nanoparticle hybrid films on anisometric colloids studied by electro-optics

Colloids and Surfaces A: Physicochem. Eng. Aspects 455 (2014) 156–163

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Polyelectrolyte/nanoparticle hybrid films on anisometric colloids studied by electro-optics Viktoria Milkova ∗ Rostislaw Kaischiew Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Hybrid polyelectrolyte/nanoparticles films on colloids.

• Stability of colloid dispersions. • Layer-by-layer self-assembly of polyelectrolytes.

• Electrical properties and film thickness of multilayers.

a r t i c l e

i n f o

Article history: Received 17 February 2014 Received in revised form 17 April 2014 Accepted 18 April 2014 Available online 30 April 2014 Keywords: Nanocomposite particles Polyelectrolytes Nanoparticles Layer-by-layer assembly Electric light scattering

a b s t r a c t In this study, electro-optics is applied for the first time for investigation of suspension stability and electrical properties of colloidal particles covered with hybrid multilayer film. The polyelectrolyte/nanoparticle hybrid film is prepared onto ellipsoidal ␤-FeOOH particles through a subsequent layer-by-layer adsorption of anionic poly(sodium 4-styrene sulfonate), cationic poly(diallyldimethylammonium chloride), and silica nanoparticles. The formation of each layer is done with fine control of the adsorption process in order to produce stable and well defined system. The adsorbed amounts of the highly charged PSS and PDADMAC have been measured by UV–vis spectroscopy, whereas the amount of the adsorbed silica nanoparticles was calculated from the thickness of the adsorbed layers. The comparison of the electrical polarizability of the PSS and PADMAC coated particles allowed us to calculate the charge balance in this bi-layer as positive. The relaxation frequency of the kilohertz electro-optical effect was found to depend on the charge density of each adsorbed component, which means that the polarization of the excess charge in the outermost layer dominate the behavior of the entire composite film. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The formation of nano-structured systems presents significant interest in the last decades because of the grate potential of such structures for applications in biotechnology and medicine. In order to realize some specific functions, the composite systems can contain units with different properties and structures such as inorganic

∗ Tel.: +359 29793922. E-mail address: [email protected] http://dx.doi.org/10.1016/j.colsurfa.2014.04.048 0927-7757/© 2014 Elsevier B.V. All rights reserved.

nanoparticles [1–3], temperature-sensitive polymers [4], proteins [5], dyes [6,7], enzymes [8], cells [9]. To produce stable multifunctional nanostructures, efficient control of the experimental condition is needed. An effective method for preparation of composite materials through subsequent adsorption of oppositely charged components is proposed by Decher and co-workers [10]. The procedure can be considered as controlled formation of a layered complex onto charged surface, stabilized with strong electrostatic forces. Since the assembly process is mainly governed by the electrostatic attractions of the oppositely charged components, overcompensation of the surface charge after deposition of each layer is needed to ensure the formation of sequent layer

V. Milkova / Colloids and Surfaces A: Physicochem. Eng. Aspects 455 (2014) 156–163

from the oppositely charged component. The key to successful multilayer film formation on colloidal particles is the re-stabilization (minimum of particle aggregation) of the suspension after each adsorption step. The electrical properties of the particles, which are closely related to the suspension stability, are of main importance also in the case of nanocomposite preparations for application in biotechnology and medicine. In this study, the electro-optics is applied for the first time for investigation of stability and electrical properties of suspension of anisometric colloidal particles, covered with hybrid films in low ionic strength solutions. Three-layer composite films are formed on positively charged ␤-FeOOH particles through a subsequent adsorption of poly(sodium 4-styrene sulfonate), poly(diallyldimethylammonium chloride), and silica nanoparticles with fine control of the concentration of each component in order to produce stable films on the particles in the stabilized suspension. The model ␤-FeOOH particles are chosen for the electro-optical investigation because of their ellipsoidal shape and narrow size distribution. The silica nanoparticles are used as a model of bioactive molecules or proteins, which can be incorporated in functional nanocomposite materials. The electric light scattering method can give information about the electrical properties and size of non-spherical colloidal particles. This allows estimation of the thickness of the layers deposited on the particle surface. The information about the variation of the particle’s size in dependence on the concentration of the oppositely charged component is used to control the stability of the suspensions. As mentioned above, the overcompensation of the particle charge after each adsorption step ensures the adsorption of the next layer from the opposite charge component. Previously, we suggested that the electro-optical behavior of the film coated colloidal particles is governed by the electrical properties of the polyelectrolyte deposited in the last adsorbed layer. In the present study, this suggestion is verified for composite multilayer films containing oppositely charged components with different charges and structure–polyelectrolyte molecules and silica nanoparticles.

