Interfaces Charged by a Nonionic Surfactant - The Journal of Physical

Subscriber access provided by University | of Minnesota Libraries

B: Fluid Interfaces, Colloids, Polymers, Soft Matter, Surfactants, and Glassy Materials

Interfaces Charged by a Nonionic Surfactant Joohyung Lee, Zhang-Lin Zhou, and Sven Holger Behrens J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b02853 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Interfaces Charged by a Nonionic Surfactant Joohyung Lee1,*, Zhang-Lin Zhou2 and Sven Holger Behrens3,* 1

Department of Chemical Engineering, Myongji University, Yongin, Gyeonggi 17058, Korea

2

Hewlett-Packard Company, 16399 W Bernardo Drive, San Diego, CA 92127, USA

3

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311

Ferst Drive NW, Atlanta, GA 30332, USA

Corresponding Authors *Joohyung Lee ([email protected]), Sven H. Behrens ([email protected])

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Highly hydrophobic, water-insoluble nonionic surfactants are often considered irrelevant to the ionization of interfaces at which they adsorb despite observations that suggest otherwise. In the present study, we provide unambiguous evidence for the participation of a water-insoluble surfactant in interfacial ionization by conducting electrophoresis experiments for surfactant-stabilized nonpolar oil droplets in aqueous continuous phase. It was found that the surfactant with amine headgroup positively charged the surface of oil suspended in aqueous continuous phase (oil/water interface), which is consistent to its chemically “basic” nature. In nonpolar oil continuous phase, the same surfactant positively charged the surface of solid silica (solid/oil interface) which is frequently considered chemically “acidic”. The latter observation is exactly opposite to what the traditional acid-base mechanism of surface charging would predict, most clearly suggesting the possibility for another charging mechanism.


Page 2 of 27

Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Oil-soluble, non-ionic surfactants lack dissociable moieties and are often thought not to participate in ionization processes;1-4 yet two types of physicochemical observations are frequently reported that suggest otherwise. First, nonpolar oils can display enhanced electric conductivity in the presence of the dissolved nonionic surfactants.5-8

Second, particles

dispersed in such nonpolar solutions of nonionic surfactants often show strong electrokinetic evidence of surface charging.9-19 It is generally believed that the charge carriers in nonpolar oils containing nonionic surfactant are inverse micellar aggregates, which lower the energy cost of charging by incorporating the charges in the polar micelle cores.7 Statistically, the occurrence of positively and negatively charged micelles can be described as equilibrium charge fluctuations around a zero mean,20-21 but the exact pathway of charge separation is still a matter of ongoing research.22-23 Similarly, the mechanism by which solid particles acquire electric surface charge in nonpolar surfactant solutions also remains unclear; currently there is no established method to predict even the sign of particle charge developed in a given system. We have recently proposed a pathway for the formation of charged inverse micelles that is consistent with observations for nonionic surfactants with basic surfactant headgroups.18-19

These basic surfactants can abstract protons from an acidic interaction

partner such as water within the micelle cores (intra-micellar acid-base interactions) and in this case charge separation would occur at the intra-micellar oil-water interface when the protonated surfactant is exchanged for an electrically neutral surfactant from another micelle (Scheme 1).18-19



Page 4 of 27

Scheme 1. Formation of charged micelles

A similar mechanism had been proposed much earlier for surfactant-mediated charging of colloidal particles. The hypothesized charging pathway, often referred to as acid-base mechanism of surface charging,9-10,

13-14, 16-17

involves three sub-

processes: i) The polar surfactant moieties form acid-base adducts with the polar particle surface groups. ii) Charges transfer among the acid-base adducts depending on the relative acidity and basicity of the surfactants and the particle surface. iii) Individual charged surfactants can be incorporated into inverse micelles, which then serve as micellar counterions to the charged particle (Scheme 2).

Scheme 2. Acid-base mechanism of surface charging.


Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Indirect support for the notion of such particle charging in nonpolar media via acidbase interaction of surfactants with the particle surface has come from electrophoretic studies on silica and metal oxide particles.10, 13-14, 17 In particular, Gacek et al.13-14 found that the particle

charge

in

these

systems

correlated

strongly

with

the

particles’

protonation/deprotonation in aqueous systems, and further depended on the acid-base character of the surfactant. By contrast, studies on surface-functionalized polymer particles showed no systematic correlation between particle charging in aqueous media, which reflects the behavior of the ionizable surface headgroups, and charging of the same particles in nonpolar solutions, which appears more closely related to with the (Lewis) acid-base character of the polymer in the particle bulk.15,

19

It thus appears that these particles’

donor/acceptor behavior can vary significantly depending on the surrounding medium. In the present study, we first establish the charging behavior, in contact with water, of an oil-soluble surfactant generally considered basic, and of (non-functionalized) silica particles generally considered acidic. Next, we show that the charging of these particles in nonpolar solutions of this surfactant is qualitatively the exact opposite of what one should expect, if charging occurred due to a direct acid-base interaction between the particle and the surfactant. Finally, we provide some considerations on how the observed behavior may be rationalized. METHOD Preparation of the Surfactant PIBS-C The synthesis and purification of the polyisobutylene succinimide (PIBS) amine surfactant PIBS-C were carried out by following the procedures described in detail in our previous studies.12,

18

Briefly, we dissolved an equimolar mixture of a polyisobutylene



succinic anhydride OLOA15500 (Mw ~ 1000 g/mol, Chevron Oronite) and N,Ndiethylpentane-1,5-diamine (97%, Mw ~ 158.2 g/mol, Matrix Scientific) in m-xylene (> 99%, Sigma Aldrich), and refluxed it at 190 °C for 20 hours to yield the polyisobutylene succinimide amine surfactant PIBS-C. The m-xylene was removed via distillation at 200 °C after the reaction. The reaction product was purified using flash chromatography, where the stationary phase was a silica gel (pore size 60 Å, 70-230 mesh, 63-200 µm, Sigma-Aldrich) treated with a Lewis base solution of 20:1 hexane (> 98.5%, VWR) / trimethylamine (> 99.5%, Sigma Aldrich), and the mobile phase was a solution of 20:1 chloroform (> 99.8%, VWR) / ethanol anhydrous (> 99.5%, Sigma Aldrich). A low polarity impurity (~30 wt. % of the total synthesis product; presumed to be unreactive polyisobutylene (PIB) from the original OLOA1550012) and the target surfactant PIBS-C were eluted from the column at the very beginning and later stages respectively, and collected separately. The solvent mixture was removed in a rotary evaporator at 45 °C with vacuum pressure of ~160 mmHg for 3 hours. A number average molecular weight Mn=903.12 with a polydispersity index PDI=1.14 was obtained for the purified surfactant by matrix assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry. The dried surfactants were dissolved in the nonpolar solvent hexane (> 98.5%, =1.89, VWR) at a concentration of 100 mM, and were again filtered through an alumina based membrane (pore size 0.02 µm, Whatman® Anotop® 10 syringe filter). Preparation of the Oil-in-Water (O/W) Emulsion Stabilized by the PIBS-C In preparation of o/w emulsion, a 1mL hexane-based solution of the surfactant PIBSC (with a surfactant concentration 100 mM) was added into 9mL deionized water (with a resistivity of 18.2 MΩ·cm, Barnstead) and the mixture was homogenized at 30,000 rpm for 1 min using a rotor-stator homogenizer (IKA Ultra-Turrax® T10 basic). Aliquots of the milky


Page 6 of 27

Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


stock emulsion were diluted heavily with aqueous salt solutions and re-dispersed by ultrasonication for 5 minutes to obtain emulsions with an oil volume fraction of ~0.6% and a continuous aqueous phase containing 1mM NaCl and varied pH (adjusted with 1M HCl and 100mM

NaOH solutions).

The

dispersed

hexane/PIBS-C

droplets maintained a

hydrodynamic diameter below 1 µm for at least 6 hours, as confirmed by dynamic light scattering (DLS) at 90 ° with ALV DLS/SLS-5022F (ALV-Laser GmBH) standard goniometer system. The electrophoretic mobility in the aqueous phase was measured within an hour after preparing the emulsion. Electrophoresis We measured the electrophoretic mobility of various dispersed phases in polar and nonpolar continuous phases by implementing phase analysis light scattering using a Zetasizer Nano ZS90 (Malvern Instruments). The concentration of silica colloids (1.01 µm, catalog #SS04N, lot #9857, Bangs Laboratories) was adjusted to ~30 ppm both in aqueous 1 mM NaCl solutions and in nonpolar, hexane-based solutions of the surfactant PIBS-C. For the latter systems, we prepared suspensions of the silica particles in hexane/PIBS-C solutions via solvent swap,11, 15, 18-19 where the particles originally received as an aqueous suspension were copiously washed with isopropanol as intermediate medium and subsequently transferred into hexane/PIBS-C solutions at a minimal PIBS-C concentration (0.5mM) by repeated centrifugation and re-dispersion (3 times each). The washed particles were finally diluted to ~30ppm in hexane/PIBS-C solutions in a range of surfactant concentration for electrophoresis. Measurements were also carried out on 1.01 µm silica particles (catalog #SS04N, lot #11145, Bangs Laboratories) that were initially received as a dry powder and were directly dispersed in hexane/PIBS-C solutions at the particle concentration ~30 ppm without the solvent swap procedure. For the nonpolar systems, a non-monotonic field dependent particle mobility was



