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Adsorption Characteristics of Monomeric/Gemini Surfactant Mixtures at the Silica/Aqueous Solution Interface Kenichi Sakai,* Kazunori Matsuhashi, Ayako Honya, Takakuni Oguchi, Hideki Sakai, and Masahiko Abe* Department of Pure and Applied Chemistry in Faculty of Science and Technology and Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Received July 16, 2010. Revised Manuscript Received August 25, 2010 The adsorption of the monomeric/gemini surfactant mixtures at the silica/aqueous solution interface has been characterized on the basis of quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) data. The gemini surfactant employed in this study was cationic 1,2-bis(dodecyldimethylammonio)ethane dibromide (12-2-12). This surfactant was mixed with monomeric surfactants (dodecyltrimethylammonium bromide (DTAB), hexadecyltrimethylammonium bromide (HTAB), and octaoxyethylenedodecyl ether (C12EO8)) in the presence of an added electrolyte (NaBr). The key finding in our current study is that the addition of the gemini surfactant (12-2-12) makes significant impact on the adsorption properties even when the mole fraction of 12-2-12 is quite low in the surfactant mixtures. This is suggested by the experimental results that (i) the QCM-D adsorption isotherms measured for the monomeric/gemini surfactant mixtures shift to the region of lower surfactant concentrations compared with the monomeric single systems; (ii) the adsorbed layer morphology largely depends on the mole fraction of 12-2-12 in the surfactant mixtures, and the increased 12-2-12 mole fraction results in the less curved surface aggregates; and (iii) the addition of 12-2-12 yields a relatively rigid adsorbed layer when compared with the layer formed by the monomeric single systems. These adsorption properties result from the fact that the more favorable interaction of 12-2-12 with the silica surface sites drives the overall surfactant adsorption in these mixtures, which is particularly obvious in the region of low surfactant concentrations and at the 12-2-12 low mole fractions. We believe that this knowledge should be important when considering the formulation of gemini surfactants into various chemical products.
1. Introduction Gemini (dimeric) surfactants consist of two monomeric surfactants linked with a spacer unit. The physicochemical properties of gemini surfactants in aqueous media have been reviewed in the previous literatures,1-3 and hence we would like to forbear referring to them here again. Instead, we focus on the spontaneous adsorption of gemini surfactants at solid/liquid interfaces. This has been studied by a number of researchers from the standpoints of e.g. *To whom correspondence should be addressed. E-mail:
[email protected]. tus.ac.jp (K.S.);
[email protected] (M.A.). (1) Zana, R.; Xia, J. In Gemini Surfactants; Synthesis, Interfacial and SolutionPhase Behavior, and Applications; Zana, R., Xia, J., Eds.; Marcel Dekker: New York, 2003; Chapter 1. (2) Zana, R. In Structure-Performance Relationships in Surfactants, 2nd ed.; Esumi, K., Ueno, M., Eds.; Marcel Dekker: New York, 2003; Chapter 7. (3) Zana, R. Alami, E. In Novel Surfactants: Preparation, Applications, and Biodegradability, 2nd ed.; Holmberg, K., Ed.; Marcel Dekker: New York, 2003; Chapter 12. (4) Esumi, K.; Goino, M.; Koide, Y. J. Colloid Interface Sci. 1996, 183, 539. (5) Chorro, C.; Chorro, M.; Dolladille, O.; Partyka, S.; Zana, R. J. Colloid Interface Sci. 1998, 199, 169. (6) (a) Li, F.; Rosen, M. J. J. Colloid Interface Sci. 2000, 224, 265. (b) Li, F.; Rosen, M. J. J. Colloid Interface Sci. 2001, 234, 418. (7) Esumi, K.; Maedomari, N.; Torigoe, K. Langmuir 2001, 17, 7350. (8) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. J. Phys. Chem. B 2003, 107, 2978. (9) Qi, L.; Liao, W.; Bi, Z. Colloids Surf., A 2007, 302, 568. (10) Tehrami-Bagha, A. R.; Holmberg, K. Langmuir 2008, 24, 6140. (11) Sakai, K.; Tamura, M.; Umezawa, S.; Takamatsu, Y.; Torigoe, K.; Yoshimura, T.; Esumi, K.; Sakai, H.; Abe, M. Colloids Surf., A 2008, 328, 100. (12) Sakai, K.; Nakajima, E.; Takamatsu, Y.; Sharma, S. C.; Torigoe, K.; Yoshimura, T.; Esumi, K.; Sakai, H.; Abe, M. J. Oleo Sci. 2008, 57, 423. (13) Zhou, Q.; Somasundaran, P. J. Colloid Interface Sci. 2009, 331, 288. (14) Manne, S.; Sch€affer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382. (15) Fielden, M. L.; Claesson, P. M.; Verrall, R. E. Langmuir 1999, 15, 3924. (16) Duval, F. P.; Zana, R.; Warr, G. G. Langmuir 2006, 22, 1143.
