ARTICLE pubs.acs.org/Langmuir
Characterization of Carboxylated Nanolatexes by Capillary Electrophoresis Farid Oukacine,† Aurelie Morel,‡ and Herve Cottet*,† †
Institut des Biomolecules Max Mousseron (IBMM, UMR 5247 CNRS-Universite de Montpellier 1-Universite de Montpellier 2), Place Eugene Bataillon, case courrier 1706, 34095 Montpellier Cedex 5, France ‡ BASF SE, GKD/P - B001, 67056 Ludwigshafen, Germany ABSTRACT: Poly(styrene-co-acrylic acid) (St/AA) and poly(styrene-co-methacrylic acid) (St/MA) nanolatexes with different acid contents were prepared by emulsion copolymerization and were analyzed by capillary electrophoresis (CE) and by laser doppler velocimetry (LDV). Due to the intrinsic differences in the methodologies, CE (separative technique) and LDV (zetametry, nonseparative technique) lead to very different electrophoretic mobility distributions. Beyond these differences, the variation of the electrophoretic mobility is a complex and nonlinear function of the hydrodynamic radius, the ionic strength, and the zeta potential. To gain better insight on the influence of the ionic strength and the acid content on the electrophoretic behavior of the nanolatexes, the electrophoretic mobility data were changed into surface charge densities using the O’Brien, White, and Ohshima modeling. This approach leads to the conclusion that the surface charge density is mainly controlled at high ionic strength (∼50 mM) by the adsorption of anionic surfactants coming from the sample. On the contrary, at low ionic strength, and/or in the presence of neutral surfactant in the electrolyte, the acid content was the main parameter controlling the surface charge density of the nanolatexes.
1. INTRODUCTION In numerous application areas, binders display the role of a glue point between pigment particles or mineral fillers. Two different types of binders are generally used in combination or alone: synthetic polymers and natural materials such as starch. Emulsion polymerization is one of the most frequently used approaches to prepare synthetic polymer binders consisting of nanoparticles. In emulsion polymerization, the added surfactants modify the interparticle interactions and produce stabilization of the system. Nonionic surfactants adsorbed onto colloidal particles can act as steric stabilizers,1,2 and adsorbed ionic surfactants cause electrostatic and steric effects.3,4 Carboxylic acids are often used as comonomers to improve the colloidal stability of the product, by increasing both the electrostatic repulsion and the steric repulsion. The control of the surface charge density of nanolatex is a key parameter to warranty the latex stability. A variety of analytical methods have been used for the characterization of latex surfaces. Several articles report on the use of NMR spectroscopy,5,6 potentiometric titration,7,8 X-ray photoelectron spectroscopy9 (XPS), and secondary ion mass spectrometry10,11 (SIMS) to determine the distribution of monomers or functional groups in polymer colloids. Scattering techniques (dynamic light scattering (DLS)),12 electron microscopy (transmission and scanning (TEM and SEM)),13,14 field-flow fractionation15-17 (FFF) were also frequently used for the determination of the size distribution of latex particles. Capillary zone electrophoresis (CE) has been also used for the separation and characterization of latex r 2011 American Chemical Society
