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Langmuir 2008, 24, 3874-3879
Tunable Synergism/Antagonism in a Mixed Nonionic/Anionic Surfactant Layer at the Solid/Liquid Interface Shaohua Lu and Ponisseril Somasundaran* Langmuir Center for Colloids and Interfaces, Columbia UniVersity, New York, New York 10027 ReceiVed October 17, 2007. In Final Form: January 14, 2008 The use of mixed surfactants for modification of solid surfaces is important for many applications, since beneficial synergism often occurs depending on the surfactant type and mixing conditions. Systematical information on the properties of surfactant mixtures at the solid/liquid interface can be helpful for optimizing the interactions between the surfactants and then their corresponding performance. In this work, a nonionic/anionic surfactant combination, n-dodecyl β-D-maltoside (DM) and sodium dodecyl sulfonate (SDS), was selected for the study of adsorption on an oxide solid, alumina. Interestingly, the mixture of the two surfactants with opposite pH-dependence of adsorption on alumina exhibits some unique synergistic or antagonistic features that were found to be tunable in the region of pH 4-10. In addition, the DM/SDS molar ratio in the adsorbed layer was found to decrease with concentration in the saturated region at all the pH and mixing ratios tested. The decrease is attributed to the monomer concentration changes in solution due to the difference in surface activities of the two surfactants. The tunable features of this mixture at the solid/liquid interface provide a way to optimize the properties by changing the mixing conditions. This can be valuable in many applications, such as enhanced oil recovery, flotation, and solubilization.
Introduction Adsorption of surfactants at solid/liquid interfaces plays a governing role in determining the surface charge, solubilization, or wettability1,2 and thus the effectiveness of many applications, such as enhanced oil recovery, flocculation, and flotation. During the last 2 decades, surfactant mixtures have received particular attention due to both beneficial and economical reasons. Amongvariouscombinationsofnonionicandionicsurfactants,3-10 mixtures of the same type in general exhibit ideal mixing behavior. The adsorption of mixed anionic/nonionic and cationic/nonionic surfactants on solids often exhibits synergistic as well as antagonistic interactions. Most work has aimed on such systems to understand the interactions involved. The adsorption of the surfactant mixture is determined by various interactions, including intermolecular interactions and the molecule/solid interactions. Generally, ionic surfactants adsorb only on the oppositely charged solid surfaces through electrostatic attraction, while the adsorption of nonionic surfactants is usually not affected by the surface charge. In a surfactant mixture, we define a surfactant that adsorbs on a solid as an active species and the one that coadsorbs as a passive species. An active species could become a passive one on the same solid when the conditions are changed. Although the interaction between the mixed surfactants is determined by * To whom correspondence should be addressed. E-mail: ps24@ columbia.edu. (1) Zhang, R.; Somasundaran, P. AdV. Colloid Interface Sci. 2006, 123, 213229. (2) Somasundaran, P.; Huang, L. AdV. Colloid Interface Sci. 2000, 88 (1-2), 179-208. (3) Zhang, L.; Somasundaran, P. J. Colloid Interface Sci. 2006, 302 (1), 2024. (4) Somasundaran, P.; Fu, E.; Qun, X. Langmuir 1992, 8 (4), 1065-1069. (5) Rao, P. H.; He, M. Chemosphere 2006, 63 (7), 1214-1221. (6) Qun, X.; Vasudevan, T. V.; Somasundaran, P. J. Colloid Interface Sci. 1991, 142 (2), 528-534. (7) Huang, L.; Maltesh, C.; Somasundaran, P. J. Colloid Interface Sci. 1996, 177 (1), 222-228. (8) Matsson, M. K.; Kronberg, B.; Claesson, P. M. Langmuir 2005, 21 (7), 2766-2772. (9) Zhang, L.; Zhang, R.; Somasundaran, P. J. Colloid Interface Sci. 2006, 302 (1), 25-31. (10) Zhou, Q.; Somasundaran, P. Synergistic Adsorption of Cationic Gemini and Sugar-Based Nonionic Surfactant Mixtures on Silica; 2005.
