Charging Mechanism for Polymer Particles in Nonpolar Surfactant

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Charging Mechanism for Polymer Particles in Nonpolar Surfactant Solutions: Influence of Polymer Type and Surface Functionality Joohyung Lee, Zhang-Lin Zhou, and Sven Holger Behrens Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00583 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 4, 2016

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Charging Mechanism for Polymer Particles in Nonpolar Surfactant Solutions: Influence of Polymer Type and Surface Functionality Joohyung Lee1, Zhang-Lin Zhou2 and Sven Holger Behrens1,* 1

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, GA 30332, USA 2

HP Inc., 16399 W Bernardo Drive, San Diego, CA 92127, USA

ABSTRACT Surface charging phenomena in nonpolar dispersions are exploited in a wide range of industrial applications, but their mechanistic understanding lags far behind. We investigate the surface charging of a variety of polymer particles with different surface functionality in alkane solutions of a custom-synthesized and purified polyisobutylene succinimide (PIBS) polyamine surfactant and a related commercial surfactant mixture commonly used to control particle charge. We find that the observed electrophoretic particle mobility cannot be explained exclusively by donor-acceptor interactions between surface functional groups and surfactant polar moieties. Our results instead suggest an interplay of multiple charging pathways, which likely include the competitive adsorption of ions generated among inverse micelles in the solution bulk. We discuss possible factors affecting the competitive adsorption of micellar ions, such as the 1

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chemical nature of the particle bulk material and the size asymmetry between inverse micelles of opposite charge. Corresponding Author: *Sven H. Behrens ([email protected]) Keywords: Colloids, Nonpolar dispersions, Surfactants, Inverse micelles, Surface charging, Electrophoresis, PALS, QCM, Acid-base, Donor-acceptor, Adhesions, PMMA, PS, PIBS.

1. INTRODUCTION Surface charging of colloidal particles in nonpolar media is important in various industrial applications such as the development of electrophoretic inks1-3 for electronic book readers and oil-based printing toners4-5, or the stabilization of asphaltene in crude oil processing.6-7 The generation of electric charge in nonpolar media such as saturated hydrocarbons (dielectric permittivity ߝ௥ ~2), although energetically disfavored,8 can be mediated by oil-borne surfactants.9-10 It is widely believed that inverse micelles formed by surfactants (both ionic and nonionic species) create locally high dielectric environments in the micelle cores, where charges can reside with the energy cost for their ionization effectively reduced, and these micellar ions are credited for raising the electric conductivity of nonpolar liquids by several orders of magnitude.9-15 However, a mechanistic understanding of particle charging mediated by surfactants (or charged inverse micelles) has not yet been achieved, despite a number of coexisting hypotheses. 2


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According to the emerging notion of an “acid-base charging mechanism”,16-22 particle charging may be attributed to polar interactions between acidic/basic (donor/acceptor) moieties of particle surface functional groups and surfactant headgroups. The mechanism hypothesizes that surfactants, when adsorbing to the particle surfaces, form acid-base adducts with surface functional groups (“head-down” orientation of surfactants at the interface is assumed);22 and as surfactants desorb from the particles, some fraction of acid-base adducts can heterolyze with a charge (proton or electron) transferred between two species depending on the species’ relative acid and base strength. As a result, some net charge remains on the surfaces, while oppositely charged surfactants are incorporated into the inverse micelles acting as counterions in the liquid bulk. It should however be noted that this hypothesis has been largely speculative.10,23-25 For example, if the sign of surface charge was measured to be negative in solutions of a certain surfactant, it was assumed that the surface might have been more acidic than the surfactants, so it could donate protons to surfactants (or accept electrons from surfactants). Experimental support for the notion of direct charge transfer between surfaces and surfactants has been limited partly due to the difficulty of characterizing the acid/base strength of surfactants on a scale consistent with any acidity/basicity metric for particle surfaces.24 Moreover, many studies have shown instances of particle charging that cannot readily be described by the acid-base mechanism.911,13,23,25-30

Surface charging phenomena not explained by a simple acid/base strength balance

between surface functionality and surfactant head-group moieties were tentatively attributed to some asymmetric adsorption of charged species.11,26,28-29 Some studies directly showed that in the presence of particles, electric conductivity caused by charged inverse micelles in the liquid bulk dropped significantly, possibly due to the adsorption of charged species to particle surfaces.13,27 3


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While the cause of the hypothesized adsorption asymmetry between oppositely charged ions (responsible for the observed net surface charge) is unknown, we have recently proposed that a systematic size asymmetry between micelle cations and anions may be an important factor, as well as selective ion-surface interactions associated with the chemistry of the interacting species.25 In an earlier study employing ionic and nonionic commercial surfactants we also showed that surface charging behavior of a series of well-defined colloidal particles deviated systematically from expectations based on the traditional acid-base charging mechanism and pointed at an interplay of multiple charging mechanisms.23 In the present study, we present new experimental support for the coexistence of multiple surface charging pathways in nonpolar dispersions. We characterize the charging behavior of a series of polymer particles under systematic variations of the particle surface properties, polymethyl methacrylate particles with sulfate functional groups (PMMA-sulfate), polystyrene particles with sulfate groups (PS-sulfate), carboxyl groups (PS-carboxyl) and amidine groups (PS-amidine). We show that the electrophoretic mobility of these particles in nonpolar dispersions containing a purified derivative of a widely employed charge control surfactant, polyisobutylene succinimide polyamine (PIBS),9-10,12,15-18,20,22,24-25,30-32 is not correlated with the acid/base character of their surface functional groups, but with the type of particle bulk material. Using a quartz crystal microbalance, we show that the actual surfactant adsorption to the respective polymer surfaces is also more closely correlated with the type of the bulk material than with the specific surface functional groups. By characterizing the charging behavior of the same particles in dispersions containing a commercial PIBS surfactant as a control experiment, we show that the acidic/basic surface functional groups might only contribute to overall particle charging if the system contains a large amount of moisture. Finally, we propose that the net 4


