Adsorption of Dianionic Surfactants Based on Amino Acids at Different

May 18, 2010 - Adsorption is studied on silica, gold, gold hydrophobized by a self-assembled ...... Denver, CO, 1995; AVS: Denver, CO, 1995; pp 1247-1...
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Adsorption of Dianionic Surfactants Based on Amino Acids at Different Surfaces Studied by QCM-D and SPR Romain Bordes,*,† J€urgen Tropsch,‡ and Krister Holmberg† †

Chalmers University of Technology, Department of Chemical and Biological Engineering, SE-412 96 G€ oteborg, Sweden, and ‡BASF, 67056 Ludwigshafen, Germany Received March 4, 2010. Revised Manuscript Received May 4, 2010

The adsorption of three dicarboxylic amino acid-based surfactants, disodium N-lauroylaminomalonate, disodium N-lauroylaspartate, and disodium N-lauroylglutamate, has been studied by surface plasmon resonance (SPR) and the quartz crystal microbalance with dissipation monitoring (QCM-D). These surfactants have high cmc values, which means that the unimer concentration is high at the plateau value of adsorption. This gives rise to a considerable “bulk effect”, which must be deducted from the observed value in order to obtain the true value of the adsorbed amount. In this article, we show how this can be done for the QCM-D technique. Adsorption is studied on silica, gold, gold hydrophobized by a self-assembled layer of an alkane thiol, and hydroxyapatite. Adsorption on hydroxyapatite differs very much among the three surfactants, with the aspartate derivative giving the strongest and the glutamate giving the weakest adsorption. This difference is explained as the difference in ability of the dicarboxylic amphiphiles to chelate calcium in the crystal lattice.

1. Introduction The adsorption of surfactants from aqueous solution is one of the most basic phenomena in surface chemistry and is a key event in most technical applications of surfactants. Above a certain concentration of surfactant in the surrounding water, the adsorption proceeds as a self-assembly process. The characteristic Langmuir-type adsorption of both ionic and nonionic surfactants at hydrophobic surfaces and the S-shaped adsorption isotherm of ionic surfactants at solid surfaces of opposite charge (and of polyoxyethylene-based nonionic surfactants on silica and related surfaces1) can be rationalized only by the self-assembly of the adsorbate.2-4 For ionic surfactants at oppositely charged surfaces, self-assembly may commence at 1 order of magnitude lower concentration than the critical micelle concentration (cmc) in the bulk. This effect is usually explained by the initial adsorption being an ion-exchange process.5,6 The counterions in the diffuse layer just outside the surface are exchanged by surfactants of the same charge. This leads to an elevated surfactant concentration close to the surface, which in turn induces micellization close to the surface at very low bulk concentrations of the amphiphile. Surfactant adsorption at a hydrophobic surface leads to a monolayer or to closely packed hemimicelles. The packing of surfactant at such a surface is relatively independent of the polarity of the surface.7 Self-assembly at hydrophobic surfaces has much in common with micellization in the bulk. For instance, values of the free energy of micellization and the free energy of adsorption at such surfaces are related. At charged surfaces, ionic surfactants of opposite sign give rise to a double layer or to closely packed micelles at the surface. *Corresponding author. E-mail: [email protected]. (1) Tiberg, F.; Lindman, B.; Landgren, M. Thin Solid Films 1993, 234, 478–481. (2) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90–96. (3) Holmberg, K.; J€onsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution, 2nd ed.; Wiley: Chichester, U.K., 2003; pp 357-387. (4) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; Wiley: New York, 2004; pp 38-59. (5) Rosen, M. J. Am. Oil Chem. Soc. 1975, 52, 431–435. (6) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 463–478. (7) Vijayendran, B. In Polymer Colloids II; Fitch, R., Ed.; Plenum Press: New York, 1980; pp 209-224.

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Many analytical tools are available nowadays for characterizing adsorbed layers at surfaces as well as for monitoring the adsorption process. In this article, we have used the quartz crystal microbalance with dissipation monitoring (QCM-D) as the main tool for studying the adsorption of a small series of dianionic surfactants at different tailormade surfaces ranging from very hydrophobic to prominently hydrophilic. The hydrophobic surface was made by the selfassembled monolayer (SAM) technique applied to gold using longchain alkane thiols as reagents. The naked gold surface was also used as a substrate. The hydrophilic surfaces were made of silica and hydroxyapatite deposited as nanometer-thick layers on the QCM gold chip. The dianionic surfactants, which all had a linear dodecyl hydrophobic tail, had high cmc values. This means that the unimer concentration is high, which gives rise to a considerable bulk effect in the QCM measurements. A methodology to separate the bulk effect from the adsorption values is presented. Surface plasmon resonance (SPR) was also used to study adsorption on the untreated gold surface, and the values obtained were compared with the values from the QCM measurements. Combining the two techniques, SPR and QCM, is often useful. SPR, which measures the change in the refractive index that occurs during adsorption, monitors the amount of organic material that builds up on the surface. QCM, however, is a gravimetric technique that measures the amount of material in the adsorbed layer (i.e., it includes the water that usually comes along with the adsorbate, either as water of hydration or as entrapped water). The dianionic surfactants were N-acyl derivatives of R-amino acids with an extra carboxyl group situated one, two, or three carbons away from the first carboxyl group. The surfactants were synthesized by acylation of the amino group with lauroyl chloride using either the unprotected or the ester-protected amino acid as the starting material. The structures, in the form of their sodium salts, are shown in Figure 1.

