6140
Langmuir 2008, 24, 6140-6145
Cationic Ester-Containing Gemini Surfactants: Adsorption at Tailor-Made Surfaces Monitored by SPR and QCM A. R. Tehrani-Bagha†,‡ and K. Holmberg*,† Department of Chemical and Biological Engineering, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden, and Institute for Colorants, Paint and Coatings, Tehran, Iran ReceiVed January 2, 2008. ReVised Manuscript ReceiVed March 28, 2008 Adsorption of a series of ester-containing cationic surfactants at a surface containing 90% methyl groups and 10% carboxyl groups was studied by two surface analysis techniques, surface plasmon resonance (SPR) and quartz crystal microbalance (QCM). Such a surface, which is at the same time hydrophobic and negatively charged, is of interest as a model for many polymeric surfaces. Two different types of ester gemini surfactants and their monomeric counterparts were included together with nonester containing surfactants of similar structure. The results show that the gemini surfactants give the same adsorbed amount at the surface as the monomeric surfactants when compared at the same bulk concentration normalized to the critical micelle concentration (cmc) in bulk. Since the cmc of the geminis is around 20 times lower than the cmc of the corresponding monomeric surfactants, the gemini surfactants are much more effective in covering the surface. The two techniques gave similar relative values but the QCM values were always higher than those from SPR, which is due to the former method taking also adsorbed water into account. The adsorption, as measured by both methods, was found to follow closely the Langmuir adsorption model.
1. Introduction Cationic surfactants, with almost 7% of the total surfactant market, have many applications, such as fabric softeners, asphalt additives, corrosion inhibitors, biocides, and textile auxiliaries. They adsorb strongly onto a wide variety of materials. The majority of minerals and many organic substrates have high energy and are hydrophilic in nature. Cationic surfactants adsorb onto negatively charged sites of the substrates almost exclusively by an ion-exchange mechanism.1–3 However, cationic surfactants in general have higher aquatic toxicity than other surfactants and are also more irritating to the skin and to the eye. The toxicity of these surfactants is believed to result from their tendency to adsorb at negatively charged surfaces.3,4 Different approaches are taken to overcome this problem. One approach is to introduce an easily cleavable bond into the surfactant structure.5,6 The search for novel surfactants with higher efficiency and effectiveness gave birth to the concept of gemini surfactants. Gemini surfactants are more surface-active and have a much lower cmc’s than do their monomeric counterparts. These surfactants are made up of two monomeric surfactant molecules with their headgroups chemically bonded together by a spacer.7–9 Since the term was coined by Menger in 1991, many different * Corresponding author. E-mail:
[email protected]. Tel: +46 31 772 2969. Fax: +46 31 16 0062. † Chalmers University of Technology. ‡ Institute for Colorants, Paint and Coatings. (1) Steichen, D. S. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; Wiley: New York, 2001; Vol. 1, p 309. (2) Steichen, D. S.; Gadberry, J. F. In New Products and Applications in Surfactant Technology; Karsa, D. R., Ed.; Sheffield Academic Press: Sheffield, U.K., 1998; p 59. (3) Cross, J. In Cationic Surfactants; Cross, J., Singer, E. J., Eds.; Marcel Dekker: New York, 1994; p 3. (4) Huber, L.; Nitschke, L. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; Wiley: Chichester, U.K., 2002; Vol. 1, p 509. (5) Stjerndahl, M.; Lundberg, D.; Holmberg, K. In NoVel Surfactants: Preparation, Applications, and Biodegradability; Holmberg, K., Ed.; Marcel Dekker: New York, 2003; p 317. (6) Tehrani-Bagha, A. R.; Holmberg, K. Curr. Opin. Colloid Interface Sci. 2007, 12, 81. (7) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (8) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (9) Rosen, M. J. CHEMTECH 1993, 23, 30.