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particle charge can be fully explained by accounting also for asymmetric adsorption of micellar ions with different sign of charge from the solution phase. 2. MATERIALS AND METHODS 2.1. Custom and Commercial Surfactants The custom surfactant PIBS-N (Scheme 1) is an example of PIBS derivatives, consisting of a polyamine headgroup and polyisobutylene (PIB) tails with a succinimide group as linker.2425

The synthesis of PIBS-N was achieved by coupling a polyisobutylene succinic anhydride

(PIBSA) and polyamine N,N-diethyldiethylenetriamine.24-25,31-32 The polyamine with either end group alkyl-substituted was chosen in order to obtain only a mono-PIB-tail species from reactive sites and avoid the generation of di-PIB-tail species. A commercial PIBSA, OLOA15500 (Mw ~ 1000 g/mol), was obtained from Chevron Oronite and N,N-diethyldiethylenetriamine (98%, Mw ~ 159.3 g/mol) was purchased from Sigma Aldrich. An equimolar mixture of these two materials was dissolved in m-xylene (> 99%, Sigma Aldrich) and refluxed at 190 ℃ for 20 hours in order to couple the two compartments via condensation reaction. After completing the reaction, the solvent was removed via distillation at 200 ℃. The desired product was isolated by silica column chromatography. A stationary phase silica gel (pore size 60 Å, 70 – 230 mesh, 63 – 200 μm, Sigma Aldrich) was preliminarily wetted with a mixture of 20:1 hexane (> 98.5%, VWR) and trimethylamine (> 99.5%, Sigma Aldrich) to prevent irreversible binding of the PIBS-N molecules. The reaction products were then loaded into the column and flashed with a mixture of 20:1 chloroform (> 99.8%, VWR) and ethanol anhydrous (> 99.5%, Sigma Aldrich). The desired product PIBS-N was collected separately from low polarity impurities including unreacted PIB. The solvent mixture was removed in a rotary evaporator at 45 ℃ with a vacuum pressure 160 5


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mmHg for 3 hours. The purified product was dissolved in a nonpolar solvent hexane (> 98.5%, ߝ௥ =1.89, VWR) and passed through an inorganic membrane filter (pore size 0.02 μm, Whatman® Anotop® 10 syringe filter). Dynamic light scattering (using a Malvern Zetasizer Nano ZS90 at 13° scattering angle) revealed a roughly constant micelle size around 4.5 nm in hexane at the surfactant concentrations used in this study. For comparison with our purified PIBS-N surfactant we also employed a commercial charging agent product, OLOA11000. This proprietary material is known to be a mixture of several PIBS derivatives and mineral oils but the exact chemical composition is poorly defined.9,32 The material was received from Chevron Oronite and used without further purification.

Scheme 1. Chemical structure of the custom surfactant PIBS-N, a derivative of commercial charging agent surfactants polyisobutylene succinimide polyamines.

2.2. Nonpolar Dispersions We employed a series of polymer particles under systematic variations of surface properties. Polymethyl methacrylate particles with sulfate functional groups were purchased from Bangs Laboratories (PMMA-sulfate, 1.1 μm, catalog #PP04N), and polystyrene particles with sulfate groups (PS-sulfate, 1.0 μm, catalog #C37498), carboxyl groups (PS-carboxyl, 1.0 6


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μ m, catalog #C37274) and amidine groups (PS-amidine, 1.0 μ m, catalog #A37322) were purchased from Life Technologies. Particles were originally received as surfactant-free aqueous suspensions, transferred into isopropanol (> 99.5%, Sigma Aldrich) as an intermediate solvent, and finally into hexane based surfactant solutions at a minimal surfactant concentration (0.5 mM). In each transfer step, particles were washed three times via centrifugation, disposal of the supernatant, and redispersion of particles in the target solvent. 2.3. Electrophoretic Mobility Measurements We employed phase analysis light scattering (PALS),33 using a Zetasizer Nano ZS90 (Malvern Instruments), to measure the particles’ electrophoretic mobility in nonpolar dispersions as an indicator of particle surface charging. A dispersion sample (diluted to ~30 ppm to avoid multiple scattering) was loaded into a glass cuvette, and a dip cell with two planar palladium electrodes spaced by 2 mm was submerged in the sample. An AC electric field was applied across the electrodes under systematic variations of field strength (2.5 to 50 kV/m) and the field dependent electrophoretic mobility34-37 of particles was measured based on phase information of light scattered by the particles (Supporting Information). To infer the particles’ equilibrium charging state in the absence of external electric fields, we extrapolate the field dependent mobility to zero field strength (zero-field mobility), following a widely adopted strategy from past studies.21-22,25,30,37-38 Prior to the measurements, the glass cuvette and dip cell were sonicated in tetrahydrofuran, wiped in a hot aqueous detergent solution, rinsed with methanol and dried with air.

