Adsorption of Anionic and Cationic Surfactants on Anionic Colloids

ARTICLE pubs.acs.org/Langmuir

Adsorption of Anionic and Cationic Surfactants on Anionic Colloids: Supercharging and Destabilization S. Ahualli,†,‡ G. R. Iglesias,‡ W. Wachter,‡ M. Dulle,‡ D. Minami,§ and O. Glatter*,‡ †

Department of Applied Physics, University of Granada, 18071 Granada, Spain Department of Chemistry, Karl-Franzens-University Graz, Heinrichstrasse 28, 8010 Graz, Austria § Department of Pure and Applied Chemistry, Tokyo University of Science, Chiba, Japan ‡

bS Supporting Information ABSTRACT: We present herein a study on the adsorption of anionic (SDS), cationic (CTAB), and nonionic (C12E5) surfactants onto anionic silica nanoparticles. The effects of this adsorption are studied by means of the static structure factor, S(q), and the collective diffusion coefficient, Dc, obtained from small-angle X-ray scattering and dynamic light scattering measurements, respectively. The effective charge on the particles was determined also from classical electrophoresis and electroacoustic sonic-amplitude measurements. The surface tension of the sample was also investigated. Of particular note is the adsorption of SDS onto the silica nanoparticles, which leads to supercharging of the interface. This has interesting repercussions for structures obtained by the layer-by-layer (LbL) technique, because emulsions stabilized with supercharged and hydrophobized silica are perfect candidates for use in a multilayer system.

’ INTRODUCTION The adsorption of ionic surfactants onto solid surfaces is of interest because of its importance in a great number of industrial and technological processes associated with colloidal stability. Like surfactants (and often in association with them), nanoparticles can be used as stabilizing additives in such dispersed systems in various fields of practical interest. The use of silica nanoparticles has been described as a means of stabilizing emulsions through adsorption (together with surfactants) at the oil
water interface (Ramsden
Pickering emulsions).1,2 The attachment of the nanoparticles is strongly influenced by their hydrophilic or lipophilic character, which in turn can be controlled by the surfactant adsorption. In the case of negatively charged silica, an obvious possibility is the use of a cationic surfactant such as cetyl trimethylammonium bromide (CTAB).3 However, this gives rise to a decrease in the effective charge and hence to a destabilization of the system. Moreover, especially for ionic surfactants, adsorption at the particle interface can result in variations in the surface charge.4
8 As a consequence, more significant particle interactions may appear in the interfacial layer; these interactions also influence the interfacial mechanical properties.9 The understanding of the interaction between nanoparticles and surfactants is required for the extensive use in industrial and technical applications associated with the colloidal stability, detergency, and functionalization of the interfaces. In this field, a number of works focused on the adsorption of nonionic surfactants onto nanoparticles10
13 which have demonstrated the adsorption of micelles of C12E5 with the r 2011 American Chemical Society

model of micelle-decorated silica beads. A very recent and interesting work14 pursued an aim similar to that in our work. They have studied the interaction of charged silica (8 nm of radius) with anionic (SDS), cationic (DTAB), and nonionic (C12E10) surfactants using small-angle neutron scattering (SANS). We will discuss the differences in the Results section. Our ultimate purpose is to extend the layer-by-layer (LbL) technique by introducing internally nanostructured Ramsden
Pickering emulsions. Therefore, the effective charge of the stabilizing colloids is of great importance. We present herein a study on particle interactions on the addition of three surfactants (anionic sodium dodecyl sulfate (SDS), cationic cetyltrimethylammonium bromide (CTAB), and nonionic pentaethylene glycol monododecyl ether (C12E5)) on anionic silica nanoparticles. The interactions were studied by using the static structure factor, S(q), and the collective diffusion coefficient, Dc, obtained from small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) measurements, respectively. The effective charge on the particles was determined also from electrophoresis and electroacoustic sonic-amplitude measurements. The hydrophobization of the samples was studied by means of interfacial surface-tension measurements. To date, there are relatively few reports on air
liquid or liquid
liquid interfacial adsorption in the presence Received: April 5, 2011 Revised: June 10, 2011 Published: June 22, 2011 9182

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of negatively charged silica together with either anionic or nonionic surfactant.15,16