microspheres. CE is a suitable analytical tool for controlling and monitoring the synthesis of copolymer latexes.18 The dependence of the CE resolution on the particle size and electrokinetic surface charge density,19,20 ionic strength,19 pH of the running buffer,21 electric field,19,20,22 and temperature22 have been investigated. Radko et al.19 demonstrated that the sizebased separation of submicrometer polystyrene latexes in CE is a function of the ionic strength according to the electrokinetic theory described by Overbeek and Booth.23 The determinant factors of selectivity and efficiency of the separation of colloidal particles are, respectively, the ionic strength of the buffer and the dispersion of electrokinetic surface charge density.20 The effect of acrylic acid13 and methacrylic acid9,24 amount on the colloidal properties of polystyrene latex, synthesized by emulsifier-free polymerization, have been reported. The packing density of COOH groups (molecules per unit surface area), at the particle surface, does not increase linearly with acid content.9 The distribution of the acid functionality in carboxylated polymer latex depends on the hydrophilicity of the carboxylic acid monomer and the pH of the reaction system.8,25 So far, there have been no reports on the study of surface charge density of carboxylated nanolatexes, synthesized by emulsion polymerization, using CE. In the present article, the electrophoretic behavior and the electrokinetic properties of poly(styrene acrylic acid) (St/AA) and poly(styrene methacrylic Received: December 6, 2010 Revised: January 20, 2011 Published: February 23, 2011 4040
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Table 1. Chemical Composition of the Polymerization Mixture and Nanolatex Hydrodynamic Radii components (g L-1) Rh
Dowfax Na2S2O8 SDS
2A1
NaHCO3
St
AA MA (nm)b PDIc
MA 6%a
0.7
28.1
24.6
0.5
336.9 0 14.7 18.5 0.06
MA 12%a MA 18%a
0.7 0.7
27.9 27.6
24.4 24.2
0.5 0.5
319.2 0 29.3 18.0 0.06 301.4 0 44.0 15.5 0.06
MA 23%a
0.7
27.4
24.0
0.5
283.7 0 58.7 14.5 0.06
MA 29%a
0.7
27.1
23.8
0.5
266.0 0 73.3 13.0 0.11
AA 7%a
0.7
17.5
34.9
0.6
337.8 11.7 0
20.5 0.07
AA 22%a
0.7
16.8
34.6
0.6
295.2 41.1 0
20.0 0.10
ST
0.7
53.2
0
0.3
355.0 0
17.5 0.04
0
a
The acid content represents the molar content of acid monomer in the reaction mixture. b Hydrodynamic radii (Rh) measured with Zetasizer equipment in a 0.05 M NaCl. c PDIs of the size distributions of the nanolatexes obtained from the Zetasizer.
(Malvern, Worcestershire, United Kingdom). The eluent was tetrahydrofuran (THF) at a flow rate of 0.8 mL min-1. The refractive index increment (dn/dc) of polystyrene was used for the calculations. 2.4. Capillary Electrophoresis. CE experiments were carried out with a 3D-CE instrument (Agilent Technologies, Waldbronn, Germany) equipped with a diode array detector. Uncoated fused silica capillaries of 33.5 cm total length (50 μm i.d., 360 μm o.d.) and 25 cm to the detection window were purchased from Composite Metal Services (Worcester, UK). New fused silica capillaries were conditioned by performing the following washes: 1 M NaOH for 20 min, 0.1 M NaOH for 15 min, and water for 10 min. Between runs, the capillary was flushed with electrolyte (5 min, 930 mbars). The temperature of the capillary cassette was maintained constant at 25 C. The samples were prepared in the deionized water and injected at a latex concentration of 0.7 g L-1 by hydrodynamic injection (17 mbar, 3 s). Ethyl 4-hydroxybenzoate (HB) was added in the sample at 0.1 g L-1 as an electrophoretic marker. Data were collected at 193 nm (bandwidth at 5 nm). The separation was carried out by applying a constant voltage of þ20 kV. Electroosmotic mobility was calculated from the migration time of mesityl oxide (neutral marker). Electropherograms were plotted in effective mobility scale (μep) using the following equation:
acid) (St/MA) copolymer nanolatexes with hydrodynamic radii between 13.0 and 20.5 nm were investigated in details. Results obtained by CE were also compared to data obtained by laser doppler velocimetry (LDV, Zetasizer).