types of gemini surfactant have been synthesized, and their physicochemical properties have been investigated.10,11 The adsorption of surfactants at model surfaces is a topic that has received steady attention in recent years. The adsorption at tailored-made solid surfaces has, for instance, been investigated by the use of the quartz crystal microbalance with dissipation (QCM-D)12–17 and by surface plasmon resonance (SPR).18–22 QCM-D is an electromechanical technique that allows simultaneous measurements of the change in frequency related to the coupled mass and the energy dissipation related to viscous losses when a molecule in solution binds to the sensor surface.23 SPR is an optical technique that is sensitive to changes in the refractive index of the medium near a metal surface. Adsorption from solution induces a change in the refractive index in the surrounding medium in close proximity to the metal surface.24 It is well known that long-chain alkanethiols adsorb from solution onto gold surfaces and form ordered, oriented monolayer films.25 By choosing alkanethiols with different functional end (10) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906. (11) Zana, R.; Xia, J. Gemini Surfactants; Marcel Dekker: New York, 2004. (12) Stålgren, J. J. R.; Eriksson, J.; Boschkova, K. J. Colloid Interface Sci. 2002, 253, 190. (13) Knag, M.; Sjoblom, J.; Øye, G.; Gulbrandsen, E. Colloids Surf., A 2004, 250, 269. (14) Boschkova, K.; Feiler, A.; Kronberg, B.; Stålgren, J. J. R. Langmuir 2002, 18, 7930. (15) Boschkova, K.; Stålgren, J. J. R. Langmuir 2002, 18, 6802. (16) Kawasaki, H.; Nishimura, K.; Arakawa, R. J. Phys. Chem. C 2007, 111, 2683. (17) Boschkova, K.; Kronberg, B.; Stålgren, J. J. R.; Persson, K.; Ratoi Salagean, M. Langmuir 2002, 18, 1680. (18) Oskarsson, H.; Frankenberg, M.; Annerling, A.; Holmberg, K. J. Surfact. Deterg. 2007, 10, 41. (19) Muller, D.; Malmsten, M.; Bergenståhl, B.; Hessing, J.; Olijve, J.; Mori, F. Langmuir 1998, 14, 3107. (20) Levchenko, A. A.; Argo, B. P.; Vidu, R.; Talroze, R. V.; Stroeve, P. Langmuir 2002, 18, 8464. (21) Sarkar, D.; Somasundaran, P. J. Colloid Interface Sci. 2003, 261, 197. (22) Tulpar, A.; Ducker, W. A. J. Phys. Chem. B 2004, 108, 1667. (23) Rodahl, M.; Ho¨o¨k, F.; Fredriksson, C.; Keller, C. A.; Krozer, A.; Brzezinski, P.; Voinova, M.; Kasemo, B. Faraday Discuss. 1997, 107, 229. (24) Liedberg, B.; Lundstro¨m, I.; Stenberg, E. Sens. Actuators, B 1993, 11, 63. (25) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, J. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
10.1021/la800009b CCC: $40.75 2008 American Chemical Society Published on Web 05/22/2008
Cationic Ester-Containing Gemini Surfactants
groups, surfaces can be tailor made with respect to the type and density of functional group. The QCM and the SPR techniques, in combination with the use of self-assembled monolayers (SAMs) of alkanethiols on gold,26 constitute powerful tools for investigating the adsorption of surfactants at solid surfaces.18 The most important difference between the QCM and the SPR techniques is that the former measures the dry mass of the adsorbed film and the latter measures the mass including coupled water.27 In this study, a series of gemini and conventional cationic surfactants have been synthesized, and their adsorption at alkanethiol SAMs coated on gold chips has been studied by means of the SPR and QCM techniques. We have been particularly interested in investigating adsorption at a hydrophobic surface (methyl-terminated) spiked with carboxyl groups because such a surface mimics the surface of many polymeric materials such as wool and nylon.28 Today there is strong interest in cleavable surfactants (i.e., surfactants that contain an easily degradable linkage). We have been active in this area for many years and have synthesized and characterized several surfactants within this category. The surfactants used in this work contain a cleavable bond, an ester linkage, between the alkyl chain and the polar headgroup. Two types of ester surfactants were prepared, one with the carbonyl carbon of the ester bond facing the hydrophobic tail and one with the opposite orientation (i.e., with the bridging oxygen of the ester bond facing the tail). The former type is referred to as esterquat surfactants, and the latter type is referred to as alkyl betainates. As reference, the corresponding monomeric and dimeric surfactants lacking the ester moiety were used. The chemical hydrolysis and the biodegradation of the estercontaining cationic surfactants have been reported in a previous communication.29