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adsorption isotherms4-13 and adsorbed layer morphologies.8,11,14-16 One of the most important topics of these studies is to understand differences in the adsorption properties between monomeric and gemini systems. The key properties relating to this are summarized as follows. First, the adsorption plateau (saturation) region appears at much lower concentrations when compared with the corresponding monomeric ones.4 This reflects the remarkably lower critical micelle concentration (cmc) of gemini surfactants than that of monomeric ones. Second, the greater adsorption density in the saturation region is expected for gemini surfactants than for monomeric ones, when we compare their adsorption data (in mol m-2) normalized by the number of hydrocarbon chains.4 Third, gemini surfactants yield less curved surface aggregates when compared with the corresponding monomeric ones14 as a result of their larger packing parameter. It has been reported that, when compared to the surfactant aggregates formed in bulk solution, surface aggregates tend to favor lower curvature but follow the same general variation with surfactant geometry.14 These properties of gemini surfactants enable us to develop environment or human friendly surface coatings in aqueous media, with additional abilities in forming closely packed and laterally smooth surfactant films at the molecular scale. We expect that this knowledge should be useful when developing functionalized dispersion systems (such as water-borne inks, cosmetics, personal care, and house maintenance/cleaning products). Although the use of gemini surfactants as an adsorption material provides superior performance over the use of monomeric ones, the cost of gemini surfactants at their synthesis and subsequent purification stages has always been problematic when considering their potential formulations in various chemical products. We suppose that there are two possibilities to reduce the total cost
Published on Web 09/29/2010
DOI: 10.1021/la1028367
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of surfactants in such products. One is the development of gemini surfactants under the reduced synthesis and purification cost strategies, and in our recent works we have provided such materials synthesized from oleic acid derivatives.17,18 The other approach is the use of mixtures of gemini surfactants with conventional monomeric ones. The aim of our current study is, therefore, to demonstrate the adsorption of monomeric/gemini surfactant mixtures at a solid/aqueous solution interface and to compare the resultant data with the corresponding data obtained from the gemini single system. Herein, we present the adsorption characteristics of monomeric/ gemini surfactant mixtures at the silica/aqueous solution interface as a function of their mixing ratio. We note that the adsorbed layer morphology of monomeric surfactant mixtures has been previously studied by using the soft-contact atomic force microscopy (AFM).19-23 For example, Ducker and Wanless19 have demonstrated a change in the adsorbed layer morphology of the cationic surfactant (dodecyltrimethylammonium bromide, DTAB) and the zwitterionic one ((dodecyldimethylammonio)propanesulfonate, DDAPS) on mica and found that the increased mole fraction of DTAB results in the gradual shape transition of the adsorbed aggregates from spheres to rods. To the best of our knowledge, however, no systematic studies focusing on the adsorption of monomeric/gemini surfactant mixtures have been reported yet. In our current study, we have mainly performed two measurements: one is the soft-contact AFM to study the adsorbed layer morphology at the silica/aqueous solution interface, and the other is the quartz crystal microbalance with dissipation monitoring (QCM-D) to estimate both the mass and viscoelastic nature of the surfactant mixtures adsorbed at the interface. The gemini surfactant used in this study was cationic 1,2-bis(dodecyldimethylammonio)ethane dibromide (12-2-12). The following three surfactant mixtures have been studied: DTAB/12-2-12, HTAB/12-2-12, and C12EO8/ 12-2-12, where DTAB and HTAB (hexadecyltrimethylammonium bromide) are cationic and C12EO8 (octaoxyethylenedodecyl ether) is nonionic.