2. Material and Methods 2.1. Materials. Lauroyl chloride (Aldrich, 98%), L-aspartic

acid (Aldrich, g98%), L-glutamic acid (Aldrich, 99%), aminomalonic acid diethyl ester hydrochloride (Fluka, g99%), sodium

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Figure 1. From left to right: disodium N-lauroylaminomalonate (C12MalNa2), disodium N-lauroylaspartate (C12AspNa2), and disodium N-lauroylglutamate (C12GluNa2). hydroxide (Fluka, g98%), tetrahydrofuran (THF) anhydrous (Aldrich, 99.9%), hydrochloric acid (Riedel de Ha€en, 37%), and 1-hexadecanethiol (Fluka, >95%) were used as purchased. Pyridine (Aldrich, 99%) was used freshly distilled in vacuo. Milli-Q water (resistance >18 MΩ cm) was used for the preparation of aqueous solutions. 2.2. Synthesis of the Surfactants. Disodium lauroyl aspartate and disodium lauroyl glutamate were synthesized via the Schotten-Baumann reaction. Disodium lauroyl malonate was synthesized by the ester protection route.

2.2.1. Synthesis by the Schotten-Baumann Reaction (Preparation of Disodium Lauroyl Aspartate, C12AspNa2, and Disodium Lauroyl Glutamate, C12GluNa2). An amino acid suspension (310 mmol) is prepared in a water/acetone (210 mL/ 150 mL) mixture in a round-bottomed flask. The pH is set to 12 with sodium hydroxide pellets. Lauroyl chloride is added dropwise under stirring at 5 °C, and stirring is continued for 90 min. The mixture is then cooled to 0 °C and stirred for 2 h while the pH is kept at 12 with an automatic titrator filled with a solution of 2.5 M sodium hydroxide. The solution is warmed to room temperature and then acidified to pH 2. A white precipitate is collected by filtration and washed with water. The product is recrystallized three times from toluene and dissolved in EtOH, and a solution of sodium hydroxide is added (2M, 2 equiv), leading to a precipitate that is isolated by filtration. The yields are 74% for both products. NMR data of the products are given in the Supporting Information.

2.2.2. Synthesis by the Ester Protection Route (Preparation of Disodium Lauroyl Aminomalonate, C12MalNa2). The diethyl ester of aminomalonic acid (47 mmol) is dissolved in pyridine (100 mL) in a round-bottomed flask. Lauroyl chloride (47 mmol) in THF (100 mL) is added under stirring at room temperature, leading to a precipitate. The suspension is stirred for 18 h. The mixture is then poured into aqueous hydrochloric acid (1.5 L, 1 M). After 2 h of stirring, the suspension is filtered. The solid diethyl ester of the dicarboxylate surfactant is filtered, washed with water, and dissolved in ethanol (150 mL) in a round-bottomed flask. Sodium hydroxide (2 M, 2 equiv) in ethanol (30 mL) is then added under stirring. A white precipitate is obtained, which is isolated by filtration in a yield of 95%. NMR data are given in the Supporting Information. 2.3. Methods. 2.3.1. NMR. NMR analyses were carried out on a Jeol 400 MHz spectrometer at 25 °C. Deuterated solvent was purchased from Armar Chemicals. 2.3.2. Contact Angle Measurements. The contact angle of a 5 μL water droplet on the surfaces was measured with a DAT 1100 contact angle tester from Fibro System (Stockholm, Sweden).

2.3.3. Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D). A QCM-D instrument (model D300) from Q-Sense (G€ oteborg, Sweden) was used. To avoid crystal perturbations during the shear oscillation of the crystals, the measurements were carried out under nonflowing conditions. The AT-cut crystals coated with a 100 nm thin gold layer were also from Q-Sense. Q-Sense also supplied the silica- and hydroxyapatite-coated crystals. The cleaning procedure, prior to use, was done as follows: UV-ozone treatment for 10 min, immersion in a 5:1:1 mixture of H2O/ammonia (25%)/H2O2 (30%) at 75 °C for 5 min, rinsing with Milli-Q water, drying with N2, and finally 10 min of UV-ozone treatment. The silica and hydroxyapatite surfaces were cleaned prior to use by a simple UV-ozone treatment. 10936 DOI: 10.1021/la100909x