2. Experimental Section 2.1. Materials. 1-Dodecanol (98%), bromoacetyl bromide (>98%), dichloromethane (>99.5%), acetone (>99.5%), N,N,N’,N’-tetramethyl-1,2-ethanediamine (99%), N,N,N’,N’-tetramethyl-1,3-propanediamine (99%), 2-dimethylaminoethanol (>99.5%), lauroyl chloride (98%), trimethylamine (99%), 2-bromoethanol, 1,3-dibrompropane (99%), 1-bromododecane (97%), and 16-mercaptohexadecanoic acid (90%) were all purchased from Aldrich. Dodecyltrimethylammonium bromide (>97%) and 1-hexadecanethiol (>95%) were obtained from Fluka. 1-Undecanethiol (99%), 11-hydroxy1-undecanethiol (99%), and 11-mercaptoundecanoic acid (99%) were purchased from Asemblon, Inc. Ethanol (99.5%) was provided by Kemetyl. H2O2 (30%) and NH3 (25%) solutions (>99%) were from Merck. 2.2. Syntheses. The procedures for the synthesis of the different cationic surfactants are provided in the Supporting Information. 2.3. PhysicochemicalCharacterizations.2.3.1. cmcMeasurements. The critical micelle concentrations (cmc’s) of the surfactants were determined by the conductivity method. The conductance as a function of surfactant concentration was measured at 25 °C. Measurements were performed with a CDM 210 conductometer (Radiometer, France) using a water bath with stirring to control the temperature. For each series of measurements, an exact volume of 10 mL Millipore water (resistivity ∼18 MΩ) was introduced into the vessel, and the specific conductivity of the water was measured. The solution was (26) Everhart, D. S. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; Wiley: Chichester, 2002; Vol. 2, p 99. (27) Ho¨o¨k, F.; Kasemo, B.; Nylander, T.; Fant, C.; Sott, K.; Elwing, H. Anal. Chem. 2001, 73, 5796. (28) Broadbent, A. D. Basic Principles of Textile Coloration; The Society of Dyers and Colourist: Bradford, 2001; p 34. (29) Tehrani-Bagha, A. R.; Oskarsson, H.; van Ginkel, C. G.; Holmberg, K. J. Colloid Interface Sci. 2007, 312, 444. (30) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (31) Vautier-Giongo, C.; Bales, B. L. J. Phys. Chem. B 2003, 107, 5398.
Langmuir, Vol. 24, No. 12, 2008 6141 then titrated with the surfactant solution, and the conductivity was measured 10 min after each addition. The concentration at which there was a break on the curve of conductivity versus surfactant concentration was taken as the cmc. 2.3.2. Micelle Ionization Degree (R). The counterion distribution in a micellar solution can be assessed from electrical conductivity versus concentration plots. The counterion binding to the micelles was determined from the ratio between the slopes above and below the cmc.30 2.3.3. Krafft Temperatures. Weighted quantities of surfactant and water were heated to 50 °C to form clear solutions of known concentrations. These solutions were placed in a refrigerator for 24 h until a precipitate of the hydrated surfactant crystals appeared. The precipitation of dodecyl betainate monomer and gemini surfactants occurred rapidly in 1 h. No precipitation was seen for dodecyl esterquat monomer and gemini surfactant solutions. The temperature of the precipitated systems was gradually raised under constant stirring. The conductance was recorded every 2 to 3 min by a CDM 210 conductometer (Radiometer, France) equipped with a thermocouple that was immersed in the solution. The Krafft temperature was taken as the temperature at which the conductance versus temperature plot showed an abrupt change in slope.31 2.3.4. Preparation of Surfaces and of Surfactant Solutions. The gold chips for SPR were obtained from Biacore AB, Uppsala, Sweden, and those for QCM were from Q-sense AB, Go¨teborg, Sweden. They were both cleaned by the following oxidizing procedure: treatment in a UV-ozone bath for 10 min, followed by washing in a mixture of 30% H2O2/25% NH4OH/H2O (1:1:5 by volume) at 70 °C for 10 min, washing with Millipore water, rinsing with ethanol (99.5%), drying with dry nitrogen, and finally again treatment in a UV-ozone bath for 10 min. The cleaned gold chips were immersed in the alkanethiol solution (1 mM in ethanol) for at least 16 h. They were rinsed with ethanol and then sonicated in ethanol for 2 × 5 min to remove loosely adsorbed alkanethiols from the surface and finally dried with nitrogen shortly before use. All surfactant solutions were made in Millipore water and filtered with Millex-GV 0.22 µm PVDF syringe filters. They were degassed by sonication prior to use. A fresh surfactant solution was prepared for each series of measurements. 2.3.5. Contact Angle Measurements. The contact angle measurements were studied by means of spreading a water droplet on each type of surface using a DAT 1100 Fibro system AB, Sweden. The volume of the drop was set to 4 µL and controlled by an automated instrument injector. A Teflon syringe was used in order to avoid liquid remaining on the tip. For each series of measurements, clean gold chips presenting different alkanethiol SAM surfaces were put in the holder and introduced into the system. The baseline was corrected according to the position of the gold chip. The spreading of the water drop was recorded by a CCD camera connected to an image analyzer. The contact angle was automatically determined from the base width and height of the individual drops. 