2. Experimental Section 2.1. Materials. The cationic gemini surfactant (12-2-12) was synthesized in our laboratory according to the procedure reported previously.4 Briefly, N,N,N0 ,N0 -tetramethylethylenediamine was refluxed with 1-bromododecane in ethanol for 48 h, and then the product obtained here was recrystallized several times from methanol/acetone mixtures. The cationic monomeric surfactants (DTAB and HTAB) were purchased from the following suppliers and used after recrystallization several times from methanol/ acetone mixtures (DTAB: Tokyo Chemical Industry (TCI); HTAB: Aldrich). The nonionic surfactant (C12EO8) was kindly supplied from Nikko Chemicals and used without further purification. The other chemicals used in this study were of analytical grade (Wako Pure Chemical Industries) and again used without further purification. Flat silica plates were prepared from silicon wafers (Nilaco): a silicon wafer was immersed in a mixed solution of H2O:NH3: H2O2 =5:1:1 (in volume) for 15 min at 80 °C, followed by a copious rinsing with deionized water to give a hydroxylated silica surface. (17) Takamatsu, Y.; Iwata, N.; Tsubone, K.; Torigoe, K.; Endo, T.; Sakai, K.; Sakai, H.; Abe, M. J. Colloid Interface Sci. 2009, 338, 229. (18) Sakai, K.; Sangawa, Y.; Takamatsu, Y.; Kawai, T.; Matsumoto, M.; Sakai, H.; Abe, M. J. Oleo Sci. 2010, 59, 541. (19) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915. (20) Davey, T. W.; Warr, G. G.; Almgren, M.; Asakawa, T. Langmuir 2001, 17, 5283. (21) Blom, A.; Duval, F. P.; Kovacs, L.; Warr, G. G.; Almgren, M.; Kadi, M.; Zana, R. Langmuir 2004, 20, 1291. (22) Blom, A.; Warr, G. G. Langmuir 2006, 22, 6787. (23) Blom, A.; Warr, G. G.; Nelson, A. Colloids Surf., A 2007, 310, 1.
17120 DOI: 10.1021/la1028367
A fresh piece of silica was used for each experiment and then discarded. The water used in the current study was filtered with a Millipore membrane filter (0.1 μm in pore size) after deionization with a Barnstead NANO pure diamond UV system. 2.2. Measurements. Static surface tension measurements were performed by using a Kr€ uss K100C Wilhelmy auto surface tensiometer with a platinum plate. Continuous measurements were carried out until a change in the surface tension becomes less than 0.01 mN m-1 per 90 s. A Q-Sense QCM-D E1 was used to assess both the mass of the surfactants adsorbed at the silica/aqueous solution interface and the viscoelastic nature (energy dissipation) of the adsorbed layer. The adsorbed mass was calculated from a change in the third overtone of the resonance frequency by applying Sauerbrey’s relationship.24 A single sensor crystal with a silica coating was used for all QCM-D experiments. The cleaning of the sensor crystal and an O-ring that seals the cell/sensor assembly has been made according to the procedure mentioned in the previous paper.25 After assembly in the QCM-D instrument, an electrolyte solution (10 mmol dm-3 NaBr) was injected into the cell, and the system was allowed to equilibrate. The flow rate of the electrolyte solution was always fixed at 0.1 cm3/min. Then, a surfactant solution at a fixed mixing ratio was injected after a stable baseline in the electrolyte solution was achieved. Again the system was allowed to equilibrate in the surfactant solution under the continuous flow, and then the solution was replaced by a new surfactant solution. Here the surfactant concentration was increased step by step to obtain the QCM-D adsorption isotherm and dissipation data at the fixed mixing ratio. In-situ imaging of the surfactant layers adsorbed on a flat silica plate was performed with a Seiko SPI3800 AFM. Cantilevers with an integral silicon nitride tip (Olympus OMCL-TR800PSA, the nominal spring constant = 0.15 N m-1) were used for all AFM experiments. The chemically oxidized silicon wafer was assembled in the AFM instrument, and the surfactant solution prepared at the desirable concentration (∼1 cm3) was injected into the AFM fluid cell. After equilibration for 1 h, images were collected using the soft-contact method26 with a scan rate of 3-4 Hz; this uses the minimum force necessary to obtain an image, thereby minimizing scanning-induced deformations of the adsorbed layer. All images presented herein are deflection images. Interaction forces between the cantilever tip and the surfactant layer adsorbed at the silica/ aqueous solution interface were also measured. All measurements reported here were performed at a constant temperature of 25 °C in the presence of 10 mmol dm-3 NaBr as a background electrolyte.