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2.4. Quartz Crystal Microbalance We compare the relative surfactant adsorption to different polymer surfaces using a quartz crystal microbalance (QCM)39 (Q-Sense E4 system, Biolin Scientific). Prior to preparing polymer surfaces, crystals plated with a bare gold electrode (purchased from Biolin Scientific) were thoroughly cleaned by being treated in an UV/ozone system for 10 min, soaked in a 5:1:1 mixture of DI water, ammonia (25%, Merck) and hydrogen peroxide (30%, Merck) at an elevated temperature (75℃) for 10 min, rinsed with DI water, dried with nitrogen gas, and cleaned in the UV/ozone system for 10 min. To prepare the polymer surfaces, the polymer particles were washed with isopropanol and dried in an oven at 90℃ for 12 hrs. The cleaned bare crystals were spin-coated with 1 wt. % chloroform solutions of the dried polymers. The spincoated crystal was dried in the oven at 80 ℃ for 30 min and cooled at room temperature. In performing QCM, the resonance frequency of polymer-coated crystal was initially obtained under alternating electric fields without any additional mass coupled to the surface. The surface was then flushed with pure hexane (background solvent), at least for 30 min, to obtain a stable baseline, and once the baseline was achieved, the surface was exposed to a parallel flow of the surfactant solution with a rate of 200 μL/min. The resonance frequency shift (∆f), which is directly proportional to the change in mass coupled to the surface,39 was monitored for 45 min to infer the relative surfactant adsorption to polymer surfaces. The surface was finally flushed again with pure hexane, and the frequency shift caused by surfactant desorption was monitored for 30 min.

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2.5. Electric Conductivity Measurements We measured the electric conductivity of nonpolar solutions of surfactant PIBS-N using the nonaqueous conductivity probe DT-700 (Dispersion Technology, Inc.). Prior to the measurements, the probe was washed with THF, wiped in a hot aqueous detergent solution, rinsed with acetone and methanol, dried with air, stabilized in a fume hood overnight, and finally flushed with pure hexane. During the measurements, coaxial cylindrical electrodes are immersed in solution samples, and a low frequency (1 Hz) AC field is applied between the electrodes. The specific conductivity is obtained from the measured electric current and cell constant for the electrode geometry. A conductivity in the order of 0.1 pS/m was found for pure hexane, and is consistent with earlier findings.14 2.6. Karl Fischer Titration We measured the residual water content of nonpolar surfactant solutions and particle dispersions using volumetric Karl Fischer titration as implemented by the commercial TitroLine KF titrator (SCHOTT). We used HYDRANAL®-Composite 5 as the titration reagent and a 1:1 mixture of chloroform and methanol as a titration medium. A water content around 30 ppm (in wt.) was found for pure hexane, consistent with the literature.11,14 We found no significant difference in water content between surfactant solutions and particle dispersion samples used for electrophoresis within the instrumental sensitivity.

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3. RESULTS AND DISCUSSION 3.1. Surface Charging Correlated with the Type of Particle Bulk Material We characterized the surface charging behavior of the polymer particles in nonpolar solutions of the surfactant PIBS-N, by measuring the particles’ electrophoretic mobility (Figure 1a). Even though aqueous charging of the same particles followed the acid/base character of their surface functional groups (Figure 1b),23 we found no evidence that such functionality played a similar role for particle charging in the nonpolar medium. For example, the PS-amidine particles, which are positively charged in water in a wide pH range (isoelectric point above 9) due to the basic character of amidine functional groups, are negatively charged in the nonpolar surfactant solution, just like PS-carboxyl and PS-sulfate particles, which owe their negative charge in water (isoelectric point below 3) to the acidic character of the functional groups. This is in stark contrast to the traditionally proposed acid-base particle charging mechanism,16-22 whereby particle charging is attributed to the direct interaction of acidic/basic moieties of surfactants with the particles’ acidic/basic surface functional groups. In the framework of the acid-base mechanism, the variation of particle charging in nonpolar media with the acidic/basic surface functional groups should correlate with the particles’ charging behavior in water, since the aqueous charging behavior is a consequence of acid/base behavior of the functional groups. For oxide particles, such a correlation has indeed been found,19-21 but for the present polymer particles, the charging in aqueous solution and nonpolar solution are qualitatively different.

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Figure 1. (a) The zero-field electrophoretic mobility of polymer particles in hexane solutions of the surfactant PIBS-N, represented as a function of surfactant concentration. (b) The electrophoretic mobility of the same particles in aqueous NaCl solutions, represented as a function of NaCl concentration (reprinted from ref. 23 with permission).

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We point out, however, that the relative charging behavior in the nonpolar solutions instead appears correlated with the type of particle bulk material, i.e. the chemical identity of the primary polymer phase, PMMA and PS, rather than the specific functional groups; all PS particles, regardless of their functionality, carry a higher negative charge than the PMMA particles. The obvious question is whether this surface charging could be a consequence of direct acid-base (donor-acceptor) interactions of surfactants with the particle bulk polymer species. In a recent review22 Gacek and Berg invoked Pearson’s “hard and soft acid–base” (HSAB) principle40 and proposed that surfactants as “soft” acids or bases, may undergo donor-acceptor interaction selectively with the “soft” bases or acids presented by the polymers, not with “hard” surface functional groups. If this were true, the observed correlation with polymer type (Fig. 1a) would suggest that PS surfaces are stronger (soft) acids than PMMA surfaces and thus form stronger donor-acceptor adducts with surfactants as (soft) bases. This conclusion, however, would be at odds with the conventional wisdom about these materials: PS is typically considered mostly nonpolar (i.e. neither significantly acidic nor basic),41-42 and to the extent that any polarity is assigned, it is usually considered basic rather than acidic because of the electron donor capacity associated with the π electrons in the aromatic ring.43-47 For further experimental clues, we compare the relative adsorption affinity of oil-borne surfactants to polymer surfaces, which is, in nonpolar media, related to the relative strength of polar (acid-base or donor-acceptor) interaction between the surfactants and surfaces.9,22,24 We employed a quartz crystal microbalance (QCM) to measure directly the mass deposition of surfactants to surfaces while surfactant solutions passed over macroscopic polymer films spincast onto the quartz crystal. As the surfaces were exposed to the parallel flow of surfactant solutions, a significantly larger decrease in the resonance frequency (∆f) was found for PMMA 12


