Streaming Potential Studies on the Adsorption of Amphoteric

We have used streaming potential measurements to determine the ζ-potential of amorphous, hydrophilic, mesoporous silica gel in aqueous nonionic−cat...
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Langmuir 2002, 18, 8447-8454

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Streaming Potential Studies on the Adsorption of Amphoteric Alkyldimethylamine and Alkyldimethylphosphine Oxides on Mesoporous Silica from Aqueous Solution Alf Pettersson* and Jarl B. Rosenholm Department of Physical Chemistry, Åbo Akademi University, Porthansgatan 3-5, FIN-20500 Åbo, Finland Received February 13, 2002. In Final Form: August 20, 2002 We have used streaming potential measurements to determine the ζ-potential of amorphous, hydrophilic, mesoporous silica gel in aqueous nonionic-cationic solutions of the amphoteric alkyldimethylamine oxide surfactants (CnDAO with n ) 8, 10, and 12) and alkyldimethylphosphine oxide surfactants (CnDPO with n ) 10 and 12). The effect of different mixtures of the nonionic and cationic (protonated) forms of the amine oxide surfactant on the ζ-potential was studied at natural pH and at pH ) 4. The nonionic phosphine oxide homologues were chosen as references. The surfactant contents ranged from dilute solutions to well above the cmc. The ζ-potential isotherms of the alkyldimethylamine oxides featured break points at the critical surface aggregation concentration (CSAC), recognized as the onset of counterion association with formed surface aggregates. The surface aggregates of all alkyldimethylamine oxide surfactants studied are charged at the cmc. At natural pH evidence of ζ-potential reversal was found as a result of the onset of formation of C12DAO hemimicelles induced by the surface with the adsorbed surfactant species. At pH ) 4 the fractional counterion binding to surface aggregates, composed of almost only cationic-nonionic C12DAO dimers, varies exponentially with the surfactant content in solution, starting from zero at about the CSAC and reaching a plateau at the cmc. As expected, the adsorption of alkyldimethylphosphine oxides studied did not change the ζ-potential of silica. Nonionic C12DAO forms probably ellipsoidal aggregates; meanwhile cationic C10DAO and C12DAO are likely to form spherical surface micelles at the aqueous solution/SiO2 interface. The adsorption mechanism of nonionic C12DAO is different from that of cationic C12DAO, and consequently depends on the degree of protonation of the surfactant.

Introduction Over the past few years, there has been notable progress in the quantitative characterization of adsorption of surfactants and increasing insights into the underlying mechanism of their cooperative self-assembly at the solid/ aqueous solution interface. In the consideration of adsorption of amphoteric amphiphiles at the hydrophilic silica surface from aqueous solution, the long ranged electrostatic interactions between headgroup and surface, together with the hydrophobic interactions between chain tails, play a key role. Surfactant adsorption to solid/ aqueous solution interfaces is a phenomenon of critical importance in many technological processes, including suspension stabilization, mineral flotation, detergency, wetting, lubrication, and fluid penetration into porous substrates. The surfactants can modify the interfacial properties significantly at very low concentrations, and they can form aggregates in solution and at the solid/ solution interface by hydrophobic interactions. In aqueous bulk solution amphiphiles have a characteristic critical micellization concentration (cmc); meanwhile in the presence of a solid surface the critical surface aggregation concentration (CSAC) depends on the local concentration of the surfactant at the interface, that is, the adsorption. The site-bound surfactant molecules at the solution/solid interface induce and constrain the surfactants to assemble at the interface below the bulk cmc. In the two-component surfactant-water system, where the accessible free space facilitates the self-assembly to occur at a higher concen* To whom correspondence should be addressed. E-mail: [email protected]. Telephone: + 358 2 215 4784. Fax: + 358 2 215 4706.

tration (cmc), the curvature of the aggregate is influenced by headgroup interactions with water and with each other. Phase studies of the binary surfactant-water systems relate the phase transitions and structures of the phases. For, for example, the dodecyl group substituted surfactants studied, the appearance of the liquid crystal phases is in the sequential order isotropic micellar solution, hexagonal, cubic, lamellar, and crystal phase at 25 °C.1-5 At the metal oxide/aqueous solution interfaces the self-assembly process is influenced by additional interactions such as surfactant-surfactant, surfactant-surface, surfactantsolvent, and solvent-surface interactions, including the free energy of adsorption, surface roughness, heterogeneity, charge, and crystallinity. At adsorption from dilute solution, monolayered aggregates (hemimicelles) often coexist with individually adsorbed surfactant molecules. At higher surfactant content in solution, bilayered structures (admicelles) or discrete globular aggregates often coexist with the hemimicelles.6-8 The formation of micellar self-assemblies by nonionic and cationic surfactants (1) Ekwall, P. In Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1975; Vol. 1, p 1. (2) Herrmann, K. W.; Brushmiller, J. G.; Courchene, W. L. J. Phys. Chem. 1966, 70, 2909. (3) Lutton, E. S. J. Am. Oil Chem. Soc. 1966, 43, 28. (4) Mol, L.; Bergenståhl, B.; Claesson, P. M. Langmuir 1993, 9, 2926. (5) Pettersson, A.; Rosenholm, J. B. Adsorption of Alkyldimethylamine and Alkyldimethylphosphine Oxides at Curved Aqueous Solution/ Silica Interfaces, Studied Using Microcalorimetry. Langmuir 2002, 18, 8436. (6) Adler, J. J.; Singh, P. K.; Patist, A.; Rabinovich, Y. I.; Shah, D. O.; Moudgil, B. M. Langmuir 2000, 16, 7255. (7) Łjtar, L.; Narkiewicz-Michałek, J.; Rudzin´ski, W.; Partyka, S. Langmuir 1994, 10, 3754. (8) Tiberg, F.; Brink, J.; Grant, L. Curr. Opin. Colloid Interface Sci. 2000, 4, 411.