their natures, we have found a general rule to estimate the interaction for adsorption of surfactant mixtures summarized in Table 1. (1) Active Nonionic + Active Ionic Surfactants. When both components in the mixed surfactant system adsorb strongly on the solids, the mixture generally exhibits synergism in the preplateau region, whereas competition for adsorption sites occurs in the plateau region. Since the electrostatic attraction for the adsorption of ionic surfactants is usually stronger than the driving force for the adsorption of nonionic surfactants, such as hydrogen bonding, anionic or cationic surfactants generally adsorb more than the nonionic surfactants in the plateau region at equal mixing ratio. For instance, in the case of adsorption of mixed n-dodecyl β-D-maltoside (DM)/sodium dodecyl sulfate3 on alumina at pH 6 and mixed sodium dodecyl sulfate/C12EO8 on kaolinite,6 strong synergistic interaction has been reported in the preplateau region, while competition for adsorption sites was observed in the plateau region. The maximum in adsorption density of each component is governed by the mixing ratio of the system. (2) Active Nonionic + Passive Ionic Surfactants. Generally, anionic or cationic surfactants show only trace adsorption on similarly charged solid surfaces; however, in the presence of nonionic surfactants, the adsorption can increase significantly, since the nonionic surfactant molecules adsorbing on the surfaces may act as anchors for the ionic surfactants to coadsorb through hydrophobic chain-chain interactions.11 Also, the nonionic surfactants screen the electrostatic repulsion and favor molecular packing in the adsorbed layer. Synergistic interaction often promotes the adsorption of the ionic components in such mixed systems, while the total adsorption of the mixture is lower than that of the nonionic surfactant alone. In the case of mixed DM/ sodium dodecyl sulfate3 on alumina at pH 11, the adsorption of the sodium dodecyl sulfate (the passive species) is promoted while that of DM (the active species) is reduced. In general, there (11) Ben-Naim, A. Hydrophobic Interactions; Plenum Press: New York, 1980; p xiii.
10.1021/la703233d CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008
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Table 1. Interactions between Mixed Surfactants at Solid/ Liquid Interfaces interaction nonionic
ionic
preplateau region
activea active passive
active passiveb active
synergism synergism synergism
plateau region
ref
antagonism antagonism synergism
3, 5, 6 3 4, 7-10
a Active: a surfactant that adsorbs. b Passive: a surfactant that coadsorbs in a mixture system.
exists synergism in the preplateau region but antagonism in the plateau region in mixed active nonionic/passive anionic surfactant systems. (3) Passive Nonionic + Active Ionic Surfactants. The adsorption of certain nonionic surfactants on solids has been found to be selective depending on the solid type. For instance, DM adsorbs on alumina but not on silica,12 whereas ethoxylated nonylphenol exhibits the opposite behavior.13 In other words, DM is passive in the case of silica but active in the case of alumina. When mixed with active anionic or cationic surfactants, the adsorption of the passive nonionic species can be increased significantly. It has been found that10 the adsorption of DM on silica was enhanced by 2 orders of magnitude in the presence of cationic Gemini surfactant, even at very low cationic surfactant ratio. Matsson et al.8 have also reported enhanced adsorption of DM on silica in the presence of dodecyldimethylamine oxide (DDAO). In the system of mixed sodium p-octylbenzene sulfonate/nonionic C12EO8 on alumina,4 the adsorption of the nonionic surfactant was enhanced by orders of magnitude and the adsorption of the anionic one was promoted as well. Similar phenomena have been reported7 in the system of mixed tetradedyltrimethylammonium chloride/nonionic pentadecylethoxylated nonylphenol on alumina at pH 10. The synergism can be attributed to the hydrophobic chain-chain interaction between the surfactants. The ionic surfactant molecules anchor the adsorbed layer (solloids) through electrostatic forces, whereas the nonionic surfactant molecules in its adsorbed layer shield the electrostatic repulsion among the surfactant molecules and favor molecular packing. In general, synergism exists in systems of mixed active anionic or cationic and passive nonionic surfactants. Although adsorption of most nonionic/ionic surfactant mixtures can be categorized into the three cases discussed above, the adsorption behavior is rather complicated in some cases. Quantifying the synergistic or antagonistic interaction is needed to formulate the mechanism for adsorption of surfactant mixtures and to predict the corresponding performance. For instance, since the driving force for adsorption of anionic surfactants on alumina continuously decreases with increase in pH, the interaction between anionic/nonionic on alumina may change from synergistic to antagonistic with an increase in pH. It will be useful to obtain systematic adsorption of surfactants on solids to elucidate the mechanisms involved. Surprisingly, the adsorption of DM, though nonionic, on alumina is pH-dependent.14 DM shows trace adsorption on alumina at pH 4, but adsorbs markedly from pH 7 to 10. On the other hand, the adsorption of sodium dodecyl sulfonate (SDS) on alumina decreases continuously by 4 orders of magnitude from pH 2 to 8.15 In other words, DM is active on alumina from (12) Zhang, L.; Somasundaran, P.; Maltesh, C. J. Colloid Interface Sci. 1997, 191 (1), 202-208. (13) Zhang, R.; Somasundaran, P. Langmuir 2005, 21 (11), 4868-4873. (14) Lu, S.; Somasundaran, P. J. Colloid Interface Sci. 2007, 316, 310-316. (15) Somasundaran, P.; Fuersten, D. W. J. Phys. Chem. 1966, 70 (1), 90-96.