2.4. Adsorption Studies. 2.4.1. Surface Plasmon Resonance (SPR). Detailed descriptions of the technique and of the flow cell setup can be found in the literature, so only a brief account is given below. Surface plasmon resonance is a phenomenon that occurs when light is reflected off of thin metal films. A fraction of the light energy incident at a sharply defined angle can interact with the delocalized electrons in the metal film (plasmon), thus reducing the reflected light intensity.26 The SPR at alkanethiol SAM surfaces is sensitive to the changes in refractive index that take place as molecules adsorb at the SAM surface. As adsorption occurs on top of the SAM surface, the angle of the minimum in reflected light is changed, and this change in angle, expressed as resonance units (RU), is proportional to the mass adsorbed at the surface according to
∆mSPR )
CSPR∆RU β
(1)
where ∆mSPR is the adsorbed amount, CSPR is a factor that contains an instrument constant, dn/dc is the variation of refractive index
6142 Langmuir, Vol. 24, No. 12, 2008
Tehrani-Bagha and Holmberg
with concentration for the adsorbent, ∆RU is the measured change in response units, and β is a factor that compensates for the decrease in the SPR signal with distance from the gold substrate. ∆RU is a dimensionless quantity that corresponds to a change in θZ, where 1000RU correspond to a change of 0.1° in contact angle. Thus, the SPR technique allows real-time measurements of surfactant adsorption. CSPR was calculated to be 0.094 ( 0.008 ng/ cm2 using an average dn/dc for 18 different surfactants,35 and β was set equal to 1, which is the case for a plain gold surface.34 The β factor will differ from 1 when the surface layer is thick. In the experiments performed in this work, the layer is very thin, and the value of β can be anticipated to be close to 1. A change in the bulk refractive index will also contribute to the recorded RU values, but in this work, the contribution from the bulk is believed to be negligible because of the low surfactant concentrations used. All SPR measurements were performed using a BIAcore 2000 system (Biacore AB, Uppsala, Sweden) in a multichannel mode. The injection volume and flow rate were 100 µL and 15 µL/min, respectively. The adsorption was monitored at 23 °C. Residual adsorption was recorded after rinsing with buffer 4 min after the end of injection. 2.4.2. Quartz Crystal Microbalance with Dissipation (QCM-D). The QCM-D technique allows for the simultaneous measurement of changes in resonance frequency and energy dissipation. The change in resonance frequency is a measure of the adsorbed amount, which can be calculated using the Sauerbrey relation36
∆m ) C
∆f n
Scheme 1. Surfactants Studied, All Based on Dodecyl Tailsa
(2)
where ∆m is the adsorbed mass, C is a constant characteristic of the crystal, in our case 17.7 ng cm-2 Hz1-, ∆f is the change in frequency, and n is the overtone number (n ) 1, 3, 5, . . .). This relation rests on the assumption that the deposited mass forms a thin, rigid film and that the adsorbed mass is uniformly distributed over the entire surface and is less than 2% of the crystal mass.12,32 A thorough discussion of mass transfer to the sensor surface can be found in the literature.23,27,33,37 A QCM instrument (model D300) from Q-sense AB (Go¨teborg, Sweden) was used. All QCM-D measurements were made under a nonflowing condition; that is, in batch mode in a cell designed to provide a fast, nonperturbing exchange of a stagnant liquid. The measurements started with Millipore water in the chamber. The volume of the T-loop in the system for thermal equilibration of the sample fluid is about 0.5 mL. For each series of measurements, 2 mL of solution was poured into the reservoir; 1.5 mL was used for washing the loop, and the rest was kept for adsorption measurements in batch mode. The solution temperature usually equilibrates in 5-10 min when measuring at room temperature. The measurements were started by filling the sensor crystal cavity with Millipore water. After obtaining a stable baseline for frequency, the reservoir refills with surfactant solution, and the same procedure follows for flowing 0.5 mL of the sample liquid through the crystal cell. Because of the adsorption of the surfactant, the frequency decreases (mass uptake) while the dissipation increases (viscoelastic changes in the surfactant film formed). The frequency and dissipation reach a plateau after a couple of minutes. Rinsing with Millipore water removes loosely bond surfactant from the substrate, and the frequency signal reaches a plateau again.