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than PS (Figure 2). Since the decrease in ∆f is directly proportional to the increase in mass coupled to the surface,39 the result indicates that more surfactant adsorbed to PMMA than to PS. Moreover, a significantly larger amount of surfactants remained on the PMMA when the surfaces were flushed subsequently with pure hexane, indicating that surfactants formed stronger adducts with PMMA than with PS. In the framework of acid-base mechanism, therefore, the propensity of charge transfer (or charge heterolysis)48 from donor-acceptor adducts should also be, in principle, more significant for PMMA than PS. Clearly, this is inconsistent with the relative surface charging behavior of these particles shown in Figure 1a, where the apparent surface charging of PS was significantly larger than PMMA. Therefore, the PS particles’ stronger negative charge cannot be simply attributed to the stronger adduct formation by a stronger “soft acid”49 (PS) with the surfactants.

Figure 2. The shift in resonance frequency (∆f) of quartz crystal microbalances coated with polymer surfaces, in response to the surfactant adsorption (mass increase) to the surfaces.

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We propose that the above seen physical phenomena in nonpolar media, both the relative surfactant adsorption to polymer surfaces (Figure 2) and the relative surface charging (Figure 1a), can be explained by the Lewis basicity (electron donicity) of the particle bulk material.25,43-47,50 The surfaces of PMMA are known to have significant electron donicity, originating from the lone electron pairs of carbonyl oxygen, and the donicity strength is often characterized to be stronger than that of PS from π electrons of the phenyl ring.23-24,43,45,47 From this perspective, it should be PMMA surfaces rather than PS that form stronger donor-acceptor adducts with surfactants (consistent with Figure 2) if surfactants have amphoteric (acidic and basic) character, specifically as the basic (electron donor) sites of solid surfaces interact with acidic (electron acceptor) sites of surfactants. As a consequence, the propensity of positive surface charging, by heterolysis of donor-acceptor adducts, should be higher for PMMA. Thus, the direct adduct formation of surface basic sites with surfactant acidic sites (and their subsequent heterolysis) could explain at least why PMMA is found to charge less negatively (= more positively) than PS (Note, however, that it does not explain why all particles acquired a net negative charge, which will be discussed in the following subsection). An example of representing the relative Lewis basicity of polymeric surfaces in a quantitative way can be found in the theory of van Oss, Chaudhury, and Good (vOCG).23-25,45-47 This approach represents the surface tension of condensed phase i as a sum of apolar and polar (acid/base) contributions, i.e. ߛ௜ = ߛ௜௅ௐ + 2(ߛ௜ା )ଵ/ଶ (ߛ௜ି )ଵ/ଶ , where ߛ௜௅ௐ is an apolar Lifshitz – van der Waals component, ߛ௜ା is an acid component, and ߛ௜ି is a base component. According to the theory, one can characterize the surface energy components of a solid surface (S), ߛௌ௅ௐ , ߛௌା ,

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and ߛௌି , by measuring the contact angle (ߠ) of a series of reference liquids (‫ܮ‬ଵ ) with known surface energy components, ߛ௅௅ௐ , ߛ௅ାభ , and ߛ௅ିభ , on the solid surface, and solving the equation భ భ

భ

భ

(1 + cosθ)ߛ௅భ = 2(൫ߛ௅௅ௐ ߛௌ௅ௐ ൯మ + ൫ߛ௅ାభ ߛௌି ൯మ + ൫ߛ௅ିభ ߛௌା ൯మ ) భ

(1)

with the measured contact angles ߠ as experimentally determined coefficients. In a previous study, we have determined ߛௌ௅ௐ , ߛௌା , and ߛௌି of the polymer materials used here, by measuring ߠ of reference liquids on macroscopic surfaces spin-cast from solutions of dissolved polymer particles (Table 1).23 This parameterization indicates that apolar (ߛௌ௅ௐ ) and acid (ߛௌା ) contribution to the total surface tension are similar for all particles, and the acid strength (electron accepticity or proton donicity) is negligibly small. It also indicates that the basicity (ߛௌି , electron donor or proton acceptor character) of PMMA is significantly larger than that of PS, as we anticipated. It is interesting that the “common sense” acid/base character of the particle surface functionality, visible in the aqueous environment (Figure 1b), is not reflected in the vOCG parameters obtained for the “dry” surfaces. We again note that the physical phenomena we actually observed in nonpolar media (Figure 1a and Figure 2) are described better with the properties of “dry” surfaces than those known from “wet” (aqueous) acid-base chemistry.

Table 1. Surface energy components, ߛௌ௅ௐ , ߛௌା , and ߛௌି , of solid polymer surfaces (in mJ/m2).23

Solids PS-amidine PS-carboxyl PS-sulfate PMMA-sulfate

Surface Energy Component Parameters / (mJ/m2) ߛௌା ߛௌି ߛௌ௅ௐ 37 0.55 ≈0 39.1 1.5 ≈0 37.1 2.05 ≈0 38.8 15.4 ≈0 15


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In the same theoretical framework, we can compare the relative donor-acceptor interaction of polymer surfaces with surfactants in a quantitative fashion, which should be directly related to the relative propensity of charge transfer as the donor-acceptor adducts ஺஻ heterolyze. This can be achieved by estimating the polar work of adhesion (ܹௌ௅ ) of polymer మ

surfaces (S) with surfactant solutions (‫ܮ‬ଶ ), given by46 ଵ/ଶ

஺஻ = −2൫ߛௌା ߛ௅ିమ ൯ ܹௌ௅ మ

− 2(ߛௌି ߛ௅ାమ )ଵ/ଶ,

(2)

using the acid/base parameters of polymer surfaces, ߛௌ௅ௐ , ߛௌା , and ߛௌି , and “surfactant solution parameters” ߛ௅௅ௐ , ߛ௅ାమ , and ߛ௅ିమ . The solution parameters ߛ௅ାమ and ߛ௅ିమ reflect the nonpolar solutions’ మ adaptive polarity exhibited by surfactants adsorbing at the interface with polar phases.24 These can be estimated by measuring the interfacial tension (ߛ௅భ ௅మ ) with a series of polar reference liquids (‫ܮ‬ଵ ), and solving the equation ଶ