2. EXPERIMENTAL SECTION 2.1. Chemicals. Poly(ethylene glycol) dodecyl ether (Brij 35), sodium tetraborate (B4Na207), and ethyl 4-hydroxybenzoate (HOC6H4CO2C2H5) were purchased from Aldrich (Steinheim, Germany). Sodium hydroxide (NaOH) was from VWR (Leuven, Belgium). Deionized water was further purified with a Milli-Q system from Millipore (Molsheim, France). 2.2. Latexes. Materials. Styrene (St), methacrylic acid (MA), acrylic acid (AA), itaconic acid (IA), fumaric acid (FA), sodium dodecyl sulfate (SDS), sodium peroxodisulfate initiator (Na2S2O8), and sodium carbonate (NaHCO3) (99%, purity) from Aldrich (Steinheim, Germany) and sodium dodecyl diphenyl oxide disulfonate (Dowfax 2A1) from Pilote Chemical Company (USA) were used as received. Water was purified using a Milli-Q Plus water purification system. Polymerization of Nanolatex. For emulsion polymerization of polystyrene nanolatexes, commercial Na2S2O8 and SDS were used as initiator and emulsifier, respectively, without further purification. Briefly, typical emulsion polymerization reactions were performed in a 5000 mL reactor under nitrogen gas at 1000 mbar. For all experiments, initiator concentration was set at 0.7 g L-1. All polymerizations were carried out at 85 C for 3 h leading to monomer conversion higher than 99%. For the synthesis of carboxylated nanolatexes (St/AA and St/MA), SDS was partially replaced by Dowfax 2A1. The formulations and hydrodynamic radii of the nanolatexes studied in this work are provided in Table 1. Hydrodynamic radii (Rh) were measured with Zetasizer equipment in a 0.05 M NaCl solution. Polydispersity indexes (PDIs) obtained from the Zetasizer were less than 0.11 (see Table 1), suggesting narrow particle size distributions for all the nanolatexes. The molar mass obtained by size exclusion chromatography (SEC) is approximately 20 000 g L-1 with a PDI of 1.5. 2.3. Size Exclusion Chromatography. SEC analyses were performed with the Viscotech TDA 302 (Malvern, Worcestershire, United Kingdom) equipped with a triple detector array: RI viscosimeter, right angle laser light scattering (RALS) and low angle laser light scattering (LALS). The characteristics of the column and pump used for SEC analysis are, respectively, TSK-Gel Tosoh Corporation GMHHR-H (7.8 mm i.d., 30.0 cm length, Tosoh Bioscience GMBH, Stuttgart, Germany) and Viscotech VE 20002 GPC eluent/sample module
μep ¼
! 1 lL tapp teo V 1
ð1Þ
where μep is the effective mobility, l is the effective capillary length to the detection point, L is the total capillary length, V is the applied voltage, tapp is the apparent detection time, and teo is the detection time of the neutral marker. 2.5. Zetasizer. Measurements of electrophoretic mobility by LDV of the suspension particles were carried out at 20 C using a Nano-ZS Zetasizer (Malvern Instruments, Orsay, France). A helium-neon laser operating at a wavelength of 633 nm was used. The power supply of the laser is 4 mW. The samples were prepared in borate buffer at an ionic strength of 30 mM. The nanolatex solution was directly injected in the cell after its preparation, just prior to data acquisition, in order to prevent bubble formation. For one given latex concentration, three to five measurements were performed.
3. RESULTS AND DISCUSSION 3.1. Particle Size. The hydrodynamic radii of the nanolatexes were determined by DLS using a Zetasizer (see section 2.5). The results are gathered in Table 1. A different behavior was observed for MA- and AA-containing nanolatexes. The hydrodynamic radius of the latexes remains almost constant from 7 to 22% AA. Above 22% AA, the latexes coagulate (concomitant increase of Rh and PDI). The incorporation of AA in the latex results in an increase of the particle size in comparison with homopolymeric polystyrene nanolatex particles without any change in the PDI of the particle size distribution. This effect is often observed when relatively hydrophilic monomers are copolymerized with hydrophobic monomers such as styrene.12,26 It can be attributed to the presence of hydrophilic regions within the particles or at the particle surface, which may swell in an aqueous environment.12 However, in the case of MA-containing nanolatexes, a slight decrease in particle size is observed with increasing MA content. This peculiar behavior can be due to the different affinity of MA with the styrene monomer during the polymerization. 3.2. Comparative Study of the Electrophoretic Mobilities Measured by LDV and CE. LDV, also called zetametry, basically provides a measurement of the electrophoretic mobility. Figure 1 displays the electrophoretic mobility distributions obtained by 4041
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Figure 1. Comparison of electrophoretic mobility distributions of 23% MA nanolatex obtained by CE (1) and LDV (2). Trace 3 displays the LDV electrophoretic distribution for SDS solution. Electrolyte: sodium borate buffer, pH 9.2 at ∼30 mM ionic strength. Experimental conditions in CE: fused silica capillary 33.5 cm (25 cm to the detector) 50 μm i.d. Sample: nanolatex 23% MA at 0.7 g L-1 in deionized water. Hydrodynamic injection: 17 mbars during 3 s. Applied voltage: þ 20 kV. UV detection at 193 nm (bandwidth, at 5 nm). HB: ethyl 4-hydroxybenzoate at 0.1 g L-1 (electrophoretic marker). Mesityl oxide at 0.1% (v/v) was used as the neutral marker. Other conditions are as described in section 2.4. Experimental conditions in zetametry: He-Ne laser. Power supply: 4 mW. Wavelength beam: 633 nm. Temperature: 20 C. Samples: 23% MA nanolatex at 0.05 g L-1 in the electrolyte (trace 2). 2.0 g L-1 SDS in the electrolyte (trace 3). Other conditions are as described in section 2.5.