Bordes et al. The hydrophobic self-assembled monolayers (SAM-CH3) were prepared by immersing the cleaned gold surfaces in 1-hexadecanethiol solution (2 mM in ethanol) for at least 16 h. The crystals were then rinsed with ethanol and sonicated in ethanol for 5 min to remove loosely adsorbed alkane thiols. Finally, the surfaces were rinsed with water and dried with nitrogen prior to use.8 The measurements were made at 20 °C with a baseline corresponding to the loading of the crystal by Milli-Q water. The surfactant solution was injected into the crystal chamber using a homemade device, going from the lowest to the highest concentration without rinsing the surface between additions. The crystal was finally rinsed with Milli-Q water to remove poorly adsorbed surfactants. The automatic injection device is based on electric valves that open different flow channels on a collector connected at the inlet of the QCM-D cell. The valves are controlled via an electronic interface using remote control by the com port of a computer. The software is developed in Python (version 2.4). Prior to use, all of the flow channels were rinsed with ethanol and water. 2.3.4. Surface Plasmon Resonance (SPR). The measurements of the variations of the surface plasmon resonance were made with an SPR Biacore X from Biacore SIA (Uppsala, Sweden). Details about the technique and the apparatus can be found elsewhere.8 To reach steady state, the surfactant solution was injected (40 μL) into the analysis chamber under a continuous flow of 25 μL/min. The gold chips from Biacore SIA (Uppsala, Sweden) were cleaned prior to use by the following procedure: UV-ozone treatment for 10 min, immersion in a 5:1:1 mixture of H2O/ammonia (25%)/H2O2 (30%) at 75 °C for 5 min, rinsing with Milli-Q water, drying with N2, and finally 10 min of UV-ozone treatment. 2.3.5. Ellipsometry. The adsorption measurements by ellipsometry were carried out on a Rudolph thin-film ellipsometer, type 436 (Rudolph Research, Fairfield, NJ), equipped with a xenon arc lamp. The wavelength and the angle of incidence were 401.5 nm and 67.7°, respectively. More details about the instruments can be found elsewhere.9,10 The silicon dioxide surfaces were cleaned with a plasma oven prior to use. The solutions of the surfactants at the cmc were directly injected into the 5 mL thermostatted quartz cuvette. 2.3.6. Molecular Modeling. Structures were optimized in vacuum with Chem3d Pro 12 (CambridgeSoft) under the MM2 forcefield. The surfactant tails were reduced to four carbon atoms. Distances expressed are taken as the distance between the two carbonyl carbons of the carboxylate group. Optimization was carried out starting from different conformations in order to assess as close as possible the lowest-energy conformation.

3. Results As mentioned in the introduction, SPR and QCM are complementary in the sense that the former technique monitors the amount of solute adsorbing at a surface whereas the latter, which is a gravimetric method, also records the amount of solvent that is included in the adsorbed layer. However, SPR is more restricted when it comes to choice of surface. The substrate needs to have a plasmon band, and in reality the choice is restricted to gold, silver, platinum, and related metals. Such surfaces modified by the self-assembled monolayer (SAM) technique have also been used as substrates.11 QCM is much more versatile when it comes to the choice of the surface. Different oxides and other materials can be applied on top of the gold-plated quartz crystal. In this work, only the naked gold surface was used for the SPR measurements and the QCM-D experiments were made with a variety of substrates. (8) Oskarsson, H.; Holmberg, K. J. Colloid Interface Sci. 2006, 301, 360–369. (9) Landgren, M.; Joensson, B. J. Phys. Chem. 1993, 97, 1656–1660. (10) Halthur, T. J.; Claesson, P. M.; Elofsson, U. M. Langmuir 2006, 22, 11065– 11071. (11) Sigal, G. B.; Mrksich, M.; Whitesides, G. M. Langmuir 1997, 13, 2749– 2755.

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Table 1. Contact Angle of Milli-Q Water on the Surfaces Used surface

gold

SAM-CH3

silica

hydroxyapatite

contact angle

88°

110°

19°

56°

3.1. Surfaces Studied. The surfaces used for the QCM-D experiments were naked gold, gold hydrophobized by the SAM procedure (SAM-CH3), silica (silicon dioxide, SiO2), and hydroxyapatite (Ca5(PO4)3(OH)). The SAM-CH3 surface was prepared by immersing the gold surface in a solution of hexadecane thiol in ethanol, a procedure that is known to anchor the alkyl chain through a Au-S bond leading to the formation of a tightly packed monolayer exposing methyl groups to the surrounding medium.11,12 Together the surfaces span a wide wettability range, and the contact angles obtained with Milli-Q water are shown in Table 1. The silica and hydroxyapatite surfaces differ not only in wettability but also in their isoelectric point. Whereas the isoelectric point of silica is low, between 2.0 and 2.5, that of hydroxyapatite is around 8.5. This means that under the relatively neutral conditions used for the adsorption measurements (i.e., pH 8.0), the silica surface will always be strongly negatively charged and the hydroxyapatite surface will carry a slight positive charge. This will have implications on the adsorption of the dianionic amphiphiles, as will be discussed below. 3.2. Surface Plasmon Resonance (SPR): Treatment of the Data. In the SPR technique, monochromatic, p-polarized light is directed through an optical unit made of a prism and a glass support coated with a thin layer of gold in contact with the solution in the flow cell. Light directed above a critical angle of the p-polarized incident light will cause an evanescent field to penetrate into the gold film. This evanescent field can couple to an electromagnetic surface wave, which is called a surface plasmon. Therefore, the reflected light will not be totally reflected, and the surface plasmon, which is excited at the gold-liquid interface, is measured by photodiodes as a minimum in reflected light. As surfactants adsorb at the gold surface, the refractive index will be altered and the conditions for SPR will be changed, which is monitored as a change in the position of the minimum intensity of the reflected light. This change of angle is in the BIAcore terminology expressed in resonance units (RUs), with 1 RU being equal to a 0.0001° change in the intensity minimum. The change in the angle of incident light, measured as RU, is related to the mass adsorbed at the surface, Δm, according to Δm ¼