3. Results and Discussion 3.1. Surface Tension. Before presenting the adsorption characteristics of the monomeric/gemini surfactant mixtures at the silica/aqueous solution interface, it is fundamentally important to see their cmc(s) as a function of mixing ratio. Figure 1 shows the mixed cmc data of the three surfactant systems (DTAB/ 12-2-12, HTAB/12-2-12, and C12EO8/12-2-12) as a function of the mole fraction of 12-2-12 (R). We note that each cmc was determined by the static surface tensiometry (see Supporting Information Figure S1): for all the surfactant mixtures, their surface tensions decrease sharply with increasing concentration (in the region of low surfactant concentrations) and attain a plateau level. The break point observed here was assumed to be the cmc of the surfactant mixtures at each mixing ratio. Also shown in Figure 1 are the cmc curves of each mixture, computed using Clint’s equation (24) Sauerbrey, G. Z. Phys. 1959, 155, 206. (25) Sakai, K.; Smith, E. G.; Webber, G. B.; Schatz, C.; Wanless, E. J.; B€ut€un, V.; Armes, S. P.; Biggs, S. J. Phys. Chem. B 2006, 110, 14744. (26) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409.
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Figure 1. Mixed cmc data of the three surfactant systems (DTAB/ 12-2-12, HTAB/12-2-12, and C12EO8/12-2-12) as a function of the mole fraction of 12-2-12 (R). These cmc data were obtained through the static surface tensiometry in the presence of 10 mmol dm-3 NaBr as a background electrolyte. The solid curves given in this figure show the mixed cmc values computed using Clint’s equation.
(assuming no significant interaction between the two surfactants).27 Clearly, the experimental cmc data are slightly lower than the computed values, suggesting that a weak attractive interaction is present between the cationic gemini surfactant and the cationic/ nonionic monomeric ones. This is not surprising when taking the knowledge reported in the previous literatures into consideration. For example, the thermodynamic studies with the combination of pyrene fluorescence I1/I3 and conductivity measurements have demonstrated the negative interaction parameters for the systems of DTAB/12-2-12 and C12EO8/12-2-12, indicating the presence of the attractive interaction between the two surfactants.28 3.2. QCM-D. We have measured the adsorption amount of (mixed) surfactants by using the QCM-D technique. Here we note that the amount of surfactants adsorbed at the silica/aqueous solution interface, estimated by the QCM-D technique, is the total mass of surfactant mixtures. This means that it is not possible to evaluate the mass of each individual component adsorbed at the interface. In addition, the QCM-D adsorbed mass includes not only the mass of active materials (surfactants) adsorbed to the interface but also the mass of solvents (water) coupled with the adsorbed layer.25 Hereafter, we use the term of the “QCM-D adsorbed mass” under the assumption that the mass of water coupled with the adsorbed layer must be included in the “adsorbed mass” estimated by Sauerbrey’s relationship, although the recent paper29 suggests that the effect of entrapped water does not make significant impact on the QCM-D adsorbed mass when taking the “actual” surface area of QCM-D sensor crystals into consideration (the actual surface area is larger than the nominal one used in the QCM-D calculation). Figure 2 shows the QCM-D adsorbed mass (in mg m-2) of (a1) DTAB/12-2-12, (b1) HTAB/12-2-12, and (c1) C12EO8/12-2-12 as a function of the total surfactant concentration. Five mole fractions were examined in each system (R=0, 0.1, 0.25, 0.5, and 1). For better clarity, the data shown in these figures are replotted in parts (a2), (b2), and (c2) of Figure 2 , where the total surfactant concentration is normalized by the mixed cmc at each mole fraction. In the cases of the cationic surfactant single systems (DTAB, HTAB, and 12-2-12), the QCM-D adsorbed mass is gradually increased with increasing concentration and attains a plateau/ saturation level around the cmc. The driving forces for this adsorption are deemed to be the combination of the electrostatic interaction between the cationic surfactants and the oppositely charged (27) Clint, J. H. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1327. (28) Alargova, R. G.; Kochijashky, I. I.; Sierra, M. L.; Kwetkat, K.; Zana, R. J. Colloid Interface Sci. 2001, 235, 119. (29) Gutig, C.; Grady, B. P.; Striolo, A. Langmuir 2008, 24, 4806.
