hydrophobic effects irrespective of the charge balance. In the systems containing positively charged TNS at pH 4, the electrophoretic mobilities showed that adsorption of like-charged PDADMAC as well as oppositely charged PSS takes place, however, the extent of the adsorption is much higher in the latter case leading to charge neutralization and charge reversal. Timeresolved DLS measurements revealed that the TNS aggrega-

(7) F


Article

Langmuir

(2) Moore, T. L.; Rodriguez-Lorenzo, L.; Hirsch, V.; Balog, S.; Urban, D.; Jud, C.; Rothen-Rutishauser, B.; Lattuada, M.; Petri-Fink, A. Nanoparticle colloidal stability in cell culture media and impact on cellular interactions. Chem. Soc. Rev. 2015, 44, 6287−6305. (3) Chen, G.; Roy, I.; Yang, C.; Prasad, P. N. Nanochemistry and nanomedicine for nanoparticle-based diagnostics and therapy. Chem. Rev. 2016, 116, 2826−2885. (4) Li, X. Z.; Liu, H.; Cheng, L. F.; Tong, H. J. Photocatalytic oxidation using a new catalyst - TiO2 microsphere - for water and wastewater treatment. Environ. Sci. Technol. 2003, 37, 3989−3994. (5) Huang, Y.; Lu, Y.; Lin, Y.; Mao, Y.; Ouyang, G.; Liu, H.; Zhang, S.; Tong, Y. Cerium-based hybrid nanorods for synergetic photothermocatalytic degradation of organic pollutants. J. Mater. Chem. A 2018, 6, 24740−24747. (6) Liu, H.; Li, X. Z.; Leng, Y. J.; Li, W. Z. An alternative approach to ascertain the rate-determining steps of TiO2 photoelectrocatalytic reaction by electrochemical impedance spectroscopy. J. Phys. Chem. B 2003, 107, 8988−8996. (7) Weir, A.; Westerhoff, P.; Fabricius, L.; Hristovski, K.; von Goetz, N. Titanium dioxide nanoparticles in food and personal care products. Environ. Sci. Technol. 2012, 46, 2242−2250. (8) Morsella, M.; d’Alessandro, N.; Lanterna, A. E.; Scaiano, J. C. Improving the sunscreen properties of TiO2 through an understanding of its catalytic properties. ACS Omega 2016, 1, 464−469. (9) Tétreault, N.; Horváth, E.; Moehl, T.; Brillet, J.; Smajda, R.; Bungener, S.; Cai, N.; Wang, P.; Zakeeruddin, S. M.; Forró, L.; Magrez, A.; Grätzel, M. High-efficiency solid-state dye-sensitized solar cells: Fast charge extraction through self-assembled 3D fibrous network of crystalline TiO2 nanowires. ACS Nano 2010, 4, 7644− 7650. (10) Rodriguez, E. F.; Chen, D.; Hollenkamp, A. F.; Cao, L.; Caruso, R. A. Monodisperse mesoporous anatase beads as high performance and safer anodes for lithium ion batteries. Nanoscale 2015, 7, 17947− 17956. (11) Osada, M.; Sasaki, T. Exfoliated oxide nanosheets: new solution to nanoelectronics. J. Mater. Chem. 2009, 19, 2503−2511. (12) Tang, Y.; Jiang, Z.; Tay, Q.; Deng, J.; Lai, Y.; Gong, D.; Dong, Z.; Chen, Z. Visible-light plasmonic photocatalyst anchored on titanate nanotubes: a novel nanohybrid with synergistic effects of adsorption and degradation. RSC Adv. 2012, 2, 9406−9414. (13) Honda, R. J.; Keene, V.; Daniels, L.; Walker, S. L. Removal of TiO2 nanoparticles during primary water treatment: Role of coagulant type, dose, and nanoparticle concentration. Environ. Eng. Sci. 2014, 31, 127−134. (14) Grzechulska, J.; Hamerski, M.; Morawski, A. W. Photocatalytic decomposition of oil in water. Water Res. 2000, 34, 1638−1644. (15) Veres, Á .; Ménesi, J.; Janáky, C.; Samu, G. F.; Scheyer, M. K.; Xu, Q.; Salahioglu, F.; Garland, M. V.; Dékány, I.; Zhong, Z. New insights into the relationship between structure and photocatalytic properties of TiO2 catalysts. RSC Adv. 2015, 5, 2421−2428. (16) Bajnóczi, É . G.; Balázs, N.; Mogyorósi, K.; Srankó, D. F.; Pap, Z.; Ambrus, Z.; Canton, S. E.; Norén, K.; Kuzmann, E.; Vértes, A.; Homonnay, Z.; Oszkó, A.; Pálinkó, I.; Sipos, P. The influence of the local structure of Fe(III) on the photocatalytic activity of doped TiO2 photocatalysts-An EXAFS, XPS and Mossbauer spectroscopic study. Appl. Catal., B 2011, 103, 232−239. (17) Dempsey, C.; Lee, I.; Cowan, K. R.; Suh, J. Coating barium titanate nanoparticles with polyethylenimine improves cellular uptake and allows for coupled imaging and gene delivery. Colloids Surf., B 2013, 112, 108−112. (18) Rouster, P.; Pavlovic, M.; Sáringer, S.; Szilagyi, I. Functionalized titania nanosheet dispersions of peroxidase activity. J. Phys. Chem. C 2018, 122, 11455−11463. (19) Beke, S.; Kő rösi, L.; Scarpellini, A.; Anjum, F.; Brandi, F. Titanate nanotube coatings on biodegradable photopolymer scaffolds. Mater. Sci. Eng., C 2013, 33, 2460−2463. (20) Liu, H.; Duan, C.; Su, X.; Dong, X.; Huang, Z.; Shen, W.; Zhu, Z. A hemoglobin encapsulated titania nanosheet modified reduced

