Forces between Glass Surfaces in Mixed Cationic−Zwitterionic

Langmuir 2004, 20, 4553-4558

4553

Forces between Glass Surfaces in Mixed Cationic-Zwitterionic Surfactant Systems William J. Lokar and William A. Ducker* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061 Received December 27, 2003. In Final Form: February 28, 2004 We report atomic force microscopy (AFM) measurements of the forces between borosilicate glass solids in aqueous mixtures of cationic and zwitterionic surfactants. These forces are used to determine the adsorption of the surfactant as a function of the separation between the interfaces (proximal adsorption) through the application of a Maxwell relation. In the absence of cationic surfactant, the zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS) undergoes little adsorption to glass at concentrations up to about 2/3 critical micelle concentration (cmc). In addition, DDAPS does not have much effect on the forces over the same concentration range. In contrast, the cationic surfactant dodecylpyridinium chloride (DPC) does adsorb to glass and does affect the force between glass surfaces at concentrations much lower than the cmc. In the presence of a small amount of DPC (0.05 mM ) cmc/300), the net force between the glass surfaces is quite sensitive to the solution concentration of DDAPS. A model-independent thermodynamic argument is used to show that the surface excess of DDAPS depends on the separation between the glass interfaces when the cationic surfactant is present and that the surface excess of the cationic surfactant is more sensitive to interfacial separation in the presence of the zwitterionic surfactant. The change in adsorption of the zwitterionic surfactant is explained in terms of an intermolecular coupling between the long-range electrostatic force acting on the cationic surfactant and the short-range hydrophobic interaction between the alkyl chains on the cationic and zwitterionic surfactants. The adsorptions of cationic and zwitterionic surfactants in mixtures were measured independently and simultaneously by attenuated total internal reflection infrared spectroscopy (ATR-IR). The adsorption of the zwitterionic surfactant is enhanced by the presence of a small amount of cationic surfactant.

Introduction Surfactants are frequently used to control the stability of colloidal particles. Consequently, the relationship between colloidal stability and adsorption to isolated solids has been studied for many years. Recently, we and others have studied the changes in adsorption of surfactant during the collision between two solids in aqueous solution.1-10 Changes in adsorption of species i, ∆Γi, are obtained experimentally from measurement of the interaction energy, Ea, between the solids as a function of the chemical potential of i, µi, as originally shown by Hall11 and by Everett and co-workers:12

( )

1 ∂Ea ∆Γ ) Γi(s) - Γi(∞) ) 2 ∂µi

librium is established, so applications are limited to low molecular weight compounds. In recent work, we13 and others14 have shown that surfactant adsorption to an isolated surface is very rapid. In previous work, we proposed that an accurate description of the adsorption regulation of cationic surfactant during particle collisions required a term in the chemical potential, RTA(θ), to account for the net attractive interactions between the surfactant molecules (particularly interactions between the alkyl chains) in water:2,3

µ ) µ0 + RT ln (1)

p,T,µj,s

The derivative is calculated at constant pressure, p, temperature, T, chemical potential of all other species, j, and separation between the surfaces, s. This equation can only be applied to situations in which adsorption equi* Corresponding author: e-mail [email protected]. (W.J.L.) e-mail [email protected]. (1) Subramanian, V.; Ducker, W. J. Phys. Chem. B 2001, 105, 1389. (2) Lokar, W. J.; Ducker, W. A. Langmuir 2002, 18, 3167. (3) Lokar, W. J.; Ducker, W. A. Langmuir 2004, 20, 378. (4) Lokar, W. J.; Koopal, L. K.; Leermakers, F. A. M.; Ducker, W. A. J. Phys. Chem. B 2004, 108, 3633. (5) Pethica, B. A. Colloids Surf. A 1986, 20, 151. (6) Pethica, B. A. Colloids Surf. A 1995, 105, 257. (7) Podgornik, R.; Parsegian, V. A. J. Phys. Chem. 1995, 99, 9491. (8) Yaminsky, V.; Jones, C.; Yaminsky, F.; Ninham, B. W. Langmuir 1996, 12, 3531. (9) Yaminsky, V. V.; Ninham, B. W.; Christenson, H. K.; Pashley, R. M. Langmuir 1996, 12, 1936. (10) Christenson, H. K.; Yaminsky, V. V. Colloids Surf. A 1997, 129130, 67. (11) Hall, D. G. J. Chem. Soc., Faraday Trans. 2 1972, 68, 2169. (12) Ash, S. G.; Everett, D. H.; Radke, C. J. Chem. Soc., Faraday Trans. 2 1973, 69, 1256.