observed (see Figure S1 and S2 in Supporting Information). RESULTS AND DISCUSSIONS We prepared the polyisobutylene succinimide amine surfactant PIBS-C,18, 24 which is a customized and purified analog of Chevron’s commercial OLOA surfactants.8-10, 12, 14, 16, 1819, 24-25

The chemical structure is shown in Scheme 3. This lipophilic surfactant is soluble in

nonpolar solvents such as saturated hydrocarbons but not soluble in polar solvents. Although the surfactant head is relatively polar and is considered chemically basic due to the presence of amine group, it is worth confirming experimentally that this surfactant indeed has the ionizability consistent with its chemical structure.

Scheme 3. The chemical structure of surfactant PIBS-C.

To this end, we studied the electrophoretic mobility of oil-in-water (o/w) emulsion droplets with the surfactant molecules adsorbed at the droplet surface. This way, evidence for the charging of the PIBS-C surfactant in contact with water could be obtained, even though this hydrophobic surfactant is water-insoluble. The results of these measurements, shown in Figure 1a, clearly indicate that the o/w interfaces were charged positively in a wide range of pH values. Given that the electrophoretic measurements of “pristine”, surfactant-free droplets of alkanes or other nonpolar solvents in water routinely show negative droplet mobility in water above pH 3-4,26-29 the positive droplet mobility found here can be attributed


Page 8 of 27

Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


unambiguously to the surfactant molecules adsorbed at the droplet interface. It thus appears that the basic polar headgroup of this oil-soluble surfactant charges positively in contact with water as one might have expected based on its chemical structure. The basic surfactant head can abstract a proton and, much like a particle with basic surface moieties, a droplet of hexane/PIBS-C solution in water retains a positive surface charge that is balanced by a diffuse layer of negative counterions in the water phase. Geometrically, the o/w system with the surfactants at the interfaces investigated in the present study may be viewed as the inverted analog of water-swollen inverse micelles in a nonpolar solvent, a dilute w/o system. Given the basicity of the surfactant, the intra-micellar interface might be positively charged with the negative counterions located in the aqueous micelle core,30 as indicated in Scheme 1 and discussed previously.18-19 Figure 1b shows the electrophoretic mobility of (unmodified) 1.01 µm silica particles (catalog #SS04N, lot #9857, Bangs Laboratories) in aqueous 1 mM NaCl solution. It indicates a negative particle charge throughout most of the probed pH window, and confirms the well-known charging behavior of silica commonly attributed to the dissociation of acidic silanol groups at the particle surface.31



Figure 1. (a) Electrophoretic mobility of PIBS-C/hexane solution droplets in aqueous 1 mM NaCl solution indicative of positive droplet charging mediated by the interfacially adsorbed basic surfactant. (b) Electrophoretic mobility of silica particles in aqueous 1 mM NaCl solution attributed to the dissociation of acidic silanol groups at the particle surface. (c) Electrophoretic mobility of the silica particles in PIBS-C/hexane solution and schematic of some hypothetical pathways for the positive particle charging.


Page 10 of 27

Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


If the basic nature of the surfactant (Figure 1a) and the acidic nature of the particles (Figure 1b) give any indication for the acid-base interaction between these particles and surfactants in nonpolar solutions, and if this interaction indeed determined the particle charge, one would expect these particles to acquire a negative surface charge in nonpolar solutions of PIBS-C as well (Scheme 2).