10.1021/la0256275 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/04/2002

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in solution is ascribed to the balance between repulsive headgroup interactions and attractive forces between hydrocarbon chains arising from a need to minimize the exposure of the hydrophobic core to water.9 In solution, counterions stabilize ionic surfactant micelles by associating with the micelle surface and screening the electrostatic repulsion between the ionic headgroups. The net charge of the ionic micellar aggregates is less than the degree of micellar aggregation, indicating that a large fraction of counterions are closely associated with the micellar structures. Rathman and Scamehorn10,11 found that the fractional counterion binding on binary mixed surfactant micelles composed of cationic hexadecylpyridinium chloride and nonionic C12DAO or C12DPO can be modeled by an electrostatic approach to predict how the binding varies with the composition.The electrostatics is the major factor in determining counterion binding, meanwhile surfactant structure has only a secondary effect. The distribution of counterions through the ionic micelle is not clear. Some of the counterions form the Stern layer at the micellar surface and are consequently assumed to also affect the ζ-potential of the silica surface, when it is layered with adsorbed ionic surfactants and self-assembled aggregates such as the hemimicelles or the surface micelles.12-15 The effect of adsorption on interfacial phenomena such as the ζ-potential can be studied by measuring the streaming potential.16,17 However, the ζ-potential isotherm correlates satisfactorily with the adsorption isotherm only in a few solid/aqueous solution systems.18 The disagreement is influenced by factors such as porosity of substrate, cooperative adsorption, layered adsorption, counterion binding, ionic strength, dissolution of solid matter, inconstant pH, and specific adsorption of ions. The alkyldimethylamine and alkyldimethylphosphine oxides are monofunctional amphoteric surfactants that change from the net cationic to the net neutral state, when going from low to high pH. In the net neutral state they may be seen either as having distinct anionic and cationic charges (zwitterionic compound) or as belonging to the semipolar subclass of the nonionic surfactants. The amine and phosphine oxide surfactants studied become by reaction with a proton a hydroxide salt or alternatively a cationic conjugate acid (eq 1). Ignoring water on each side of the equation, the equilibrium of the hydrophilic headgroup of the amine oxides used (and the phosphine oxides used as well) is expressed by19-21 (9) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley-Interscience: New York, 1980. (10) Rathman, J. F.; Scamehorn, J. F. J. Phys. Chem. 1984, 88, 5807. (11) Rathman, J. F.; Scamehorn, J. F. Langmuir 1987, 3, 372. (12) Wanless, E. J.; Davey, T. W.; Ducker, W. A. Langmuir 1997, 13, 4223. (13) Huibers, P. D. T. Langmuir 1999, 15, 7546. (14) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548. (15) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 4447. (16) Somasundaran, P.; Zhang, L. In Adsorption on Silica Surfaces; Papier, E., Ed.; Surfactant Science Series 90; Marcel Dekker Inc.: New York, 2000. (17) Hough, D. B.; Rendall, H. M. In Adsorption from Solution at the Solid/Liquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: London, 1983; p 247. (18) Pettersson, A.; Marino, G.; Pursiheimo, A.; Rosenholm, J. B. J. Colloid Interface Sci. 2000, 228, 73. (19) Lomax, E. G. In Amphoteric Surfactants; Lomax, E. G., Ed.; Surfactant Science Series 59; Marcel Dekker Inc.: New York, 1996. (20) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: Chichester, 1999. (21) Laughlin, R. G. In Cationic Surfactants; Rubingh, D. N., Holland, P. M., Eds.; Surfactant Science Series 37; Marcel Dekker Inc.: New York, 1991; pp 1-40.