ߛ௅భ ௅మ = ൣ(ߛ௅௅ௐ )ଵ/ଶ − (ߛ௅௅ௐ )ଵ/ଶ ൧ + 2ൣ(ߛ௅ାభ )ଵ/ଶ − (ߛ௅ାమ )ଵ/ଶ ൧ൣ(ߛ௅ିభ )ଵ/ଶ − (ߛ௅ିమ )ଵ/ଶ ൧ భ మ

(3)

with the measured ߛ௅భ ௅మ as an experimentally determined coefficient (for details, see ref. 24). The parameters for hexane solutions of the surfactant PIBS-N have been characterized in a range of surfactant concentrations in our previous study,24-25 and the results for acidity (ߛ௅ାమ ) and basicity (ߛ௅ିమ ) strength are summarized in Figure 3.

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Figure 3. Effective (a) acidity (ߛ௅ାమ ) and (b) basicity (ߛ௅ାమ ) parameter of nonpolar solutions of the surfactant PIBS-N, in a range of surfactant concentrations.24-25

Using the acid/base parameters of solid surfaces and surfactant solutions, the two terms on the right hand side of Eq. 2 were calculated and plotted separately in Figure 4. The first term ଵ/ଶ

of the right-hand side of Eq.2, −2൫ߛௌା ߛ௅ିమ ൯

, represents the free energy gain by combining the

surfaces’ acidic sites (with strength ߛௌା ) with the surfactant solutions’ basic moieties (with strength ߛ௅ିమ ) into adducts, the heterolysis of which would cause negative surface charging (Figure 4a). The second term, −2(ߛௌି ߛ௅ାమ )ଵ/ଶ , represents the free energy gain by adduct formation between the surfaces’ basic sites (with strength ߛௌି ) and the solutions’ acidic moieties (ߛ௅ାమ ), which is related similarly to the propensity of positive surface charging (Figure 4b). The result suggests that the adduct formation is dominated by the interaction between the surfaces’ basic sites and solutions’ acidic moieties, indicating that the acid-base or donor-acceptor interaction of surfaces and surfactants in the current system is mostly relevant to charging the 17


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surfaces more positively (or less negatively, top right of Scheme 2). Moreover, the magnitude of this interaction is significantly larger for the PMMA surface than for the PS surfaces, all of which have very similar adduct formation energies despite the big difference in their specific functional groups. This lack of sensitivity to surface functional groups is consistent with the charging behavior seen in Figure 1a.

Scheme 2. Multiple surface charging pathways which may determine the net surface charge of colloidal particles in nonpolar dispersions: i) direct acid-base (donor-acceptor) interaction of particle bulk with surfactants, ii) preferential adsorption of inverse micellar ions (formed in the liquid bulk by intra-micellar acid-base interaction of moisture and surfactants), and iii) ionization of surface functionality promoted by excess moisture.

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3.2. Preferential Adsorption of Inverse Micellar Ions Even though the considerations of direct acid-base interaction between the particles and surfactants may provide some idea about the relative surface charge, an obvious question has not yet been addressed. Why do the particles have a net negative sign of charge? To demonstrate the origin of negative ions, we point out the fact that nonpolar surfactant solutions do contain a third species: moisture.9-11,13-14,18,21,25 Even pure alkanes11,14 and nominally “dry” surfactants (unless partly decomposed)51 contain trace water, the complete removal of which can be experimentally unachievable.10 Thus, the inverse micelles are usually swollen with water;13 and we note that the molar concentration of water is typically larger than that of charged inverse micelles in common nonpolar surfactant solutions.14-15,18,21 The presence of residual water is considered important for micelle charging because it contributes to a (locally) high dielectric environment in the micelle core, where ions can thus reside at a reduced energy cost (Born energy).9-10,14,21 The participation of intra-micellar water in acid-base interactions, however, has rarely been discussed.25 Water molecules incorporated in inverse micelle cores, form nanoscale (intra-micellar) interfaces52 with the surrounding surfactant solution and associate with the micelles’ polar surfactant headgroups. The primary driving force for this self-assembly in a nonpolar continuous phase is the polar interaction or acid-base (donor-acceptor) adduct formation of surfactant polar groups with water molecules.9 Therefore, we can in principle estimate the free energy gain by donor-acceptor adduct formation of surfactant solutions with water molecules to assess the propensity of charge transfer between the water pool and the surfactant solution, in a manner analogous to the way we have estimated the charging propensity of solid surfaces in contact with 19


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the surfactant solutions. In the framework of the vOCG model, as an example, we calculated ା ି 46 = 25.5 mJ/m2 = ߛௐ ) and the each term of Eq. 2 using the acid/base parameters of water (ߛௐ

solution parameters (Figure 3). Figure 5 shows the interfacial energy benefit for the adduct formation of acidic water moieties with basic moieties of the surfactant solution exposed at the liquid interface (Figure 5a) and for the opposite process of adduct formation between basic water moieties and acidic moieties of the surfactant solution (Figure 5b). It is clear that the former process is energetically favored over the latter. Therefore, the water phase is likely to charge negatively by donating protons to the basic surfactant moieties via donor-acceptor adduct formation and subsequent heterolysis.48

Figure 5. (a) The adduct formation energy of the water’s acidic (electron acceptor or proton donor) moieties with the surfactant solution’s basic (electron donor or proton acceptor) moieties and (b) The adduct formation energy of the water’s basic (electron donor or proton acceptor) moieties and the surfactant solutions’ acidic (electron acceptor or proton donor) moieties.