157

Poly(sodium 4-styrene sulfonate), NaPSS, of molecular weight 70 kDa, poly(diallyldimethylammonium chloride), PDADMAC, of molecular weight 100–200 kDa, and 8-hydroxypyrene-1,3,6trisulfonic acid trisodium salt, HPTS, are products of Sigma–Aldrich. The polyelectrolytes are used without further purification. The stock solutions of polyelectrolytes with concentration 1 g dm−3 are prepared with double distilled water. The stock concentration of the dye solution (HPTS) is 2 × 10−3 M. 2.2. Deposition of polyelectrolyte and silica nanoparticles layers Hybrid films were prepared at pH6 in order to achieve irreversible adsorption of well charged silica nanoparticles. The first layer from NaPSS is adsorbed by adding the suspension of ␤-FeOOH particles to a solution of polyelectrolyte with required concentration and stirring for 20 min. At enough high concentration (where strong overcompensation of the particle charge and stabilization of the suspension against flocculation is achieved), the excess polyelectrolyte is removed by centrifugation of the solution at 4500 rpm (3147 g) for 45 min. The settled particles are re-dispersed by sonication for 20 s in water. This procedure is repeated by adding the suspension to a solution of cationic PDADMAC and centrifugation of the stabilized suspension of ␤-FeOOH particles with adsorbed PSS/PDADMAC bi-layer. The third adsorption step is performed by mixing equal volumes of suspension of the PSS/PDADMAC coated particles and suspension of the silica nanoparticles with different concentrations (to achieve different number concentration ratio, R, defined as the number of silica nanoparticles to the number of polymer coated particles) and stirring for 1 h. The pH6 of both solutions is adjusted before mixing. The excess nanoparticles in the solution are removed by centrifugation of the suspension at 4500 rpm for 45 min. The obtained hybrid particles are re-dispersed by sonication for 20 s in water with defined pH.

2.3. Preparation of PDADMAC/HPTS complexes 2. Materials and methods 2.1. Materials ␤-FeOOH particles of narrow size distribution are prepared by acid hydrolysis of a 1.8 × 10−2 M FeCl3 solution containing 10−5 M HCl over a period of 3 weeks at room temperature [11]. The extraneous ions are removed by centrifugation in double-distilled water. This procedure gives particles with average dimensions 285 ± 56 and 72 ± 14 nm for the major a and minor b axes of the particle, respectively. These dimensions are determined by electron microscopy and they are slightly different from ones obtained from the electro-optics a = 300 and b = 75 nm. The discrepancy may find its origin in size polydispersity. Since the oxide suspension is not completely devoid of any (though tiny) size polydispersity, the hydrodynamic data may reflect somewhat more the dimensions of the largest particles. In acidic medium, ␤-FeOOH particles are positively charge due to the interaction of protons with hydroxyl groups on the particle surface. The concentration of particles in suspension is 8 × 10−3 g dm−3 (ca. 2 × 1012 particles dm−3 ). The spherical silica nanoparticles are product of Sigma–Aldrich—Ludox LS 30 (30 wt% suspension in water, LSNPs) and Ludox TM 40 (40 wt% suspension in water, TM-NPs). The diameters of both silica samples are determined by dynamic light scattering to be 16 and 34 nm for the LS-NPs and TM-NPs, respectively. The silica nanoparticles are negatively charged at pH above 3 (the isoelectric point is about pH 2.6 [12]). The pH of the solutions is adjusted by adding NaOH or HCl (Merck).

To measure the adsorbed amount of PDADMAC by spectroscopy, a complex is prepared in advance with a suitable concentration of the dye HPTS. For preparation of PDADMAC/HPTS complex, in which 0.83% and 10% of the PDADMAC charges are neutralized by HPTS, 0.129 or 1.55 ml from the stock dye solution is added to 20 ml of the PDADMAC (1 g dm−3 ) solution. The mixtures are stirring for 20 min. The above concentrations are calculated having in mind that the total molar number of the negative charges in the HPTS molecules is ndye = Vdye Cdye Ndye , where Vdye is the volume of the dye solution added, Cdye is the molar concentration of the solution (2 × 10−3 M) and Ndye is the number of negative charged groups on each due molecule (4) [13]. The total number of positive charges on the PDADMAC is given by nPE = VPE CPE /Mmonomer , where VPE is the volume of the polyelectrolyte solution, CPE is the concentration of the solution (1 g dm−3 ) and Mmonomer is the molecular weight of the polyelectrolyte monomer (161.5 g mol−1 ). In the present experiments, the ratio ϕ = ndye /nPE is 0.0083 (noted as ϕ0.0083 ) or 0.1 (noted as ϕ0.1 ). 2.4. Methods 2.4.1. Electric light scattering method The orientation of anisotropic particles by an externally applied electric field results from the interaction between the particle electric moments (permanent and induced) and the orienting field. As a consequence of the particle orientation, the light scattered by the suspension is changed. The steady-state electro-optical effect