Pettersson and Rosenholm H3O+

R(CH3)2N+-OH w\ x R(CH3)2NfO H OH

R(CH3)2N+-O- (1) The headgroup of the two resonance forms of the net neutral compound is depicted with an arrow pointing toward the negative end of the bond (the semipolar group in the middle), or as a zwitterionic (to the right). The PfO group is less strongly hydrophilic than the NfO group.22 The longer chain alkyldimethylamine oxides have been used in various technological fields such as detergent formulation, cosmetics, viscosity builder, wetting agent, emulsifier, foam stabilizer, and antistat agent. The adsorption of zwitterionic surfactants to surfaces of powder precursors has wider interest because of its potential use in processing of ceramics.23,24 The adsorbent silica gel used is a noncrystalline, granular, sol-gel derived solid. Silica gel may be considered as a coherent, rigid three-dimensional network of contiguous particles of colloidal silica, and classified as microamorphous.25 The surface of silica gel consists of silanol groups (tSi-OH) and generally unreactive siloxane groups (tSi-O-Sit). The hydroxyl (silanol) groups, which have been identified by IR and NMR studies, can be divided into four different classes, that is, single isolated, geminal, vicinal, and hydrated vicinal silanols. The total number of hydroxyl groups of a fully hydrolyzed silica gel surface is about 5 per nm2, of which 0.3 are isolated non-hydrogen-bonded tSi-OH or dSi(OH)2 groups and 4.7 are bridged, Η-bonded silanol groups per nm2. The ratio for the surface concentration of geminal silanols dSi(OH)2 relative to tSi-OH is about 0.29.26-29 The site density of fully hydrated colloidal silica (with physically bound water) with a high specific surface area is believed to be higher and set at 8 sites nm-2.30 The exact surface structure of amorphous silica is unknown. In the presence of water, the silanol groups ionize, producing mobile protons that associate-dissociate with the surface to impart an electrical conductivity to the surface. As these groups dissociate, hydronium ions are produced which diffuse from the surface to develop a pHdependent surface charge and potential. However, in the pH range studied, the net ζ-potential and the relative surface charge density are very low and about constant (see below), making it a suitable model substrate. Silica has extensively been used as a support in heterogeneous catalysis, in membranes for separations, in microelectronic fabrication, and as an important component in several biomaterials, such as the bioactive glass-ceramics. Experimental Section Materials. The N,N-dimethyldecylphosphine-N-oxide and N,N-dimethyldodecylphosphine-N-oxide were kindly supplied by Dr. Robert G. Laughlin, The Procter & Gamble Co, Cincinnati, (22) Laughlin, R. G. In Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1978; Vol. 3, p 41. (23) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994. (24) Chavez, P.; Ducker, W.; Israelachvili, J.; Maxwell, K. Langmuir 1996, 12, 4111. (25) El Shafei, G. M. S. In Adsorption on Silica Surfaces; Papier, E., Ed.; Surfactant Science Series 90; Marcel Dekker Inc.: New York, 2000. (26) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface; Studies in Surface Science and Catalysis 93; Elsevier Science B. V.: Amsterdam, 1995. (27) Van Der Voort, P.; Gillis-D’Hamers, I.; Vansant, F. J. Chem. Soc., Faraday Trans. 1990, 86, 3751. (28) Zhravlev, L. T. Langmuir 1987, 3, 316. (29) Maciel, G. E.; Sindorf, D. W. J. Am. Chem. Soc. 1980, 102, 7606. (30) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; John Wiley & Sons: New York, 1979.

Adsorption of Amphoteric Oxides on Mesoporous Silica

Figure 1. Schematic illustration of the measuring cell used for determining the ζ-potential of the silica gel particles. OH, in their pure solid form.The N,N-dimethyloctylamine-Noxide (purity >99%), the N,N-dimethyldecylamine-N-oxide (>99%), and the N,N-dimethyldodecylamine-N-oxide (>98%) used were all supplied by Fluka. They were not further purified, but due to hygroscopicity they were dried in a vacuum and stored over P2O5. The SiO2 used was Davisil Silica gel and supplied by Aldrich. The manufacturer provided the following specifications: grade 644, 100-200 mesh, average pore diameter 150 Å, purity 99+%, specific surface area 300 m2 g-1, pore volume 1.15 cm3 g-1, pH (5% slurry) ) 7.0. The samples were dried in an oven at 150 °C for 4 h before use. Double checking using an Asap 20100 Sorptometer (by Micromeritics, Norcross, GA) gave the specific surface area 266 m2 g-1, which is used throughout the calculations. The pores of the granular silica gel can be featured as open spaces between the clustered primary sol particles. The water used in all experiments (F g 107 Ω cm, pH ) 5.7) was double distilled and subsequently passed through a Milli-Q water purification system. CMC Determination. The cmc’s of the surfactants used at natural pH, at which all phosphine oxide and almost all of the amine oxide is in the nonionic form (see pK1’s below), were determined from surface tension measurements using the Du Nou¨y ring method at 298 K. Determination of the ζ-Potential of Granular Silica Gel Using Streaming Potential Measurements. The ζ-potential of the SiO2 particles were measured using a PAAR EKA electrokinetic analyzer (Anton Paar K. G., Graz, Austria) equipped with a glass cylindrical cell, having filters at both ends keeping the plug of the particular sample in place (see Figure 1). Using the capillary bundle model by Fairbrother and Mastin,31 the ζ-potential for a capillary system is given by

UpηκB ζ) ∆P0

of the granules and to a smaller degree affected by the ζ-potential of the surface of the pores. However, the chemical properties of the exterior and the interior surfaces of the granule are supposed to be the same. Since the adsorption of surfactant and surface charge determining species to the surface of the mesopores is diffusion controlled, the instrument was operated in the manual mode, allowing equilibrium of adsorption to be reached in the porous silica plug before measurement was taken. The experiments were carried out by at first letting fresh aqueous solution flow through the cell. The volume of solution was 800 mL during measurement, and the amount of silica gel sample was approximately 260 m2 on average. Consequently, the lowering of the surfactant content of the solution due to adsorption was infinitesimal.5 For the same reasons, the increase in ionic strength due to dissolution of species from the silica surface at different pH values of solution is overlooked. The pH of the surfactant solutions (monitored with two decimals) did not drift during the experiments. During all streaming potential measurements, the ionic strength was fixed at I ) 0.001 mol dm-3 KCl. Determination of the Surface Charge Density. The relative surface charge density of the silica gel used was determined by the potentiometric titration method first developed by Bolt,32 and Parks and de Bruyn.33 Aqueous solutions of C ) 0.1 mol dm-3 HCl and KOH were used as standard solutions. The relative surface charge density versus pH was determined from the net adsorption densities Γi (mol m-2) by

[(

) (

(2)

)]

b b nH+ nH nOH- nOH + si si si si

σ0 ) F(ΓH+ - ΓOH-) ) F

(3)

where nH+ and nOH- are the total number of moles of H+ and OHb b added to the suspension, nH + and nOH- are the number of moles added to the blank (supernatant) to give the same pH, si is the powder surface area in each individual experiment, and F is the Faraday constant ()96 489 C mol-1). Since the surface of silica gel is highly heterogeneous on a local scale, all methods mentioned use sample sizes that well represent the system studied. All experiments in this study were carried out at T ) 25 °C, unless otherwise indicated.