CE (1) and LDV (2) for St/MA copolymer nanolatex containing 23% MA, together with the electrophoretic distribution obtained by LDV (3) for an SDS solution. The exact chemical compositions of copolymer nanolatexes are given in Table 1. For a better comparison, the time-scale electropherograms in CE were converted into effective mobility scale using eq 1. The electrophoretic mobility distribution obtained by LDV is much broader than that obtained by CE. The standard deviations calculated on the electrophoretic mobility distribution (σμ) are about 16 times higher for LDV than for CE (σμ∼20.6 10-9 m2 V-1 s-1 vs 1.3 10-9 m2 V-1 s-1). Indeed, in CE, all the compounds present in the sample are separated from each other, providing that their mobilities are different. Therefore, the electrophoretic mobility distribution obtained in CE represents the distribution of the latex in given electrolyte conditions (pH, ionic strength) and after separation from the other components of the sample. In LDV, there is no separation, and the distribution can be different from that obtained in CE due to interactions between the latexes and the other components (such as ionic or nonionic surfactants, residual ions, etc.). For instance, the distribution obtained in LDV for the 23% MA nanolatex (Figure 1, trace 2) clearly displays some contribution from SDS surfactant (see Figure 1, trace 3 for the distribution of SDS alone), which is present in the latex sample. The average electrophoretic mobility is also higher (in absolute value) in CE than in LDV, as already reported in the literature.27 3.3. Influence of the Ionic Strength on the Electrophoretic Mobility of Carboxylated Nanolatex. Figure 2 displays the effect of ionic strength on the electrophoretic mobility distributions of St/MA nanolatexes at 12 and 29% MA contents (see Table 1 for chemical composition of latexes). The acid content represents the molar content of the acid monomer in the reaction mixture. Since monomer conversion is higher than 99%, we assume
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Figure 2. Electropherograms in effective mobility scale of St/MA latexes containing 12% and 29% MA. Electrolyte: sodium borate buffer, pH 9.2, ionic strength as indicated on the graph. Other conditions are as described in section 2.4. See Table 1 for the chemical composition of latexes. HB: ethyl 4-hydroxybenzoate (electrophoretic marker).
that the acid contents in the reactor and in the final nanolatex are identical. Similar electropherograms were obtained for St/MA nanolatexes at 6 and 23% MA content and for nanolatex carboxylated with AA (results not shown). The effective mobility data were plotted versus the ionic strength in Figure 3A (St/MA latexes) and B (St/AA latexes). The bars represent the dispersion of the peak in mobility ((1 standard deviation centered on the average mobility). At low acid content (6-7%) the electrophoretic mobility tends to increase with the ionic strength. For higher acid content, the behavior is more complex with U-shape curves. The electrophoretic mobility behavior of the latex with the ionic strength is a complex (and nonlinear) function of the size and the zeta potential of the particle.1-4 To gain better insight into the influence of the ionic strength on the physicochemical properties of the nanolatex, it was found more appropriate to change the electrophoretic mobility into surface charge density using O’Brien and White modeling.3 Surface charge density seems to be the more relevant parameter to characterize nanoparticles having an intrinsic constant charge density. Indeed, in the absence of adsorption or desorption at the surface of a particle, the surface charge density is independent of the ionic strength. However, this is not the case for the zeta potential. This was clearly pointed out by Makino et al.28 on gold nanoparticles. Nevertheless, the determination of surface charge density requires the preliminary calculation of ζ potential. 3.4. Determination of Nanolatex ζ Potential. There is no simple equation available for the calculation of the ζ potential from the electrophoretic mobility. Taking into account the electrophoretic retardation and the relaxation effect, numerical calculation 4042
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Figure 3. Effect of the ionic strength on the electrophoretic mobility of St/MA (A) and St/AA (B) latexes. Electrophoretic conditions are as described in Figure 2. The data points represent the average electrophoretic mobility value, and each bar represents (1 standard deviation of the electrophoretic mobility distribution (Figure 2).