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is defined as the difference between the intensities of the scattered light in the presence of an electric field, IE , and without field, IO [14]: ˛=

IE − I0 I0

(1)

The electro-optical effect presents a square function of the electric field strength when the energy of particle orientation is lower than the Boltzmann energy kT [15]: ˛=

A (Ka, Kb) I0 (Ka, Kb)



p 2 E2 + (a − b ) kT 4kT



˛t = ˛0 exp (−6Dr t)

(3)

For prolate ellipsoids, this relation is given by the Perrin’s equation [20]:

Dr =



  −1 +

4 p4 − 1

2p2 − 1



2p

p2 − 1

ln

2.4.3. Spectroscopy measurements T60 UV–vis Spectrophotometer is used in this study for determination of the adsorbed amount from the polyelectrolytes in the shell.

(2)

,

where A(Ka, Kb) and I0 (Ka, Kb) are optical depending on  functions  the particles sizes and on K = 2/sin /2 (where  = 547 nm is the wavelength of the incident light and  is the angle of observation); p ,  a , and  b are the permanent dipole moment and the electrical polarizability with respect to the long and transverse axes of the particle. For a stable system (when the optical functions are constant), the initial slope of ˛ versus E2 dependence directly reflects changes in the particle electric polarizability, assuming that p = 0. Two effects are generally observed in colloidal suspensions when a sinusoidal electric field is applied [16]. The first one appears in the range of particle rotation (at hertz frequencies) and seems to be related to the “permanent” dipole moment of the particle. The second electro-optical effect (at kilohertz frequencies) is related to polarization of a diffuse part of the electrical double layer of the particle. The mobility of ions in the diffuse layer is accepted equal to that of free ions in a salt solution. In a suspension stabilized by adsorption of highly charged polyelectrolyte, an additional (third) effect appears near the range of particle rotation. It is attributed to polarization of a layer from condensed counterions on the polyion surface [17]. The mobility of the condensed counterions is suggested lower than the free ion’s mobility because of the strong attraction to the polyion surface [18]. The transient process of Brownian particle disorientation after switching the electric field off permits determination of the rotational diffusion coefficient, relative to the particle dimensions [19]:

kTp2

2.4.2. Microelecrophoresis The overcompensation of the surface charge density of the particles was measured by Rank Brothers II Aparatus with a flat quartz cell at 25 ◦ C.

p+ p−

 

p2 − 1 p2 − 1

 (4)

where p is the axial ratio a/b,  is the viscosity of the suspending medium, and = ab2 /6 is the volume of the particle. From the change in the time of the particles disorientation after switching electric field off and the axial ratio p = (a + 2LH )/(b + 2LH ) before and after deposition of each layer, the hydrodynamic thickness of a polyelectrolyte layer, LH , can be calculated. The hydrodynamic thickness is sensitive to the “loops” and “tails” in the adsorbed layer and might be twice as great as the average layer thickness [21]. The electro-optic responses of the investigated samples are measured on a conventional electric light scattering apparatus [16]. The measuring cell is of volume 8 cm3 and diameter 18 mm. Platinum plate electrodes of radius 5.5 mm and distanced at 2.6 mm are incorporated in the cell. The electric pulses are applied perpendicularly to the observation plane. White unpolarized light is incident on the sample. The intensity of the scattered light is measured at angle 90◦ to the incident beam, using a photomultiplier/oscilloscope registration.