ΔRU  CSPR β

where CSPR is a factor containing an instrument constant and dn/dc (the variation of refractive index with concentration) of the adsorbent and β is a factor compensating for the decrease in the SPR signal with distance from the gold substrate. CSPR was calculated to be 0.094 ( 0.008 ng/cm2 using an average dn/dc for 18 different surfactants, and β was set to 1, which is the case for a plain gold surface.13 The factor β will differ from 1 when the surface layer is thick. In this work, the layer is very thin and the value of β can be anticipated to be close to 1. The change in refractive index measured by the SPR technique can be caused not only by the adsorption of a solute at the gold surface but also by variations of the refractive index of the solution in close proximity (within ca. 200 nm) to the surface. The adsorption of surfactants from solutions of high concentrations will give rise to variations in the refractive index of the solution, (12) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 111, 7155– 7164. (13) Larsson, C.; Rodahl, M.; Hook, F. Anal. Chem. 2003, 75, 5080–5087.

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Figure 2. Mass adsorbed of the three dianionic amino acid-based surfactants on a gold surface determined by SPR at 20 °C.

which in turn will give a substantial contribution to the RU value recorded. To discriminate between this bulk contribution and surface adsorption, Whitesides et al. have shown that the linear increase after the cmc is due only to the bulk effect, assuming that a plateau is reached at the cmc, which is normally the case for surfactant adsorption.11 This slope will be the same when the measurements are carried out on a nonadsorbing surface, such as a surface coated with poly(ethylene glycol). The real surface adsorption can then be obtained by subtracting the bulk contribution retrieved from the slope above the cmc from the value recorded. The recorded data were processed this way (i.e., using the following equations) Δmcorrected ¼ Δmmeasured - Δmbulk where Δmbulk is Δmbulk ¼ slope 

C cmc

3.3. SPR at the Gold Surface. Figure 2 shows the mass adsorbed as determined by SPR versus the concentration of the dianionic surfactants normalized to the cmc. From the slope of the curves and from the plateau values, it can be concluded that the driving force for adsorption and the adsorbed amount increase with the length of the spacer between the two carboxyl groups (i.e., with the molecular weight of the amphiphile). It is interesting that for all three surfactants the adsorbed layer seems to be fully developed at a concentration of 80% of the cmc. 3.4. Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D): Treatment of Data. QCM-D is a gravimetric method based on monitoring the resonance frequency of a quartz crystal. When adsorption occurs, the amount adsorbed induces a change in the oscillation and the loss of energy due to the adsorbed layer is measured through the dissipation. A detailed description can be found elsewhere.14,15 The frequency variation was monitored at the third, fifth, and seventh overtones. For simple adsorption, the values obtained are within 5% deviation when normalized versus the overtone number. The adsorbed mass can then be determined using the (14) Tehrani-Bagha, A. R.; Holmberg, K. Langmuir 2008, 24, 6140–6145. (15) Hook, F.; Kasemo, B.; Nylander, T.; Fant, C.; Sott, K.; Elwing, H. Anal. Chem. 2001, 73, 5796–5804.

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Figure 3. Changes in adsorbed mass, Δm, (left) determined by the Sauerbrey equation of C12MalNa2 on a gold surface monitored by QCM-D at the third (O), fifth (4), and seventh (0) overtones at 20 °C, and in dissipation, ΔD (right). The Δm values are normalized by the overtone numbers. The lines are drawn as guide for the eyes.

Sauerbrey relation,16 assuming homogeneous coverage on the surface and the amount adsorbed being less than 2% of the mass of the crystal Δm ¼ -

Δf  C n

where Δm is the adsorbed mass, Δf is the variation of frequency observed at overtone n, and C = 17.7 ng.Hz-1.cm-2, a constant that is characteristic of the equipment. The dissipation, which corresponds to the decay of the intensity of the oscillation versus time due to the loss of energy, was also monitored. The dissipation can be defined as D ¼

Edissipated 2πEstored

where Edissipated and Estored are the dissipated and the stored energy of the adsorbed layer, respectively. During the measurements, the baseline was first recorded in pure water and then solutions of surfactant with increasing concentrations were injected into the measurement cell, leading to a step-by-step response of the adsorption. The surfactants adsorbed will add to the total mass, thus giving rise to a decrease in the oscillation frequency. The adsorption will also increase the dissipation factor due to an increase in the viscoelasticity of the adsorbate film on the surface. Both the dissipation and the frequency will be affected if the viscosity of the surrounding solution increases as an effect of an increase in solute concentration. This is referred to in the literature as a bulk effect.17 This effect is small when QCM is used to monitor the adsorption of surfactants with low cmc value. For adsorption of typical nonionic surfactants, for instance, which have cmc values on the order of (1-10)  10-3 M, the bulk effect is very small and is usually neglected.17 The surfactants used in this work have high cmc values of 50, 73, and 74 mM for C12MalNa2, C12AspNa2, and (16) Sauerbrey, G. Z. Phys. 1955, 155, 206. (17) Knag, M.; Sj€oblom, J.; Øye, G.; Gulbrandsen, E. Colloids Surf., A 2004, 250, 269–278. (18) Bordes, R.; Tropsch, J.; Holmberg, K. J. Colloid Interface Sci. 2009, 338, 529–536.

