Article pubs.acs.org/IECR
Impact of Monovalent and Divalent Cations on the Colloidal Stability of Negatively Charged Latex Particles Decorated with Poly(ethylene glycol) Larissa H. N. Nukui,† Leandro R. S. Barbosa,‡ and Denise F. S. Petri*,† †
Instituto de Química, Universidade de São Paulo, P.O. Box 26077, São Paulo, SP 05513-970, Brazil Instituto de Fı ́sica, Universidade de São Paulo, São Paulo, Brazil
‡
S Supporting Information *
ABSTRACT: The present work reports results from the interactions of monovalent (Na+, NH4+, and K+) and divalent (Ca2+ and Sr2+) cations, nitrate was the counterion, with negatively charged poly(methyl methacrylate) particles decorated with poly(ethylene glycol) (PMMA/PEG). Particle characterization was performed with dynamic light scattering, ζ-potential measurements, and scanning electron microscopy (SEM). Phase separation analyses were performed in the salt concentration range of 0.01 to 1.0 mol L−1. Among the monovalent cations at 1.0 mol L−1, the colloidal stability order was Na+ > K+ > NH4+ and only NH4+ bound specifically to PEG oligomers, causing the largest instability for concentrations higher than 0.75 mol L−1. In the case of divalent cations, three different situations were observed: (i) at 0.01 mol L−1 the dispersions were very stable, (ii) in the concentration range of 0.1 to 0.5 mol L−1 the particles aggregated due to charge screening, and (iii) at 1.0 mol L−1 the stability was larger than at the intermediate salt concentration because free divalent cations tend to chelate with PEG oligomers, building flocs. After phase separation all dispersions could be redispersed by manual shaking, recovering the original colloidal state. The colloidal behavior under experimental conditions of centrifugation, freeze/thaw cycle, and heating was investigated in the presence of 1.0 mol L−1 salt. The colloidal behavior of PMMA/PEG particles was discussed with basis on the high molecular density of PEG oligomers on the particle surface and specific ion binding.
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behavior is well reported in the literature.11−14 Lutter and coworkers 13 investigated the interactions of a series of monovalent (Li+, Na+, K+, Rb+, Cs+, and NH4+) and divalent (Mg2+, Ca2+, Sr2+, Ba2+, Co2+, Ni2+, Cu2+, and Cd2+) cations with a triblock copolymer, poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO−PPO− PEO), by determining the phase transition temperatures of the polymer in the presence of chloride salts. They proposed a model, where the cations might interact with polymer by three different ways: (i) direct binding to the oxygen in the polymer chains, (ii) sharing one water molecule with the polymer in their first hydration layer, and (iii) interacting with the polymer second hydration layer. In situations i and ii the polymer solubility in water increases because the charge density increases, whereas in situation iii the competition between polymer and cation for water molecules causes dehydration of polymer chains and phase separation, because the interaction between cation and water dipole is stronger than the H bonding between polymer and water dipole. Each situation depends mainly on the charge density, enthalpy of hydration, and polarizability of cations.15 For instance, in the presence of PEG, Ca2+ ions, which have high enthalpy of hydration, promote the fusion of anionic and neutral phospholipids.16,17
INTRODUCTION Most colloids are charged due to ionization of functional groups present on the surface or due to the emulsifiers used or due to the adsorption of charged species on the surface.1−3 Charges are important because they promote electrostatic repulsion among the particles, avoiding aggregation. Thus, the impact of ions dissolved in aqueous medium on the stability of colloidal dispersions can be dramatic, depending on their type and concentration.4 For instance, commercial oil in water (O/ W) emulsions are stabilized by a cocktail of surfactants, including those negatively charged (sulfate terminated, for instance) and uncharged (e.g., ethylene oxide polar group). However, if the emulsions are diluted with tap water (ionic strength ∼ 2.0 mmol L−1),5 the cations dissolved there tend to adsorb on the O/W interface, leading to phase separation.6,7 In drug delivery formulations the colloidal carriers should be stable at the physiological ionic strength of NaCl 0.15 mol L−1. In paint formulations, which should have a shelf life of at least 1 y, the ionic strength can reach ∼NaCl 2.5 mol L−1.8,9 The ionic strength of cleaners for modern acrylic paint films should be carefully evaluated to provide efficient film cleaning and to avoid film damaging.10 Poly(ethylene glycol) (PEG) is a water-soluble polymer with −(CH2−CH2−O)− as repetitive units. Usually when its molecular weight is larger than 10 000 g mol−1, it is renamed to poly(ethylene oxide) (PEO). PEG or PEO is widely used in biotechnological and biomedical applications, where generally ions of different types are present. The effect of specific interaction between ions and PEO or PEG on solution phase © 2015 American Chemical Society