3. Results and discussion 3.1. Deposition of oppositely charged components onto ellipsoidal ˇ-FeOOH particles 3.1.1. Formation of PSS/PDADMAC bi-layer Fig. 1A and B shows the dependence of the electrophoretic mobility and time of particle disorientation after switching off the electrical field for ␤-FeOOH particles as a function of the concentration of NaPSS added to the suspension. At low concentrations of the polymer, all molecules adsorb on the particle surface due to the electrostatic attraction between the oppositely charged polyelectrolyte monomers and surface. The experimental data show that the electrophoretic mobility of ␤-FeOOH particles gradually decreases with increasing concentration of the polymer as a result of neutralization of the particle charge upon the NaPSS adsorption (Fig. 1A). Simultaneously, the time of the particle disorientation after switching off the electrical field (which is related to the particle dimensions as = 1/6Dr ) increases due to the partial adsorption onto the surface and the particle aggregation (Fig. 1B). The full aggregation in the suspension occurs at a concentration about 10−4 g dm−3 (isoelectric point), where neutral complexes stick to each other due to the strong short-range attraction. The peak value of the time for particle disorientation as a function of polyelectrolyte concentration (Fig. 1B) illustrates this aggregation. The coincidence of the concentration for maximum aggregation with the isoelectric points (IEP) indicates that the interaction between polyelectrolyte molecules and ␤-FeOOH particles is mainly electrostatic. The further increase of NaPSS concentration leads to an increase in the electrophoretic mobility of the particles, but with opposite sign. This shows overcompensation of the particles’ surface charge, which causes re-stabilization of the oxide suspension against flocculation at polyelectrolyte concentration ca. 10−2 g dm−3 . The suspension stabilization results from the increased repulsion between isolated colloidal particles covered with PSS layer. In this suspension, the thickness of the adsorbed layer is calculated from the difference in the times of particle’s disorientation before and after the adsorption (Eq. (4)) to be ca. 7 nm. In the following, the concentration of NaPSS 10−2 g dm−3 is used for preparation of stable suspension of ␤-FeOOH particles. After centrifugation in order to remove the small excess of PSS (not adsorbed or loosely attached to the particles surface), the experiment is continued with adding of positively charged PDADMAC to ensure new stabilization of the suspension. The dependence of the electrophoretic mobility and relaxation time of the particles after switching off the electric field are presented in Fig. 1C and D. The results show similar behavior of the system in dependence of the PDADMAC concentration as in the case of NaPSS. Full aggregation in the suspension occurs at a polyelectrolyte concentration ca. 4 × 10−4 g dm−3 and the stabilization occurs at a concentration 10−2 g dm−3 . The thickness of the adsorbed PDADMAC layer in the stabilized suspension is ca. 13 nm.

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Fig. 1. Dependence of the electrophoretic mobility, Uef , and relaxation time of particle disorientation, , as a function of concentration of NaPSS (A and B) and PDADMAC (C and D) added to the suspension.

3.1.2. Interaction between PSS/PDADMAC covered ˇ-FeOOH particles and silica nanoparticles Fig. 2 shows the variation in the electrophoretic mobility and the time of particle disorientation after switching off the electrical field for ␤-FeOOH particles covered with PSS/PDADMAC bi-layer as a function of the concentration of silica nanoparticles (presented as ratio of the number of silica nanoparticles to the number of PSS/PDADMAC coated ␤-FeOOH particles, R). The results demonstrate similar adsorption behavior in this system as in the case of polyelectrolyte adsorption. The full aggregation in the suspension

Fig. 2. Dependence of the electrophoretic mobility, Uef , (A) and relaxation time of particle disorientation, , (B) for the “soft” ␤-FeOOH particles (covered by PSS/PDADMAC bi-layer) as a function of particle number concentration ratio, R, upon adsorption of TM-NPs () and LS-NPs (♦) silica nanoparticles.

occurs at particle number concentration ratio R about 102 . At high enough concentration of the silica particles (R ∼ 3.5 × 104 ), the electrophoretic mobility and the relaxation time of disorientation of the particles reach constant values. This corresponds to the stabilized suspension. It is seen that the electrophoretic mobility and IEP significantly depend on the particle size. The slightly higher value of the Uef for the smaller silica nanoparticles (in the stabilized suspension, at lgR ∼ 5) might be a result from larger adsorbed amount of these particles on the “soft” particle surface. The thickness of the adsorbed silica layer is calculated from the difference in the relaxation times of the particles in stabilized suspensions before and after the silica adsorption. The estimated thicknesses are close to the particle diameters (ca. 30 nm and 10 nm for the TM-NPs and LS-NPs, respectively). Having in mind that the number of charges on the silica surface is about 270 (for TM-NPs) and 28 (for LS-NPs) [22], the results in Fig. 3 can also be presented as a function of lg(Nsilicaparticles Qsilica ), where N is the number of the silica particles and Q is number of charges per particle.