pH 7 to 10 but passive at pH 4, whereas SDS is active from pH 4 to 7 but passive above pH 9. In this work, the adsorption of mixed DM/SDS on alumina was determined here systematically at various pH and mixing ratios. Particularly, pH 4, 7, and 10 were selected, since the adsorption of mixed DM/SDS on alumina can be passive/active, active/active, or active/passive at these three pHs. In other words, the adsorption of these two surfactants is tunable. The change of adsorption of each surfactant in the system was determined in both the preplateau region and the plateau region. The synergism and antagonism interactions were correlated with pH and the mixing ratio. Experimental Section Materials. Surfactants. DM was obtained from Calbiochem and used as received. The critical micellar concentration of DM measured by surface tension experiments is 0.18 mM. Anionic SDS of g99.0 purity purchased from TCI Chemicals was used as received. Solid. AKP-50 R-alumina obtained from Sumitomo had a mean diameter of 0.2 µm. The specific surface area obtained using BET was 10.8 m2/g and the isoelectric point (iep) was 8.9. Other Reagents. HCl and NaOH, used for pH adjusting, were of ACS grade certified (purity >99.9%), from Fisher Scientific Co. Water used in all the experiments was triple distilled with a specific conductivity of less than 2.5 µΩ-1 and was tested for the absence of organics using surface tension measurements. NaCl purchased from Fisher Scientific Co. was ACS grade certified and used to adjust the ionic strength (IS) of the solutions to be 0.03 mol/L. Adsorption Test. Adsorption experiments were conducted in capped 20 mL vials by mixing 2 g of the solids with 10 mL of triple distilled water for 2 h at room temperature. The pH was adjusted and then 10 mL of the surfactant solution of desired concentration was added, and the samples were equilibrated further for 16 h with pH adjustment. The solution pH was adjusted till the pH was stable ((0.1) in 1 h. The samples were centrifuged for 30 min at 5000 rpm and the clear supernatant was pipetted out for concentration analysis. Adsorption density was then calculated from surfactant depletion from the solution. In the cases of surfactant mixtures, the two surfactants were mixed by their molar ratio (mol/mol) and the adsorption density was then determined by the corresponding concentration depletion. The amount of surfactant adsorbed is calculated as Γ ) (C0 - Ce)V/(mAS) where C0 is the initial surfactant concentration, Ce is the equilibrium surfactant concentration, V is the volume of the surfactant solution, m is the mass of the solid, and AS is the specific surface area of the solid. Analytical Techniques. The residual concentration of sugarbased surfactant was determined by colorimetric method through phenol-sulfuric acid reaction.16 The residual concentration of SDS was measured using a two-phase titration method.17 A Shimazu total organic carbon analyzer was used to measure the total organic carbon for verifying the total surfactant concentration. The accuracy of the measurement of the residual concentration determines the total error of the adsorption tests. The error of the colorimetric method is (1% and that of the two-phase titration method is (3%. The overall error is within (5%.