Received: Revised: Accepted: Published: 606
October 30, 2015 December 21, 2015 December 31, 2015 December 31, 2015 DOI: 10.1021/acs.iecr.5b04103 Ind. Eng. Chem. Res. 2016, 55, 606−614
Article
Industrial & Engineering Chemistry Research
After 3 h the system was cooled to room temperature and dialyzed (dialysis membrane 14000 MW, Viskase Corporation, USA) against water with changes until the conductivity of dialysis water reached 5 μS cm−1. Particle Characterization. Particle characterization was performed with dialyzed dispersions. Dynamic light scattering (DLS) and ζ-potential measurements were performed in a commercial instrument Zetasizer NanoZS (Malvern, UK) for stock dispersions of PMMA/PEG diluted 100 times in Milli-Q water. A He−Ne laser was used as a light source with wavelength λ = 633 nm. Concerning the DLS experiments, the intensity of light scattered was recorded at an angle of 90° with an avalanche photodiode detector. The Zetasizer Software 6.2 (provided by Malvern) was used to determine the particle size distribution. In few words, the software uses the correlation function to obtain the distributions of the decay rates, and hence, the apparent diffusion coefficients. Finally, the distributions of the hydrodynamic radius of the scattering particles in solution are calculated via Stokes−Einstein equation,25 which is only valid for spherical particles. ζPotential measurements were conducted on sample-filled capillary cells and data were analyzed with the Helmholtz− Smoluchowski equation2,26 ξ = μη/ε, where μ is the electrophoretic mobility, η is the medium viscosity, and ε the medium dielectric constant. It is important to mention that this equation is valid for sufficiently thin double layers, or if the particle radius, r, is much larger than the Debye length, κ−1, i.e., κr ≫ 1, it does not require any specificity on the particle shape, only on the ionic cloud surrounding the particle. Scanning electron microscopy (SEM) analyses were performed to determine the morphology and mean diameter of dried particles using SEM-FEG JEOL 7401F equipment. The stock dispersions of PMMA/PEG were diluted 100 times in Milli-Q water; then droplets were deposited onto clean Si wafers. The water evaporated slowly at room temperature. The dried dispersions were coated with gold (2 nm) prior to the analyses. The mean diameter and diameter distribution were determined measuring over 400 dried particles without interaction with salts, using ImageJ free software. The solid content was determined by gravimetric analysis upon drying 1 mL dispersion. The conversion of monomer into polymer (polymerization yield) was calculated based on the monomer content in 1 mL of the initial emulsion (mI) and dried solid content in 1 mL of the final dispersion (mF). The relation mF/mI gives the polymerization yield. The mean particle number density (NP) was calculated considering the particle mean diameter obtained by DLS and the solid content, as described elsewhere.9,20 Colloidal Stability Tests. The stability of the PMMA/PEG dispersions in the presence of NaNO3, KNO3, NH4NO3, Ca(NO3)2, and Sr(NO3)2 was studied using a separation analyzer LUMiReader416-1 Separation Analyzer (L.U.M. GmbH, Germany). Only salts containing the same anion, namely NO3−, were used in order to investigate the effects of cation type on the colloidal stability. Dispersions and salt solution were mixed so that final NP was 1.4 × 1011 particles mL−1, and the measurements were carried out at (30 ± 2) °C. The phase separation behavior was monitored by the SEP View 4.01 software, which registered the normalized integral light transmission as a function of time. Cuvettes made of glass with optical path length of 10 mm and 50 mm long were used for the experiments. At the beginning, the dispersion is homogeneous and the transmitted light through the cuvette