(1 -θ θ) + zF ψ(s) + RTA(θ) A

(2)

where θ is the fractional coverage of the surface by the surfactant, z is the valence of the surfactant, FA is Faraday’s constant, ψ(s) is the separation-dependent surface potential, and A(θ) accounts for the short-range interactions between the surfactant molecules. Inclusion of the RTA(θ) term is consistent with the Frumkin (and more complex) adsorption isotherms that are commonly used to describe surfactant adsorption to isolated surfaces.15 Here we examine forces and adsorption in aqueous solutions containing both a net uncharged zwitterionic surfactant (N-dodecyl-N,N-dimethyl-3-ammonio-1propanesulfonate or DDAPS) and a cationic surfactant (dodecylpyridinium chloride or DPC). Our aim is to examine how forces and adsorption are affected by interactions between alkyl chains on different types of surfactants. Earlier work from our laboratory showed that the addition of a small amount of cationic surfactant to (13) Clark, S.; Ducker, W. A. J. Phys. Chem. B 2003, 107, 9011. (14) Atkin, R.; Craig, V. S. J.; Biggs, S. Langmuir 2001, 17, 6155. (15) Prosser, A. J.; Franses, E. I. Colloids Surf. A 2001, 178, 1.

10.1021/la036459z CCC: $27.50 © 2004 American Chemical Society Published on Web 04/21/2004

4554

Langmuir, Vol. 20, No. 11, 2004

Lokar and Ducker

DDAPS solution caused a large increase in repulsive forces on a mica sheet.16 All of our experiments are performed for a small surface area compared to the solution volume, so the chemical potential is very nearly constant during adsorption. Experimental Section Reagents. Dodecylpyridinium chloride (Aldrich, Milwaukee, WI) was recrystallized three times from a 10:1 mixture of HPLC-grade acetone and methanol. N-Dodecyl-N,N-dimethyl3-ammonio-1-propanesulfonate (Sigma, St. Louis, MO) was recrystallized three times from 2-propanol (HPLC-grade). There was no minimum in a plot of surface tension vs concentration for either surfactant. Water was prepared by an EASYpure UV system (model D7401, Barnstead Thermolyne Co., Dubuque, IA). The water had a conductivity of 18.3 MΩ/cm at 25 °C and a surface tension of 72.3 mN/m at 22.5 °C. By the time that we finished preparing solutions, CO2 from the air had equilibrated in the solution, so all our experiments are at pH ≈ 6. KCl (Aldrich, Milwaukee, WI) and NaOH (Fischer Scientific, Pittsburgh, PA) were each roasted in air at 300 °C to decompose organic impurities. HCl (EM Science, Gibbstown, NJ) was used as received. D2O (99.9%, Cambridge Isotope Laboratory) used for ATR-IR experiments was distilled before use. Force Measurements. Forces between a ∼3 µm radius borosilicate glass particle and a flat borosilicate glass sheet were measured by atomic force microscopy (AFM) at 22 °C as described previously.3 All measurements were performed in a background of aqueous 1 mM KCl. Adsorption to “Isolated” Surfaces. Adsorption to “isolated” surfaces was measured by attenuated total reflectance infrared (ATR-IR) spectroscopy at 25 °C as described previously.13 The spectra were recorded on a Nicolet Nexus 670 Fourier transform infrared spectrometer (Madison, WI). The conditions for these measurements were not the same as for the AFM measurements. All isotherms were measured in D2O rather than in H2O, so as to avoid the overlap between OH and CH absorption bands. Furthermore, the solid substrate was a silicon internal reflection element (IRE) (Wilmad, Buena, NJ) with a native layer of silicon oxide, rather than the glass surfaces used in the AFM experiments. The silicon IRE was prepared by rinsing with a large volume of ethanol (100%, Aaper, Shelbyville, KY) and a large volume of water until the water wet the silicon IRE. The crystal was dried with a stream of nitrogen and each side of the crystal was exposed to UV light from a UV Pen Ray lamp (UVP, Upland, CA) for 30 min. Adsorption to the silicon IRE may differ slightly from that on the glass surfaces used in the force measurements because of the slightly different solid and solvent. Therefore, we restrict our discussion to the general trends in the adsorption isotherms obtained with ATR-IR. The isotherms were measured by sequential addition of a stock surfactant solution to a recirculating reservoir. Each concentration was permitted to equilibrate for 45 min in a temperaturecontrolled ATR cell before spectra were taken. Each spectrum consisted of 256 scans at 2 cm-1 resolution at 25 °C. When there was only one surfactant present, the adsorption was obtained from integration of the aliphatic CH stretch in the range 30002800 cm-1. Figure 1 shows sample spectra for adsorption from a surfactant mixture. In a mixture of surfactants, the adsorption of DPC was calculated from integration of the pyridinium CH stretch in the range 3160-3035 cm-1. The known adsorption of DPC was used to calculate the DPC contribution to the aliphatic CH stretch. The remainder of the aliphatic CH stretch absorption was then attributed to DDAPS and used to calculate the DDAPS adsorption. Quantitation of the area of the weak pyridinium CH stretch was complicated by absorption by a trace amount of H2O. We were able to account for this absorption with measurements in the absence of surfactant and in DPC-only solutions. The calculation of surface excess is described in detail elsewhere,13 so only the main assumptions are stated here. To obtain a surface excess of surfactant from the absorption, it is (16) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915.