Results Surface Chemical Properties of Silica. Ignoring water on each side of the equation, the equilibrium of the proton adsorption/desorption reactions for singly coordinated surface Si groups can be formulated as H3O+

OH-

OH

H3O

SiOH2+ w\ x SiOH y\ z SiO+

where Up is the streaming potential, η is the dynamic viscosity of the solution, ∆P is the pressure difference between the inlet and outlet of the capillary system,  is the permittivity of the test solution, 0 is the permittivity of free space, and κB is the specific electrical conductivity of the solution outside the sample. A measurement is carried out in such a range where the relation between UP and ∆P is linear. The quotient UP/∆P represents the slope of the pressure ramp. Because the specific electrical conductivity is the total of the conductivity of the solution and the surface conductivity of the sample, the results were corrected by carrying out measurements on the same systems but filled with a concentrated KCl solution at the ionic strength I ) 0.1 mol dm-3, suppressing the influence of the surface conductivity. The absolute values of the ζ-potentials corrected for surface conductivity were about 10% higher than the uncorrected ones. As illustrated in Figure 1, the particular silica gel is held in place as a tortuous plug between two filters in the sample cell. Because during an experiment solution mainly flows between the porous granules of the plug and to a negligible degree through the meso- or microchannels of the granules, the measured streaming potential is mainly attributed to the exterior surface (31) Fairbrother, F.; Mastin, H. J. Chem. Soc. 1924, 75, 2318.

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(4)

The association constant for the reaction to the left is at about log K ) -1.9, and it is at about log K ) -7.2 for the protonation of the oxo group to the right (eq 4).34 The estimated log K for the protonation of the Si-O-Si siloxane link is extremely low, log K ) -16.9, meaning that this group can be considered inert toward the process of protonation.35 Relative Surface Charge Density of SiO2 Studied. Figure 2 shows the net ζ-potential and relative surface charge density of the silica used versus the pH of the aqueous phase (adjusted using HCl or KOH). In comparison with other metal oxides, the silica gel used features a broad characteristic pH range from 3 to 9, over which the relative surface charge density and the net ζ-potential are very low and about constant.36 However, the negative (32) Bolt, G. H. J. Phys. Chem. 1957, 61, 1166. (33) Parks, G. A.; de Bruyn, P. L. J. Phys. Chem. 1962, 66, 967. (34) Hiemstra, T.; De Wit, J. C. M.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1989, 133, 105. (35) Hiemstra, T.; Van Riemsdijk, W. H.; Bolt, G. H. J. Colloid Interface Sci. 1989, 133, 91. (36) Persello, J. In Adsorption on Silica Surfaces; Papier, E., Ed.; Surfactant Science Series 90; Marcel Dekker Inc.: New York, 2000.

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Figure 3. Conductivity (left axes) and pH (right axes) of prepared C8DAO solutions vs surfactant content at 298 K. Figure 2. ζ-Potential and relative surface charge density of silica vs pH of aqueous phase (adjusted using HCl or KOH) at I ) 0.001 mol dm-3 KCl. The inset is the expansion of the axes around the IEP and PZC, respectively. Table 1. Measured CMC and pK1 of CnDAO Monomers and pKa of CnDPO Used5 surfactant C8DAO C10DAO C12DAO C10DPO C12DPO a

cmc (25 °C), mol dm-3 0.19a

1.9 × 10-2 2.0 × 10-3 4.2 × 10-3 4.4 × 10-4

pK1 (25 °C) 4.2 4.8 4.9

pKa

-1.5b -1.5b

Reference 41. b Reference 22.

ζ-potential features a local minimum at about pH ) 4.7 and a local maximum at about pH ) 5.5. The origins of these inflection points are not exactly known, but they are assumed to be related to the association constants of protonation of different surface groups, the partial solubility of silica, and the possible redeposition of dissolved matter. The point of zero charge, pHPZC ) 3.0, is somewhat lower than the IEP, pHIEP ) 3.5, indicating that the K+ cations of the background electrolyte, added during the streaming potential measurements, adsorb more specifically to the negatively charged surface than the Cl- anions do to the net positively charged surface, and so shift the IEP to a higher pH than the pHPZC.37 The affinity of silica for the K+ counterion results from the ability of large cations to lose their water of hydration and form polar bonds with the structural oxygen of silica. This has been found to increase the surface charge density and result in a decrease in the PZC and consequently in an increase in the IEP.36,37 It is well-known that silica is slightly soluble at low pH and considerably soluble at high pH.30 The time scales of the PZC and the IEP measurements were however not exactly the same, implying that redeposition of dissolved material is possible. Purity, CMC, and Ionizability of the Surfactants Studied. The cmc values determined, with the exception of that for C8DAO, are summarized in Table 1. The absence of an obvious minimum in the surface tension curves measured and the agreement of determined cmc values with literature data confirmed the purity and the accuracy of prepared solutions of the surfactants.38-41 The protonation of the amphoteric alkyldimethylamine oxide (37) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: London, 1995; Vol. 2.