tion processes are sensitive to the PSS dose applied and the stability regimes correlate well with the charging behavior. Accordingly, charge neutralization induced rapid particle aggregation, while the dispersions were restabilized upon charge reversal. The major interparticle forces were of DLVO origin, however, patch-charge effects were observed at low surface coverage by the PSS. Once the pH was shifted to 10, the negatively charged TNS did not interact with the like-charged PSS due to the hydrophilic character of the surface and to the low molar mass of the polyelectrolyte. The oppositely charged PDADMAC adsorbed strongly and the charge of the nanosheets changed from negative to zero and to positive by increasing the PDADMAC concentration. A saturated polyelectrolyte layer formed on the surface of the nanosheets at high polyelectrolyte loadings. Such a behavior again resulted in a destabilization−restabilization phenomenon, which showed a good qualitative agreement with the DLVO theory. The resistance of the bare and polyelectrolyte-functionalized TNS against salt-induced aggregation was different in the individual systems and the TNS of saturated PDADMAC layer on the surface at pH 10 formed the most stable dispersion. The CCC values were also calculated within the DLVO model. Comparison with the experimental data revealed that the interparticle forces are of DLVO origin in the majority of the systems, however, additional attraction and repulsion was detected for the positively charged bare TNS and for the PDADMAC-coated TNS at pH 10, respectively. In the latter case, the joint effect of steric and double layer repulsions gave rise to highly stable dispersions, which could be destabilized only at high ionic strengths. The results shed light on the importance of the interfacial processes of titanium-oxide particles in the presence of likecharged or oppositely charged polyelectrolytes. Such processes are responsible for the aggregation mechanism and subsequent colloidal stability of the samples. Once the nature of the interparticle forces is known, the colloidal stability can be predicted. Therefore, fundamental studies similar to the present one are needed in order to build comprehensive models to predict dispersion stability relevant in applications of titanium oxide particles, where polyelectrolytes of the same or opposite sign of charge are present.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

István Szilágyi: 0000-0001-7289-0979 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by the Lendület program of the Hungarian Academy of Sciences (grant number: 96130) and by the Ministry of Human Capacities, Hungary through the project 20391-3/2018/FEKUSTRAT.

■

REFERENCES

(1) Patzke, G. R.; Zhou, Y.; Kontic, R.; Conrad, F. Oxide nanomaterials: Synthetic developments, mechanistic studies, and technological innovations. Angew. Chem., Int. Ed. 2011, 50, 826−859. G