The intra-micellar donor-acceptor complexes can heterolyze (Scheme 2) e.g. through molecular surfactant exchange with another micelle.9,53-55 The precursor micelle from which the (positively) charged surfactant separates, is likely to contain an atypically large amount of water that promotes the initial charge separation, and thus becomes a large micellar anion through the 20


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surfactant exchange. On the other hand, the micelle into which the (positively) charged surfactant is inserted will typically be an average micelle in sufficiently close proximity to the precursor micelle for the molecular exchange to occur. As a result, the pair of inverse micelle ions formed can have a significant size asymmetry, where the anion, on average, tends to be larger than the cation.25 Here, the “size” also implies a difference in aggregation number between the larger anionic micelles and smaller (less swollen) cationic micelles, since a larger water pool is likely to attract a larger number of surfactant molecules.11,56 Note that in dynamic equilibrium, where spontaneous fusion of inverse micelles as well as the exchange of individual molecules between an inverse micelle and the solution bulk are allowed,9,53-55,57 a significant width of the micelle size distribution is expected,15,58 and the common characterization of the micelle size by a single average value may be problematic.25 The generation of micellar ions in (particle-free) surfactant solutions is confirmed by the increase in electric conductivity upon the addition of surfactants (Figure 6a). The notion of a size asymmetry between micelle ion pairs is supported by electrophoretic light scattering in particle-free micelle solutions (Figure 6b). A net negative electrophoretic mobility is systematically observed in micelle solutions. For Rayleigh scattering59-60 by the small oppositely charged micelles the scattering intensity is proportional to square of their mass or the sixth power of their size.61 The mobility measured by phase analysis light scattering is determined by a phase difference (of the scattered light) accumulated over a time interval τ and weighted by the intensity amplitude.62 This intensity weighting implies that the most massive scatterers have a disproportionately large influence the measured electrophoretic mobility. Therefore the negative net mobility observed in our system suggests that micellar anions are larger than their cationic counterparts, scattering the incident light more strongly25 (Note that an equal number of positive and negative charges are present, as dictated by 21


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electroneutrality and the prohibitively large self-energy of multivalent ions in nonpolar media.)11,13

Figure 6. (a) Electric conductivity in nonpolar surfactant solutions mediated by inverse micellar ions and (b) field dependent electrophoretic mobility in (particle-free) nonpolar surfactant solutions, indicating a size asymmetry between oppositely charged inverse micelles. The data was reproduced from ref. 25.

In competitive adsorption of the charged inverse micelle ions (Scheme 2), this size asymmetry can be an entropic driver63 for asymmetric adsorption to the solid surface;13,26,28 the adsorption of larger micelle anions being favored because it minimizes the overall translational entropy loss associated with confining the adsorbate on the surfaces.25,63 This may explain how hydrophobic and predominantly basic polymer surfaces can acquire net negative surface charge. In contrast to the assumptions made by proponents of the traditional acid-base particle charging mechanism,17,22 we propose that the ionization of micelles in the liquid bulk and their asymmetric adsorption to particles surfaces can affect the charging of polymer particles in nonpolar surfactant solutions more strongly than direct surfactant-particle interaction. By comparing the total polar work of adhesion of surfactant solutions with polymer surfaces (Figure 4) and with water (Figure 5), we conclude that the formation of donor-acceptor adducts is 22


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energetically more favorable in the liquid bulk. Moreover, we note that the oil-borne moisture (~30ppm in pure hexane, 40~80ppm in solutions of the PIBS-N),25 as dispersed and confined in inverse micelle cores with a few nanometers in size, can provide a significantly larger interfacial area than the total surface area of micron-size colloidal particles dispersed in nonpolar dispersions (~30ppm in our study, comparable to the concentrations in other studies13,2223,25,30,32,52

at which electrophoresis measurements were carried out). Overall, the free energy gain

by donor-acceptor adduct formation (and by extension the propensity of charge transfer and ionization) of surfactants can be significantly larger with the third species water than with the polymer surfaces, and therefore, the net surface charge is likely to be governed by the partitioning of inverse micelle ions generated in the liquid bulk.25 We also note that the net surface charge corresponding to the small electrophoretic particle mobility in nonpolar dispersions is only in the order of a few tens of elementary charges, consistent with a small asymmetry in the adsorption of oppositely charged micelles.13,38 In a similar vein, the positive influence to net surface charging, which was tentatively attributed to the charge transfer between surfaces and electrically neutral surfactants, might also involve the contribution of micellar cation adsorption, favored chemically by basic surfaces (Scheme 2). Note that this type of ion-surface interaction13 can also be considered an “acid-base” interaction, although the mechanism is different from the charge transfer between two electrically neutral species.64 From this perspective, qualitative correlations19-22 between relative surface charging behavior and acidity/basicity strength for a series of particles do not by themselves provide compelling evidence for charge creation by direct donor-acceptor interaction of neutral surfaces with surfactants. For example, a predominantly acidic (electrophilic) surface

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might charge negatively either by undergoing donor-acceptor interaction with basic moieties of neutral surfactants or by preferentially adsorbing micellar anions. 3.3. Ionization of Specific Surface Functional Groups Although we have not observed any clear impact of the particles’ specific surface functionality on the particle charge in nonpolar solutions discussed above, the charging behavior of the same particles in solutions of the closely related commercial surfactant product OLOA11000 (Figure 7) does show the impact of surface functionality and may suggest under what conditions the functional groups matter. The electrophoretic mobility of PMMA-sulfate, PS-sulfate, and PS-carboxyl particles in these solutions appears similar to that in the PIBS-N solution (Figure 1a). However, the mobility of PS-amidine is significantly smaller than that of PS-sulfate and PS-carboxyl. It is not possible to explain this particular difference with only a combination of charging pathways suggested above.