’ EXPERIMENTAL SECTION Materials. The interaction between particles was studied in moderately concentrated silica dispersions that were obtained by dilution with water or surfactant solution of the commercial colloidal-silica dispersion Levasil 300 (in the following, denoted as Le300), kindly supplied by H. C. Starck/Bayern. Levasil 300 is a colloidal dispersion at 18.3% in volume of spherical silica nanoparticles, the particle density and specific surface area of which are 2.1 g/cm3 and 300 m2/g, respectively. The surfactants used in this work (without further purification) in association with silica nanoparticles were C12E5 (Sigma Aldrich, g 98%), SDS (Sigma Aldrich, g 99.0%), and CTAB (Fluka, Germany, g 99.0%). The water utilized was produced by a Millipore purifier system (Milli-Q) using doubly distilled water. Two series of samples were prepared: One of them consists of a fixedvolume fraction of silica (5%) suspended in water with various surfactant concentrations between 0.01 mM and the corresponding critical micelle concentration (cmc) for each surfactant (and even values clearly above the cmc for SDS due to the unexpected results). The pH was approximately 10 for all systems containing 5% nanoparticles. The other series was made by varying the volume fraction of particles from 0.5% to 15% while the concentration ratio of surfactant to particles was kept constant. Small Angle X-ray Scattering (SAXS). The SAXS equipment comprised a SAXSess camera (Anton Paar, Austria), connected to a PW 1730/10 X-ray generator (Phillips) with a sealed-tube anode (Cu
KR; wavelength 0.154 nm).17 The generator was operated at 40 kV and 50 mA. The SAXSess camera is equipped with focusing multilayer optics and a block collimator for an intense, monochromatic primary beam with low background. A semitransparent beam stop enables measurement of an attenuated primary beam for the exact definition of the zero scattering vector and transmission correction. All samples were enclosed in the same vacuum-tight, reusable 1 mm quartz capillary to attain exactly the same scattering volume and background contribution. The sample temperature was controlled with a thermostatted sample-holder unit (TCS 120, Anton Paar). The 2D scattering pattern was recorded with a CCD detector and integrated into a 1D scattering function with SAXSQuant software from Anton Paar, Graz, Austria. The CCD camera from Princeton Instruments, which is a division of Roper Scientific Inc. (Trenton, NJ), was equipped with a PI-SCX fused 1:1 fiber-optic taper. The CCD camera features a 2048  2048 array with pixel size of 24  24 μm (chip size: 50  50 mm) at a sample
detector distance of 311 mm. All 2D scattering patterns were converted to 1D scattering curves as a function of the magnitude of the scattering vector q = (4π/λ) sin(θ/2), where θ is the total scattering angle. All scattering patterns were transmission-corrected by setting the attenuated scattering intensity at q = 0 to unity, and were corrected for the scattering of the sample cell and the solvent. In order to get the scattering patterns on an absolute scale, water was used as a secondary standard.18 Samples were equilibrated at the desired temperature of 25 °C for each measurement. The samples were exposed to X-rays (five times) for 60 s, and the integrated scattering profiles were averaged. The scattering patterns were evaluated by using the generalized indirect Fourier transformation (GIFT) method.19
22 This method is based on the simultaneous determination of the form factor and the structure factor with a minimum of a priori information. In the case of monodisperse, globular particles, the total scattering intensity can be expressed by IðqÞ ¼ nSðqÞ PðqÞ

ð1Þ

with the number density n of the particles. This factorization allows a separation of inter- and intraparticle effects by means of the GIFT

technique which is an extension of the indirect Fourier transformation (IFT). The IFT transforms the scattering curve into real space resulting in a pair distance distribution function (PDDF) including correction of instrumental broadening effects. The PDDF is the convolution square of the electron-density distribution, and it represents a histogram of the distances inside the particle weighted with the electron-density differences relative to the solvent. It becomes zero at the maximum particle dimension. This transformation is model free, and there are no constraints except the maximum dimension of the particles. The IFT approximates the PDDF, p(r), by a linear combination of a finite number of cubic B-spline functions. The expansion coefficients are the unknown parameters. The PDDF is connected to the form factor P(q) by the spatially averaged Fourier transformation. The GIFT technique uses the same model free approach for the PDDF, which corresponds in this case only to the form factor. At the same time, a model for the structure factor is assumed and fitted to the data according to the eq 1. This simultaneous approximation of the form and structure factor is only possible due to the different mathematical properties of the two functions. Nevertheless, the problem is highly nonlinear and has to be solved in an iterative way to find the parameters leading to a global minimum of the mean deviation. The structure factor, S(q), is determined with a model for charged and monodisperse spheres assuming Yukawa potential and the effective and widely used hypernetted chain closure (HNC). The free parameters used in the calculation were the charge and the interaction radius, while the volume fraction and ionic concentrations were known input data. The Boltzmann simplex simulated annealing algorithm used in the GIFT technique needs to calculate several thousand different structure factors, so the process can lead to notable calculation time. Because of the different contrast scenarios of silica
water and surfactant
water, the measured form factor is hardly influenced by the surfactants; therefore, our study focuses on the changes in the structure factor under the addition of different types and amounts of surfactants. Dynamic Light Scattering (DLS). DLS was used to determine the collective diffusion coefficient. The equipment was composed of a goniometer and a diode laser (Coherent Verdi V5, λ = 532 nm, maximum output of 5 W) with single-mode fiber-detection optics (OZ from GMP, Z€urich, Switzerland). The data acquisition was performed with a photomultiplier, which was combined with pseudo cross correlation and an ALV 5000/E correlator with fast expansion (ALV, Langen, Germany). This allowed a minimum time interval of 12.5 ns for the correlation function. The ALV-5000/E software package was used to record and store the correlation functions. Correlation functions were collected 20 times, for 40 s each, and averaged. From these functions, an average diffusion coefficient D was obtained by using cumulant analysis.23 All experiments were carried out at 25 °C and a scattering angle of 90°. The laser power for the experiments was between 10 and 150 mW, depending on the scattering power of the sample. Correlation functions that clearly showed some kind of flaw (e.g., from large aggregates passing through the beam) were discarded. Surface Tension. The interfacial surface tension data were obtained using the drop-shape method with a Dataphysics OCA-20 analyzer. This instrument consists of a CCD video camera with resolution of 768  576 pixels and a dosing/microsyringe unit with internal diameter of 1.36 mm. The drop-shape image was processed by an automatic feedback that calculates the interfacial area during the experiment. The surface tension obtained for distilled water was 71.89 mN/m at 25 °C. Electrophoretic Measurements. Static electrophoretic measurements were performed at T ≈ 25 °C using a “ZetaPlus” zetapotential analyzer (Brookhaven Instruments, 632.8 nm, 10 mW, 15°). Dynamic mobility measurements were performed for frequencies 9183