Article

Langmuir graphene oxide nanocomposite as a mediator-free biosensor. Sens. Actuators, B 2014, 203, 303−310. (21) Damodaran, V. B.; Bhatnagar, D.; Leszczak, V.; Popat, K. C. Titania nanostructures: a biomedical perspective. RSC Adv. 2015, 5, 37149−37171. (22) Rehman, F. U.; Zhao, C.; Jiang, H.; Wang, X. Biomedical applications of nano-titania in theranostics and photodynamic therapy. Biomater. Sci. 2016, 4, 40−54. (23) Katagiri, K.; Inami, H.; Koumoto, K.; Inumaru, K.; Tomita, K.; Kobayashi, M.; Kakihana, M. Preparation of hollow TiO2 spheres of the desired polymorphs by layer-by-layer assembly of a water-soluble titanium complex and hydrothermal treatment. Eur. J. Inorg. Chem. 2012, 2012, 3267−3272. (24) Wang, G. M.; Feng, H. Q.; Hu, L. S.; Jin, W. H.; Hao, Q.; Gao, A.; Peng, X.; Li, W.; Wong, K. Y.; Wang, H. Y.; Li, Z.; Chu, P. K. An antibacterial platform based on capacitive carbon-doped TiO2 nanotubes after direct or alternating current charging. Nat. Commun. 2018, 9, 2055. (25) Horváth, E.; Kukovecz, Á .; Kónya, Z.; Kiricsi, I. Hydrothermal conversion of self-assembled titanate nanotubes into nanowires in a revolving autoclave. Chem. Mater. 2007, 19, 927−931. (26) Rouster, P.; Pavlovic, M.; Szilagyi, I. Improving the stability of titania nanosheets by functionalization with polyelectrolytes. RSC Adv. 2016, 6, 97322−97330. (27) Muráth, S.; Sáringer, S.; Somosi, Z.; Szilágyi, I. Effect of ionic compounds of different valences on the stability of titanium oxide colloids. Colloids Interfaces 2018, 2, 32. (28) Papa, A.-L.; Boudon, J.; Bellat, V.; Loiseau, A.; Bisht, H.; Sallem, F.; Chassagnon, R.; Bérard, V.; Millot, N. Dispersion of titanate nanotubes for nanomedicine: comparison of PEI and PEG nanohybrids. Dalton Trans. 2015, 44, 739−746. (29) Horváth, E.; Grebikova, L.; Maroni, P.; Szabó, T.; Magrez, A.; Forró, L.; Szilagyi, I. Dispersion characteristics and aggregation in titanate nanowire colloids. ChemPlusChem 2014, 79, 592−600. (30) Szabó, T.; Tóth, V.; Horváth, E.; Forró, L.; Szilagyi, I. Tuning the aggregation of titanate nanowires in aqueous dispersions. Langmuir 2015, 31, 42−49. (31) Pavlovic, M.; Adok-Sipiczki, M.; Horváth, E.; Szabó, T.; Forró, L.; Szilagyi, I. Dendrimer-stabilized titanate nanowire dispersions as potential nanocarriers. J. Phys. Chem. C 2015, 119, 24919−24926. (32) Horváth, E.; Szilágyi, I.; Forró, L.; Magrez, A. Probing titanate nanowire surface acidity through methylene blue adsorption in colloidal suspension and on thin films. J. Colloid Interface Sci. 2014, 416, 190−197. (33) Szilagyi, I.; Trefalt, G.; Tiraferri, A.; Maroni, P.; Borkovec, M. Polyelectrolyte adsorption, interparticle forces, and colloidal aggregation. Soft Matter 2014, 10, 2479−2502. (34) Niecikowska, A.; Krasowska, M.; Ralston, J.; Malysa, K. Role of surface charge and hydrophobicity in the three-phase contact formation and wetting film stability under dynamic conditions. J. Phys. Chem. C 2012, 116, 3071−3078. (35) Kuscer, D.; Stavber, G.; Trefalt, G.; Kosec, M. Formulation of an aqueous titania suspension and its patterning with ink-jet printing technology. J. Am. Ceram. Soc. 2012, 95, 487−493. (36) Snoswell, D. R. E.; Duan, J.; Fornasiero, D.; Ralston, J. Colloid stability of synthetic titania and the influence of surface roughness. J. Colloid Interface Sci. 2005, 286, 526−535. (37) Kirby, G. H.; Harris, D. J.; Li, Q.; Lewis, J. A. Poly(acrylic acid)-poly(ethylene oxide) comb polymer effects on BaTiO3 nanoparticle suspension stability. J. Am. Ceram. Soc. 2004, 87, 181− 186. (38) Liu, W.; Sun, W.; Borthwick, A. G. L.; Ni, J. Comparison on aggregation and sedimentation of titanium dioxide, titanate nanotubes and titanate nanotubes-TiO2: Influence of pH, ionic strength and natural organic matter. Colloids Surf., A 2013, 434, 319−328. (39) Parks, G. A. The isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems. Chem. Rev. 1965, 65, 177−198.