in the case of spherical colloidal particles was first done by Overbeek et al.2 and further developed by O’Brien and White.3 Numerical solutions have been reported for some values of κRh.4 In 2001, Ohshima29 proposed the following equation, which is valid for a symmetrical electrolyte z:z and for moderate ζ values (ζ e 100 mV):5,29 " 2εr ε0 ζ zeζ 2 f1 ðkRh Þ μep ¼ f3 ðkRh Þ 3η kB T # mþ þ m- zeζ 2 f4 ðkRh Þ ð2Þ kB T 2 where εr is the relative electric permittivity, ε0 is the electric permittivity of vacuum, η is the viscosity of the electrolyte, κ is the Debye-H€uckel parameter, e is the elementary electric charge, z is the charge number of electrolyte ions, Rh is the hydrodynamic particle radius, kB is the Boltzmann constant, and T is the absolute temperature. f1, f3, and f4 are functions of κRh according to the following equations: f1 ðkRh Þ ¼ 1 þ
1 2½1 þ 2:5=fkRh ð1 þ 2e-kRh Þg3
ð3Þ
f3 ðkRh Þ ¼
kRh ðkRh þ 1:3e-0:18kRh þ 2:5Þ 2ðkRh þ 1:2e-7kRh þ 4:8Þ3
ð4Þ
f4 ðkRh Þ ¼
9kRh ðkRh þ 5:2e-3:9kRh þ 5:6Þ 8ðkRh þ 1:55e-0:32kRh þ 6:02Þ3
ð5Þ
Figure 4. Reduced mobility versus reduced ζ potential curves derived from Ohshima modeling and used to calculate the ζ potential of St/MA (A) and St/AA (B) latexes. The hμ = f(ζ h) curve was plotted using eq 2 in section 3.4.
For a symmetrical electrolyte, the dimensionless ionic drag coefficients (mþ and m-) are easily accessible from the limiting conductances of the cation Λ0þ and the anion Λ0- in the electrolyte. m( ¼
2εr ε0 kB TNA 3ηzΛ0(
ð6Þ
The Debye-H€uckel parameter κ, for a 1:1 electrolyte is given by7 !1=2 2NA e2 I1000 k¼ ð7Þ ε0 εr kB T where NA is the Avogadro number and I is the ionic strength. Electrophoretic mobilities and ζ potentials are often expressed in 8 their dimensionless forms (μ h and ζh), defined as μ ¼
3ηe μ 2ε0 εr kB T ep
ð8Þ
eζ ð9Þ kB T Equation 2 is valid for a rigid particle, the surface of which is maintained at a constant electrical potential. Strictly speaking, the nanolatexes used in this work are soft particles with a hard core and a porous layer (corona). Nevertheless, the scale length of the corona, due to surfactant adsorption, is in the order of nanometers, and therefore is 15 to 20 times smaller than the hard core. In addition, the modeling of soft particle electrophoretic mobility is much more complex and only recently published (especially if ζ ¼
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Figure 5. Effect of the ionic strength on the ζ potential of St/MA (A) and St/AA (B) latexes. Electrophoretic conditions are as in Figure 2. ζ potentials were obtained from mobility values using Figure 4.
one takes into account relaxation effect; see eq 20 in ref 30). This model requires fitting parameters such as the electrophoretic softness (1/λ) that cannot be easily obtained experimentally. Therefore, we consider that, in our case, the hard core particle modeling is a good approximation. ζ potentials of the nanolatexes were determined by plotting the curve μ h = f(ζ h), using eq 2 for different values of κRh. The reduced ζ potential is obtained from the intercept of the experimental reduced mobility value with the curve corresponding to the experimental κRh value (see the black dots in Figure 4A,B in the case of 6% MA and 7% AA nanolatexes). After the calculation of the ζ potential, it is then possible to plot the ζ = f(I) curves for all the nanolatexes (see Figure 5A,B). ζ potential values found for the carboxylated nanolatexes are in good agreement with values usually reported in the literature.9 It should be noted that an increase of the electrophoretic mobility with the ionic strength does not necessarily lead to an increase of ζ potential. 3.5. Determination of the Surface Charge Density σ at the Shear Plane. The surface charge density, σ, of a charged planar surface can be determined from the surface potential, ψ, using the Gouy-Chapman theory of a planar double layer in a 1:1 electrolyte solution:8 σ ¼
2ε0 εR kB Tk eψ sinh e 2kB T
ð10Þ
The surface potential is difficult to access experimentally. Practically, this surface potential is usually estimated from the electrokinetically relevant ζ potential, which occurs at the electrokinetic plane of shear next to the charged interface.8 For a symmetrical electrolyte, Loeb et al.10 have proposed the following relationship
Figure 6. Effect of the ionic strength on the surface charge density of St/ MA (A) and St/AA (B) latexes. Electrophoretic conditions are as in Figure 2. ζ values were converted into σ values using eq 11.