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Figure 1. Scattered intensity (a) and structure factor (b) for 5% silica at various concentrations of CTAB: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1 mM indicated by the different colors.

Figure 2. Effective charge of Le300 after the addition of CTAB at various concentrations, as determined from the SAXS data. ranging from 1 to 18 MHz in an Acoustosizer II (Colloidal Dynamics) apparatus. This instrument is based on the electrokinetic sonic amplitude (ESA) technique, in which an alternating electric field is applied to the suspension, and the amplitude and phase of the generated acoustic wave is measured and transformed into a dynamic mobility frequency spectrum, ue(ω). Determinations were repeated at least 15 times for each sample, and the average spectrum was used for further calculations. The model elaborated by Ahualli et al.24 was used to fit the mobility spectra. The fitting was carried out by using the size determined from the SAXS data. The fitting consisted of varying the charge to minimize the squared differences between calculated and measured mobility data using the real part of each frequency.

’ RESULTS Structure Factor from SAXS Data. Effect of Addition of the Cationic Surfactant CTAB. The scattered intensities for moder-

ately concentrated silica suspensions have been measured. The particle concentration was kept constant at 5% v/v. The scattered intensities (Figure 1a) were obtained at various concentrations of the positively charged surfactant CTAB between 0 and 1 mM (cmc). The structure factor (Figure 1b) shows the lowest interaction at the highest CTAB concentrations. From the fitting process, we obtain two interaction parameters: the interaction radius, which

is almost constant and equal to 7.8 nm, and the effective charge for each concentration, as shown in Figure 2. For the silica without surfactant, we obtained an effective charge of 14 e
. This effective charge shows the expected trend: as the surfactant concentration increases, CTAB is increasingly adsorbed onto the surface of the particles, which reduces the effective charge, as can be seen from Figure 2. The same conclusion was obtained by Schmitt-Rozieres et al.25 by means of electrophoretic measurements. Effect of Addition of the Nonionic Surfactant C12E5. The intensity hardly changes when the nonionic surfactant, C12E5, is added at concentrations below cmc (0.07 mM) to 5% of Le300 (see Figure 3). It is possible that either the amount of nonionic surfactant is not sufficient to induce any change in the effective charge or, due to its uncharged nature, the added surfactant does not modify the electrical properties of the particles. Effect of Addition of the Anionic Surfactant SDS. The behavior of the anionic silica was carefully monitored in the presence of increasing levels (even above the cmc) of the equally charged SDS (see Figure 4). The experimental data was analyzed under two assumptions: • The form factor for silica is unchanged upon addition of surfactant. (The absolute intensity at q = 0 of the plain surfactant is lower than that of 5% silica particles dispersion by 3 orders of magnitude!) In order to reduce the number of free parameters and hence increase the precision of method, we decided to keep the form factor constant at the value obtained from the Le300 in plain water and to fit the structure factor for the whole range of SDS. • For each concentration of SDS, the same concentration of sodium ions is present in the bulk, due to dissociation of SDS. The fitting process for the effective charge takes into account the [Na+] corresponding to each concentration of SDS. We can see a clear increase in the fitted effective charge with the SDS concentration. Although the particle and surfactant both have negative charges, we can conclude that at least part of the SDS molecule is adsorbed on the surface of the particles. Note that the trend in structure factor (Figure 5a) does not correspond to the typical behavior of those corresponding to high repulsion due to a strongly increasing charge. The reason is two simultaneous effects: First, when we add a certain amount of surfactant, the same number of Na+ ions surrounds the particles; this forms part of the double layer and decreases the repulsive 9184

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Figure 3. Scattered intensities (a) and effective charge (b) for Le300 5% at 0, 0.01,0.02, 0.05, and 0.1 mM C12E5.

Figure 4. (a) Scattered intensity of 5% silica at various concentrations (shown by different colors) of SDS: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 3.5, 5, 6.5, 8, 10, 20, and 35 mM. (b) Pair
distance distribution function, p(r), for 5% silica in water with no SDS.