(40) Cakara, D.; Kleimann, J.; Borkovec, M. Microscopic protonation equilibria of poly(amidoamine) dendrimers from macroscopic titrations. Macromolecules 2003, 36, 4201−4207. (41) Smits, R. G.; Koper, G. J. M.; Mandel, M. The influence of nearest-neighbor and next-nearest-neighbor interactions on the potentiometric titration of linear poly(ethylenimine). J. Phys. Chem. 1993, 97, 5745−5751. (42) Holthoff, H.; Egelhaaf, S. U.; Borkovec, M.; Schurtenberger, P.; Sticher, H. Coagulation rate measurements of colloidal particles by simultaneous static and dynamic light scattering. Langmuir 1996, 12, 5541−5549. (43) Chen, K. L.; Mylon, S. E.; Elimelech, M. Enhanced aggregation of alginate-coated iron oxide (hematite) nanoparticles in the presence of, calcium, strontium and barium cations. Langmuir 2007, 23, 5920− 5928. (44) Ehrl, L.; Jia, Z.; Wu, H.; Lattuada, M.; Soos, M.; Morbidelli, M. Role of counterion association in colloidal stability. Langmuir 2009, 25, 2696−2702. (45) Elimelech, M.; Gregory, J.; Jia, X.; Williams, R. A. Particle Deposition and Aggregation: Measurement, Modeling, and Simulation; Butterworth-Heinemann Ltd.: Oxford, 1995. (46) Park, S. Y.; Bruinsma, R. F.; Gelbart, W. M. Spontaneous overcharging of macro-ion complexes. Europhys. Lett. 1999, 46, 454− 460. (47) Rouster, P.; Pavlovic, M.; Horváth, E.; Forró, L.; Dey, S. K.; Szilagyi, I. Influence of protamine functionalization on the colloidal stability of 1D and 2D titanium oxide nanostructures. Langmuir 2017, 33, 9750−9758. (48) Ghimici, L.; Nichifor, M. Separation of TiO2 particles from water and water/methanol mixtures by cationic dextran derivatives. Carbohydr. Polym. 2013, 98, 1637−1643. (49) Fazio, S.; Guzmán, J.; Colomer, M. T.; Salomoni, A.; Moreno, R. Colloidal stability of nanosized titania aqueous suspensions. J. Eur. Ceram. Soc. 2008, 28, 2171−2176. (50) Evans, D. F.; Wennerstrom, H. The Colloidal Domain; John Wiley: New York, 1999. (51) Verwey, E. J. W.; Overbeek, J. T. G. Theory of Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (52) Feick, J. D.; Velegol, D. Measurements of charge nonuniformity on polystyrene latex particles. Langmuir 2002, 18, 3454−3458. (53) Miklavic, S. J.; Chan, D. Y. C.; White, L. R.; Healy, T. W. Double layer forces between heterogeneous charged surfaces. J. Phys. Chem. 1994, 98, 9022−9032. (54) Leong, Y. K. Interparticle forces arising from an adsorbed strong polyelectrolyte in colloidal dispersions: charged patch attraction. Colloid Polym. Sci. 1999, 277, 299−305. (55) Popa, I.; Papastavrou, G.; Borkovec, M. Charge regulation effects on electrostatic patch-charge attraction induced by adsorbed dendrimers. Phys. Chem. Chem. Phys. 2010, 12, 4863−4871. (56) Gillies, G.; Lin, W.; Borkovec, M. Charging and aggregation of positively charged latex particles in the presence of anionic polyelectrolytes. J. Phys. Chem. B 2007, 111, 8626−8633. (57) Wiese, G. R.; Healy, T. W. Coagulation and electrokinetic behavior of TiO2 and Al2O3 colloidal dispersions. J. Colloid Interface Sci. 1975, 51, 427−433. (58) Rouster, P.; Pavlovic, M.; Szilagyi, I. Destabilization of titania nanosheet suspensions by inorganic salts: Hofmeister series and Schulze-Hardy rule. J. Phys. Chem. B 2017, 121, 6749−6758. (59) Delgado, A. V.; González-Caballero, F.; Hunter, R. J.; Koopal, L. K.; Lyklema, J. Measurement and interpretation of electrokinetic phenomena. J. Colloid Interface Sci. 2007, 309, 194−224. (60) Grolimund, D.; Elimelech, M.; Borkovec, M. Aggregation and deposition kinetics of mobile colloidal particles in natural porous media. Colloids Surf., A 2001, 191, 179−188. (61) Somosi, Z.; Pavlovic, M.; Pálinkó, I.; Szilágyi, I. Effect of polyelectrolyte mono- and bilayer formation on the colloidal stability of layered double hydroxide nanoparticles. Nanomaterials 2018, 8, 986. H


Article

Langmuir (62) Fritz, G.; Schädler, V.; Willenbacher, N.; Wagner, N. J. Electrosteric stabilization of colloidal dispersions. Langmuir 2002, 18, 6381−6390. (63) Illés, E.; Tombácz, E. The effect of humic acid adsorption on pH-dependent surface charging and aggregation of magnetite nanoparticles. J. Colloid Interface Sci. 2006, 295, 115−123.

I


Regulation of the Stability of Titania Nanosheet Dispersions with

Recommend Documents