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C12GluNa2, respectively,18 which means that the concentration of unimer in the solution is high at the point when the adsorption reaches the plateau value. This high unimer concentration gives rise to a considerable bulk effect that needs to be taken into account. The QCM technique has, in fact, been used as a sensitive tool for measuring the microviscosity of solutions.19 Figure 3 illustrates the point. The adsorbed mass, which is obtained from the variation of the frequency, Δf, as described above, does not level out around the cmc, as would be the case if there was no contribution from the surrounding liquid. Part of the recorded Δf is most likely caused by the viscosity of the surfactant solution, and the influence of the viscosity (i.e., the bulk effect) becomes more important the higher the concentration. Evidence of a bulk effect can be seen from the fact that the recorded Δm values at different concentrations vary with the overtone numbers. When there is no bulk effect, the plots of Δm normalized by the overtone number versus the surfactant concentration normalized by the cmc should be identical. In Figure 3 (left), the curves are overtone-dependent with the values decreasing with an increase in overtone number. This is the expected trend if there is a contribution from the viscosity of the bulk because the depth of the probing is overtone-dependent. The higher the overtone, the smaller the distance away from the surface that is being taken into account in the measurement, which means that the bulk effect should be most important for the third overtone and least important for the seventh overtone. Very strong dissipation was also obtained (Figure 3, right). This is characteristic of measurements with a strong contribution from the bulk effect. Kanazawa has derived equations to assess how changes in the viscosity and the density of the surrounding solution affect the resonance frequency and the dissipation without any surface adsorption taking place.20 Here we will show how the bulk effect can be separated from adsorption. By doing so, we will demonstrate that the QCM-D technique is also useful for monitoring the adsorption of surfactants (and other solutes) that give rise to substantial changes in the viscosity of the solution surrounding the quartz crystal. (19) Ash, D. C.; Joyce, M. J.; Barnes, C.; Booth, C. J.; Jefferies, A. C. Meas. Sci. Technol. 2003, 1955. (20) Kanazawa, K.; Gordon, J. G. Anal. Chim. Acta 1985, 175, 99–105.

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Figure 4. QCM-D measurements of adsorption at 20 °C for C12MalNa2 at different concentrations normalized to the cmc. Upper left: change in frequency, Δf, recorded at the third (O), fifth (4), and seventh (0) overtones. Lower left: change in dissipation, ΔD, recorded at the same overtones. Upper right: change in frequency normalized by the square root of the overtone number. Lower right: change in dissipation multiplied by the square root of the overtone number.

To quantify the bulk effect, experiments with surfactant solutions were carried out using a surface onto which the surfactants do not adsorb. The principle is the same as previously used by Whitesides to assess the bulk effect in SPR measurements.11 Silica was chosen as a nonadsorbing surface. It is known from the literature that anionic surfactants adsorb very little, if at all, on silica if the pH of the solution is such that both the silica surface and the surfactant carry a negative charge.21 To confirm that silica was also nonadsorbing in this case, the adsorption of the three dicarboxylic surfactants, C12MalNa2, C12AspNa2, and C12GluNa2, was studied by ellipsometry at a concentration corresponding to the cmc. No adsorption was recorded. Next, adsorption measurements were carried out with the QCM-D technique using a silica-coated quartz crystal as the surface. The results obtained with C12MalNa2 are presented in Figure 4. Very similar results were obtained for C12AspNa2 and C12GluNa2. Figure 4 shows plots of the change in frequency normalized by the square root of the overtone number versus surfactant concentration normalized by the cmc overlap. The same applies for the plots of change in dissipation multiplied by the square root of the overtone number versus surfactant concentration normalized by the cmc. This is a strong indication that the values recorded are caused entirely by the bulk effect and that the adsorption of C12MalNa2 at the silica surface is so small that it can be neglected. Thus, the values can be used to deduce the bulk effect from the Δf and ΔD values obtained on surfaces onto which the surfactants adsorb. In the subsequent QCM measurements performed on several different surfaces, the Δf values were corrected at every overtone according to Δfcorrected ¼ Δfmeasured - Δfbulk Changes in adsorbed mass, Δm, were obtained from the Δf values as described above. (21) Penfold, J.; Staples, E.; Tucker, I.; Thomas, R. K. Langmuir 2002, 18, 5755– 5760.