The structure of negatively charged biomolecules is also affected by the presence of cations. The G-triplex human genomic DNA structure is stabilized by cations in the following sequence of binding affinity: Ca2+ > K+ > Mg2+ > Na+.18 The binding order of cations to carboxylate groups of elastin-like polypeptide was Zn2+ > Ca2+ > Ba2+ > Sr2+ > Mg2+ for divalent cations and NH4+ > Li+ > Na+ > NMe4+ > K+ > Rb+ ≥ Cs+ for monovalent cations.19 The general trend is that strongly hydrated cations bind more tightly to negatively charged surfaces than those weakly hydrated, and the hydration increases with the increase of charge/radius ratio. Negatively charged poly(methyl methacrylate) latex particles decorated with PEG oligomers, coded as PMMA/PEG, were successfully applied for the development of quick agglutination tests for dengue fever detection and for the immobilization of BSA.20 The stability of PMMA/PEG particles in the presence of anions F−, Cl−, Br−, NO3−, and SCN−, sodium as counterion, followed the same order as the Hofmeister series:21 (more stable) SCN− > NO3− > Br − > Cl− > F− (more instable), as reported in a previous work.22 The SCN− ions (high polarizability) adsorbed on the particle PEG shell and, consequently, increased particles charge density, whereas the high charge density of F− ions attracted water molecules to their own hydration layer, causing dehydration of the PEG layer and phase separation. In the present study, the influence of mono- (NH4+, K+, Na+) and divalent cations (Ca2+ and Sr2+) on the colloidal stability of PMMA/PEG dispersions was investigated by means of dynamic light scattering (DLS), ζpotential, scanning electron microscopy (SEM), and turbidity. Moreover, the influence of additional factors, such as centrifugal force, freeze/thaw cycle, and heating, in the presence of 1.0 mol L−1 salts was also evaluated. Noteworthy, nitrate was used as the anionic counterion throughout this study. One should bear in mind that several biomedical and technological applications involving PEG-terminated23 or negatively charged particles are in contact with aqueous medium containing ions, thus it is important to evaluate, systematically, their influence on the colloidal stability of PMMA/PEG particles.
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EXPERIMENTAL SECTION Materials. Methyl methacrylate (MMA; Fluka, Buchs, Switzerland), sodium persulfate (Na2S2O8; Merck Munich, Germany), and poly(ethylene glycol) sorbitan monolaurate (Tween-20, Sigma-Aldrich Co., Milwaukee, WI, USA) were used in the polymerization. Tween-20 molecule carries 20 ethylene glycol monomers distributed in the main structure and in four branches located at the headgroup, as schematically shown in the Supporting Information (SI1). Sodium nitrate (NaNO3), potassium nitrate (KNO3), ammonium nitrate (NH4NO3), calcium nitrate (Ca(NO3)2), and strontium nitrate (Sr(NO3)2), supplied by Labsynth Ltda, São Paulo, Brazil, were used in the colloidal stability tests. All reagents were used as received. Synthesis of PMMA/PEG Particles. The particles were synthesized following a typical emulsion polymerization recipe.24 Tween-20 concentration was 60 mg L−1 (4.8 × 10−5 mol L−1), which is close to Tween-20 critical micelle concentration (cmc). The medium was purged with N2 during the whole process, and the polymerization was carried out under reflux and mechanical stirring at 500 rpm. The temperature was set to (75 ± 2) °C, and then, 10 mL of MMA were added and the temperature was brought up to (80 ± 2) °C. Afterward, 1.0 g of Na2S2O8, the initiator, was added. 607
DOI: 10.1021/acs.iecr.5b04103 Ind. Eng. Chem. Res. 2016, 55, 606−614
Article
Industrial & Engineering Chemistry Research is very low. As time goes by, the dispersion starts to destabilize and fluctuations in concentration cause particles to nucleate and to aggregate, increasing the transmission of light through the liquid. The influence of NH4NO3, Ca(NO3)2, and Sr(NO3)2 concentration in the range of 0.01 to 1.0 mol L−1 on the colloidal stability was studied, as described in Table 1.
defrosting at room temperature for 14 h and redispersed using a vortex before the stability analyses. The freeze/ thaw cycle was made only once. (4) The samples were heated at 60 °C during 3 h and cooled in water bath at ∼20 °C during 20 min before the stability analyses. Statistical Analyses. All data values were expressed as mean values with the corresponding standard deviations. One way analysis of variance (ANOVA) with post hoc test was used to evaluate the differences of variables among groups. A value of p < 0.05 was considered a significant difference. Analyses were carried out in Excel 2013 for Windows (Microsoft Office Home and Student, 2013).