Figure 1. Sample ATR-IR spectra in D2O on an oxidized silicon crystal in the presence of 0.75 mM DDAPS, 1 mM KCl, and either 16.0 mM DPC (upper curve) or 6.0 mM DPC (lower curve). The absorption bands at 3160-3025 cm-1 are the CH stretching bands for the pyridinium ring. The bands in the range 30002800 cm-1 are the symmetric (νs) or asymmetric (νas) CH stretching bands for the methyl and methylene groups in the hydrocarbon chain.

Figure 2. Adsorption of DDAPS or DPC to the thin oxide layer on a silicon ATR crystal as a function of surfactant concentration in 1 mM KCl in D2O. The plateau adsorption values are 4.1 molecules/nm2 for DDAPS and 3.1 molecules/nm2 for DPC. Arrows indicate the cmc of each surfactant in 1 mM KCl. necessary to subtract the absorption that would occur if the bulk concentration of surfactant continued up to the solid. This subtraction was made by use of the measured molar absorption coefficient of the surfactant and the calculated profile of electrical field magnitude as a function of depth into the solution, z. The decay of this electric field is about 200 nm. Because the electric field decays as a function of distance into the solution, the absorption due to the surface excess depends in principle on the density of surfactant as a function of z. We assume that the surface excess is concentrated in the region immediately adjacent to the solid (z , 200 nm) and we use the magnitude of the electric field at the interface (z ) 0). Furthermore, the surface excess is calculated per cross-sectional area at the interface. This is the same as the interfacial area for a perfectly smooth surface but can lead to an overestimate of the surface excess when the surface is rough.

Results and Discussion Figure 2 shows the adsorption to the oxidized surface of a silicon ATR element from D2O solutions for DDAPS only and DPC only. All solutions also contain 1 mM KCl. DPC and DDAPS have the same alkyl chain length (C12), so differences in adsorption can be attributed to the surfactant headgroups. The adsorption of DPC begins at much lower concentrations because of the attractive force between the positively charged surfactant and the negatively charged surface. For example, a surface excess of 0.5 molecule/nm2 is obtained in 0.1 mM DPC, whereas 2 mM DDAPS is required to reach the same surface excess.

Forces between Glass Surfaces in Mixed Surfactants

Figure 3. Adsorption of DDAPS and DPC to the thin oxide layer on a silicon ATR crystal as a function of DDAPS concentration in D2O, 0.05 mM DPC, and 1 mM KCl. The isotherm of DDAPS in the absence of DPC is given for comparison. The adsorption of DDAPS is greater in the presence of DPC and the step shifts to lower DDAPS concentrations. The adsorption of DPC increases near the step in the DDAPS isotherm and then decreases near the cmc (2.8 mM). The decrease in adsorption of DPC above the cmc is probably because of solubilization of the DPC in the micelles in bulk.

Figure 4. Forces between borosilicate glass surfaces as a function of separation in 1 mM KCl and the following DDAPS concentrations: 0.25, 0.5, 0.75, 1.0, 1.5, and 2.0 mM. At separations greater than 3.0 nm, the dependence on DDAPS concentration cannot be resolved.