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Figure 5. Changes in mass (Δm) determined with QCM-D vs surfactant concentration normalized by the cmc for the adsorption of C12MalNa2, C12AspNa2 and C12GluNa2 on gold at 20 °C. The values shown represent the third (O), fifth (4), and seventh (0) overtones. Dashed lines are uncorrected values, and solid lines are corrected values.

Figure 6. Changes in mass (Δm) determined with QCM-D vs surfactant concentration normalized by the cmc for the adsorption of C12MalNa2, C12AspNa2, and C12GluNa2 on hydrophobized gold at 20 °C. The values shown represent the third (O), fifth (4), and seventh (0) overtones. Dashed lines are uncorrected values, and solid lines are corrected values.

3.5. QCM-D at the Gold Surface. Figure 5 shows the adsorption curves, expressed as the change in adsorbed mass, Δm, corrected and uncorrected at the third, fifth, and seventh overtones for the three dianionic surfactants. After the correction, the values obtained are very similar regardless of the overtone. This is in agreement with the adsorption leading to a rigid film on the surface and proves the efficiency of the correction that is implemented. 3.6. QCM-D at Hydrophobized Gold. The SAM-CH3 surface, obtained by treatment of the gold surface with hexadecane thiol, was used as the substrate. The results of the QCM-D measurements are shown in Figure 6. DOI: 10.1021/la100909x

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Figure 7. Changes in mass (Δm) determined with QCM-D vs surfactant concentration normalized by the cmc for the adsorption of C12MalNa2 (O), C12AspNa2 (4), and C12GluNa2 (0) on hydroxyapatite at 20 °C. The values represent the seventh overtone. Dashed lines are uncorrected values, and solid lines are corrected values.

3.7. QCM-D at Hydroxyapatite. A thin layer of hydroxyapatite was applied to the gold-coated quart crystal. Such a surface is of particular interest in the adsorption of dicarboxylic surfactants because of the possibility of complex formation with lattice calcium. Figure 7 shows the adsorption curves for C12MalNa2, C12AspNa2, and C12GluNa2. Only the curves for the seventh overtone are given

4. Discussion Use of the QCM-D technique to study the adsorption of surfactants with high cmc values constitutes a problem.17 The high unimer concentration around the cmc gives rise to an increase in solution viscosity that will severely affect the resonance frequency of the quartz crystal. When QCM-D measurements are performed on a nonadsorbing surface, the contribution from the viscosity can be quantified and the values obtained can then be deducted from the Δf values recorded for experiments on surfaces on which the surfactants adsorb. We show here that this procedure seems to give reliable values of the adsorbed amount as a function of the solution concentration of surfactant. This procedure allows a more general use of the QCM-D technique to monitor the adsorption of surfactants at solid surfaces. The technique as such is attractive because the variety of surfaces that can be studied is large, as is demonstrated in this work. The contact angle recorded on the gold surface, 88°, is surprisingly high. This value was obtained after thorough cleaning of the surface with UV-ozone and wet oxidative treatment (H2O2/NH3). The issue of whether the gold surface is hydrophilic or hydrophobic is subject to considerable controversy in the literature, however. It is well known that a gold surface cannot be completely wetted by water under ambient conditions,22 but the values reported differ depending on the history of the surface. Ron et al.23 obtained a value of below 10° after UV irradiation in air. Subsequent treatment with ethanol raised the contact angle to 70°. The very low contact angle was attributed to the formation of gold oxide at the surface. (22) Smith, T. J. Colloid Interface Sci. 1980, 75, 51–55. (23) Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116–1121. (24) King, D. E. Oxidation of Gold by Ultraviolet Light and Ozone at 25°C. Proceedings of the 41st National Symposium of the American Vacuum Society, Denver, CO, 1995; AVS: Denver, CO, 1995; pp 1247-1253.