3. Results and Discussion 3.1. Physicochemical Characterization of the Surfactants. The syntheses of the monomeric and the dimeric (gemini) cationic (32) Larsson, C.; Rodahl, M.; Ho¨o¨k, F. Anal. Chem. 2003, 75, 5080. (33) Caruso, F.; Serizawa, T.; Furlong, D. N.; Okahata, Y. Langmuir 1995, 11, 1546. (34) Stenberg, E.; Persson, B.; Roos, H.; Urbaniczky, C. J. Colloid Interface Sci. 1991, 143, 513. (35) Oskarsson, H.; Holmberg, K. J. Colloid Interface Sci. 2006, 301, 360. (36) Sauerbrey, G. Z. Phys. 1959, 155, 206. (37) Ho¨o¨k, F.; Vo¨ro¨s, J.; Rodahl, M.; Kurrat, R.; Bo¨ni, P.; Ramsden, J. J.; Textor, M.; Spencer, N. D.; Tengvall, P.; Gold, J. Colloids Surf., B 2002, 24, 155.
a From top to bottom: dodecyl betainate monomer, dodecyl betainate gemini, esterquat monomer, esterquat gemini, dodecyltrimethylammonium bromide, and the 12-2-12 gemini surfactant. The systematic names of the surfactants are given in Supporting Information.
surfactants have been presented in Supporting Information. These four ester-containing surfactants, as well as their nonestercontaining counterparts, are shown in Scheme 1. The critical micelle concentration (cmc) and the degree of micelle ionization (R) were determined at 25 °C by conductivity measurements, and the values obtained are reported in Table 1. As can be seen, the cmc values for the gemini surfactants are nearly 20 times smaller than those of the corresponding monomers. This illustrates the strong tendency of gemini surfactants to selfassemble, and similar data have been reported before by several authors.10,38 3.2. Characterization of the Surfaces. The SAMs used were characterized with respect to wetting by water. Table 2 shows the advancing contact angles recorded. It can be seen that the surface that presents only methyl groups is very hydrophobic, indicating successful formation of the SAM. Also, our target surface, the 90:10 methyl/carboxyl surface, is quite hydrophobic. 3.3. Adsorption Behavior. Before investigating adsorption at the target surface that exposes 90% methyl groups and 10% carboxyl groups, we used the SPR technique to monitor the (38) Zana, R. AdV. Colloid Interface Sci. 2002, 97, 203.
Cationic Ester-Containing Gemini Surfactants
Langmuir, Vol. 24, No. 12, 2008 6143
Table 1. Critical micelle concentration (cmc), micelle ionization degree (r), and Krafft temperature of the cationic surfactants determined by conductivity measurements
Table 2. Contact Angle Measurements of Alkanethiol SAMs on Gold Using Millipore Watera alkanethiol SAM
contact angle (deg)
HS-(CH2)10-CH3 HS-(CH2)11-OH HS-(CH2)10-COOH HS-(CH2)10-CH3 (90%)-HS-(CH2)10-COOH (10%)
103 ( 2 53 ( 5 28 ( 5 90 ( 6
a
Values are the averages of at least 10 measurements.