Figure 5. Structure factor (a) and effective charge (b) for Le300 5% at various concentrations of SDS.

interparticle effect of the charge. The structure factor that one should obtain under these conditions is shown in Figure 6a. Second, an increase in the effective charge without any variation in ionic concentration should give S(q), as shown in Figure 6b. The main features of Figure 6 are summarized in Figure 7. The structure factor at zero q indicates the compressibility of the system, so if the system is charged in plain water, the compressibility is lowered, as Figure 6b shows, but the decrease with charge is less pronounced than the increase with ionic concentration (Figure 6a). The S(0) values obtained from our

experiment have the trend corresponding to the increasing ionic concentration but reduced by the increasing charge effect (Figure 7a). The same behavior is found for the q value for the position of the first peak of the structure factor (Figure 7b). This is due to two effects: First, a denser double layer arising from the added surfactant induces the particles to be closer to each other, as the maximum shifts to higher q (the first-neighbors distance decreases). Second, an increase in the charge only generates a slight increase in the distance and therefore a slight decrease in qmax. 9185

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Figure 6. (a) Theoretical prediction of the structure factor for ionic concentration from 1  10
5 up to 3.5  10
3 M at a fixed effective charge equal to 14 e
(fitted for silica alone). (b) Theoretical prediction of the structure factor for each value of the fitted effective charge (see Figure 5b or the Supporting Information), but only in water without sodium ions. The remaining parameters are the same as those for the silica particles.

Figure 7. (a) S(q) at q = 0 obtained from the experimental data (squares) captured from Figure 5a, theoretical predictions (triangles) when only the ion concentration changes (Figure 6a), and when only the charge varies (circles; data from Figure 6b). (b) Position of the first maximum qmax in the structure factor against the same experimental and theoretical data used in (a).

By studying the behavior of the effective charge for silica in presence of these three surfactants, we can conclude that the effective charge decreases, as expected, in the case of CTAB due to the adsorption of positive charge on negative particles. Below the cmc, C12E5 does not show a significant effect. More importantly, and somewhat unexpected, is the fact that part of the SDS molecule is adsorbed on the particle, which increases the effective charge and hence the repulsion between particles. These results show that some dodecyl sulfate ions are adsorbed on the silica nanoparticles (possibly because only part of the interaction between them is electrostatic), which gives rise to a supercharged system. It is worth it to comment on the difference between our results and those obtained by Kumar and Aswal.14 While they have studied silica samples at 1% wt (0.4% vol), where the interparticle structure S(q) can be neglected, the concentrations of our samples are almost 10 times higher, the reason why we can study the effective charge by means of the structure factor. So, the small amount of molecules of SDS adsorbed (also having a much lower difference electron density than silica) is not able to modify the form factor of the concentrated systems, but any change the effective charge is very well reflected in the structure factor. With respect to the cationic surfactant, the results coincide about the

formation of aggregates due to the decrease of the effective charge as we have shown. We cannot conclude anything about the structures of C12E5 adsorbed as micelle-decorated silica beads due to the fact that the intensity of 5% of silica is much bigger than that coming from the surfactant. We can conclude only about the effective charge: this is not modified by C12E5, at least at the ratio (surfactant concentration)/particle used in this work, possibly due to their electrical neutrality. Collective Diffusion Coefficient from DLS Data. DLS experiments provide an important method to investigate colloidal dispersions, and are mostly used for particle sizing in dilute, noninteracting systems. However, the time-dependent correlation function of scattered light is determined not only by single-particle properties, but also by the interactions between the particles. A further step in the generalization of Einstein theory was made by Batchelor, who combined hydrodynamical and statistical methods to develop a general approach to the Brownian motion of interacting spherical particles.33 Applying these methods to colloidal systems with hard-sphere interactions, he obtained the first order correction of the collective diffusion coefficient with respect to the volume fraction of the particles, Dc ¼ DSE ð1 þ λϕÞ 9186

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The collective diffusion coefficient of Le300 without surfactant is presented in Figure 8. It has the features described above, that is, increasing Dc with volume fraction with the typical slope (straight line in Figure 8) for charged repulsive systems up to a maximum, and a subsequent decrease. To quantify the particle charge, we use an approximate model. Petsev et al.30 obtained a simple formula for the collective diffusion coefficient, based on the force balance of a given particle in the framework of the cell model: 2aM

Figure 8. Collective diffusion coefficient for Le300 without surfactant as a function of particle content from 0.5% to 16%. The straight line is a visual guide and corresponds to the typical slope shown by charged repulsive systems at low volume fraction.