10940 DOI: 10.1021/la100909x

The generation of gold oxide at a gold surface exposed to UV irradiation has been demonstrated by XPS.24 The gold oxide was reduced back to metallic gold by ethanol treatment, as also shown by XPS. It has been reported that gold kept under ultrahigh vacuum is hydrophilic.22 It is also known that a gold surface can bind halide ions, in particular, bromide and iodide, by chemisorption,25,26 and such a surface becomes negatively charged. The mode of surfactant adsorption at a gold surface will therefore depend on the history of the gold surface as well as on the type of surfactant used. The adsorption of anionic and nonionic surfactants is likely to lead to either a monolayer or closely packed hemimicelles, as is the normal mode of adsorption of amphiphiles at hydrophobic surfaces. Cationic surfactants may adsorb either as a monolayer/hemimicelles or as a double layer/closely packed micelles, depending on the type of counterion.27 We have previously used a combination of QCM-D and SPR to study the adsorption of two anionic amino acid-based surfactants, sodium N-lauroylglycinate and sodium N-lauroylsarcosinate, at gold. The sarcosine-based surfactant produced bilayer adsorption, and the glycine derivative adsorbed as a monolayer.28 This difference in the adsorption pattern was attributed to a stronger interaction between the surfactant headgroup and the gold surface for the sarcosinate than for the glycinate surfactant. The sarcosine headgroup is known to interact strongly with many inorganic surfaces.29 Also in this work we combine QCM-D and SPR for the adsorption studies at the gold surface. For both techniques, we subtract the influence from the solution surrounding the adsorbing surface. For SPR, where the bulk effect is caused by the contribution of the surfactant solution to the change in refractive index, we have used the procedure proposed by Whitesides et al.11 For QCM-D, where the bulk effect is due to an increase in the viscosity of the surfactant solution, we have adjusted the values according to the procedure presented above. The two methods of monitoring surfactant adsorption give somewhat different results. The SPR plots show a very sharp kink in the curves at concentrations somewhat below the cmc and the plateau adsorption is highest for the glutamate surfactant and lowest for the aminomalonate. The QCM-D curves are not very distinct, but a comparison of the corrected plots shows that the aminomalonate surfactant produces higher plateau adsorption than the other two surfactants, which are approximately equal. The values of the adsorbed amount are considerably higher for QCM than for SPR. This is in accordance with previous work in which the two techniques have been compared as tools for monitoring surfactant adsorption,28 and it is usually attributed to QCM taking into account the water of hydration (i.e., water bound to the polar headgroups facing the bulk, which SPR does not do). The surfactants studied in this work contain very powerful polar headgroups with two carboxylate groups and an amide bond. It is likely that such surfactants in a self-assembled state incorporate a considerable amount of water. When the results from the two methods are compared, it is also important to realize that the gold surfaces are not identical. The roughness of the two types of chips differs, and this property is known to play a particularly role in QCM measurements.30 (25) Tao, N. J.; Lindsay, S. M. J. Phys. Chem. 2002, 96, 5213–5217. (26) Wandlowski, T.; Wang, J. X.; Magnussen, O. M.; Ocko, B. M. J. Phys. Chem. 1996, 100, 10277–10287. (27) Jaschke, M.; Butt, H. J.; Gaub, H. E.; Manne, S. Langmuir 1997, 13, 1381– 1384. (28) Bordes, R.; Tropsch, J.; Holmberg, K. Langmuir 2009, 26, 3077–3083. (29) Salensky, G. A.; Cobb, M. G.; Everhart, D. S. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 133–140. (30) Daikhin, L.; Gileadi, E.; Katz, G.; Tsionsky, V.; Urbakh, M.; Zagidulin, D. Anal. Chem. 2002, 74, 554–561.

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To our knowledge, the effect of the surface topology on the SPR response has not been investigated. The values obtained from the QCM measurements point to the formation of a monolayer at the surface. A QCM response of 20 Hz has been reported for a surfactant bilayer on SiO2,31 which is equivalent to an adsorbed amount of 400 ng cm-2. A monolayer would be roughly half of this value (i.e., 200 ng cm-2). This is approximately what we obtain in this work. By applying Gibbs’ adsorption equation to tensiometric data, we have previously shown that, at the cmc, C12MalNa2 has a larger area per molecule at the air-water interface (101 A˚2) than C12AspNa2 and C12GluNa218 (71 and 88 A˚2, respectively). Thus, C12MalNa2 gives a less tightly packed monolayer at the surface than do the other two surfactants. This can be explained by the close proximity of the carboxyl groups in this surfactant rendering close alignment less favorable than for surfactants with somewhat longer spacer units, which provide possibilities for orienting the carboxyl groups away from each other. This is the same trend that we find here in the SPR experiments with values of 47 A˚2 for C12MalNa2, 43 A˚2 for C12AspNa2, and 39 A˚2 for C12GluNa2. The almost reverse trend that is found in the QCM data can tentatively be explained by C12MalNa2, which has the shortest spacer and the least tight packing, having more water of hydration in the headgroup layer than do the other two surfactants. There is more room for bound water in a loosely packed monolayer than in a monolayer composed of tightly aligned amphiphiles. The QCM data for the adsorption of the three dicarboxylic surfactants at the SAM-CH3 surface resemble the values on gold, which is not surprising. The higher adsorbed amount of C12MalNa2 found on the gold surface was not seen on the hydrophobic organic surface, however. The plateau values of adsorption are similar for the three surfactants. We have no explanation of why C12MalNa2 incorporates less water into the surfactant monolayer when aligned at a very hydrophobic SAM surface than when formed on the less hydrophobic gold. The mode of adsorption of the dicarboxylic surfactants at hydroxyapatite is more difficult to predict compared to adsorption at the gold and SAM-CH3 surfaces. The surfactants may adsorb tail down, as on a hydrophobic surface, but the surface-induced self-assembly may also result in a different adsorption pattern. If the headgroup interaction with lattice calcium is strong, then a double layer may form. Sarcosine-based surfactants are examples of a class of anionic amino-acid based surfactants that interact strongly with calcium-containing surfaces. The use of sodium N-lauroyl sarcosinate as a collector for apatite flotation is an important practical example where this is utilized.32 We have previously reported that the distance between the two carboxyl groups in the series of dicarboxylic amino acid-based surfactants (i.e., the length of the spacer unit) plays a crucial role in the interaction of the headgroup with divalent cations in solution.18 For instance, although the aminomalonate- and the aspartate-based surfactants precipitate readily in the presence of calcium ions in solution, the glutamate surfactant remains in solution over a wide range of calcium ion concentration. This was attributed to C12MalNa2 and C12AspNa2 forming intramolecular chelates with the divalent calcium ion, leading to six- and seven-membered rings. C12GluNa2 has a three-carbon spacer between carboxyl groups and would form an eight-membered ring with Ca2þ. Eight-membered rings are less favorable, and an intermolecular binding of Ca2þ is (31) Cho, N.-J.; Cho, S.-J.; Hardesty, J. O.; Glenn, J. S.; Frank, C. W. Langmuir 2007, 23, 10855–10863. (32) Ravishankar, S. A. In Encyclopedia of Surface and Colloid Science, 2nd ed.; Somasundaran, P., Ed.; Taylor & Francis: Boca Raton, FL, 2006; pp 6147-6161.