giving σ as a function of ζ for a spherical particle: 2ε0 εR kB Tk eζ 2 eζ sinh þ tanh σ ¼ e 2kB T kRh 4kB T
ð11Þ
The second term (in tanh) on the right-hand side of eq 11 is a firstorder correction for spherical particles. For a 1:1 electrolyte, Loeb et al.10 compared the results of the numerical treatment with those obtained with eq 11. For 0.5 < κRh < ¥, the maximum deviation is only 5% (independently of ζ values). ζ potentials of the nanolatexes were then converted into σ values using eq 11. Figure 6 presents the effect of the ionic strength on the surface charge density for St/MA (Figure 6A) and St/AA (Figure 6B) nanolatexes. As shown in Figure 6, the surface charge density monotonically increases with the ionic strength, whatever the nature (MA and AA) and percentage of the acid in the nanolatex. This behavior can be explained by the adsorption of surfactants at the surface of the nanolatexes. It is worth noting that the surfactants come from the synthetic process (see Table 1 for the chemical composition of the reaction medium), and they interact strongly with the nanolatex surface due to the weakly acidic conditions of polymerization. Once injected in the capillary, a certain quantity of surfactants can remain on the latex surface during the electrophoretic process. This can happen even at pH 9.2, depending on the electrophoretic conditions (ionic strength). Indeed, the adsorption of surfactants has been recognized as a major parameter in the control of the surface charge density of particles and colloids.11 The main adsorption mechanism onto the latex surface is controlled by a balance between the hydrophobic attraction (nonpolar tail of the surfactant 4044
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Figure 7. Schematic representation of the effect of MA content and ionic strength on the surface charge density of carboxylated nanolatex.
interacting with the hydrophobic regions of the latex surface) and the electrostatic repulsion of the latex surface and the anionic surfactant head (as for SDS).13,24 In this context, a decrease of the ionic strength can facilitate the desorption of the anionic surfactant from the latex surface, due to higher electrostatic repulsions. On the contrary, higher ionic strength tends to stabilize the adsorption of the surfactants onto the surface, maintaining a high surface charge density.31 Looking at the variation of the surface charge density with the acid content, Figure 6 clearly demonstrates that σ is a decreasing function of the acid content at high ionic strength (50 mM). This behavior is supported by a stronger anionic surfactant adsorption onto the latexes with the lowest acid contents due to higher hydrophobicity. On the contrary, at low ionic strength (5 mM), σ slightly increases with the acid content, since at this low ionic strength, there is very low surfactant adsorption and the surface charge density is directly related to the acid content. Figure 7 schematically depicts the surface charge of the latexes as a function of the ionic strength and the acid content. Electropherograms displayed in Figure 2 clearly indicate that the dispersion of the electrophoretic mobility distribution increases with the ionic strength and at lower acid contents. The main sources of dispersion in CE are the molecular diffusion, the size distribution and the charge density distribution. Since the dispersion in size of the nanolatexes is low (PDI ∼ 0.1) and the molecular diffusion is expected to be similar for all the nanolatexes, this suggests that the electrophoretic dispersion is correlated with the distribution in charge density. Since the largest dispersion was observed for the highest ionic strength and the lowest acid content, this can be explained by the higher
content of surfactants at the surface of the nanolatex in these nonequilibrium conditions that leads to higher electrophoretic mobility dispersion. A comparison of panels A and B in Figure 6 shows that, for the same acid content, the St/MA latex presents slightly higher surface charge density than the St/AA latex, whatever the ionic strength. For instance, the surface charge densities of St/MA 23% and St/AA 22%, at 50 mM ionic strength are, respectively, -5.09 and -3.75 μC cm-2. This is due to the higher hydrophilicity of AA as compared to MA,15 leading to lower surfactant adsorption onto the St/AA latex surface. 