Table 1. Concentration of Salts in the Presence of PMMA/ PEG Dispersions (mol L−1) for Four Different Treatments for Colloidal Stability Analysesa (1)
(2)
(3)
(4) heating
salt
without treatment
centrifugal
freeze/ thaw
NaNO3 KNO3 NH4NO3
0.10, 0.50, 0.75, 1.0 0.10, 0.50, 0.75, 1.0 0.01, 0.1, 0.25, 0.50, 0.75, 1.0 0.01, 0.1, 0.25, 0.50, 0.75, 1.0 0.01, 0.1, 0.25, 0.50, 0.75, 1.0
1.0 1.0 1.0
1.0 1.0 1.0
1.0 1.0 1.0
1.0
1.0
1.0
1.0
1.0
1.0
Ca(NO3)2 Sr(NO3)2
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RESULTS Particle Characterization. Figure 1a shows a typical SEM image obtained for dried PMMA/PEG particles without any treatment, where approximately 400 spherical particles were used to estimate the mean diameter D as (665 ± 94) nm. The particle diameter distribution determined by DLS and SEM is presented in Figure 1b. The former indicated broader size distribution than the later, but the difference between Dh and D is below 10%. The DLS measurements performed for PMMA/ PEG particles evidenced a bimodal size distribution; the mean hydrodynamic diameter Dh of the larger population of particles was (623 ± 42) nm, while the smaller one (67 ± 8) nm. Similar effect was also observed for PEG decorated polystyrene particles27 and was explained by Ostwald ripening, where at the beginning of the emulsion polymerization there are large and small droplets of MMA dispersed in the aqueous medium. Considering that the solubility of MMA in water is 15 g L−1, the monomers in small droplets tend to migrate to large drops to minimize the chemical potential differences. For any curved surface, the smaller the radius of curvature (r), the greater the pressure difference (ΔP = Pinside − Poutside) between the inner side and external surface, in accordance to Laplace equation ΔP = 2γ/r, where γ is the interfacial tension between the aqueous and monomer phases. The negative ζ-potential value of −(65.1 ± 0.5) mV stems from Na2S2O8, the free radical initiator used for the polymerization. Dispersion solid content, polymerization yield, and NP values amounted to (28 ± 1) g L−1, (31 ± 1)%, and (2.4 ± 0.2) × 1011 particles mL−1, respectively.
The mean particle number density (NP) was fixed at 1.4 × 1011 particles mL−1. a
Additionally systematic analyses were performed combining the effects of salts at 1.0 mol L−1 with centrifugation, freeze/thaw, and heating processes (Table 1). A period of 4 h was arbitrarily taken for each measurement. Details of each treatment are listed in Table 1: (1) The stability tests were carried out immediately after mixing the dispersion and salt solution. After phase separation, the samples with salt at 1.0 mol L−1, still inside the cuvette, were shaken manually for a few seconds, obtaining a homogeneous dispersion, and the redispersed system was analyzed again. (2) Prior to the stability tests the dispersions were mixed with the salt solution, and the system was centrifuged at 3600 rpm, during 30 min, followed by redispersion using a vortex until obtaining a homogeneous dispersion. (3) Immediately after mixing the dispersion with salt solution, the system was put into a freezer, where it was maintained for 12 h at −18 °C. It was followed by
Figure 1. (a) Typical scanning electron micrograph obtained for PMMA/PEG particles after drying in the air. The scale bar stands for 1 μm. (b) Normalized frequency of the diameter distribution determined by SEM (dots pattern bars) and DLS (diagonal stripes pattern bars). 608
DOI: 10.1021/acs.iecr.5b04103 Ind. Eng. Chem. Res. 2016, 55, 606−614
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Industrial & Engineering Chemistry Research
Figure 2. (a) Normalized integral light transmission determined as a function of time at (30 ± 2) °C for aqueous PMMA/PEG dispersions (NP = 1.4 × 1011 particles mL−1) in the presence of monovalent salts NH4NO3, KNO3, and NaNO3 (1.0 mol L−1) and respective redispersions, coded as red. (b) Photograph of PMMA/PEG dispersions (Np = 1.4 × 1011 particles mL−1) in the presence of NH4+ at 1.0 mol L−1 after the first run. The transmittance profiles represent mean values of duplicates for each system; the corresponding standard deviations (SD) were NH4+ > K+. The D, Dh, and PDI values found for redispersed particles were statistically similar (p > 0.05) to the original values, except for the PDI values determined for redispersed particles