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instead preferred. This difference could be seen in the closer packing of C12GluNa2 compared to that of C12MalNa2 and C12AspNa2 at the air-water interface in the presence of calcium ions in the solution. The calcium ions trigger the close packing of C12GluNa2 but not of C12MalNa2 and C12AspNa2. Precipitation was, however, favored by the intramolecular binding of calcium ions. Against this background, it was of interest to investigate the adsorption of the three dicarboxylic surfactants at hydroxyapatite. The isoelectric point of hydroxyapatite is around 8.5.33 Thus, the surface should carry a slightly positive charge under the conditions used for the experiments. Contrary to the results on the gold and SAM-CH3 surfaces, there is a considerable difference in the adsorbed amount among the three surfactants. On hydroxyapatite, adsorption increases in the order C12GluNa2 , C12MalNa2 < C12AspNa2. One can also note that the adsorption curve for C12GluNa2 is flat, indicating a very poor driving force for adsorption. These substantial differences in the adsorption pattern are interesting in view of the fact that the surfactants differ only with respect to the length of the spacer. The weak adsorption of the glutamate surfactant on the hydroxyapatite surface can most likely be explained by the same argument as used above to explain the good tolerance to calcium ions for this surfactant compared to that of the aminomalonate and aspartate surfactants: its inability to form strong intramolecular complexes due to too long of a spacer between carboxyl groups and thus to chelate ions from the crystal lattice. Following this line of reasoning, the stronger adsorption of C12AspNa2 compared to that of C12MalNa2 can be assumed to be due to better complexing of the aspartate surfactant with lattice calcium. The effective radius of the calcium ion in crystals is 1.34 A˚, which yields a diameter of 2.68 A˚.34 The distances between the two carboxyl groups of the dicarboxylic amino acid-based surfactants were calculated using simple molecular mechanics, and the following values were obtained: C12MalNa2, 2.64 A˚; C12AspNa2, 3.97 A˚; and C12GluNa2, 4.73 A˚. It seems reasonable that the weaker adsorption of C12MalNa2 compared to that of C12AspNa2 is due to the malonate surfactant having too short a distance between the carboxyl groups to chelate the calcium ion effectively. The plateau value of adsorption obtained for the most efficient surfactant, C12AspNa2, of around 350 ng cm-2, corresponds to a fully developed and closed-packed bilayer. Similar values have been recorded for lipid bilayers on a hydrophilic surface.31

5. Conclusions In this work, the adsorption of a small series of amino acidbased dicarboxylic surfactants with varying distances between carboxyl groups has been studied. The surfactants have high cmc values, which means that the unimer concentration is high at the plateau value of adsorption. This gives rise to a large bulk effect for the two techniques used to assess the adsorption, SPR and QCM-D. For the SPR data, we have used a method developed by Whitesides to separate the bulk effect from the true adsorption. For the main technique used in this work, QCM-D, we have worked out a similar procedure, and we demonstrate that the results obtained in terms of adsorption seem reliable. The QCM-D data show that on silica there is no adsorption at all. This can be explained by the fact that there is very little driving force for the adsorption of a dianionic surfactant on a strongly negatively charged surface. On gold and on gold covered by a hydrophobic self-assembled alkane thiol layer, the surfactants adsorb in a monolayer and the amounts adsorbed are relatively similar. The adsorbed (33) Kothapalli, C. R.; Wei, M.; Shaw, M. T. Soft Matter 2008, 4, 600–605. (34) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751–767.

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amount differs with respect to hydroxyapatite, however. The aspartate surfactant, which has two carbons between the carboxyl groups, adsorbs as a fully developed bilayer, the aminomalonate surfactants with one carbon between the charged groups absorbs slightly less, and the glutamate surfactant, which has a three-carbon spacer, yields only weak adsorption. The difference is explained by differences in the ability to chelate calcium in the crystal lattice.

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Acknowledgment. Professor Tommy Nylander (Lund University) and Dr. Tobias Halthur (Lund University) are thanked for their valuable help with the ellipsometry measurements. Supporting Information Available: NMR data of C12MalNa2, C12AspNa2, and C12GluNa2. This material is available free of charge via the Internet at http://pubs.acs.org.

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