One possible explanation is that the

functional groups’ ionizability might have been activated in this solution for some reason, and that protonation of the amidine functional groups rendered the net surface charge less negative.

Figure 7. The zero-field electrophoretic mobility of polymer particles in hexane solutions of the commericial surfactant product OLOA11000, represented as a function of surfactant concentration.


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A likely factor promoting the ionization of surface functionality is excess moisture,9,23 which might directly partition to the particle surface65 and behave as an “aqueous bulk”66-67 rather than molecules confined in inverse micelle cores.68-69 In such cases, the specific surface functional groups would be solvated more efficiently by water molecules and the locally high dielectric environments would promote their ionization. We measured the water content in nonpolar solutions of OLOA11000 using Karl Fischer titration, and found that these solutions, prepared with the product as received (without further purification or drying) included a significantly larger amount of water than the PIBS-N solutions (Figure 8). We note that the measured water content in OLOA11000 solutions is roughly in the same order of magnitude as in solutions of another hygroscopic surfactant, aerosol OT, where some evidence for the ionization of particle functional groups was previously observed.23 However, we stress that the ionization of surface functional groups, which is related to their acid/base characters, appears to be just one of several contributions to net surface charging (Scheme 2), not the only governing parameter – as is clear from the fact that PS-amidine still acquires more negative charge than PMMA-sulfate (Figure 7).

Figure 8. Water content in hexane solutions of the commercial surfactant product OLOA11000 and custom surfactant PIBS-N.


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In an earlier study of nonpolar dispersions containing the commercial surfactant Span85, we had also found some hints at the interplay between multiple surface charging pathways.23 At low concentrations of this surfactant (below the critical micelle concentration, CMC), the relative charging of PS-amidine, PS-carboxyl, and PS-sulfate particles was strongly correlated with the particles’ aqueous surface charging. We hypothesize that in this dilute regime residual water cannot be effectively solubilized by the extremely hydrophobic surfactant Span8514,21,23 and therefore partitions mostly to the particle surface, where it solvates the functional groups and promotes their ionization. At high surfactant concentrations (above the CMC), on the other hand, water might have been more effectively scavenged by inverse micelles instead, leaving the surface functional groups “dry” and apparently inactive: in this regime all PS particles acquired positive charge of similar magnitude. Moreover, PMMA-sulfate acquired significantly larger positive charge than any of PS particles. The charging behavior at high Span 85 concentrations thus appeared more correlated with the polarity of particle bulk than with the specific surface functional groups, in close analogy to our current systems containing the purified surfactant PIBS-N. 4. CONCLUSION Having recently studied particle charging in nonpolar surfactant solutions by varying the surfactant chemistry,25 we have now observed the charging behavior of a series of polymer particles, PMMA-sulfate, PS-sulfate, PS-carboxyl, and PS-amidine in nonpolar solutions of the single, purified surfactant PIBS-N. Our data indicate no correlation between surface charging and the acid/base chemistry of the functional surface groups. Instead, we found that charging appeared somewhat correlated with the Lewis basicity of the particle bulk, as the more basic 26


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PMMA surfaces display a more positive (less negative) charge than any of the PS surfaces; but even acid-base interaction between the surfactant and the polymer cannot explain the net negative surface charge observed throughout this study. This observed charging is, however, consistent with a different charging mechanism we recently hypothesized25 that involves the preferential adsorption of charged inverse micelles formed in the solution bulk. According to this hypothesis, acid-base driven charge exchange between surfactant molecules and water in the cores of the most water-swollen micelles, followed by the inter-micellar exchange of surfactant molecules, can generate micellar ion pairs in which the anions are larger, on average, than their cationic counterparts (Scheme 2). This size asymmetry between negatively and positively charged micelles not only explains the negative electrophoretic mobility (Figure 6b) of PIBS-N micelles, but also causes the adsorption of negatively charged micelles to be favored entropically. While this entropic preference applies to the adsorption at all particle surfaces equally, the more basic PMMA surfaces plausibly have a higher chemical affinity for positively charged ions than the PS surfaces, which may partly offset the entropic bias for anion adsorption and explain the correlation of the particles’ charging characteristic with the polymer’s acid-base behavior. The competitive nature of chemical and entropic charging pathways is consistent with experimental observations shown in our previous article25 where PMMA acquired a net positive surface charge in some surfactant solutions with little or no micellar ion size asymmetry. Finally, we found that only in the system with a commercial surfactant mixture of high moisture content, did ionization of the surface functional groups seem to contribute to surface charging, presumably because the excess moisture can solvate the specific functional groups more efficiently and reduce the energy cost for their ionization. In general, it appears that surfactant-mediated surface charging in nonpolar dispersions can involve multiple charging pathways, the balance of which may hinge 27