after interaction with KNO3, which increased from 0.06 ± 0.01 to 0.15 ± 0.04. After the first run the dispersions containing monovalent ions showed the typical feature shown in Figure 2b. High concentration of particles compactly deposited in the bottom of the cuvette was observed, due to the destabilization of the colloidal system. The centrifugal treatment did not cause any significant effect on the colloidal behavior compared to the ones observed heretofore. The redispersion after centrifugation promoted the return to the initial situation, and there was no considerable difference in results between the runs in which there was no previous treatment of the dispersion (Supporting Information Figure SI2). One hypothesis was that the centrifugal force drove the particles to the bottom of the tube, but probably the electrolytes remained mainly in the supernatant so that the screening effects on the particle surface were minimized. In order to investigate about the hypothesis, the ionic conductivity values were measured for the salts solutions at 1.0 mol L−1 before adding to the colloidal dispersion and compared to the ionic conductivity values of the corresponding supernatants (after centrifugation). The ionic conductivity values determined for the supernatants were slightly larger (3 ± 1%) than those determined for the corresponding salt solutions. The differences were very small, indicating that the centrifugal force caused efficient separation between polymer particles and electrolytes to the supernatant, as suggested. Regardless of the salt type freeze/thaw cycle did not cause significant changes in the colloidal behavior with respect to the first run, as shown in Supporting Information Figure SI3. Contrarily, the heating treatment (Supporting Information Figure SI3) induced a significant stability increase. Particularly in the presence of NH4NO3, after 4 h the integral transmittance was only 40% of the value corresponding for the first run. One possible explanation for this effect is a partial loss of NH4+ ions by evaporation of NH3. In the case of KNO3 and NaNO3, after 4 h the integral transmittance was approximately 70% of the
Colloidal stabIlity in the Presence of Monovalent Cations. Dialyzed PMMA/PEG dispersions (NP = 1.4 × 1011 particles mL−1) are stable over at least 6 months under room conditions, i.e., (23 ± 2) °C and 1 atm. The high stability of the dispersion occurs by electrostatic and steric repulsion, arising from the high ζ-potential value and the presence of PEG on the particle surface, respectively. The normalized integral light transmission as a function of time determined for PMMA/PEG particles in the presence of monovalent salts, NH4NO3, KNO3, and NaNO3 at 1.0 mol L−1, and their redispersions, are presented in Figure 2a. Regardless of the salt type, the aggregation observed at the first run was reversible, indicating particles flocculation, since simple manual shaking allowed the redispersion and retrieval of homogeneous systems. However, the curves presented in Figure 2 indicate that the kinetics of destabilization depends on the type of salt used. Colloidal stability of particles (first run and redispersion) in the presence of salts followed the order: NH4NO3 < KNO3 < NaNO3, according to the Hofmeister series. After the first run the mean ζ-potential value and size of the redispersed particles were determined, as shown in Table 2. The ζ-potential values of the particles decreased in modulus after interacting with the salts Table 2. Mean ζ-Potential, Hydrodynamic Diameter (Dh), and Polydispersity Index (PDI) Values Calculated from DLS Data, Diameter (D) Determined by SEM, of PMMA/PEG Particles before and after Interaction with Different Nitrate Salts ζ-potential (mV)
D (nm)
Dh (nm)
−65.1 ± 0.5
665 ± 94
+
−63.6 ± 0.7
682 ± 181
67 ± 8 623 ± 42 655 ± 40
0.15 ± 0.04
+
−49.1 ± 0.4
674 ± 67
681 ± 37
0.076 ± 0.001
+
−41.5 ± 0.3
647 ± 76
652 ± 12
0.05 ± 0.01
+
−47.2 ± 0.2
613 ± 99
661 ± 19
0.07 ± 0.04
+
−40.9 ± 0.2
691 ± 159
664 ± 19
0.05 ± 0.04
PMMA/PEG PMMA/PEG KNO3 PMMA/PEG NH4NO3 PMMA/PEG NaNO3 PMMA/PEG Ca(NO3)2 PMMA/PEG Sr(NO3)2
PDI 0.06 ± 0.01
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DOI: 10.1021/acs.iecr.5b04103 Ind. Eng. Chem. Res. 2016, 55, 606−614