Results and Discussions Adsorption of mixed nonionic DM and anionic SDS on alumina was investigated at various mixing ratios and pHs. The composition of the adsorbed layer was analyzed as a function of the adsorption density. (16) Dubois, M.; Gilles, K. A.; Hamilton, J. K.; Rebers, P. A.; Smith, F. Anal. Chem. 1956, 28 (3), 350-356. (17) Li, Z.; Rosen, M. J. Anal. Chem. 1981, 53 (9), 1516-1519.
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Figure 1. Adsorption of DM alone and from its mixtures with SDS on alumina at pH 7 (IS, mol/L; S/L, solid/liquid ratio, g/g; DM/SDS, mixing ratio, mol/mol).
Figure 2. Adsorption of SDS on alumina from its mixtures with DM at pH 7 (IS, mol/L; S/L, g/g; DM/SDS, mol/mol).
(1) Adsorption of DM/SDS Mixture on Alumina at pH7 (Active Nonionic/Active Anionic). The results obtained for the adsorption of mixed DM/SDS on alumina at pH 7 and 0.03 M NaCl are shown in Figures 1-3. The adsorption isotherms of DM at various mixing ratios are plotted in Figure 1. In the case of DM/SDS 1:0 ratio, the adsorption isotherm exhibits a typical three-stage “S”-shaped isotherm:12 a very low adsorption at low concentrations, a sharp increase at concentrations below its critical micelle concentration (cmc) and a plateau region at concentrations above its cmc. It is seen that the sharply rising regions shift to low concentrations following the order DM/SDS 1:0, 3:1, 1:1, and 1:3. At pH 7, nonionic DM adsorbs on alumina through hydrogen bonding12,14 and anionic SDS adsorbs through electrostatic attraction15 at this pH; therefore, the adsorbed SDS molecules promote the formation of surface aggregates through hydrocarbon chain-chain interaction in the sharply rising region. It was also observed that the adsorption of DM in the plateau region decreases with a decrease in DM/SDS mixing ratio, suggesting competition between the surfactants for adsorption sites. The adsorption isotherms of SDS from the mixtures at pH 7 are plotted in Figure 2. The shape of the isotherms is different in the sharply rising region compared to those of DM adsorption isotherms. The sharply rising region covers a wider concentration region due to the electrostatic repulsion between the headgroups, which obstructs the formation of surface aggregates.
Lu and Somasundaran
Figure 3. Total adsorption of DM/SDS mixtures on alumina at pH 7 (IS, mol/L; S/L, g/g; DM/SDS, mol/mol).
Figure 4. DM molar ratio in adsorbed layer on alumina at pH 7 (IS, mol/L; S/L, g/g; DM/SDS, mol/mol).
In addition, it was noticed that the total adsorption density of DM and SDS in the plateau region remains fairly constant at mixing ratios 3:1, 1:1, and 1:3, as shown in Figure 3. This can be attributed to the facts that the total adsorption area on the solid surface is fixed and the parking areas for DM and SDS molecules on the surface at pH 7 are fairly close. Also, the onset of the plateau region at which the adsorption isotherm reaches the plateau increases with the ratio of SDS, which is attributed to the increase in the mixed cmc with the DM/SDS mixing ratio. In Figure 3, the plateau adsorption of this surfactant mixture at pH 7 was determined to be ∼7.0 × 10-6 mol/m2, based on which the average parking area of the molecules in the adsorbed layer was estimated to be ∼23 Å2/molecule. Since the parking areas for DM and SDS were determined to be 46 and 58 Å2/molecule by surface tension measurements,18 it can be predicted that the adsorbed layer is a well-packed bilayer at pH 7. On the basis of the results shown in Figures 1-3, it can be concluded that the adsorption of DM and SDS in this mixture is determined mainly by the mixing ratio, as both species are active on the alumina surface at this pH. To quantitatively compare the adsorption activity of the surfactants in this mixture, the composition of the adsorbed layer was calculated. The DM molar ratios in the adsorbed layer are plotted as a function of the total adsorption density in Figure 4 to show the compositional change with surfactant concentration and mixing (18) Lu, S.; Somasundaran, P. Manuscript in preparation, 2007.
Synergism/Antagonism in a Mixed Surfactant Layer
Figure 5. Adsorption of DM/SDS mixture on alumina at pH 4. The adsorption density of the individual components is plotted as a function of the residual concentration of the components (IS, mol/L; S/L, g/g; DM/SDS, mol/mol).