adsorption of the different surfactants at the three homogeneous SAM surfaces (i.e., surfaces presenting either methyl, hydroxyl, or carboxyl groups). Figure 1a-c shows the so-called response curves for five different surfactants at the three model surfaces. Out of the six synthesized surfactants (Scheme 1), dodecyl betainate gemini could not be included because of its high Krafft point. In principle, one can run SPR at elevated temperature, but using a surfactant solution just above its Krafft point could result in precipitation and clogging of the narrow channels of the instrument. We therefore refrained from this experiment. The injecting concentration of each surfactant was half the cmc, and the flow rate was set to 15 µL/min. The rinsing was done with Millipore water at the same flow rate. The plateau obtained after rinsing should be seen as the value of true adsorption. In the Figures, these come as response units (RUs) that can be transformed according to eq 1 to the adsorbed amount of surfactant per surface area. The average values of three to five experiments and the standard deviations are summarized in Table 3. The Table shows that there is substantially less adsorption at the OH-presenting surface than at the CH3- and COOH-presenting surfaces and there is no clear trend that gemini surfactants adsorb more (or less) than the monomeric surfactants. It is noteworthy that hydroxyl groups are so much more efficient than carboxyl groups at preventing adsorption. A probable explanation of this effect is that the COOH groups are partially deprotonated and that the resulting carboxylate groups attract cationic surfactants. A systematic study on the effect of pH on the adsorption of the cationic surfactants at the COOH-functional surface would be a logical extension of the work, although one must keep in mind that the ester-containing surfactants degrade rapidly at only slightly alkaline pH.29 As seen from the Table, there are distinct differences in adsorbed amount between the different surfactants, and these differences are not easy to explain. On the CH3-presenting surface, the two surfactants that lack ester bonds, DTAB and the 12-2-12 gemini, adsorb substantially less whereas at the COOH-presenting surface the two monomeric ester-containing surfactants adsorb
Figure 1. Adsorption isotherms for DTAB (full line), 12-2-12 (b), dodecyl betainate monomer (0), dodecyl esterquat monomer (×), and dodecyl esterquat gemini (2) at 23 °C on (a) the HS-(CH2)10-CH3 SAM, (b) the HS-(CH2)11-OH SAM, and (c) the HS-(CH2)10-COOH SAM. The surfactant was injected at a concentration of half the cmc, and the flow rate was set to 15 µL/min. Rinsing was done with Millipore water with the same flow rate. The plots are the averages of three to five measurements.
less than the others. The values for these outliers are the means of five experiments, so they seem to be statistically verified. When comparing the values, it is important to remember that the concentrations of the different surfactant solutions have varied greatly. All experiments have been carrried out at a concentration of half the cmc of the surfactant, and because the cmc values of the gemini surfactants are much lower than the values for the monomeric surfactants (Table 1), the concentrations of the injecting solutions have varied correspondingly. Thus, the adsorption values obtained are a confirmation that gemini surfactants are efficient in that they cover a surface at much lower absolute concentration than do ordinary surfactants. We then turned our attention to the surface consisting of 90% methyl groups and 10% carboxyl groups. SAMs consisting of mixtures of alkanethiols may have the terminal groups either homogeneously distributed or present in patches. Bertilsson and Liedberg have shown by a combination of infrared reflectionabsorption spectroscopy and X-ray photoelectron spectroscopy that mixing a methyl-terminated and a hydroxyl-terminated alkanethiol of the same chain length gives a homogeneously
6144 Langmuir, Vol. 24, No. 12, 2008
Tehrani-Bagha and Holmberg
Table 3. Adsorbed Amount of Cationic Surfactants at Gold Chips Covered by Different Alkanethiol SAMs Recorded by the SPR Technique at 23 °Ca
a
The reported values are the means of three to five measurements.
Figure 3. Adsorption isotherms for DTAB (full line), 12-2-12 (b), dodecyl betainate monomer (0), dodecyl esterquat monomer (×), and dodecyl esterquat gemini (2) at the HS-(CH2)10-CH3 (90%)-HS-(CH2)10COOH (10%) SAM measured by the QCM-D technique at 23 °C with adsorbed values expressed in (a) ng/cm2 and (b) nmol/cm2.