The slope of the adsorption-concentration graph is much less for the DPC at a surface excess greater than 0.5 molecule/nm2 because electrostatic repulsion between the charged headgroups of DPC hinders further adsorption.17 The plateau adsorption values are somewhat greater than values measured by solution depletion.18-20 This could be due to a difference in surface chemistry or to a difference in surface area determination. Solution depletion results are usually normalized by the area that is accessible to a N2 molecule (rather than a surfactant), whereas our analysis of ATR-IR results assumes that the interface is perfectly smooth. The cmc of each solution was determined from the plateau in the surface excess. The cmc in DPC solution was confirmed by measurement with a surfactant-selective electrode. We also measured the adsorption from mixed DDAPS/ DPC solutions that were relevant to the surface force measurements. The independent measurement of the (17) Goloub, T. P.; Koopal, L. K.; Bijsterbosch, B. H.; Sidorova, M. P. Langmuir 1996, 12, 3188. (18) Zajac, J.; Chorro, C.; Lindheimer, M.; Partyka, S. Langmuir 1997, 13, 3, 1486. (19) Partyka, S.; Lindheimer, M.; Faucompre, B. Colloids Surf. A 1993, 76, 267. (20) Goloub, T. P.; Koopal, L. K. Langmuir 1997, 13, 673.

Langmuir, Vol. 20, No. 11, 2004 4555

Figure 5. Forces between borosilicate glass surfaces as a function of separation in 1 mM KCl for a series of DDAPS concentrations >2 mM. The forces increase with concentration at s - s0 < 15 nm. A mechanical instability occurs at 1.5 < (s - s0) < 2.5 nm (indicated by arrows) depending upon concentration. The inset shows the region at 5 nm < (s - s0) < 7 nm more clearly so that the trend with concentration can easily be seen.

Figure 6. Forces between borosilicate glass surfaces as a function of separation in 1 mM KCl for a series of DPC concentrations. The forces decrease with concentration at s s0 < 20 nm. A mechanical instability occurs at 2 < (s - s0) < 5 nm (marked by the solid arrows) depending upon the concentration. Each curve is labeled with its DPC concentration in millimolar units. The van der Waals force, shown for comparison, was calculated by use of the Hamaker constant for silica-water-silica of 0.8 × 10-20 J25 and does not account for the adsorbed surfactant.

adsorption of the two surfactants is possible by use of ATR-IR. Figure 3 shows that the addition of 0.05 mM DPC to a DDAPS solution causes a change in the plateau surface excess to a slightly lower concentration (2.8 vs 3.2 mM) and therefore the cmc is decreased to the same extent. The DDAPS adsorption is greater at all concentrations below the plateau, which we explain by the existence of a glass-water interface that is more hydrophobic in the presence of adsorbed DPC. The addition of DDAPS causes an increase in the adsorption of DPC by the same mechanism. In summary, net attractive interactions between the alkyl chains in water promote adsorption of each surfactant in the presence of the other surfactant. This is consistent with the use of an expression for the surface chemical potential of each surfactant that is a function of the surface density of both surfactants:

µi ) µi0 + RT ln

(

)

θi + ziFAψ(s) + 1 - θ i - θj RTA(θi, θj) (3)

Compared to eq 2, the fourth term in eq 3 also accounts

4556

Langmuir, Vol. 20, No. 11, 2004

Lokar and Ducker

Figure 7. Forces between borosilicate glass surfaces as a function of separation in 0.05 mM DPC and 1 mM KCl and a series of DDAPS concentrations. (a) [DDAPS] < 1.5 mM. The forces decrease with concentration at s - s0 < 20 nm. A mechanical instability occurs at 2 < (s - s0) < 5 nm (marked by the arrows) depending upon concentration. Each curve is labeled with its respective DDAPS concentration in millimolar units. The silica-water-silica van der Waals force (O) is the same as in Figure 6. The following surface potentials at infinite separation were obtained by fitting an exact numerical solution of the Poisson-Boltzmann equation to the measured force: 0.05 mM DPC (-78 mV), 0.1 mM DDAPS (-73 mV), 0.2 mM (-66 mV), 0.5 mM (-58 mV), 0.75 mM (-46 mV), 1.0 mM (-30 mV), and 1.5 mM (-25 mV). (b) [DDAPS] > 1.5 mM. The forces are approximately constant for s - s0 > 4 nm and the force barrier increases with concentration above [DDAPS] ) 1.5 mM. For comparison, the solid line shows the van der Waals force calculated with a Hamaker constant for hydrocarbon-water-hydrocarbon, 0.5 × 10-20 J,25 that has its origin at s - s0 ) 3.2 nm. This is an approximate estimate of the van der Waals force between dense surfactant layers.