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Figure 7. Adsorption of DM on alumina from its mixtures with SDS at various pH values (IS, mol/L; S/L, g/g; DM/SDS, mol/mol).
Figure 8. DM molar ratio in adsorbed layer on alumina (IS, mol/L; S/L, g/g; DM/SDS, mol/mol). Figure 6. Adsorption of DM/SDS mixture on alumina at pH 10. The adsorption density of the individual components is plotted as a function of the residual concentration of the components (IS, mol/ L; S/L, g/g; DM/SDS, mol/mol).
ratios. Below the cmc, above which the surface is saturated, the adsorption of each surfactant is independent. In the case of mixing ratio 1:1, the DM molar ratio in the adsorbed layer is much higher than the overall ratio 0.5 at low adsorption density (10-7 mol/m2), while it remains fairly close to 0.5 above that and decreases continuously from 0.5 to 0.35 in the plateau region. The high DM ratio at low adsorption density is attributed to the high adsorption activity of DM at low concentrations, although the experimental error is large in the low concentration region. In the case of mixing ratio 3:1, the DM molar ratio in the adsorbed is lower than 0.75 below the plateau region, while in the case of mixing ratio 1:3, it is higher than 0.25. The results suggest that the low molar ratio component is favored in the preplateau region. Interestingly, it is seen that the DM molar ratio in the adsorbed layer decreases sharply in the plateau regions at the three mixing ratios tested. It suggests the replacement of DM molecules by SDS molecules with an increase in the concentration of surfactant mixtures when the alumina surface is saturated. This compositional change in the adsorbed layer can be attributed to the decrease in DM monomer concentration. In the past, the adsorption of surfactant mixtures has been correlated to the monomer concentration of each component19,20 (19) Fu, E. Dissertation, Columbia University, 1987.
in the bulk, which was proposed to determine the adsorption density. The monomer concentration of the surfactant mixture can be calculated using regular solution theory21,22 or determined experimentally using an ultrafiltration technique.23 Above the cmc, the monomer concentration of the more surface active component usually decreases in surfactant mixtures, while the monomer concentration of the other component increases, as Rubingh21 and Huang23 have reported for nonionic/nonionic and nonionic/cationic surfactant mixtures, respectively. For nonideal mixtures, the monomer concentration may undergo abnormal changes with concentrations due to the interactions between the surfactants. An interaction parameter is often used to indicate the deviation of a system from an ideal mixture.20 On the basis of the surface tension results, the interaction parameter β obtained for the mixed DM/SDS system was found to lie between -2.2 and -3.2, depending on the mixing ratio.18 These values suggest synergistic interactions between these two surfactants, which indicate the formation of mixed solloids on the surface and accounts for the adsorption increase of surfactant mixtures in the sharply rising region. Although the synergistic interactions between DM and SDS may affect the monomer concentrations, the DM monomers (20) Thibaut, A.; Misselyn-Bauduin, A. M.; Grandjean, J.; Broze, G.; Jerome, R. Langmuir 2000, 16 (24), 9192-9198. (21) Rubingh, D. N. Solution Chem. Surf. [Proc. Sect. 52nd Colloid Surf. Sci. Symp.] 1979. (22) Clint, J. H. J. Chem. Soc. Faraday Trans. 1 1975, 71 (6), 1327-1334. (23) Huang, L.; Somasundaran, P. Langmuir 1996, 12 (24), 5790-5795.
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Figure 9. Illustration of adsorption of DM/SDS mixture on alumina at pH 4, 7, and 10 in the three adsorption regions: I, the low adsorption region; II, the sharply rising region; III, the plateau region.