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silica surface sites and the hydrophobic interaction between surfactant molecules. This adsorption is very typical and has similarly been reported in the previous papers.4,7 In the case of the nonionic surfactant single system (C12EO8), a different behavior is observed from the cationic surfactant systems: the QCM-D adsorbed mass is measured to be very low at concentrations below its cmc, then starts to increase steeply as the concentration exceeds the cmc, and finally attains a plateau/ saturation level around 3 cmc. Since the adsorption plateau/ saturation is usually seen at a concentration around the cmc,30-34 the adsorption isotherm data obtained here are not always consistent with the general trend. An exact reason for this is not clear, but sometimes in the previous papers we can see such an increased adsorbed amount above the cmc.7,35 For example, Esumi et al.7 have studied the adsorption of the nonionic surfactant hexaoxyethylenedecyl ether (C10EO6) on silica by using the classical depletion method. The adsorption isotherm reported in this earlier work highly resembles our current QCM-D adsorption isotherm in both their shape and concentration dependency (normalized by the cmc). The authors have suggested that the combination of the following two factors drives the adsorption: one is the hydrogen bonding between the oxyethylene units and the surface silanol groups, and the other is the intermolecular hydrophobic interaction. A further rearrangement of surfactants adsorbed on silica has also been suggested to occur when the surfactant concentration is increased in order to form a closely packed adsorbed layer.7 It is interesting to estimate the degree of hydration for the surfactant single systems. The adsorbed amounts of DTAB, HTAB, and 12-2-12 (at their saturation level) on silica in the presence of 10 mmol dm-3 NaBr as a background electrolyte are reported to be 3.41 μmol m-2 (= 1.05 mg m-2), 4.80 μmol m-2 (= 1.75 mg m-2), and 2.87-3.40 μmol m-2 (= 1.76-2.09 mg m-2), respectively.4,7 We assume that the adsorbed amount reported in these papers corresponds to the mass of surfactants adsorbed at the silica/aqueous solution interface (i.e., “dry” mass, Γdry). Since the QCM-D adsorbed mass includes the mass of water coupled with the adsorbed surfactant layer (i.e., “wet” mass, ΓQCM-D), our rough calculation yields the degree of hydration (= (ΓQCM-D - Γdry)/ ΓQCM-D 100),25 as follows: ca. 58% for DTAB, ca. 38% for HTAB, and ca. 25-36% for 12-2-12. It has been reported that the degree of hydration largely depends on the adsorbed layer morphology,36 and hence we discuss our current results later again, with the soft-contact AFM data. It should be noted that an additional feature of our QCM-D measurements is the simultaneous monitoring of energy dissipation. The voltage driving the oscillation of the sensor crystal is turned off, and the decay of the oscillation is monitored. For a rigid layer, the decay is relatively slow, while for a viscoelastic layer the decay is fast due to the dampening effect of the adsorbed layer. Hence, it is possible to assess the relative rheological properties of the adsorbed layer on the basis of the measured dissipation data. In our current case, the dissipation values are measured to be lamellar bilayer (R=0.5) for the HTAB/12-2-12 mixtures (see Supporting Information Figure S3). In all the mixtures investigated, the increased 12-2-12 mole fraction results in the less curved surface aggregates on silica. The force-distance data obtained through the AFM measurements give further insights about the layer structure of the surfactant mixtures adsorbed on silica. Figure 6 shows the force curve data obtained for the (a) DTAB/12-2-12, (b) HTAB/12-2-12, and (c) C12EO8/12-2-12 mixtures. Again, for all the mixtures the mole fractions of 12-2-12 (R) are set to 0, 0.1, 0.25, 0.5, and 1. In the cases of the DTAB and HTAB single systems, a relatively longrange repulsive