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on the amount of residual water and its partitioning between the particle surfaces and the micelle cores. Supporting Information The field dependent electrophoretic mobility of the charged polymer particles in hexane-based dispersions is available free of charge at http://pubs.acs.org/. Funding Sources This research was funded by the National Science Foundation (NSF) through award # 1160138. REFERENCES (1) Comiskey, B.; Alvert, J. D.; Yoshizawa, H.; Jacobson, J. An Electrophoretic Ink for AllPrinted Reflective Electronic Displays. Nature 1998, 394, 253-255. (2) Chen, Y.; Au, J.; Kazlas, P.; Ritenour, A.; Gates, H.; McCreary, M. Flexible Active-Matrix Electronic Ink Display. Nature 2003, 423, 136-136. (3) Heikenfeld, J.; Drzaic, P.; Yeo, J.-S.; Koch, T. Review Paper: A Critical Review of the Present and Future Prospects for Electronic Paper. J. Soc. Inf. Disp. 2011, 19, 129-156. (4) Pearlstine, K.; Page, L.; Elsayed, L. Mechanism of Electric Charging of Toner Particles in Nonaqueous Liquid with Carboxylic Acid Charge Additives. J. Imaging Sci. 1991, 35, 55-58. (5) Jenkins, P.; Basu, S.; Keir, R. I.; Ralston, J.; Thomas, J. C.; Wolffenbuttel, B. M. A. The Electrochemistry of Nonaqueous Copper Phthalocyanine Dispersions in the Presence of a Metal Soap Surfactant: A Simple Equilibrium Site Binding Model. J. Colloid Interface Sci. 1999, 211, 252-263. (6) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Asphaltenes: Structural Characterization, SelfAssociation, and Stability Behavior. Energy & Fuels 2000, 14, 6-10. (7) Gonzelez, G.; Neves, G. B. M.; Saraiva, S. M.; Lucas, E. F.; Sousa, M. A. Electrokinetic Characterization of Asphaltenes and the Asphaltenes-Resins Interaction. Energy & Fuels 2003, 17, 879-886. (8) Van der Hoeven, P. C.; Lyklema, J. Electrostatic Stabilization in Non-Aqueous Media. Adv. Colloid Interface Sci. 1992, 42, 205-277. (9) Morrison, I. D. Electric Charges in Nonaqueous Media. Colloids Surf., A 1993, 71, 1-37. (10) Smith, G. N.; Eastoe, J. Controlling Colloid Charge in Nonpolar Liquids with Surfactants. Phys. Chem. Chem. Phys. 2013, 15, 424-439. (11) Hsu, M. F.; Dufresne, E. R.; Weitz, D. Z. Charge Stabilization in Nonpolar Solvents. Langmuir 2005, 21, 4881-4887. (12) Kim, J; Anderson, J. L.; Garoff, S.; Schlangen, L. J. M. Ionic Conduction and Electrode Polarization in a Doped Nonpolar Liquid. Langmuir 2005, 21, 8620-8629. 28


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Langmuir

Table of contents figure 243x181mm (240 x 240 DPI)


Langmuir

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Scheme 1. Chemical structure of the custom surfactant “PIBS-N”, a derivative of commercial charging agent surfactants polyisobutylene succinimide polyamines. 127x41mm (150 x 150 DPI)


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Langmuir

Figure 1. (a) The zero-field electrophoretic mobility of polymer particles in hexane solutions of the surfactant PIBS-N, represented as a function of surfactant concentration. (b) The electrophoretic mobility of the same particles in aqueous NaCl solutions, represented as a function of NaCl concentration (reprinted from ref. 23 with permission). 770x1240mm (96 x 96 DPI)


Langmuir

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Figure 2. The shift in resonance frequency (∆f) of quartz crystal microbalances coated with polymer surfaces, in response to the surfactant adsorption (mass increase) to the surfaces. 688x561mm (96 x 96 DPI)


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Langmuir

Figure 3. Effective (a) acidity (γL2+) and (b) basicity (γL2−) parameter of nonpolar solutions of the surfactant PIBS-N, in a range of surfactant concentrations.24-25 273x209mm (300 x 300 DPI)


Langmuir

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Figure 4. (a) The adduct formation energy of the solid surfaces’ acidic (electron acceptor or proton donor) sites with the surfactant solution’s basic (electron donor or proton acceptor) sites and (b) The adduct formation energy of the solid surfaces’ basic (electron donor or proton acceptor) sites and the surfactant solutions’ acidic (electron acceptor or proton donor) sites. 149x60mm (240 x 240 DPI)


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Langmuir

Scheme 2. Multiple surface charging pathways which may determine the net surface charge of colloidal particles in nonpolar dispersions: i) direct acid-base (donor-acceptor) interaction of particle bulk with surfactants, ii) preferential adsorption of inverse micellar ions (formed in the liquid bulk by intra-micellar acid-base interaction of moisture and surfactants), and iii) ionization of surface functionality (marked X) promoted by excess moisture. 338x262mm (150 x 150 DPI)


Langmuir

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Figure 5. (a) The adduct formation energy of the water’s acidic (electron acceptor or proton donor) moieties with the surfactant solution’s basic (electron donor or proton acceptor) moieties and (b) The adduct formation energy of the water’s basic (electron donor or proton acceptor) moieties and the surfactant solutions’ acidic (electron acceptor or proton donor) moieties. 147x59mm (240 x 240 DPI)


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Langmuir

Figure 6. (a) Electric conductivity in nonpolar surfactant solutions mediated by inverse micellar ions and (b) field dependent electrophoretic mobility in (particle-free) nonpolar surfactant solutions, indicating a size asymmetry between oppositely charged inverse micelles. The data was reproduced from ref. 25. 1441x610mm (96 x 96 DPI)


Langmuir

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Figure 7. The zero-field electrophoretic mobility of polymer particles in hexane solutions of the commericial surfactant product OLOA11000, represented as a function of surfactant concentration. 273x209mm (300 x 300 DPI)


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Langmuir

Figure 8. Water content in hexane solutions of the commercial surfactant product OLOA11000 and custom surfactant PIBS-N. 258x201mm (300 x 300 DPI)


Charging Mechanism for Polymer Particles in Nonpolar Surfactant

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