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Figure 3. Normalized integral light transmission determined as a function of time at (30 ± 2) °C for aqueous PMMA/PEG dispersions (NP = 1.4 × 1011 particles mL−1) in the presence of (a) Ca(NO3)2 and (b) Sr(NO3)2 (1.0 mol L−1) and respective treatments of centrifugation (a and b), freeze/ thaw cycle (c and d), and heating (e and f). The transmittance profiles represent mean values of duplicates for each system; the corresponding standard deviations (SD) were Ba2+, indicating that the affinity
Table 3. Hydrate Ionic Radii28 and Hydration Enthalpy (ΔHhyd)29 Reported for NH4+, K+, Na+, Ca2+, and Sr2+ Cations Cations (NO3− as Counterion) hydrated radius (Å) cation ΔHhyd (kJ mol−1)
Na+
K+
NH4+
Ca2+
Sr2+
3.58 −416
3.30 −334
3.31 −329
4.14 −1602
4.14 −1470
data for the hydrate ionic radii28 and cation hydration enthalpy (ΔHhyd).29 Colloidal stability of PMMA/PEG particles (first run and redispersion) in the presence of monovalent cations at 1.0 mol L−1 followed the order Na+ > K+ > NH4+ (Figure 2) and the decrease of ζ-potential values in modulus (Δζ) was Na+ > NH4+ > K+ (Table 2). These trends indicated that the interaction between Na+ and particle surface was the strongest 612
DOI: 10.1021/acs.iecr.5b04103 Ind. Eng. Chem. Res. 2016, 55, 606−614
Article
Industrial & Engineering Chemistry Research between alginate, a polyanion containing many hydroxyl groups, was Ba2+ > Sr2+ > Ca2+.30 The colloidal stability as a function of Ca2+ or Sr2+ concentration (Figure 5) revealed a curious behavior. At lower concentrations of Ca2+ or Sr2+ (0.01 mol L−1) the formation of gelatinous flocs did not take place and the colloidal systems were very stable due to electrosteric repulsion (Figure 6a). In the concentration range of 0.10 to 0.50 mol L−1, Ca2+ and Sr2+ ions caused the largest instability (Figures 5 and 6b), probably due to charge screening and insufficient amount of free divalent cations to chelate with PEG oligomers. Upon increasing the salt concentration to 1.0 mol L−1 the PMMA/ PEG dispersion presented higher stability than at intermediate salt concentration. A similar effect was observed by Berg and co-workers31 for polystyrene latex with sulfate terminal groups coated PEO−PPO−PEO copolymers, where the length of PEO exposed to the electrolytic medium varied from 22 to 48 repetitive units. In that work,31 the dispersions were 100 times more diluted than the dispersions reported in the present work, but “restabilization” was observed (i) in the presence of Ba2+, but not in the presence of Ca2+, for PEO chains with 22 or 26 monomers, but for long PEO chains (48 monomers) no phase separation could be observed, and (ii) in the presence of Li+, Na+, K+, and Ba2+ for PEO chains with 26 monomers; by increasing the PEO length, the length of hydrophobic PPO block also increased, making the distance between two PEO blocks larger. In the present study the PEG oligomers exposed to the electrolytic medium have in average five monomers and there are no hydrophobic PPO blocks because the PEG oligomers on PMMA/PEG particles stem from the uncharged surfactant (Tween-20) used in the emulsion polymerization; the “restabilization” was only observed for Ca2+ or Sr2+, but not for Na+, K+, or NH4+. Maybe not only the PEG length, but also the PEG chain density on the surface of the particles plays an important role on the specific interaction with cations and on the “restabilization” effect. The PEG chain density on PMMA/ PEG particles was estimated considering that all surfactant molecules used in the polymerization are on the particles surfaces. Therefore, the mean number of Tween-20 molecules per particle is ∼3 × 105 [= (4.8 × 10−5)(6 × 1023)/1.4 × 1014]. The mean particle area calculated from D = 665 nm (Table 2) is ∼1.4 × 106 nm2. Thus, the mean molecular density of Tween-20 at each polymer particle can be estimated as 3 × 105/1.4 × 106 nm2, which yields ∼0.21 molecules nm−2. The estimate indicates that PEG chains on PMMA/PEG are densely packed on the particle surface; for the sake of comparison, in the case of protein adsorption, the PEG grafting density of 0.1 molecule nm−2 is the minimum required to repel proteins.32 Although the density of PEO blocks on the PS particles was not provided in ref 31, this parameter cannot be neglected. The high molecular density of PEG oligomers on the surface provides a huge number of chelating sites for Ca2+ or Sr2+, and for the building of gelatinous flocs.