where λ = 1.45, that is, the linear-order coefficient, which in general depends on the interparticle interactions. DSE is the diffusion coefficient of spherical particles at infinite dilution given by the Stokes
Einstein relation. DSE ¼

kT 6πηa

ð3Þ

where kT is the thermal energy, η is the solvent viscosity, and a is the particle radius. For hard spheres, the second-order virial expansion gives the following results:26
 Dc ¼ DSE 1 þ 1:454ϕ
0:45ϕ2 þ Oðϕ3 Þ ð4Þ This expression describes a weak initial increase in Dc when ϕ is increased. However, the situation is much more complex due to the necessity of using a screened Coulomb potential in superposition with the hard-sphere potential. For example, the experimental results in ref 27 showed a great increase in the collective diffusion coefficient, with a decrease in electrolyte concentration in the case of charged bovine serum albumin molecules. At finite concentration, Dc depends on direct interactions between particles, and is expressed by the interparticle potential and indirect hydrodynamics interactions which are mediated by the solvent. The short-time collective diffusion coefficient is given by28 Dc ¼ DSE

HðqÞ SðqÞ

ð5Þ

where S(q) = S(q,t = 0) is the static structure factor and H(q) accounts for hydrodynamic interaction. For charged spheres, the initial increase in Dc(ϕ) at small volume fraction is typically far steeper than that for uncharged hard spheres. Moreover, on further increasing ϕ, Dc passes through a maximum. This maximum, due to the competing influences of the structure factor and the hydrodynamic interaction, increases and shifts to larger values of ϕ for growing particle charge and, oppositely, decreases and shifts to smaller values of ϕ for increasing salt content.26,29 Our aim in the DLS experiments was to explore how the diffusion coefficient behaves as a function of silica-particle content on the addition of various types and amounts of surfactant, which brings about different charges.

∂fel ∂ðϕ1=3 Þ ∇ϕ ¼
kT þ 4πηa jðϕÞ V ϕ ∂ðϕ1=3 Þ ∂x

ð6Þ

where M is the product M = γ0m cos2 θ that characterizes the specific geometry of local ordering in the colloidal suspension, and could be regarded as a free parameter in the interpretation of experimental data. In this case, we fit the data using the values of a simple cubic-packing geometry (γ0 = 0.806, m = 1, and cos θ = 1). The left-hand side of eq 6 is the interparticle force given by the electrostatic force, which is obtained by solving the Poisson
Boltzmann equation for a spherical cell and the Hamaker equation for the van der Waals attractive potential. Since the particle-volume fraction of interest is lower than 0.20, the van der Waals attractive interactions are ignored in this model because of the relatively large interparticle distances. For the determination of the derivative ∂f/∂(ϕ1/3), a specific form of the direct-interaction potential is needed. Alexander et al.31 solved the Poisson
Boltzmann equation numerically, and showed that the interaction energy between the sphere inside the cell and the spheres outside the cell has a Yukawa form. Thus, Petsev et al.30 could find the derivative of the electrostatic force with respect to (ϕ1/3), " ! ∂fel z2 λkTka 2kaγ0 2kaγ0 ¼ 2 þ 1 ∂ðϕ1=3 Þ 2a γ0 ð2kaγ0 =ðϕ1=3 ÞÞ ðϕ1=3 Þ ðϕ1=3 Þ !# ! 2kaγ0 2kaγ0 þ þ 2 exp
1=3 ð7Þ ðϕ1=3 Þ ðϕ Þ where z is the particle-charge number and λ is the Bjerrum length (λ = e2/(εkT), where e is the elementary charge, ε is the dielectric constant of the solvent), and k is the inverse of the Debye length. The total outer force (right-hand side of eq 6) acting on a sphere moving with velocity V in the suspension is the superposition of the Brownian force (first term) and the viscous drag (second term). It is possible to obtain the diffusion coefficient from eq 6 by means of Fick’s law: ! Dc 3 2aM ∂fel 1 þ ¼ ð8Þ 3kT ∂ðϕ1=3 Þ DSE 2jðϕ1=3 Þ For the function j(ϕ1/3), which accounts for the hydrodynamic interactions, the expression given by Happel was used: jðϕ1=3 Þ ¼

3 þ 2ðϕ1=3 Þ5 2
3ðϕ1=3 Þ þ 3ðϕ1=3 Þ5
2ðϕ1=3 Þ6

ð9Þ

Fits to eq 8 were made by a least-squares method (routine lsqcurvefit in the Matlab program), which provided the charge, ze, and the radius of the particle, a, as parameters. There is good agreement between the experimental data and the analytical formula prediction, as can be seen in Figure 9. Moreover, the 9187

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Langmuir fitted parameters used here agree very well with those obtained from SAXS data, as will be shown in Figure 13. In the following, the results for particles after the addition of surfactant are presented in such a way that for each volume fraction we add various amounts of surfactant that keep the

Figure 9. Collective diffusion coefficient for Le300 without surfactant (symbols) as a function of the particle content and its corresponding theoretical prediction (solid line).

Figure 10. Diffusion coefficient for Le300 with C12E5 as a function of Le300 concentration at a constant ratio of surfactant to silica: [C12E5]/[Le300] = 1.7  10
3 (which means [C12E5] = 0.1 mM for 5% of Le300).