3.6. Effect of Neutral Surfactant (Brij 35) Adsorption on the Surface Charge Density. Figure 8A displays the effect of MA content on the electrophoretic mobility of St/MA nanolatexes for different neutral surfactant (Brij 35) concentrations in the electrolyte at 30 mM ionic strength. The critical micelle concentration (CMC) of Brij 35 is 0.09 mM.32 As shown in Figure 8A, the addition of neutral surfactant dramatically decreases the electrophoretic mobility, even for surfactant concentrations lower than the CMC, demonstrating that the free surfactant interacts strongly with the latex surface. For a better comparison, the electrophoretic mobility data were converted into surface charge density in Figure 8B. As discussed previously, in the absence of surfactant in the electrolyte, the surface charge density is a decreasing function of the MA content. Reversely, in the presence of neutral surfactant, the surface charge density increases with the MA content. This can be explained if one considers that the neutral surfactant has completely replaced the anionic surfactant. In these conditions, the surface charge density is directly related to the acid content (and not to the quantity of 4045
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understand the complex adsorption mechanism that controls the surface charge density, it has been necessary to change the electrophoretic mobility values into surface charge density via the calculation of ζ potential using O’Brien, White, and Ohshima modeling. This transformation from electrophoretic mobility to surface charge density is a prerequisite to discuss the influence of the ionic strength and the acid content since the electrophoretic mobility is a nonlinear and complex function of the hydrodynamic radius, the surface charge density, and the ionic strength. In this context, this work demonstrates that the surface charge density directly follows the acid content of the latex at low ionic strength (∼5 mM) for which the anionic surfactants are weakly adsorbed onto the latex surface. On the contrary, at higher ionic strength (50 mM), the surface charge density decreases with the acid content. In this case, the surface charge density is controlled by the quantity of anionic surfactant adsorbed onto the surface which decreases with the acid content because of the increasing hydrophilicity. In the presence of neutral surfactant at 50 mM ionic strength, the surface charge density drops because of the replacement of the anionic surfactant by the neutral surfactant. The surface charge density then becomes an increasing function of the acid content.
’ AUTHOR INFORMATION Corresponding Author
Figure 8. Effect of MA content on electrophoretic mobility (A) and surface charge density (B) of St/MA latexes in the presence of neutral surfactant (Brij 35). Electrolyte: 60 mM sodium borate buffer, pH 9.2, containing Brij 35 as indicated in the figure. Other conditions are as described in section 2.4.
anionic surfactant adsorbed onto the surface). Surface charge densities are also much lower in the presence of neutral surfactant, demonstrating that the neutral surfactant takes the place of the anionic surfactant on the latex surface. The replacement of SDS by neutral surfactant has been observed previously and is favored in our work by the electrophoretic separation and absence of SDS in the electrolyte. Porcel et al.24 have shown that when Triton X-100 (nonionic surfactant) is adsorbed on anionic latex first, the preadsorbed surfactant causes a “blocking” of the adsorption of SDS added later. However, when SDS is added first, it does not block the adsorption of Triton X-100; in addition, the nonionic surfactant replaces the ionic SDS on the surface of the anionic latex, and this replacement is enhanced in the case of anionic latex due to electrostatic repulsion between SDS and the latex surface.24 By using Triton X-405, the adsorption has been reported to be extremely strong, and the removal of SDS from the surface was observed by Bolze et al.27 using small-angle X-ray scattering (SAXS) and by Colombie et al.33 using filtration/colorimetry and proton NMR. Thus, in the presence of Brij 35, the surface charge density of carboxylated nanolatex increases linearly with the acid content since the anionic surfactant is desorbed from the nanolatex surface. In addition, the surface sulfate end-groups and the carboxylic acid groups are buried by the hydrophilic copolymer layer of nonionic surfactant, which contributes to the fall in surface charge density.