concentration is expected to decrease with concentration above the cmc, since DM is more surface active than SDS. For this system, the monomer concentration was estimated using the regular solution theory, and a correlation between the DM ratio in the adsorbed layer and that in solution was observed.18 In the plateau adsorption region, the DM monomer ratio was found to decreases with the surfactant concentration at DM/SDS ratios of 3:1, 1:1, and 1:3, which accounts for the compositional changes in the adsorbed layer. (2) Adsorption of DM/SDS Mixture on Alumina at pH 4 (Passive Nonionic/Active Anionic). At pH 4, the mixture of DM/SDS can be categorized into the third type in Table 1, active ionic/passive nonionic. This surfactant mixture exhibits synergism in both the preplateau region and the plateau region, and the adsorption is promoted significantly by the synergistic interaction. The isotherms of DM and SDS and the total adsorption from the mixture with 1:1 mixing ratio are shown in Figure 5. Since SDS is active on alumina at pH 4, it adsorbs more than DM. The total adsorption reaches a plateau at concentration around 2 × 10-4 M, which is close to the cmc of DM alone. Also, the isotherm of total adsorption exhibits a typical S shape with a sharp increase region. The shape is similar to that of DM alone and suggests that the mixed DM/SDS system at pH 4 behaves as a nonionic surfactant with respect to the adsorption on alumina. (3) Adsorption of DM/SDS Mixture on Alumina at pH 10 (Active Nonionic/Passive Anionic). In contrast to the situation at pH 4, DM is active on alumina at pH 10, while SDS is passive. The case can be categorized into the second type in Table 1: active nonionic/passive ionic. The adsorption behavior of this mixture at pH 10 was expected to be opposite to that at pH 4, as shown in Figure 6. SDS has a negatively charged head group and the alumina surface is also negatively charged at pH 10; therefore, SDS alone shows only trace adsorption on alumina due to the electrostatic repulsion. In the case of the mixtures, the adsorption of SDS is promoted by the presence of DM molecules on the surface. Since the adsorption of nonionic surfactant DM does not affect the surface charge of alumina, the driving force for SDS adsorption is proposed to be due to the hydrocarbon chain-chain interactions between the two surfactants. Although synergism was observed in the preplateau region, antagonism was seen in the plateau region. The adsorption of SDS is promoted significantly due to the formation of mixed solloidal aggregates, while the adsorption of DM is reduced due to antagonistic
interaction between the surfactants in the adsorbed layer. Below the concentration at which the adsorption reaches a plateau, DM adsorbs more than SDS, which is opposite to the case at pH 4. Also, the total adsorption isotherm reaches the plateau region at a concentration around 1 × 10-3 M. The shape of the total adsorption isotherm is close to that of a typical adsorption isotherm of anionic surfactants, as the sharply rising region covers a wide concentration due to the electrostatic repulsion among the surfactant molecules, which suggests that the formation of mixed solloids is dominated by the association of anionic SDS molecules. Furthermore, the plateau adsorption of the mixture at pH 10 is lower than that at pH 4 or 7, indicating that the molecules in the adsorbed layer are not well packed due to the electrostatic repulsion between the adsorbed SDS and the alumina surface. (4) Effects of the Synergistic or Antagonistic Interaction on the Adsorption of DM. The adsorption of DM on alumina has been found to be pH dependent,14 although it remains constant from pH 7 to 10.12 As shown in Figure 7, the plateau adsorption density of DM alone on alumina at pH 4 is only 2% of that at pH 7. The pH dependence has been correlated to the concentration changes of surface hydroxyl groups, which were proposed to account for the formation of hydrogen bonding between DM molecule and the surface.14 Besides the adsorption isotherms of DM alone at pH 4 and 7, the isotherms of DM from its mixtures with SDS at pH 4, 7, and 10 at a mixing ratio of 1:1 are plotted in Figure 7. Interestingly, the adsorption isotherms of DM from its mixtures at pH 4 and 7 on alumina are almost identical, even though the adsorption isotherms of DM alone under same conditions are significantly different. The mixture exhibits strong synergism at pH 4 due to the formation of mixed surfactant aggregates at the solid/liquid interface. At pH 4, the adsorbed SDS molecules act as anchors for DM molecules to coadsorb through hydrocarbon chainchain interaction, even though the direct association between DM and the surface is very weak. In addition, the presence of DM molecules screens the electrostatic repulsion between SDS molecules and promotes the formation of mixed aggregates and thus enhances the total adsorption. At pH 10, the saturation adsorption of DM from its mixture at a mixing ratio 1:1 is onethird of that in the case of DM alone (the isotherm at pH 10 is identical to that at pH 7), suggesting an antagonistic interaction. (5) Compositional Changes in the Adsorbed Layer on Alumina with pH. The pH dependence of the adsorption of