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surfactant/particle concentration ratio constant. In the case of the nonionic surfactant, C12E5 (Figure 10), we can confirm the SAXS results: the diffusion coefficient is practically the same as that for silica alone, which means that the charge is not altered. The behavior of the diffusion coefficient when CTAB is added to Le300 is very interesting (Figure 11). When CTAB is added at a low ratio, we can see the same tendency as for plain Le300, and we also find a similar effective charge from the fitting process. However, the whole curve is shifted to lower values of Dc, which indicates an increment in size, as we can see by the extrapolation of Dc to ϕ = 0 and as confirmed by the fitted value of radius (a = 10 nm). For the highest ratio of CTAB to Le300, the maximum disappears and an almost linear decay is found. The system has a lower charge, and an attractive interaction behavior is indicated by the immediate decrease in diffusion coefficient with the volume fraction. We can also observe that the addition of CTAB destabilized the system (right part of Figure 11), presenting big aggregates (also the fitted average radius is larger a = 11.8 nm) that probably arise from partial clustering of particles with lower charge. With SDS as additive, we observe totally different behavior (Figure 12). We expected an increase in the slope at low volume fraction with concentration, provided that the effective charge increases, as was predicted by SAXS. However, with the exception of the lowest SDS concentration, we could not see a clear

Figure 12. Diffusion coefficient for Le300 with SDS at concentrations for three different constant ratios [SDS]/[Le300]: low = 2.7  10
4, medium = 1.4  10
3, and high = 5.5  10
3 (i.e., [SDS] = 0.1, 0.5, and 2 mM for 5% of Le300, respectively).

Figure 11. Diffusion coefficient for Le300 with CTAB at concentrations for two different constant ratios [CTAB]/[Le300]: low = 3.5  10
4 and high = 1.7  10
3 (i.e., [CTAB] = 0.1 and 0.5 mM for 5% of Le300, respectively). On the right side, Le300 with (I) and without (II) surfactant. 9188

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Langmuir increment in the initial slope. The reason is, as discussed above, that the diffusion coefficient depends not only on the effective charge but also on the ion concentration in the sample, and there are as many sodium ions as dodecyl sulfate ions in solution. By considering the sodium ions present, we found from the fitting process that the effective charge increases with the amount

Figure 13. Summary of fitted effective charge obtained from SAXS (empty symbols and dashed lines) and DLS (full symbols and full lines) for Le300 alone (hexagons), with SDS (circles), CTAB (squares), and C12E5 (triangles).

Figure 14. Surface tension of water
air interface at various surfactant concentrations (O, SDS; 0, CTAB; 4, C12E5) for pure surfactant (empty symbols) and for surfactant with 5% of Le300 (full symbols).

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of SDS. Moreover, the trend and the values agree very well with those obtained from SAXS data, as shown in Figure 13. Surface Tension. Finally, we expect an increase in the hydrophobicity upon addition of these different types of surfactants on the nanoparticles’ surface; this would provide a driving force for their attachment at the air
water or oil
water interfaces. Therefore, we compared the surface tensions and interfacial tensions against 1-octadecene in systems containing only surfactants with those where the solutions also contain 5% silica nanoparticles. Figure 14 shows the surface tension of aqueous solutions of the surfactants (SDS, CTAB, and C12E5) with and without addition of particles. In the former case, we found that the surface tension decreases with increasing surfactant concentration up to the cmc, beyond which it remains fairly constant. At 25 °C, the literature cmc values of SDS, C12E5, and CTAB aqueous solutions are 8.2, 0.07, and 1 mM, respectively (dashed vertical lines in Figure 14); our determinations are in good agreement with these values. Note that, according to our data in Figure 14, at very low surfactant concentration the inclusion of nanoparticles has a negligible influence on the free energy of the air
water interface, as was previously reported by Ravera et al.,3 who offer the following reasonable explanation: in the absence of surfactants, the utilized particles are strongly hydrophilic and then completely wetted so that they do not prefer the interface. For larger concentrations of surfactants (see Figures 14 and 15), rather significant changes occur when particles are present. Thus, in the case of positively charged surfactant, the surface tension in the presence of particles, especially at the air
water interface (Figure 14), is greater than that corresponding to the plain CTAB solutions and is practically independent of concentration. The explanation lies in the fact that, due to a favorable electrical attraction, most of the CTAB is adsorbed onto the particles, leaving almost no free surfactant in the bulk (and, therefore, at the interface), which brings about surface-tension values close to those of water with no surfactant at all. A similar effect is found at the air
octadecene interface (Figure 15). However, the other surfactants show quite unexpected behavior: when nanoparticles are present, the surface tension of SDS (and, to a lesser extent, of the nonionic surfactant C12E5) drops more steeply at concentrations below the cmc, so that the surface or interfacial tension is lower for almost all surfactant concentrations when compared to the system without nanoparticles.

Figure 15. Surface tension of water
1-octadecene interface at various surfactant concentrations (O, SDS; 0, CTAB) for pure surfactant (empty symbols) and for surfactant with 5% of Le300 (full symbols). (f) Samples after washing and filtration, placed at the initial SDS concentration. Both graphs present the same data but in linear and log scale for better visibility of the different regions. 9189

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Figure 16. Electrophoretic mobility (O, SDS; 0, CTAB) as a function of surfactant concentration, and zeta potential as a function of SDS concentration.