monomers used was recently studied using potentiometric titration and was reported as pK1 values (index 1 refers to surfactant monomer). In Table 1 are cited some data necessary to discuss the present study.5 The alkyldimethylphosphine oxides studied are nonionic surfactants at the pH values studied. Their pKa values are very low and found in the literature to be ) -1.5 for both C10DPO and C12DPO.22 Influence of Adsorbed Amphoterics on the ζ-Potential of SiO2. It is well established that one of the major factors influencing the adsorption of surfactant ions at any interface is that of electrical interactions within the environment of an electrical double layer. Among the ionic species that may adsorb specifically are the counterions which are recognized to play a critical role in the adsorption mechanism of ionic surfactants and the shape formation of the surface aggregates. In this study the solutions of the alkyldimethylamine oxides, from which the adsorption occurs, consist of more or less mixtures of the nonionic and cationic forms of the surfactant. It was previously found that all the cationic alkyldimethylamine oxide species and dissociated silanol groups did not loose all their counterions and hydrated water when interacting electrostatically with each other.5 As a property of the interface at the solid/aqueous solution slipping plane, the ζ-potential reflects the electrostatic interactions of the adsorption process such as the degree of ionization of surface aggregates of different structure, and the counterion binding to the surface layer. Effect of C8DAO Adsorption. The adsorption of shortchain C8DAO onto the silica/aqueous solution interface at natural pH was previously studied using microcalorimetry. It was found that the incomplete adsorption isotherm had a local plateau at a formed submonolayer much below the cmc. The displacement enthalpy was exothermic in the whole concentration range studied, indicating hydrogen bondings between headgroups and the energetically heterogeneous silica gel surface.5 At natural pH, less than 2% of C8DAO is ionized in the most dilute solutions, and this amount drops considerably when the content is increased and consequently pH increases (see Figure 3). The decrease in the degree of ionization of (38) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; Nat. Stand. Ref. Data. Ser.; National Bureau of Standards (U. S.): Washington, DC, 1970. (39) Herrmann, K. W. J. Phys. Chem. 1962, 66, 295. (40) Imaishi, Y.; Kakehashi, R.; Nezu, T.; Maeda, H. J. Colloid Interface Sci. 1998, 197, 309. (41) Corkill, J. M.; Herrmann, K. W. J. Phys. Chem. 1963, 67, 934.

Adsorption of Amphoteric Oxides on Mesoporous Silica

Figure 4. ζ-Potential of silica against the content of C8DAO in the liquid phase at natural pH (5.88-7.75) and at pH ) 4 (adjusted using HCl). The ionic strength was 0.001 mol dm-3 KCl. At natural pH, less than 2% of C8DAO is cationic in solution, and at pH ) 4, about 61% of C8DAO is cationic in solution.

the C8DAO seems to have only a minor effect on the conductivity of the surfactant solutions studied (Figure 3). Adsorption of C8DAO at natural pH in the low-affinity region affects the ζ-potential of the silica surface by only a slight change toward a positive value in Figure 4. At constant pH ) 4 (adjusted with HCl) about 61% of C8DAO is protonated whereas the silica surface is weakly net negatively charged (Figure 2). Both the cationic and the nonionic C8DAO adsorb. The cationic C8DAO species adsorb readily onto the oppositely charged silica surface due to long ranged electrostatic interactions, with the headgroups orientated toward the surface. The negatively charged surface sites act as counterions to some adsorbed cationic species. This results in, when increasing the concentration from dilute solution, a neutralization of the potential at the slipping plane and reversal of the sign followed by an enhancement of the positive ζ-potential with a local plateau of 2.2 mV at the content of about 0.016 mol kg-1 in solution. Since the cmc of C8DAO increases when the pH is lowered (due to electrostatic repulsion between cationic headgroups of the micelle), the adsorption is expected to reach its saturation at a much higher bulk solution molality than 0.016 mol kg-1. At pH ) 4, it therefore seems to be some kind of specific counterion binding above the bulk molality of 0.016 mol kg-1 (See Figure 4). Effect of C10DPO and C12DPO. The surface excesses at saturated adsorption of nonionic C10DPO and C10DAO at natural pH’s were previously found to be about the same, that is, Γmax ) (7.4-7.5) × 10-6 mol m-2.5 Since the pKa of the cationic phosphine oxide headgroup is very low, C10DPO and C12DPO are totally net neutral at moderate pH, and the CMC’s respectively are not affected by a moderate decrease in pH. The effect of the adsorption of C10DPO and C12DPO on the ζ-potential of silica is seen in Figure 5, where the ζ-potential is unaffected at constant pH, with the levels correlating with the ζ-potential values of SiO2 in Figure 2. At natural pH the ζ-potential of the silica/aqueous solution interface versus the C12DPO molality features a local maximum at m ) 0.0002 mol kg-1, which is recognized as the local maximum in ζ-potential of SiO2 in Figure 2 in the pH range 5-6. Effect of C10DAO Adsorbing at pH ) 4. The pK1 of the amphoteric C10DAO monomer is about 4.8 (see Table 1), at which pH half of the number of surfactant monomers

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Figure 5. ζ-Potential of silica vs the content of C10DPO in solution at pH ) 4 and at pH ) 7.5 (lower x-axes), and vs the content of C12DPO in solution at natural pH (upper x-axes). pH was adjusted using HCl or KOH. Electrolyte content was 0.001 mol dm-3 KCl.