Fig. 3. Dependence of the electrophoretic mobility of “soft” ␤-FeOOH particles as a function of number of the silica particles in solution (Nsilica particles. ) and number of charges per particle (Qsilica ) for TM-NPs () and LS-NPs (♦).

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3.2. Adsorbed mass of the components in stabilized suspensions 3.2.1. Number of the adsorbed PSS chains The adsorption of PSS onto the oppositely charged ␤-FeOOH particles is performed in the absence of low molecular salt. In this case, one can assume rod-like conformation of the highly charged chains because of the strong electrostatic repulsion between likely charged monomers of the polyelectrolyte. Due to high charge density of NaPSS, on the other hand, most of the PSS counterions are expected to condense near to the polyion chain [23]. (When charge density on a linear polyelectrolyte with monovalent counterion exceeds a critical value  > 1, some of its counterions “condense” near the chain to such an extent that the Coulomb repulsion energy of two adjacent charged groups becomes equal to the thermal energy kT. The charge density parameter is defined as  = e2 /εkTd, where e is the electronic charge, ε is the bulk dielectric constant, and d is the polyelectrolyte charge spacing. For NaPSS and PDADMAC molecules, with average distance between two charges along the polyion chain of 0.26 nm and 0.52 nm, the charge density parameter values  are 2.75 and 1.37, respectively.) According to the Manning approximation, a value of 0.64 is calculated for the fraction of condensed counterions of NaPSS [24]. Assuming that the fraction of the condensed counterions remains almost unchanged during its adsorption onto low charged ␤-FeOOH surface [25], one can calculate the number of the adsorbed PSS chains from the electrical polarizability of the particles in the stabilized suspension. Details on the procedure are given in [26]. Briefly, the electrical polarizability of the polyelectrolyte-coated particle is determined experimentally from the initial slope of the effect dependence on the electric field strength (Eq. (2)). The polarizability of one PSS molecule is calculated according to the Manning’s theory for polarization of condensed counterions along a rod-like polyelectrolyte in solution [27]: M =

1 −  −1 e2 L 3   × 12kTd 1 − 2  − 1 ln d

(5)

where  M is the electrical polarizability, L is the polyion length, and is the Debye–Hückel screening parameter. When divided the polarizability of the polymer-coated particle by the polarizability of one polyelectrolyte chain, one can obtain the number of the polymer chains adsorbed on the particle surface. The calculation is made with assumption for additive contribution of the polarizability of adsorbed molecules in the layer. For the number of the adsorbed PSS chains (70 kDa), such calculation gives ca. 32 chains per one particle, or 1.23 ␮g m−2 [26]. In this study, the number of the adsorbed NaPSS chains per one particle, estimated from the electrical polarizability, is compared with spectroscopic data. In order to determine the adsorbed amount of NaPSS, the suspension is centrifuged after the polyelectrolyte adsorption and the supernatant is followed by using UV–vis spectroscopy. (These experiments are performed at four times larger concentrations of the polymer and particles in order to obtain reliable signal.) As shown in Fig. 4, the maximum of NaPSS absorbance is located at 260 nm. The number of the adsorbed NaPSS molecules per one ␤-FeOOH  particle is determinated using the relation NPSS = nPSS − nsup NA /Nparticles , where nPSS and nsup are the amount of the polyelectrolyte (in mol) added to the suspension and its amount in the supernatant after centrifugation, NA is the Avogadro number and Nparticles is the number of the ␤-FeOOH particles in the suspension (ca. 2 × 1012 particles dm−3 at concentration −3 −3 8 × 10 g dm  ). The adsorbed amount is estimate by using PSS = nPSS − nsup Mw /Sparticles , where Sparticles is the total surface in the suspension (ca. 5.7 m2 at concentration 8 × 10−3 g dm−3 ).

Fig. 4. Absorbance from suspension of ␤-FeOOH particles coated with NaPSS layer. Inset: absorbance from solution of NaPSS at different concentrations.