Figure 17. Left: Real part of the electrophoretic mobility as a function of the frequency at various concentrations of SDS (symbols, experimental data; solid line, theoretical predictions). Right: Zeta potential obtained from the theoretical predictions for each concentration of SDS (the solid line is a visual guide).

To determine whether the presence of both silica and the remaining free SDS are responsible for this behavior, we almost completely eliminated the remaining surfactant with an ultrafiltration cell by repeatedly washing with water until we obtained a value of 2.7% of particles. After this washing step, we still obtain surface tensions below the corresponding values for SDS alone, as we can see in Figure 15. We can therefore conclude that particles are partially coated with SDS and are thus capable of influencing the interfacial properties, at least at this concentration. Other authors3,7,8 have also explored this issue. Ma et al.’s explanation for this behavior is, in summary, that the repulsive electrostatic force between the particles and the SDS promotes the surfactant diffusion toward the interface, which leads to a decrease in the surface tension, while Ravera et al. argue that increasing the CTAB concentration implies an increase in the surface coverage of the nanoparticles and, as a consequence, in their hydrophobicity. This means a greater affinity of the particles for the fluid interface. Thus, the number of particles at the interface rises with surfactant concentration. Such particles contain a large amount of surfactant that can be released to the fluid interface; that is, the particles can act as carriers of CTAB toward the interface. This effect is even more significant when the sample is in contact with 1-octadecene. This could be because the partially hydrophobized particles are more strongly attached to the oil
water interface.

However, these arguments alone cannot fully explain our results. In the case of SDS, we found that the surface tension decreases even after complete elimination of free surfactant, and it was still lower than that of water. This reduction could be due to SDS release from the particles but could also be due to the effect of SDS-coated particles on the surface tension. In the case of CTAB, this behavior is not reproduced, probably because CTAB promotes aggregation between particles, as we have shown above (Figure 11). This prevents the particles’ approach to the interface and thus hinders any effect on the surface tension. Electrophoretic Mobility. Electrophoresis was used as a complementary technique to confirm the changes in the effective charge. Both classical electrophoresis and electroacoustic measurements give mobilities that increase with SDS concentration. However, strong fluctuations in the data were found. This is either due to the fact that in classical determination of mobility we were working at the detection limit of the device due to the small size of the particles, or we get different values for the electrophoretic velocities, as not all the particles are coated with the same amount of surfactant. In any case, a clear trend is observed in the electrophoretic mobility: an increase in the negative mobility for SDS (super charging) and a trend toward charge reversal (less negative mobility) for CTAB (Figure 16). Due to the strong scattering of the experimental data, electroacoustic measurements were subsequently carried out to confirm 9190

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Langmuir the trend. The electric sonic amplitude is proportional not only to the mobilities but also to the difference in the medium and particle densities and the volume fraction of the sample. Thus, we have obtained a series of more reliable measurements. The dynamic mobility at zero frequency corresponds to the classical electrophoretic mobility. It stays constant at low frequencies for small particles, like those in our study, and reaches a maximum in the high-frequency region (beyond the range of our instrument). The plateau value of the mobility increases with surfactant concentration, which is in good agreement with the prediction by SAXS and DLS of the increment in the effective charge. We obtained the zeta potential at each concentration of SDS that fit the mobility as a function of frequency (Figure 17). In this way, we could confirm an increasing trend for the zeta potential with the anionic surfactant, thereby verifying the conclusions obtained from SAXS and DLS measurements.

’ CONCLUSIONS We have studied the adsorption of various types of surfactant on the surface of silica nanoparticles. The more favorable surfactant for having a charge opposite to that of the particle, CTAB, gives good adsorption, as shown by the surface tension data, but decreases effective charge. This brings about aggregation, as the DLS predicted, by an increasing size and attractive potential, with increasing CTAB concentration and a subsequent destabilization of the system. The effective charge on the particles does not show any appreciable change when nonionic surfactant is added, at least not at the concentrations used in this study. The case of SDS gave unexpected results. We observed that, despite the equal sign of the charge, part of the SDS molecule is adsorbed on the silica, bringing about a supercharged system by increasing the effective charge of the silica. We confirmed this behavior by means of two different, independent experiments (SAXS and DLS). The zeta potential also shows the same trend, which confirms the increasing electric repulsion between particles. Silica particles partially coated with SDS are capable of influencing the interfacial properties; that is, they decrease the surface tension at the air
water and oil
water interfaces. A potential application of this study is the formulation of Ramsden
Pickering emulsions for incorporation into layer-bylayer systems. This requires well-charged particles with increased hydrophobicity to bind at the oil
water interface. At this point, we can disclose that very stable internally nanostructured emulsions with a mean diameter of 400 nm were obtained by using particles in combination with SDS just below its cmc.32 Our results are clearly different from similar works published until now, probably due to the different surfactant/particle concentration regime used in other works and/or due to different sensitivity of the methods, especially when colloidal systems are compared with macroscopic surfaces. Recently, we have studied the effect of addition of surfactants to large, half-micrometer sized silica particles.32 Future work will involve adsorption of a combination of various types of surfactant onto nanoparticles. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figure showing the size distribution of silica particles from the experimental scattered intensity obtained by SAXS, and tables showing the fitting parameters

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from SAXS and DLS experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Telephone: +43 316 380 5433. Fax: +43 316 380 9850. E-mail: [email protected].