for interactions between surfactant molecules of different types. Thus, the charge regulation when two surfaces interact should be dependent on the concentration of both DDAPS and DPC. Figures 4 and 5 show the forces between a glass sphere and a flat glass plate as function of their separation at various DDAPS concentrations in the presence of 1 mM KCl. The force is normalized by the radius of the particle and divided by 2π so that it is equal to the energy per unit area for one flat surface interacting with another infinite flat surface (by use of the Derjaguin approximation).21 The separation axis is the distance minus the zero distance. The zero distance was defined by the region of constant compliance, by use of the usual AFM convention.22 Our analysis of proximal adsorption relies on the establishment of adsorption equilibrium, so we always confirmed that the forces were the same at a particular concentration during approach and separation (except for mechanical instabilities) and also when the concentration was raised and lowered. At low DDAPS concentrations ([DDAPS] < 2.0 mM), the forces are not a function of DDAPS concentration, with the exception of small deviations below s - s0 ) 3.0 nm. This is not surprising, as the adsorption isotherm shows little surfactant adsorbed to the surface. From eq 1, the invariance of the interaction energy means that the surfactant adsorption is not changing as a function of s. This is consistent with the weak dependence of adsorption on the chemical potential at low DDAPS concentrations (Figure 2) and the weak influence of the approaching surface on the zwitterionic surfactant. Figure 5 shows that the force gradually increases in the range 2.0 mM < [DDAPS] < cmc. From eq 1, this means that DDAPS desorbs as the surfaces approach. The force is also larger at separations up to 10 nm, which may be because the adsorption of a dense layer of (zwitterionic) dipoles moves the plane of net charge further into solution. For a cationic surfactant, the behavior is quite different. Figure 6 shows that, at low DPC concentrations in 1 mM KCl, the force decreases as the surfactant concentration (21) Derjaguin, B. V. Kolloid-Zeitschrift 1934, 69, 155. (22) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831.

increases. This means the surfactant adsorbs as the glass surfaces approach each other. Adsorption of the cationic surfactant reduces the magnitude of the long-range repulsive double-layer force between the surfaces. In earlier work1-3 we showed that, in fact, adsorption of the cationic surfactant occurred in excess of the amount required to regulate the surface charge, so there must be some accompanying desorption of K+ or adsorption of counterions (Cl- in this case). In common with our earlier work, Figure 6 shows that, under some conditions, the force is more attractive than our estimate of the van der Waals force. This provides further support to the idea that, in addition to the double-layer and van der Waals forces that are considered in classical DLVO theory, there may be additional attractive forces between surfaces coated with long-chain (hydrophobic) amphiphiles.23 Figure 7a shows the forces in a variety of DDAPS concentrations in the presence of 0.05 mM DPC and 1 mM KCl. The addition of just a small amount of DPC (cmc/300) makes the force very sensitive to the DDAPS concentration. At low DDAPS concentrations, the force decreases as the DDAPS concentration increases. The surface potential at infinite separation, obtained from fitting an exact numerical solution of the PoissonBoltzmann equation to the measured force at s - s0 > Debye length, is now a strong function of the DDAPS concentration. The potential changes from -78 to -25 mV when the concentration of a zwitterionic (net-neutral) surfactant is changed from 0 to 1.5 mM. The mechanism for this process can be understood with the aid of eq 1. DDAPS adsorbs as the solid-solid separation decreases (Figure 8). DDAPS does not have a net charge, so it should not, in general, feel the approach of another charged solid. DPC, on the other hand, does have a charge, so it does feel the approach of another charged surface. DPC adsorbs to reduce the increase in magnitude of the electrostatic potential. The chemical potentials of DPC and DDAPS on the surface, however, are coupled (eq 3). The adsorption of DPC lowers the chemical potential of adsorbed DDAPS, which causes (23) Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984, 98, 500.