The estimation gives ca. 34 adsorbed polyelectrolyte chains per particle or ca. 1.34 ␮g m−2 , which means that the electro-optics can be successfully used for the estimation of the number of the adsorbed chains from highly charged polyelectrolytes (with condensed counterions). 3.2.2. Number of the adsorbed PDADMAC chains The adsorbed amount of PDADMAC is determined after complexation of polyelectrolyte molecules with 8-hydroxypyrene1,3,6-trisulfonic acid trisodium salt (HPTS) in order to be examined by spectroscopy. As mentioned above, the PDADMAC–HTPS complexes are formed at conditions where part of the polyelectrolyte charge, ϕ, is neutralized by HPTS charges. The dye amount in the complex must be such high as to ensure reliable absorbance, but not to change the structure of the PDADMAC molecule. The experiments are performed at HPTS concentrations of 1.3 × 10−5 M and 1.4 × 10−4 M, leading to 0.83% and 10% neutralization of the polyion charge, respectively. The PDADMAC–HPTS complexes are adsorbed onto ␤-FeOOH particles covered with PSS layer. The complex concentration for preparation of stable suspension is determined by electro-optics and electrophoresis. Fig. 5 shows that stabilization of the suspension is achieved at concentration of the PDADMAC–HPTS complex ca. 2 × 10−2 g dm−3 and this concentration is used for spectroscopic measurements. The layers from PDADMAC–HPTS are formed by adding to the suspension of ␤-FeOOH particles, covered with layers from PSS at pH6, and stirring for 20 min. (The concentration of the ␤-FeOOH particles is about four times higher than that in the stability experiments in order to obtain reliable absorbance from HPTS.) After the adsorption, the suspension is centrifuged and the supernatant is examined by spectroscopy. To prove that HPTS remains stable in the PDADMAC–HPTS complexes, frequency dependences of the electro-optical effect from suspension of PSS particles covered with PDADMAC layer are compared to that of particles covered with PDADMAC–HPTS complexes (Fig. 6). The results show that the effect value decreases with increasing of the dye content in the complex, which can be related to the partial neutralization of the PDADMAC charge by HPTS. The experiments are performed at 460 nm, where the absorbance of HPTS has maximum [28]. The number of the adsorbed PDADMAC molecules per particle is determined only for PDADMAC–HPTS complexes with ϕ0.1 because of the low signal at ϕ0.0083 . The number of the adsorbed molecules from PDADMAC per particle is determined by using the relation  NPDADMAC = 4NA Mmonomer ndye − nsup /ϕMw Nparticles , where ndye and nsup are the amount of the dye added to the suspension and in the supernatant after centrifugation, respectively (in mol), Mmonomer is the molecular weight of the PDADMAC monomer (161.5 g mol−1 ), Mw is the molecular weight of PDADMAC (200 kDa)

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Fig. 5. Dependence of the electrophoretic mobility and relaxation time of particle disorientation as a function of the concentration of PDADMAC–HPTS complexes at pH 6: ϕ0.0083 (A and B) and ϕ0.1 (C and D).

(Figs. 1–3). Taking into account that the silica nanoparticles have well defined spherical shape and size, the number of the adsorbed silica nanoparticles on the PSS/PDADMAC covered ␤-FeOOH particles is calculated to be ca. 4000 (from TM-NPs) and 18 000 (from LS-NPs), corresponding to adsorbed amount of ca. 22 and 17 mg m−2 for TM-NPS and LS-NPS (lgR ∼ 3.6 and 4.3, respectively). Repulsion between the silica nanoparticles and their partial dipping into the “soft” particles are not taken into account in these calculations (LH values of ca. 10 and 30 nm for LS-NPs and TM-NPs, respectively, corresponds to one adsorbed layer in both cases). In spite of the approximate character of the estimations, the obtained values of lgR correspond satisfactorily to the stabilized suspensions (Figs. 1–3). Fig. 6. Frequency dependence of the electro-optical effect for particles with adsorbed layer from PDADAMAC () and PDADMAC–HPTS complexes with ϕ0.0083 () and ϕ0.1 (). The electric field strength is 2.3 × 10−4 V m−1 . Inset: Dependence of the electro-optical effect as a function of the quadratic of electric field strength. The frequency of the applied electric field is 1 kHz.

and Nparticles is number of ␤-FeOOH particles in the suspension, ϕ is 0.1. The number of the adsorbed PDADMAC chains is determined to be ca. 15 chains or ca. 1.7 ␮g m−2 . 3.2.3. Number of the adsorbed silica nanoparticles Table 1 shows the hydrodynamic thickness of the layers in suspensions containing ␤-FeOOH particles, covered with PSS, PDADMAC, and both samples of silica nanoparticles. The thicknesses are determined from the relaxation times of the particles after deposition of each layer in the stabilized suspensions Table 1 Hydrodynamic thickness, LH , of the adsorbed layers. Adsorbed layer