As experiments summarized in Figure 1c clearly show, that

expectation would be wrong. In order to study the characteristically low electrophoretic particle mobility in the low dielectric media, phase analysis light scattering (PALS) was implemented.32 The mobility was found to depend on the strength of the applied electric field (see Supporting Information S1), as is often observed for nonpolar dispersions11, 13-19, 32-34 and has recently been explained theoretically.35 To infer the particles’ equilibrium charging state in the absence of the applied field, we used a widely-adopted method of extrapolating the field dependent mobility to zero field strength – this zero field mobility is plotted in Figure 1c. Note that the magnitude of electrophoretic mobility in nonpolar dispersions is generally much smaller than that in aqueous dispersions due to the much lower dielectric constant ( ≈ 2 for nonpolar media vs. ≈ 80 for aqueous media; accordingly the mobility for particles with similar zeta potential in both types of media would differ by a factor of ~40.) Figure 1c clearly shows that the silica particles were charged positively in nonpolar hexane/PIBS-C solutions. We also measured the electrophoretic mobility of 1.01 µm silica particles (catalog #SS04N, lot #11145, Bangs Laboratories) that were initially received as the dry powder than the aqueous suspension, and dispersed directly in PIBS-C/hexane solutions without the solvent swap procedure (see Supporting Information S2). The measured fielddependent electrophoretic mobility for the unwashed silica particles appear qualitatively similar to that for the washed silica particles with the identical sign for the zero-field mobility, indicating that the observed particle charging behavior is not a consequence of some unexpected change in the particle surface composition during sample preparation.



Together, the data of Figure 1, which constitute the central finding of this study, show that the ionization of untreated silica particles and surfactants in water does not give a straightforward indication of the surfactant-mediated particle charging in nonpolar solutions. We cannot offer a rigorous explanation of the observed behavior, but will use the remainder of this paper to discuss a semi-heuristic approach we find useful for rationalizing the experimental evidence. We propose that the particles’ propensity for acquiring positive charge may be related to the Lewis basicity36-39 of the particle surface. It was previously observed that in nonpolar solutions containing “dry” surfactants, the charging behavior of a series of polymer colloids with well-defined acidic or basic surface functionalities correlated only with the Lewis basicity of the particle bulk polymer materials (not with surface functionality), whereas charging in solutions containing “hygroscopic” surfactants, and thus more residual water, appeared to be influenced also by the particles’ functional surface headgroups.15, 19 We note that a water content below 50ppm measured by Karl Fischer titration in PIBS-C/hexane solutions (up to 20mM)18 indicates the relative dryness or hydrophobicity of the PIBS-C compared to some commercial surfactants (cf. a water content up to ~300ppm found in OLOA11000/hexane19 and up to ~400 ppm in AOT/heptane14 in the corresponding surfactant concentration range). Based on the previous observations for polymer particles, we conjecture that in the dry PIBS-C/hexane solutions the poorly solvated silanol groups on the silica particle surface cannot deprotonate as they do in water, and that the (Lewis) acidity and basicity of the silica itself become more relevant for particle charging. A number of methods to characterize the acid-base properties of solid surfaces are known,40 but there is no generally accepted way of assessing within the same conceptual framework the acidity and basicity of solids and that of


Page 12 of 27

Page 13 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


oil-soluble surfactants interacting with the surfaces in nonpolar solutions. We recently proposed a semi-heuristic method for doing so,24 that borrows its notation and conceptual framework from the theory by van Oss, Chaudhury, and Good (vOCG) for the surface and interfacial energy of pure phases.36 This theory expresses the surface tension γ of condensed phase i as a sum of apolar and polar (acid/base) contributions, such that γ = γ + / / 2(γ (γ ) , where γ is an apolar Lifshitz-van der Waals component, γ ) is an acid 2 component, and γ is a base component (all in the unit of surface tension, mJ/m ). The

surface energy component of a solid (S), γ , γ , and γ can be found, according to the

theory, by measuring the contact angle (θ) of a series of reference liquids (L1) with known surface energy components, γ , γ , and γ on the solid surface and solving the equation / / / (1 + cosθ)γ = 2((γ + (γ + (γ ) for the work of adhesion γ ) γ ) γ )

between a solid and liquid. Values for the acidity and basicity parameter of the silica surface are found in the literature37 with γ = 1.97 mJ⁄m and γ = 40.22 mJ⁄m within the

framework of this theory, suggesting the presence of the surface basicity which is not immediately appreciable by solely performing aqueous electrophoresis for colloidal silica. (Note that the direct comparison of γ versus γ is not meaningful because the each

quantity is based on the somewhat arbitrary stipulation that

γ %&'() = γ%&'() (=

25.5 mJ⁄m );36 whereas relative differences between two materials with respect to the same parameter do reflect differences in the material properties, independently of the chosen reference values for water.) A set of effective “parameters γ , , γ, , and γ, for nonpolar

surfactant solutions”, which reflect the interfacial response of the solution to contact with a polar phase, can also be obtained by utilizing the vOCG framework in a heuristic fashion.24 These solution parameters are determined by measuring the interfacial tension γ , of the nonpolar surfactant solutions (L2) with a series of polar reference liquids (L1) of known