Forces between Glass Surfaces in Mixed Surfactants

Figure 8. Changes in surface excess for DDAPS as a function of separation in 0.05 mM DPC and 1 mM KCl. DDAPS adsorbs with decreasing separation. ∆Γ is a maximum at 0.75 mM DDAPS. The calculation of ∆ΓDDAPS requires the assumption that µDPC is constant when the concentration of DDAPS is varied. This assumption was verified (within experimental error) by measurement of µDPC with a surfactant-selective electrode.

Figure 9. Schematic of proximal adsorption in a mixed surfactant system. On the left, a mixture of cationic and zwitterionic surfactants is adsorbed to an isolated interface (infinite separation). The approach of a second, charged interface without a change in adsorption would increase the magnitude of the negative electrostatic potential and thus would decrease the chemical potential of the cationic surfactant at the surface. (1) The cationic surfactant adsorbs in response, thereby reducing the magnitude of both the potential and the double-layer energy. (2) The surface hydrophobicity increases and drives further adsorption of the zwitterionic surfactant. (3) The adsorption of the cationic surfactant is greater than in the absence of the zwitterionic surfactant because the zwitterionic surfactant also increases the surface hydrophobicity.

DDAPS adsorption, which in turn lowers the chemical potential of adsorbed DPC, causing further adsorption. In other words, DDAPS amplifies the adsorption of DPC. This effect is shown schematically in Figure 9 and will be described in the following section. The changes in adsorption are largest at [DDAPS] ) 0.75 mM, where the isotherm in Figure 3 begins to rise. This is consistent with previous work with ionic surfactants.3 Furthermore, the adsorption changes extend out to (s - s0) ) 3κ-1. Therefore, despite the absence of a net charge on DDAPS, adsorption regulation of DDAPS occurs over the same range as the electric double-layer force. Figure 7b shows the effect of adding 0.05 mM DPC on the forces at higher DDAPS concentrations (2.0 mM < [DDAPS] < 2 cmc). The forces in this concentration range shows a monotonic attraction for s - s0 > 4 nm, which is independent of DDAPS concentration, and a repulsion at smaller separations, which increases with DDAPS concentration. The initial attraction is consistent with a van der Waals force between the surfaces coated in a hydrocarbon film. At small separations, the increase in repulsion with concentration shows that it is becoming more difficult to displace the surfactant film from the solid.

Langmuir, Vol. 20, No. 11, 2004 4557

Figure 10. Forces between borosilicate glass surfaces as a function of separation in 1 mM KCl and 0.75 mM DDAPS for a series of DPC concentrations. The forces decrease with concentration at s - s0 < 20 nm. A mechanical instability occurs at 2 < (s - s0) < 5 nm (solid arrows) depending upon concentration. Each curve is labeled by its DPC concentration in millimolar units. The van der Waals force (O) for silicawater-silica is shown for comparison. The ionic strength varies slightly with surfactant concentration.

Figure 11. Adsorption of DPC and DDAPS to the thin oxide layer on a silicon ATR crystal as a function of DPC concentration in D2O, 0.75 mM DDAPS, and 1 mM KCl. The isotherm of DPC in the absence of DDAPS is given for comparison. The adsorption of DPC is greater in the presence of DDAPS and the step shifts to lower DPC concentrations. The adsorption of DDAPS increases with increasing DPC adsorption and then decreases above the cmc (15.5 mM), probably because of solubilization of DDAPS in bulk micelles.

This set of curves is of interest for colloidal processing. With elimination of the long-range double-layer force, two colliding particles will fall into the soft secondary minimum. This secondary minimum is ideal for producing a high-density yet workable slurry.24 The ease of collapse into the primary minimum can be tuned through the concentration of DDAPS without changing the character of the secondary minimum. We will now examine the effect of DDAPS on the regulation of DPC. To do so, the chemical potential of DDAPS must be held constant. Since the maximum proximal adsorption and step in the mixed isotherm occur at 0.75 mM, we use a fixed DDAPS concentration of 0.75 mM and we assume that the chemical potential of the DDAPS is unaffected by changes in DPC concentration. Figure 10 shows the forces measured in DPC solutions in 1 mM KCl with a constant DDAPS concentration of 0.75 mM. (24) Ducker, W. A.; Luther, E. P.; Clarke, D. R.; Lang, F. F. J. Am. Ceram. Soc. 1997, 80, 575. (25) Hough, D. B.; White, L. R. Adv. Colloid Interface Sci. 1980, 14, 3-41.