Hydrodynamic thickness LH [nm]

PSS PDADMAC LS-NPs TM-NPs

7 13 10 30

3.2.4. Electrical properties of the composite film It is generally accepted that the electro-optical effect at kilohertz frequencies, ˛kHz , is related to polarization of ions in a diffuse part of the electrical double layers of colloidal particles [14,15]. The relaxation frequency of this effect, cr , defined as a frequency for a twofold decrease in the effect value, is related to the mobility of the counterions responsible for creation of the effect. With Cl− as a predominant counterion of the positively charged ␤-FeOOH particles, cr can be calculated according to the following equation: cr =

4D0 i

a2

(6)

where Di0 is the translational diffusion coefficient of the Cl− ions in solution and a is the particle length [29]. With Di0 = 2 × 10−5 cm2 s−1 and a = 285 nm for the long axis of the ellipsoidal ␤-FeOOH particle, this value is evaluated to be ca. 30 kHz. This value is close to the experimental one (Fig. 7), which confirms that the kilohertz electrooptical effect arises from the motion of free Cl− ions along the long particle axis [30]. NaPSS and PDADMAC have constant charge density. Both polyelectrolytes are highly charged and according to Oosawa [23] and Manning [27] considerable part of their counterions must condense near the polyion chains. The mobility of the condensed counterions is found to be lower than the one of the free ions in a solution or of

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film is also calculated from the number of charged monomers per chain, determined from the spectrophotometery experiments. This estimation gives for the ratio of adsorbed charges ca. 0.62, confirming again that the electro-optics can be successfully applied for determination of the number of adsorbed charged units. 4. Conclusions

Fig. 7. Frequency dependence of the electro-optical effect for bare ␤-FeOOH particles (*) and for particles with adsorbed layer from NaPSS (䊉), PDADMAC () and TM-NPs () at pH 6. The electric field strength is 2.3 × 10−4 V m−1 . The arrows show cr . Inset: dependence of the electro-optical effect as a function of the quadratic of electric field strength at low energy of orientation. The frequency of the applied electric field is 1 kHz.

the ions in the diffuse part of the polyelectrolyte ionic atmosphere [18]. The electro-optical behavior of the particles covered with layers from PSS and PDADMAC is different from that of the bare ␤-FeOOH particles. Fig. 7 shows the dependence of the electrooptical effect for the bare oxide particles and particles with last adsorbed layer from PSS, PDADMAC, and TM-NPs. The critical frequency of relaxation of the kilohertz electro-optical effect from the particles with adsorbed polyelectrolytes is lower in comparison to the effect of particles without adsorbed polyelectrolyte. Previously, we explained the decrease in the relaxation frequency of the effect from the PSS covered particles with a decrease in the mobility of ions, responsible for creation of the electro-optical effect [30]. The relaxation frequency of the effect from particles covered with highly charged polyelectrolyte can be estimated according to Eq. (6) if one knows the diffusion coefficient of the condensed counterions and the polyion contour length. In a previous work, we found that the experimentally obtained frequency of relaxation of the effect for NaPSS molecules with condensed Na+ counterions is close to the one calculated from the mobility of condensed c ions along the contour length of the molecule (ca. 3 kHz, at DNa + = −11 2 −1 9.8 × 10 m s and L ∼ 90 nm) [26]. The experimental value of the critical frequency of relaxation for the particles with last PDADMAC layer is about 4 kHz. Because of the branched structure of the PDADMAC molecule, however, it is not possible to compare experimental and theoretical values in this case. The relaxation frequency of the electro-optical effect for the particles with last adsorbed layer from TM-NPs is close to the one of the bare ␤-FeOOH particles (Fig. 7). This is in accordance with the mobility (diffusion) of free Na counterions along the whole particle surface because of the low surface charge density of the silica nanoparticles at pH6 (no condensation). The results in Fig. 7 show that the electrical properties of the last deposited layer define the electro-optical behavior of the entire film also in the case of hybrid multilayer films [31]. To determine the charge stoichiometry in the PSS/PDADMAC bilayer, the electrical polarizability of the particles is compared. The experimental values of the polarizability are 1.8 × 10−30 F m−2 (for PSS layer) and 2.6 × 10−30 F m−2 (for PDADMAC layer) (calculated according Eq. (6)). Therefore, the PSS/PDADMAC polarizability ratio is ca. 0.69, which means that some excess of positive charges occurs in this bilayer. The result is in line with our previous work [32] and with the last results of Schlenoff et al. [33], which found larger excess of positive charges on the film surface upon PDADMAC adsorption. The charge stoichiometry in the PSS/PDADMAC