freeze/thaw cycle had no additional effect on the already observed stability in the presence of monovalent cations, whereas heating destabilized the dispersions to a smaller extent. This is relevant particularly for practical applications, where the formulations experience drastic temperature changes during storing or processing. Divalent Ca2+ and Sr2+ cations in the concentration range of 0.1 and 0.5 mol L−1 caused phase separation mainly due to charge screening. However, upon increasing the cations concentration the specific binding to PEG layer led to the formation of gelatinous flocs, which was more favored in the presence of Sr2+ and high density of PEG oligomers on the particle surfaces. One important remark is that all situations tested here covered a broad range of ionic strength and after phase separation all dispersions could be redispersed achieving in most cases the original colloidal state. The reversibility is achieved because the molecular density of PEG oligomers is high, providing many sites for specific ion binding, especially for divalent cations, and building flocs, which can be easily perturbed by manual shaking. Thus, the results presented here demonstrated that bringing together charged species and densely packed PEG oligomers on the particles surfaces is a strategy to provide high colloidal stability.
CONCLUSIONS Aqueous dispersions of PMMA/PEG particles are very stable colloidal systems. The concentration of monovalent cations required to destabilize the dispersions is relative high, 1.0 mol L−1 of KNO3 or NaNO3 and >0.75 mol L−1 for NH4NO3. Particularly NH4+ ions were more efficient to destabilize the colloidal dispersions due to specific association to PEG oligomers on the particles surface, causing interparticle aggregation. External parameters such as centrifugation and
Corresponding Author
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04103. Schematic representation of Tween-20 molecule (SI1); normalized integral light transmission determined as a function of time at (30 ± 2) °C for aqueous PMMA/ PEG dispersions (NP = 1.4 × 1011 particles mL−1) in the presence of (a) NH4NO3 (1.0 mol L−1), (b) KNO3 (1.0 mol L−1), and (c) NaNO3 (1.0 mol L−1), first run (black) and after centrifugation at 3600 rpm, during 30 min, followed by redispersion using a vortex (red) (SI2); normalized integral light transmission determined as a function of time at (30 ± 2) °C for aqueous PMMA/ PEG dispersions (NP = 1.4 × 1011 particles mL−1) in the presence of (a) NH4NO3 (1.0 mol L−1), (b) KNO3 (1.0 mol L−1), and (c) NaNO3 (1.0 mol L−1), first run (black), after redispersion (red), freezing (blue), and heating (green) (SI3); normalized integral light transmission determined as a function of time at (30 ± 2) °C for aqueous PMMA/PEG dispersions (NP = 1.4 × 1011 particles mL−1) in the presence of NH4NO3 at crescent concentrations (0.01, 0.10, 0.25, 0.50, 0.75, and 1.0 mol L−1) (SI4); and photographs taken for aqueous PMMA/ PEG dispersions (NP = 1.4 × 1011 particles mL−1) after a 4 h stability test in the presence of divalent Ca2+ and Sr2+ ions (SI5) (PDF)
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AUTHOR INFORMATION
*E-mail:
[email protected]. Tel.: 0055-11-30913831. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by research grants from Brazilian funding agencies São Paulo Research Foundation (FAPESP) Grants 2010/51219-4 and 2012/25096-8 and National Council 613
DOI: 10.1021/acs.iecr.5b04103 Ind. Eng. Chem. Res. 2016, 55, 606−614
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Industrial & Engineering Chemistry Research
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for Scientific and Technological Development (CNPq) Grants 305178/2013-0, 448497/2014-0, and 303048/2012-3.
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DOI: 10.1021/acs.iecr.5b04103 Ind. Eng. Chem. Res. 2016, 55, 606−614