4558

Langmuir, Vol. 20, No. 11, 2004

Figure 12. Changes in surface excess for DPC as a function of separation in 0.75 mM DDAPS and 1 mM KCl. DPC adsorbs with decreasing separation. The Debye length varies slightly with concentration.

Figure 13. Comparison of the changes in surface excess for DPC and DDAPS as a function of separation in 1 mM KCl. Proximal adsorption of DPC is greater in the presence of 0.75 mM DDAPS at [DPC] ) 0.05 mM, and the total proximal adsorption is much larger in the presence of DDAPS.

Comparison with Figure 6 reveals two effects. First, DPC changes the force at a lower concentration in the presence of DDAPS. In addition, the interaction goes from purely repulsive to attractive over a narrower concentration regime. Therefore, with DDAPS present, more DPC adsorbs during a collision. The same effect occurs for a single (isolated) surface. Figure 11 shows the adsorption of DPC in the presence of 0.75 mM DDAPS. The DDAPS increases the surface excess of DPC and shifts the step in the isotherm to lower concentrations. In Figure 12 the proximal adsorption analysis of the force data reveals that the maximum changes in surface excess occur at [DPC] ) 0.03 mM, whereas in the absence of DDAPS, the maximum occurred at [DPC] ) 0.08 mM. This shift in proximal adsorption mirrors the shift in the adsorption to the isolated surface (Figure 11). Once again the maximum proximal adsorption occurs at the step in the isotherm for the isolated surface. We note that the use of ATR-IR to measure simultaneously the adsorption of two surfactants in a mixture

Lokar and Ducker

allows us to detect some interesting subtleties in the adsorption. For example, we observe that as we increase the concentration of the more concentrated species in a mixture, the adsorption of the more dilute component passes through a maximum just below the cmc (Figures 3 and 11). This effect is explained by the solubilization of the dilute species in the micelles. Finally, we compare the magnitudes of the proximal adsorption for the two surfactants at a given solution composition: 0.75 mM DDAPS, 0.05 mM DPC, and 1 mM KCl. Figure 13 shows that ∆Γ of each surfactant is greater in the mixed system than in each single surfactant system, and therefore ∆Γtotal in the mixed surfactant system greatly exceeds ∆Γ for each of the single surfactant systems. In summary, the presence of DDAPS in solution alters the adsorption of DPC both at infinite separation and as a second surface approaches. This is the mechanism by which DDAPS alters the long-range electrostatic force. Conclusions Only a small amount of the zwitterionic surfactant adsorbs to glass surfaces at concentrations less than 2/3 cmc. Furthermore, at these concentrations, the zwitterionic surfactant is not useful for changing the surface forces. The presence of a small amount of cationic surfactant makes the adsorption of zwitterionic surfactant much more sensitive to perturbations in conditions. The zwitterionic surfactant adsorbs at lower concentrations and can now be used to control the surface forces. We explain this phenomenon in terms of proximal adsorption. The approach of another surface causes the cationic surfactant to adsorb to reduce the magnitude of the surface charge. Because the chemical potentials of the surfactants on the surface are linked through the net-attractive shortrange hydrophobic effect, the adsorption of the cationic surfactant promotes further adsorption of the zwitterionic surfactant. Likewise, the coupling between the chemical potentials makes adsorption of the cationic surfactant greater during a particle collision in the presence of the zwitterionic surfactant. We show an example in which the total proximal adsorption of surfactant is about 3 times larger in the presence of a zwitterionic surfactant than it is in a solution of cationic surfactant alone. Coupling between the long-range electrostatic force and the short-range hydrophobic effect can produce interesting changes in surface forces. This coupling can either be intramolecular, when the alkyl chain in covalently bound to the charge, or intermolecular, when the alkyl chain is on another molecule. ATR-IR is a useful tool to measure adsorption from solutions containing mixtures of surfactants. Acknowledgment. This work is based on research supported by the National Science Foundation Grant CHE-0203987. We thank S. P. Clark for training us in the use of his ATR-IR apparatus and software. In addition, we thank Dr. T. C. Ward for use of the IR spectrometer and B. Pethica and C. McKee for useful discussions. LA036459Z