Figure 2. Adsorption isotherms for DTAB (full line), 12-2-12 (b), dodecyl betainate monomer (0), dodecyl esterquat monomer (×), and dodecyl esterquat gemini (2) at the HS-(CH2)10-CH3 (90%)-HS-(CH2)10COOH (10%) SAM measured by the SPR technique at 23 °C with adsorbed values expressed in (a) ng/cm2 and (b) nmol/cm2.
mixed monolayer rather than patches on a gold surface.39 Also, Whitesides and co-workers have demonstrated that “islands” are normally not formed.40,41 Thus, we anticipate that our surface consists of carboxyl groups relatively homogeneously distributed in the methyl group layer. Figure 2a,b shows adsorption isotherms for the series of cationic surfactants at the mixed methyl-carboxyl surface recorded by SPR. Adsorption was monitored over a range of concentrations from 0.1 to 3.2 times the cmc, and each injection of surfactant (39) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141. (40) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (41) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097.
solution was followed by rinsing with buffer. In Figure 2a, the adsorbed amount is expressed in ng/cm2 as a function of surfactant concentration normalized by the cmc. Figure 2b shows the same in nmol/cm2. It is clear from the Figures that the amount adsorbed is approximately the same for all of the surfactants at the same relative concentration, which means that the geminis adsorb less when calculated as nmol/cm2. Again, it is important to recall that the gemini surfactants have much lower cmc’s, which means that the plateau value of adsorption is reached at substantially lower absolute surfactant concentration. Next, the adsorption of the series of cationic surfactants at the mixed methyl-carboxyl surface was recorded by the QCM technique. Whereas SPR is an optical technique that measures changes in refractive index at the surface and therefore only records the adsorption of substances other than water, QCM measures the change in the resonance frequency of a crystal, which is a function of the total adsorbed amount (i.e., with water bound to the adsorbate included). The difference in measuring principles can be utilized to assess the amount of bound water incorporated into the adsorbed layer. Figure 3a,b shows adsorption isotherms for the series of cationic surfactants at the HS-(CH2)10-CH3 (90%)-HS-(CH2)10-COOH (10%) SAM surface expressed in ng/cm2 and nmol/cm,2 respectively. As with the SPR experiments, a series of measurements were performed with surfactant solutions below, at, and above the cmc, and the surfactant concentrations are all normalized to the cmc. In general the QCM measurements confirm the picture seen with SPR. There are no clear differences between the gemini surfactants and the monomeric surfactants in terms of adsorbed amount, measured in ng/cm2. It is known that the QCM technique can give erroneous results when recording surfactant adsorption if the viscosity or the density of the bulk is varied.13,33 This may be the case at high surfactant concentration. Here, relatively low
Cationic Ester-Containing Gemini Surfactants
Langmuir, Vol. 24, No. 12, 2008 6145
concentrations have been used, and we do not expect that the results have been affected by such parameters. The values of the adsorbed amount of the different surfactants are consistently higher when recorded by QCM than with SPR. This is the expected behavior and is due to QCM also taking into account bound water. The polar headgroup of the surfactant brings water into the adsorbed layer. The amount of water incorporated in the adsorbate is the difference between the values from the two techniques. The chemisorption of the thiol group (or rather the thiolate, i.e., R-S-) on gold is the main driving force for the formation of SAMs, but chemisorption alone is not sufficient to generate an organized monolayer. Also, van der Waals interactions between the alkyl chains are needed to stabilize the structure.26 The optimum chain length for the formation of well-ordered SAMs is C16-C24. The results presented in Figure 13 are all based on undecanethiol and derivatives of this compound (i.e., on SAMs composed of C11 chains). To deterimine if chain length is of importance, the target surface, the HS-(CH2)n-CH3 (90%)-HS(CH2)n-COOH SAM, was created with C16 alkane chains (i.e., by mixing hexadecanethiol and 16-mercaptohecadecanoic acid). The adsorption of the different cationic surfactants on this surface was monitored by SPR. Adsorption isotherms that are very similar to those obtained with the C11 mixed SAM and shown in Figure 2a,b were obtained (Supporting Information). Thus, the C11 SAMs seem to be stable enough for our purpose. 3.4. Adsorption Isotherm Models. There are different isotherms for the adsorption of a solute at a solid surface. We have tested the Langmuir, Freundlich, and Tempkin adsorption isotherms for adsorption at the HS-(CH2)10-CH3 (90%)-HS(CH2)10-COOH (10%) SAM surface. In the Langmuir theory, the basic assumption is that the sorption takes place at specific homogeneous sites within the adsorbent.42 This adsorption equation can be written as