Three-layer polyelectrolyte/nanoparticle films have been formed through a subsequent adsorption of two oppositely charged polyelectrolytes and negatively charged silica nanoparticles onto colloidal particles of ␤-FeOOH. The electrostatic self-assembling of PSS, PDADMAC, and two kinds of silica nanoparticles with different sizes has been studied by electro-optics and electrophoresis. Both techniques allowed determination of the concentration, which is needed for stabilization of the suspension after deposition of each new charged component. The adsorbed amounts of the highly charged PSS and PDADMAC have been measured by means of spectrophotometery, whereas the amount of both kinds of silica nanoparticles was calculated from the thickness of the adsorbed layers. The relaxation frequency of the kilohertz electro-optical effect was found to depend on the charge density of each adsorbed component, which means that the polarization of the excess charge in the outermost layer dominate the behavior of the entire composite film. The adsorbed mass of NaPSS was calculated also using electric light scattering method and the result was found close to the one obtained by means of spectrophotometry. In spite of the approximately character of the electro-optical estimation, based on the assumption for polarization of condensed counterions, this result showed that most counterions of the highly charged NaPSS remain condensed when the polyelectrolyte chain approaches the oppositely (weakly) charged surface of the ␤-FeOOH particle. The comparison of the electrical polarizability values of the PSS and PDADMAC coated ␤-FeOOH particles has allowed us to estimate the charge balance in this bi-layer. The excess of positive charge, which has also been confirmed by spectrophotometry, means that the interaction between PSS and PDADMAC does not lead to formation of complexes with 1:1 stoichiometry of charges. References [1] C. Peng, Y.S. Thio, R.A. Gerhardt, Effect of precursor-layer surface charge on the layer-by-layer assembly of polyelectrolyte/nanoparticle multilayers, Langmuir 28 (2012) 84–91. [2] C. Peng, Y.S. Thio, R.A. Gerhardt, H. Ambaye, V. Lauter, pH-promoted exponential layer-by-layer assembly of bicomponent polyelectrolyte/nanoparticle multilayers, Chem. Mater. 23 (2011) 4548–4556. [3] T. Chen, P. Somasundaran, Preparation of novel core–shell nanocomposite particles by controlled polymer bridging, J. Am. Ceram. Soc. 81 (1998) 140–144. [4] J.E. Wonga, A.K. Gaharwara, D. Müller-Schultec, D. Bahadurb, W. Richteringa, Layer-by-layer assembly of a magnetic nanoparticle shell on a thermoresponsive microgel core, J. Magn. Magn. Mater. 311 (2007) 219–223. [5] K. Uto, K. Yamamoto, N. Kishimoto, M. Muraoka, T. Aoyagi, I. Yamashita, Electrostatic adsorption of ferritin, proteins and nanoparticle conjugate onto the surface of polyelectrolyte multilayers, J. Mater. Chem. 18 (2008) 3876–3884. [6] C.R. Zamarreno, J. Bravo, J. Goicoechea, I.R. Matias, F.J. Arregui, Response time enhancement of pH sensing films by means of hydrophilic nanostructured coatings, Sens. Actuators, B 128 (2007) 138–144. [7] E. Kharalampieva, S.A. Sukhishvili, Release of a dye from hydrogen-bonded and electrostatically assembled polymer films triggered by adsorption of a polyelectrolyte, Langmuir 20 (2004) 9677–9685. [8] F. Caruso, D. Trau, H. Mohwald, R. Renneberg, Enzyme encapsulation in layer-by-layer engineered polymer multilayer capsules, Langmuir 16 (2000) 1485–1488. [9] A. Diaspro, D. Silvano, S. Krol, O. Cavalleri, A. Gliozzi, Living cell encapsulation in nano-organized polyelectrolyte shell, Langmuir 18 (2002) 5047–5050. [10] G. Decher, J.D. Hong, J. Schmitt, Buildup of ultrathin multilayer films by a self-assembly process: III. Consecutively alternating adsorption of anionic and cationic polyelectrolytes on charged surfaces, Thin Solid Films 210/211 (1992) 831–835. [11] H. Zocher, W. Heller, Schillerschichten als Reaktionsprodukte der langsamen Eisenchlorid-Hydrolyse, Z. Anorg. Allg. Chem. 186 (1930) 75–96.

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