4. CONCLUSIONS In this work, CE has been used for the determination of the electrophoretic mobility distribution of nanolatexes of different acid contents and at different ionic strengths. To better
*Tel þ33 4 6714 3427. Fax: þ33 4 6763 1046. E-mail:
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’ REFERENCES (1) Radko, S.; Stastna, M.; Chrambach, A. Electrophoresis 2000, 21, 3583–3592. (2) Overbeek, J. T. G., Wiersema, P. H. Electrophoresis: Theory, Methods, and Applications; Academic Press: New York, 1967; pp 1-52. (3) O’Brien, R. W.; White, L. R. J. Chem. Soc. Faraday Trans. 1978, 77, 1607–1626. (4) Hunter, R. J. Zeta Potential in Colloid Science: Principles and Applications; Academic Press: London, 1981; pp 11-58, 98-123. (5) Kimura, K.; Takashima, S.; Ohshima, H. J. Phys. Chem. B 2002, 106, 7260–7266. (6) Pyell, U. Electrophoresis 2008, 29, 576–589. (7) Ohshima, H.; Furusawa, K. Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications; Marcel Dekker, Inc.: New York, 1998; pp 1-18. (8) Paruchuri, V. K.; Nguyen, A. V.; Miller, J. D. Colloids Surf., A 2004, 250, 519–526. (9) Vanhoenacker, G.; Goris, L.; Sandra, P. Electrophoresis 2001, 22, 2490–2494. (10) Loeb, A. L.; Overbeek, J. Th. G.; Wiersema, P. H. The Electrical Double-Layer around a Spherical Colloid Particle; MIT Press: Cambridge, MA, 1960. (11) Kandori, S. L.; Ishiguro, H.; Kon-no, K.; Kitahara, A. Langmuir 1989, 5, 1258–1261. (12) Davies, M. C.; Lynn, R. A. P.; Hearn, J.; Paul, A. J.; Vickerman, J. C.; Watts, J. F. Langmuir 1996, 12, 3866–3875. (13) Jodar-Reyes, A. B.; Ortega-Vinuesa, J. L.; Martín-Rodríguez, A. J. Colloid Interface Sci. 2005, 282, 439–447. (14) Shirahama, H.; Suzawa, T. Polym. J. 1984, 16, 795–803. (15) Polpanich, D.; Tangboriboonrat, P.; Elaïssari, A. Colloid Polym. Sci. 2005, 284, 183–191. (16) Stockes, J. C.; Jonhson, M. E. Microchem. J. 2004, 76, 121–129. (17) Petersen, S. L.; Ballou, N. E. J. Chromatogr. A 1999, 834, 445– 452. (18) Anik, N.; Airiau, M.; Labeau, M. P.; Bzducha, W.; Cottet, H. Langmuir 2009, 26, 1700–1706. 4046
dx.doi.org/10.1021/la1048562 |Langmuir 2011, 27, 4040–4047
Langmuir
ARTICLE
(19) Ducatte, G. R.; Ballou, N. E.; Quang, C.; Petersen, S. L. J. Microcolumn Sep. 1996, 8, 403–412. (20) Quang, C.; Petersen, S. L.; Ducatte, G. R.; Ballou, N. E. J. Chromatogr. A 1996, 732, 377–384. (21) Petersen, S. L.; Ballou, N. E. Anal. Chem. 1992, 64, 1676–1681. (22) Romero-Cano, M. S.; Martin-Rodriguez, A.; Chauveteau, G.; De las Nieves, F. J. Colloids Surf., A 1998, 140, 347–356. (23) Evers, M.; Garbow, N.; Hessinger, D.; Palberg, T. Phys. Rev. E 1998, 57, 6774–6784. lvarez, R.; (24) Porcel, R.; Jodar, A. B.; Cabrerizo, M. A.; Hidalgo-A Martín-Rodríguez, A. J. Colloid Interface Sci. 2001, 239, 568–576. (25) Fuerstenau, D. W. J. Phys. Chem. 1956, 60, 981–985. (26) Rembaum, A.; Yen, S. P. S.; Cheong, E.; Molday, R. S. J. Macromol. Sci., Chem. 1979, A13, 603–632. (27) Bolze, J.; Horner, K. D.; Ballauf, M. Colloid Polym. Sci. 1996, 274, 1099–1108. (28) Makino, K.; Ohshima, H. Langmuir 2010, 26, 18016–18019. (29) Ohshima, H. J. Colloid Interface Sci. 2001, 239, 587–590. (30) Ohshima, H. Colloids Surf., A, DOI: 10.1016/j.colsurfa.2010. 09.012 (31) Sefcik, J.; Verduyn, M.; Storti, G.; Morbidelli, M. Langmuir 2003, 19, 4778–4783. (32) Zheng, Z.; Obbard, J. P. Water Res. 2002, 3, 2667–2672. (33) Colombie, D.; Landfester, K.; David Sudol, E.; El-Aasser, M. S. Langmuir 2000, 16, 7905–7913.
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