Synergism/Antagonism in a Mixed Surfactant Layer
mixed DM/SDS on alumina makes this system a unique mixture in terms of the interactions involved. In other words, the interaction between these two surfactants at the solid/liquid interface is tunable with pH. At pH 4, this mixture exhibits as passive nonionic/active anionic; at pH 7, it becomes active nonionic/ active anionic; and at pH 10, it behaves as active nonionic/ passive anionic. The DM molar ratios in the adsorbed layer at pH 4, 7, and 10 are shown as a function of total adsorption density in Figure 8 with a mixing ratio 1:1. Although the mixing ratio remains constant, the DM ratios in the adsorbed layer are very different. At pH 4, the DM molar ratio in the adsorbed layer approaches 0.5 and then decreases in the plateau region. The DM ratio is lower than 0.5 in the whole isotherm region, since the adsorption of DM is dependent on that of SDS. At pH 7, the DM ratio is close to 0.5 in the preplateau region, since both DM and SDS are active. At pH 10, the DM ratio in the adsorbed layer is greater than 0.5 in the preplateau region, since the adsorption of SDS is dependent on DM. Interestingly, the DM molar ratios in the adsorbed layer decrease in the plateau region at all three pHs, suggesting that this compositional change is independent of whether the interaction is synergistic or antagonistic. As DM is more surface active than SDS, the DM monomer concentration is expected to be lower than that of SDS in the concentration above the cmc. It is hypothesized that the molar ratio in the adsorbed layer in the plateau region is determined mainly by the monomer concentrations of the components in the mixture. The performance of surfactant mixtures in many applications can be optimized on the basis of this hypothesis. For instance, the adsorption of an active surfactant species, which is usually more costly, can be reduced by introducing a passive species with low surface activity.
Summary In this study, the adsorption of mixed nonionic DM and anionic SDS on alumina was investigated systematically as a function of mixing ratio and pH. Even though DM is a nonionic surfactant, DM alone has trace adsorption on alumina in the acidic pH region and high adsorption above pH 7, while the adsorption of SDS decreases with pH monotonically. When mixed, the adsorption of DM and SDS on alumina shows interesting pH-
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dependence, as the mixture behaves as passive nonionic/active anionic, active nonionic/active anionic, or active nonionic/passive anionic at pH 4, 7, and 10, respectively. Due the difference in adsorption density and interactions involved, the structure of the adsorbed layers is expected to vary significantly with pH, as illustrated in Figure 9. At pH 4, DM is passive, while SDS is active. The mixture exhibits strong synergism, as the preadsorbed SDS molecules act as anchors for DM to adsorb through hydrophobic chainchain interaction. The formation of surface aggregates is dominated by the association of nonionic DM molecules, as the total adsorption isotherm is close to a typical isotherm of nonionic surfactants with an S shape. At pH 7, both DM and SDS are active. In the preplateau region, the formation of mixed aggregates promotes the adsorption with a synergistic interaction, while in the plateau region, competition for the adsorption sites occurs and the mixture exhibits antagonism. At pH 10, DM is active, whereas SDS is passive. The adsorption of SDS is dependent on the adsorbed DM through hydrophobic chain-chain interaction. The formation of mixed solloids is dominated by the association of SDS molecules, as the total adsorption isotherm is close to that of the anionic surfactant with the sharply rising region covering a wider concentration region. In addition, the DM molar ratio in the adsorbed layer was observed to decrease in the plateau region at the pH and mixing ratios tested. The compositional changes in the adsorbed layer can be attributed to the monomer concentration change in the bulk, which is related to the surface activity. In this mixture system, the DM molar ratios of monomers is expected to decrease due to its higher surface activity in the concentration above the cmc and this decrease accounts for the compositional change in the adsorbed layer. The study yields unique information for understanding synergistic and antagonistic interactions in adsorption of surfactant mixtures. Acknowledgment. The authors acknowledge the financial support of the Department of Energy (DE-FC26-03NT15413) and the Industrial/University Cooperative Research Center (IUCRC) for advanced studies on novel surfactants at Columbia University. LA703233D