Γ)
( )
ΓmaxKC 1 1 1 1 + f ) 1 + KC Γ Γmax KΓmax C
(3)
where Γ is the adsorbed amount, K is the equilibrium constant, Γmax is the adsorption at full coverage, and C is the equilibrium surfactant concentration. The Freundlich isotherm is derived by assuming a heterogeneous surface with a nonuniform distribution of heat of adsorption over the surface.43 The Freundlich isotherm can be expressed as
Γ ) KCm f ln(Γ) ) ln(K)m + ln(C)
(4)
This isotherm assumes an exponential decrease in adsorption enthalpy. The relation between the logarithm of surface excess and the logarithm of concentration is linear.13 The Tempkin model can be regarded as a modified Langmuir isotherm that assumes linearly decreasing adsorption enthalpy. The model will give a linear dependence between the surface excess and the logarithm of concentration:
Γ ) K1 ln(C) + K2
(5)
As can be seen in Tables 4 and 5, very high regression coefficients for the adsorption of the different surfactants were obtained with the Langmuir model using values obtained both from the SPR and from the QCM measurements. Considerably lower regression coefficients were obtained with the other models. The Same result was obtained for the mixed methyl-carboxyl (42) Langmuir, I. J. Am. Chem. Soc. 1917, 39, 1848. (43) Freundlich, H. M. F. Z. Phys. Chem. 1906, 57A, 385. (44) Tehrani-Bagha, A. R.; Bahrami, H.; Movassagh, B.; Arami, M.; Amirshahi, S. H.; Menger, F. M. Colloids Surf., A 2007, 307, 121.
Table 4. Regression Coefficients for Different Isotherm Models for Adsorption of the Cationic Surfactants at the HS-(CH2)10-CH3 (90%)-HS-(CH2)10-COOH (10%) SAM Surfacea isotherm models surfactants
Langmuir Freundlich Tempkin
dodecyl esterquat monomer dodecyl esterquat gemini (s ) 3) dodecyl betainate monomer DTAB 12-2-12 a
0.9968 0.9807 0.9802 0.9832 0.9912
0.8715 0.7832 0.9083 0.9210 0.9526
0.9515 0.8583 0.9460 0.9393 0.9783
Monitored by SPR.
Table 5. Regression Coefficients for Different Isotherm Models for Adsorption of the Cationic Surfactants at the HS-(CH2)10-CH3 (90%)-HS-(CH2)10-COOH (10%) SAM Surfacea isotherm models surfactants
Langmuir Freundlich Tempkin
dodecyl esterquat monomer Dodecyl esterquat gemini (s ) 3) dodecyl betainate monomer DTAB 12-2-12 a
0.9800 0.9722 0.9661 0.9424 0.9552
0.8919 0.7029 0.7081 0.8402 0.8878
0.9226 0.7285 0.6982 0.8537 0.9025
Monitored by QCM.
surface based on C16 alkane chains, as shown in Supporting Information. These results confirm that the adsorption is of the Langmuir type, and they also serve as an indirect indication of the validity of the techniques that we have used to monitor surfactant adsorption.
4. Conclusions This work shows that gemini surfactants and the corresponding monomeric surfactant, when used at the same concentration normalized to the cmc, adsorb to approximately the same extent on a model surface composed of 10% carboxyl groups and 90% methyl groups. This surface serves as a model for the surfaces of many polymeric materials. The adsorption isotherms on this hydrophobic yet negatively charged surface were similar when measured by SPR as by the QCM technique, although the adsorbed amount was higher with QCM because this technique also takes into account the water that is incorporated into the adsorbed layer. The adsorption data obtained from both the SPR and the QCM measurements were found to correlate very well with the Langmuir model of adsorption but not with the Freundlich and Tempkin models. The gemini surfactants have a much lower cmc than the corresponding monomeric surfactants. This means that the plateau value of adsorption is reached at lower concentrations. In other words, the geminis are considerably more efficient in covering the surface. This behavior is useful for technical applications, where there is a constant desire to reduce the amount of chemicals added in the process.44 We have also demonstrated that the estercontaining geminis, which were developed as readily degradable surfactants,29 are at least as efficient as their nonester-containing counterparts. Acknowledgment. A.R.T.-B. is grateful to the Iran Ministry of Science, Research and Technology and to the Swedish Institute for grants that enabled him to study at Chalmers University of Technology. Supporting Information Available: Syntheses of compounds in section 2.2, adsorption isotherms, and regression coefficients for different isotherm models. This material is available free of charge via the Internet at http://pubs.acs.org. LA800009B