’ ACKNOWLEDGMENT S.A. acknowledges financial support by the University of Granada. Partial financial support was also provided by COST action D43 “Colloid and Interface Chemistry for Nanotechnology”. ’ REFERENCES (1) Ramsden, W. Proc. R. Soc. London 1903, 72, 156–164. (2) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001–2021. (3) Ravera, F.; Santini, E.; Loglio, G.; Ferrari, M.; Liggieri, L. J. Phys. Chem. B 2006, 110, 19543–19551. (4) Wang, W.; Gu, B.; Liang, L.; Hamilton, W. A. J. Phys. Chem. B 2004, 108, 17477–17483. (5) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. J. Colloid Interface Sci. 2003, 206, 236–244. (6) Penfold, J.; Staples, E.; Tucker, I. Langmuir 2002, 18, 2967–2970. (7) Thibaut, A.; Misselyn-Bauduin, A. M.; Grandjean, J.; Broze, G.; Jer^ome, R. Langmuir 2000, 16, 9192–9198. (8) Penfold, J.; Tucker, I.; Thomas, R. K. Langmuir 2005, 21, 11757–11764. (9) Paunov, V. N.; Binks, B. P.; Ashby, N. P. Langmuir 2002, 18, 6946–6955. (10) Lugo, D.; Oberdisee, J.; Karg, M.; Schweins, R.; Findenegg, H. Soft Matter 2009, 5, 2928–2936. (11) Oberdisse, J. Curr. Opin. Colloid Interface Sci. 2007, 12, 3–8. (12) Despert, G.; Oberdisse, J. Langmuir 2003, 19, 7604–7610. (13) Sharma, K. P.; Aswal, V. K.; Kumaraswamy, G. J. Phys. Chem. B 2010, 114, 10986–10994. (14) Kumar, S.; Aswal, V. K. J. Phys.: Condens. Matter 2011, 23, 035101. (15) Ma, H.; Luo, M.; Dai, L. Phys. Chem. Chem. Phys. 2008, 10, 2207–2213. (16) Whitby, C.; Fornasiero, D.; Ralston, J. J. Colloid Interface Sci. 2009, 329, 173–181. (17) Bergmann, A.; Orthaber, D.; Scherf, G.; Glatter, O. J. Appl. Crystallogr. 2000, 33, 869–875. (18) Orthaber, D.; Bergmann, A.; Glatter, O. J. Appl. Crystallogr. 2000, 33, 218–225. (19) Brunner-Popela, J.; Glatter, O. J. Appl. Crystallogr. 1997, 30, 431–442. (20) Weyerich, B.; Brunner-Popela, J.; Glatter, O. J. Appl. Crystallogr. 1999, 32, 197–209. (21) Bergmann, A.; Fritz, G.; Glatter, O. J. Appl. Crystallogr. 2000, 33, 1212–1216. (22) Fritz, G.; Bergmann, A.; Glatter, O. J. Chem. Phys. 2000, 113, 9733–9740. (23) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814–4820. (24) Ahualli, S.; Delgado, A.; Miklavcic, S. J.; White, L. R. Langmuir 2006, 22, 7041–7051. (25) Schmitt-Rozieres, M.; Kr€agel, J.; Grigoriev, D. O.; Liggieri, L.; Miller, R.; Vincent-Bonnieu, S.; Antoni, M. Langmuir 2009, 25, 4266–4270. (26) Banchio, A. J.; N€agele, G. J. Chem. Phys. 2008, 128, 104903–20. (27) Anderson, J. L.; Rauh, F.; Morales, A. J. Phys. Chem. 1978, 82, 608–616. (28) N€agele, G. Phys. Rep 1996, 272, 215. 9191

dx.doi.org/10.1021/la201242d |Langmuir 2011, 27, 9182–9192

Langmuir

ARTICLE

(29) Genz, U.; Klein, R. Phys. A 1991, 171, 26–42. (30) Petsev, D. N.; Starov, V. M.; Ivanov, I. B. Colloids Surf., A 1993, 81, 65–85. (31) Alexander, S.; Chaikin, P. M.; Grant, P.; Morales, G. J.; Pincus, P. J. Chem. Phys. 1984, 80, 5776. (32) Iglesias, G. R.; Wachter, W.; Ahualli, S.; Glatter, O. Soft Matter 2011, 7, 4619–4622. (33) Batchelor, G.K. J. Fluid Mech. 1972, 52 , 245.

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