Figure 6. ζ-Potential of silica against the content of C10DAO in solution at pH ) 4 (adjusted using HCl) and at I ) 0.001 mol dm-3 KCl. About 86% of the C10DAO monomers in solution are cationic.

are protonated to cationic C10H21(CH3)2N +-OH. At pH ) 4 about 86% of the C10DAO are cations, and the silica surface is slightly net negatively charged (see Figure 2). The ζ-potential of the silica surface against the C10DAO molality in solution at pH ) 4 is shown in Figure 6. The ζ-potential behavior resembles that of C8DAO in Figure 4; the main difference is the plateau value of about 3.9 mV, compared to 2.2 mV of C8DAO. The aqueous solutions of the amphoterics studied are Brønsted alkaline in nature. The measured natural pH and the conductivities of prepared aqueous C10DAO solutions are shown in Figure 7. The conductivity has a maximum at about pH ) 6.75, at which only 1% of the species in solution are present in the protonated form. Increasing the C10DAO content up to the cmc results in an increase in pH and consequenly a higher proportion of nonionic species and a slight lowering of the conductivity. Neat water has a conductivity of 0.01 mS m-1. The natural pH’s of the C10DAO solutions are within the pH range at which the ζ-potential and surface charge density of the silica studied are about constant and very low (Figure 2).

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Figure 7. Conductivity (left axes) and pH (right axes) of prepared C10DAO solutions vs the surfactant content at 298 K.

Figure 9. Influence of pH on the ζ-potential of the silica surface vs the content of C12DAO in the liquid phase. At natural pH the content of ionized C12DAO in dilute solution was about 11%, and it was 0.8% at the cmc. At pH ) 4 (adjusted using HCl) the content of ionized C12DAO was about 90%. The different regions of both graphs, denoted I-IV (natural pH) and 1-2 (pH ) 4), are discussed in the text. The ionic strength was 0.001 mol dm-3 KCl in both experiments.

Figure 8. Conductivity (left axes) and pH (right axes) of prepared aqueous C12DAO solutions vs surfactant content at 298 K.

Effect of C12DAO Adsorption. Increasing the chain length further with -C2H4- does not result in a substantial change in the pH and conductivity profile of aqueous C12DAO solutions compared to C10DAO solutions (Figures 7 and 8). The conductivity features a maximum below the cmc, when the pH is about two units higher than the pK1 of C12DAO, after which the conductivity decreases very slowly. At around the cmc, the pH changes to higher values, indicating that nonionic C12DAO picks up H+ from the bulk solution upon micellization (Figure 8). A small degree of protonation is probably favorable and results in Η-bonded headgroups of cationicnonionic C12DAO surfactant couples (tN+-O-H‚‚‚ONt).42-44 It was previously found that these exothermic intramicellar H-bondings contributed considerably to the measured displacement enthalpies of C12DAO adsorbing and aggregating at the aqueous solution/silica interface.5 It was previously found that, at natural pH, the adsorption isotherm of C12DAO at the silica/aqueous solution interface was more complex than the corresponding isotherm of C10DAO.5 Figure 9 shows the wavelike ζ-potential of the silica slipping plane versus (42) Brackman, J. C.; Engberts, J. B. F. N. Langmuir 1992, 8, 424. (43) Maeda, H. Colloids Surf., A 1996, 109, 263. (44) Tereda, Y.; Maeda, H.; Odagaki, T. J. Phys. Chem. B 1997, 101, 5784.

the C12DAO molality in solution at natural pH (see Figure 8) and at pH ) 4 (adjusted using HCl). At natural pH the degree of protonation R1 ) 11% at the initial adsorption; meanwhile R1 ) 0.8% at the cmc due to the natural rise in pH. Over the natural pH range of C12DAO solutions studied (Figure 8), the silica surface is weakly net negatively charged (Figure 2). Because of adsorption of cations from dilute solution, the initial ζ-potential of -1.2 mV is neutralized at about the molality of 0.0005 mol kg-1, but it changes back to negative ζ-potential as the adsorption continues. After having reached a local minimum of about -1.2 mV at about 0.001 mol kg-1 in solution, the ζ-potential passes the zero point due to the adsorption of cationic C12DAO and levels off at about 1.2 mV due to saturated adsorption at about the cmc. At pH ) 4, when the silica surface is weakly net negatively charged (Figure 2) and C12DAO is considerably protonated (90% of the amine oxide groups), the electrostatic interactions are more extended, and the ζ-potential is enhanced about 5-fold compared to that for adsorption at natural pH, but it starts to drop when the molality of CD12AO exceeds 0.0015 mol kg-1 in solution (See Figure 9). At pH ) 4 the cmc of C12DAO is about 5.8 × 10-3 mol dm-3, at which the surface excess is assumed to level off. 39 Discussion Effect of the Ionic Strength I ) 0.001 mol dm-3 KCl. The addition of salt reduces the electrostatic repulsion between headgroups through the shielding effect and lowers the curvature of the aggregate,12 resulting in a lowering of the cmc of nonionic and mixed nonioniccationic amine oxide surfactant solutions,39,45 an increase of the radii46 and the aggregation number of mixed (45) Ikeda, S.; Tsunoda, M. A.; Maeda, H. J. Colloid Interface Sci. 1979, 70, 448.