Page 14 of 27

/ / vOCG parameters and solving the equation set γ , = -(γ . (γ / + ) , )

/ / / / 2-(γ . (γ / -(γ . (γ / . The parameters for PIBS-C/hexane solutions ) , ) ) , )

have been studied in a range of surfactant concentrations in our previous study,24 and the results for γ , and γ, are summarized in Figure 2. They indicate an increasingly basic

solution behavior with increasing surfactant concentration, but no significant acidity at any concentration, suggesting that the surfactant molecules adsorbed at interfaces with polar media may act as monopolar bases (proton acceptors or electron donors). Therefore, the polar 12 / / work of adhesion36 W = .2(γ . 2(γ characterizing the strength of the γ, ) γ, ) ,

acid-base adduct formation between the silica surface and the adsorbed surfactants, is /

dominated by the term .23γ γ, 4

for the adduct formation of the surfactant basic sites

with surface acidic sites, favoring negative surface charging. In other words, the direct (Lewis) acid-base interaction between the silica surface and the surfactant as described in the present

Figure 2. Acid (56 ) and base (56 ) parameters of the hexane-based solutions of the surfactant PIBS-C in contact with polar phases. The data were reproduced from ref 24.

framework fails to explain the positive charging of the colloidal silica observed in Figure 1c, as does the surfactant interaction with the silanol surface groups responsible for particle


Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


charging in water. One possible mechanism for the positive particle charging is the preferential adsorption5,

15, 18-19, 25, 41-43

of the cationic reversed micelles from the solution. These

positively charged (reversed) micelles, formed alongside their negatively charged counterpart in nonpolar surfactant solutions, are indeed potential proton donors (“acids”) to the basic particle surface.18-19

The presence of micelle ions in PIBS-C/hexane solutions was

witnessed in our earlier study18 with increasing conductivity up to ~50pS/m for increasing surfactant concentration up to 20mM, compared to a conductivity around ~0.1pS/m in pure hexane. Micellar ion pairs may be generated when ionogenic impurities dissociate in a polar micelle core and the dissociated charges partition into two separate micelles. A typical candidate for an ionogenic impurity is moisture, which is indeed ubiquitous in nonpolar surfactant solutions.7, 18-19, 43-44 (Even nominally “pure” nonpolar solvents as well as “dried” surfactants contain residual water whose molar concentration already exceeds the typical number of ions in nonpolar surfactant solutions.) Thus, many inverse micelles likely contain at least some water.44 In our recent studies, we proposed that moisture in the micelle cores would not only create a local high dielectric environment reducing the energetic cost of charging (Born energy) but could play an active role in charging via the pathway represented in Scheme 1.18-19 It was proposed that surfactants in the micellar aggregates would charge via formation of acid-base adducts with water at the intra-micellar interfaces, and that pairs of micellar ions could be generated when a charged surfactant is exchanged between two initially neutral micelles. We note that the proposed mechanism for micelle charging closely resembles the classical acid-base particle charging mechanism (Scheme 2), except that the interaction partner of the adsorbed surfactant is water instead of the particle. In either case (of the particle or water interacting with the surfactant) one might expect the generation of charges to correlate with the total energy of adduct formation between the (monopolar basic)



Page 16 of 27

surfactant and the respective polar phase, given by the product W 712A 7 , where W 712 is the polar work of adhesion per unit interfacial area, and A 7 is the total interfacial area. Not only is

W12 ≈ ,

/

.2 9γ γ : ,

for water (L1) and the PIBS-C/hexane solutions (L2) larger in

12 magnitude than the corresponding term W for silica (S) as the polar phase (W12 ≈ , ,

.14

;

Interfaces Charged by a Nonionic Surfactant - The Journal of Physical

Recommend Documents