interaction is seen from an apparent separation of ca. 7 nm, and then the AFM tip breaks through the surfactant layer adsorbed on the silica surface. It seems likely that the repulsive interaction observed here originated from the electrostatic interaction, as is generally reported for many cationic monomeric surfactant systems.16,39 It has been reported that the push-through distance provides a measure of the adsorbed layer thickness,41 and in our current case the force instability occurs at 3.0 nm for DTAB and at 3.6 nm for HTAB. In the case of the C12EO8 single system, such a long-range electrostatic repulsion is not detected within the resolution of our force curve measurements, but the force instability caused by the “squeezed out” of the adsorbed surfactant layer is still observed at an apparent separation of 2.5 nm (see also Supporting Information Figure S4). Interestingly, the force curve data obtained for the 12-2-12 single system are significantly different in their shape from the data obtained for the monomeric surfactant single systems. (41) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 3207.
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One can see a relatively long-range electrostatic repulsion from an apparent separation of ca. 8 nm, followed by an additional steeply repulsive interaction beginning at an apparent separation of 2.1 nm. Then, a final push-through instability into adhesive contact is seen at an apparent separation of 1.4 nm. Similar force curve data are reported in the previous papers,16,42 where such two instabilities are observed in (i) the adsorption of 12-2-12 on silica in the presence of 100 mmol dm-3 NaCl at pH 4 at a concentration above its cmc42 and (ii) the adsorption of 1,8-bis(dodecyldimethylammonium)octane dibromide (12-8-12) on mica at 2 cmc in the absence of added electrolytes.16 It seems likely that the additional steep repulsion observed in the short-range (in other words, the second barrier against the normal pressure applied from the tip) results from the surfactant layer formed at the silica/aqueous solution interface, whereas the first push through is ascribed to the collapse of the layer weakly adsorbed on the tip.16 Under this assumption, it is possible to note that the steric repulsion against the normal pressure during the compression is significantly greater for the 12-2-12 layer adsorbed on silica than for the monomeric DTAB, HTAB, and C12EO8 ones. This is a remarkable feature of the 12-2-12 layer adsorbed at the silica/aqueous solution interface and is consistent with its low degree of hydration, low-energy dissipation, and high microviscosity (suggested by the ESR measurements4). The final push-through distance observed at 1.4 nm is significantly shorter than the thickness expected for the fully extended 12-2-12 bilayer. This may suggest the intercalation of the hydrocarbon chains to occur within the 12-2-12 bilayer as a result of the normal pressure during the compression. As is seen in Figure 6, the general shape of the force curve data obtained for the mixtures of 12-2-12 with the monomeric surfactants is quite similar to that obtained for the 12-2-12 single system, and the steep repulsion again appears in the short-range. This indicates that the addition of 12-2-12 to the monomeric surfactant systems makes significant impact not only on the adsorbed layer morphology but also on the inward force curve data. In particular, the mixtures of DTAB/12-2-12 and HTAB/12-2-12 give the steeply repulsive interaction (i.e., the second barrier) even at R = 0.1, suggesting that the adsorption of 12-2-12 occurs preferentially on silica from its low mole fractions. In the case of the mixtures of C12EO8/12-2-12, on the other hand, the steep repulsion is only observed at R = 0.5, although the AFM imaging data (Figure 5) demonstrate the significant shape transition of the adsorbed C12EO8 layer to occur even at R = 0.1. Again, these results are consistent with their low dissipation data (