Adsorption of Amphoteric Oxides on Mesoporous Silica

nonionic-cationic micelles,39,45 and a closer packing and hence a smaller effective cross-sectional area per headgroup of adsorbed ionic surfactant.20 However, when surfactant ions compete with salt coions for the oppositely charged surface sites (charge reversal), the effect of the salt addition gives rise to a decrease in surfactant adsorption.47 Because of the low level of KCl background electrolyte added to the solutions used (I ) 0.001 mol dm-3) in the streaming potential experiments, most of the mentioned effects are safely overseen in the discussions. Nevertheless, the possible adsorption of the electrolyte makes the interpretation of the measured ζ-potentials more difficult. Effects of C10DAO Adsorbing at pH ) 4. We previously found by combined calorimetry and adsorption studies at natural pH indications that nonionic C10DAO and C10DPO form globular surface micelles at the same silica/aqueous solution interface as in this work, except for the content of 0.001 M KCl in this study.5 The cmc of C10DAO at natural pH is 0.019 mol dm-3 (Table 1). Eads and Robosky48 reported the cmc of C10DAO, at pH ) 3, to be 0.0326 mol dm-3. Hence, at pH ) 4 the adsorption is expected to saturate at the molality of solution between the two values. In Figure 6 the ζ-potential is neutralized, the sign is reversed, and it increases due to the adsorption of C10DAO cations. The slope of the ζ-potential starts to change at the molality 0.0075 mol kg-1, which is interpreted as the CSAC and the onset of counterion binding to form surface self-assemblies. (Since for dilute solutions the numerical values of molarity and molality are very close, we use the abbreviation CSAC in the following.) The proportion of cationic C10DAO is about 86%. Cationic quaternary ammonium surfactants are believed to form spherical micelle-like aggregates at the silica/solution interface;15,49,50 meanwhile nonionic alkylpoly(ethylene oxide) and C10DAO surfactants with short and medium chain lengths have been reported to form aggregates such as spherical, ellipsoidal, or larger bilayer-type structures.51-55 At the aqueous solution/polystyrene latex interface, both nonionic and cationic C12DAO form monolayers at saturated adsorption, with the headgroups facing the solution.56 Kira´ly and Findenegg55 reported the adsorption of C10DAO on silica glass to be a reversible process in the whole range from dilute solution to well over the cmc at natural pH. The same is assumed for all surfactant solution/silica systems studied in this work. We believe that the ζ-potential isotherm in Figure 6 indicates formation of spherical C10DAO micelles at pH ) 4. Influence of pH on the ζ-Potential of SiO2 in Aqueous C12DAO Solutions. In our previous paper, evidenced by sorption calorimetry measurements, we reported probable formation of spherical C12DAO aggregates at the same solution/silica interface, with the exception of the background electrolyte added to the streaming potential experiments in this paper. Since the (46) Maeda, H.; Muroi, S.; Kakehashi, R. J. Phys. Chem. B 1997, 101, 7378. (47) Mehrian, T.; de Keizer, A.; Lyklema, J. Langmuir 1991, 7, 3094. (48) Eads, C. D.; Robosky, L. C. Langmuir 1999, 15, 2661. (49) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (50) Liu, J.-F.; Min, G.; Ducker, W. A. Langmuir 2001, 17, 4895. (51) Levitz, P.; Van Damme, H. J. Phys. Chem. 1986, 90, 1302. (52) Tiberg, F. J. Chem. Soc., Faraday Trans. 1996, 92, 531. (53) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288. (54) Rutland, M. W.; Senden, T. J. Langmuir 1993, 9, 412. (55) Kira´ly, Z.; Findenegg, G. H. Langmuir 2000, 16, 8842. (56) Alargova, R. G.; Vakarelsky, I. Y.; Paunov, V. N.; Stoyanov, S. D.; Kralchevsky, P. A.; Mehreteab, A.; Broze, G. Langmuir 1998, 14, 1996.

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Figure 10. Proposed schematic molecular depiction (snapshots) of the different regions of the ζ-potential isotherm and adsorption of C12DAO at the silica gel/aqueous solution interface at natural pH to the left, and at pH ) 4 to the right. The regions marked I-III and 1-2 are described in the text and seen in Figure 9 as well. O refers to a net nonionic, and b refers to a protonated headgroup.

solution molality at the sudden onset of hydrophobic cooperative interactions of adsorbates at the solution/solid interface did not coincide with the solution molality at the onset of high-affinity adsorption, we concluded the occurrence of two different CSAC’s.5 Figure 10 depicts the molecular interpretation of the different regions of the ζ-potential isotherm of C12DAO in Figure 9 and the adsorption process, at natural pH to the left (Roman numerals) and at pH ) 4 to the right (Arabic numerals). It was previously evidenced that all cationic surfactant species and dissociated silanol groups do not lose all their counterions and hydrated water when interacting electrostatically with each other.5 In dilute solutions at natural pH the ζ-potential of SiO2 increases due to the adsorption of C12DAO cations through electrostatic interactions, with the counterions of some surfactant species staying in the diffuse part of the double layer of the solid (region I, Figures 9 and 10). The negatively charged surface sites act as counterions to some of the adsorbed cations. The nonionic species adsorb also, but through Η-bonding. At around the molality m ) 0.0005 mol kg-1, when the ζ-potential becomes slightly positive, the surface aggregation onsets and the ζ-potential reverses its sign and increases back to about the initial negative ζ-potential () -1.2 mV) at about m ) 0.001 mol kg-1. It was previously shown that, in this low-affinity adsorption interval (region II), hemimicelles are suddenly formed by hydrophobic interactions induced by the silica surface with the adsorbed species.5 This inflection point in Figure 9 is recognized as the first CSAC. According to Figure 8, the proportion of cationic species in solution decreases at this interval (since the natural pH increases), after which both remain about constant up to the cmc. The charge reversal or ζ-potential reversal at about the molality m ) 0.0005 mol kg-1 occurs probably due to the adsorption of Cl- counterions to cationic headgroups of adsorbed C12DAO species connected with the formation of the hemimicelles (Figure 10, II).57 The local minimum in Figure 9 is recognized as the second CSAC and arises due to the change of the adsorption mechanism. It was previously shown that interval III in Figure 9 is a high-affinity adsorption region, at which surface micelles are likely to be formed.5 In this region the proportion of cationic C12DAO remains about constant (Figure 8). Generally, in a micellar solution of cationic surfactant the net charge of the aggregates of random shape is less, however not zero, than the degree (57) Fuerstenau, D. W.; Colic, M. Colloids Surf., A 1999, 146, 33.

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of micellar aggregation, indicating that a large fraction of counterions are associated with the micellar structures.20 In mixed nonionic-cationic micelles the counterion binding is expected to be less.11 The formed dimers of cationic-nonionic pairs of C12DAO are supposed to behave as cationic species. Because of the extended adsorption, including cations, the ζ-potential increases and obtains the same values as those in region I, but the mechanism of the associative adsorption in region III is different and remains the same when the ζ-potential is passing zero, increases, and levels off at around the cmc.5 Since it is likely that the cationic C12DAO species are unevenly distributed in the globular surface aggregates, a flattened shape such as an ellipsoid is probable. Above the cmc the ζ-potential remains about constant in region IV in Figure 9 despite the increase in the pH of the solution (Figure 8) and the decrease in the degree of protonation of C12DAO. The effect may be explained by a reduced counterion bonding due to interactions between neighboring C12DAO globular surface aggregates. The formation of a bilayered structure having the headgroups of the second templated layer facing the liquid phase (Figure 10, III) is considered improbable, since in our previous paper it was found that the constraint on the second layer would be very strong.5 At pH ) 4 the electrostatic headgroup-solid interactions are extensive, since the silica surface is weakly net negatively charged (Figure 2) and the proportion of cationic monomers in solution R1 ) 90%. The ζ-potential isotherms of C12DAO (Figure 9) and C10DAO (Figure 6) are similar with respect to the behavior in dilute solutions, as they both pass beyond zero to positive values without featuring hemimicelle formation. Figure 9 shows a maximum of 5.45 mV at m ) 0.0015 mol kg-1, which is somewhat higher than the plateau value of C10DAO in Figure 6. In contrast to the ζ-potential isotherm of C10DAO, the ζ-potential of C12DAO drops to a level of about 2.4 mV at about the cmc. The breakpoint at about m ) 0.001 mol kg-1 is assumed to be the CSAC, and it reflects the change in the counterion bonding. Imaishi et al.40 found that, in aqueous nonionic-cationic micellar solutions of C12DAO in all pH range, the degree of ionization of the species composing the micelles (RM) is lower than the degree of ionization of the monomers (R1). At pH ) 4, for example, they reported RM ≈ 0.46 and R1 ≈ 0.85, meaning that the micelles consist roughly of only cationic-nonionic dimers. The net charge of such aggregates is less, however not zero, than half the number of the micellar aggregation. Assuming formation of discrete micellar-type surface aggregates of C12DAO in region 2 in Figure 9, the aggregation number is supposed to increase with increasing adsorption. It seems reasonable to believe that the exponential decline of the ζ-potential indicates an increase in the degree of counterion binding to the micellar structures at the solution/solid interface. In this way the counterions stabilize ionic surfactant micelles by binding to the micelle surface and screening the electrostatic repulsions between the ionic headgroups (Figure 10, 2).20

Pettersson and Rosenholm

Concluding Remarks In the present article we study the ζ-potential of granular silica gel interacting with aqueous solutions of amphoteric alkyldimethylamine oxides (CnDAO with n ) 8, 10, and 12) and alkyldimethylphosphine oxides (CnDPO with n ) 10 and 12). The nonionic phosphine oxide homologues were chosen as references. The ζ-potential was determined by measuring the streaming potential across the capillary of the plug of sample. The effects of adsorbing the nonionic as well as the cationic character of the amine oxide surfactants were studied. The ζ-potential isotherms of the alkyldimethylamine oxides studied featured a break point recognized as the CSAC and indicated the onset of increased counterion binding to form surface aggregates. The ζ-potential of SiO2 leveled off at about the cmc. Because of mixed nonioniccationic solutions of the alkyldimethylamine oxide surfactants studied, all the surface aggregates at the cmc are positively charged. The two previously observed CSAC’s of C12DAO at the solution/SiO2 interface at natural pH, measured using sorption calorimetry, are recognized in the ζ-potential isotherm, having however somewhat higher values. At natural pH the ζ-potential reversal is initiated at about the first CSAC as a result of the onset of hemimicelle formation. The second CSAC in the ζ-potential isotherm is an inflection point at which the adsorption mechanism changes. The fractional counterion binding to surface aggregates, composed of assumedly almost only cationicnonionic C12DAO dimers at pH ) 4, varies exponentially with the surfactant content in solution, starting from zero at about the CSAC and approaching a plateau at the cmc. Nonionic C12DAO forms probably globular ellipsoidal aggregates; meanwhile cationic C10DAO and C12DAO form spherical surface micelles at the aqueous solution/ SiO2 interface. The adsorption mechanism of nonionic C12DAO is different from that of cationic C12DAO, and consequently depends on the degree of protonation of the surfactant. The use of the EKA instrument for the application in this work has proved to be a sensitive enough method to measure the small ζ-potential of mesoporous silica over the pH range 3-8 and record the small changes in streaming potential across the sample when changing the content of cationic surfactant species in solution. However, the ζ-potential measurements should be combined with measurements of adsorption isotherms, calorimetry, and a structure sensitive method like AFM in order to attain a more complete picture of the induced surface selfassembly, aggregate structures, and adsorption mechanisms. Acknowledgment. We thank Dr. Robert G. Laughlin, The Procter & Gamble Company, Cincinnati, OH, for kindly supplying the samples of phosphine oxides. This work was supported by The Foundation of Åbo Akademi University through the grant to A.P.. LA0256275