Separation of Bulk Effects and Bound Mass during Adsorption of

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Separation of Bulk Effects and Bound Mass during Adsorption of Surfactants Probed by Quartz Crystal Microbalance with Dissipation: Insight into Data Interpretation Romain Bordes*,† and Fredrik Ho¨o¨k‡ Chalmers University of Technology, Department of Chemical and Biological Engineering and Department of Applied Physics, SE-412 96 Go¨teborg, Sweden The assessment of adsorbed surfactant mass by quartz crystal microbalance with dissipation (QCM-D) monitoring is often complicated due to large bulk responses, particularly for surfactants with high critical micelle concentration (CMC). We present in this work means to interpret QCM-D data that enables the response from the bulk contribution to be separated from the response originating from adsorbed mass. Adsorption of two surfactants, Triton X100 and C12AspNa2 with low and high CMCs, respectively, at the gold-liquid interface surface has been evaluated. Two different approaches to quantify the bulk response are compared. The first approach involves the use of a nonadsorbing surface (silica), yielding a calibration curve for the concentration dependent bulk response. The second method is based on the fact that the overtone-dependent QCM-D response that originates from changes in the bulk differs from that induced by the adsorbed layer of the surfactants. Under the reasonable assumption that the bulk solution and the adsorbed surfactants can be treated as a Newtonian liquid and an acoustically rigid film, it is demonstrated that the bulk contribution can be quantified without control measurements involving inert surfaces. An excellent agreement between the two methods is reported. The adsorption of surfactants at the solid-liquid interface found applications in numerous industrial applications, including detergency, stabilization of dispersion, etc. A common trend in the analysis of surfactant adsorption is the replacement of classical techniques, such as the serum replacement method, which typically yields equilibrium or end-point data,1 by surface-sensitive methods, such as ellipsometry2 or surface plasmon resonance3-5 * To whom correspondence should be addressed. E-mail: bordes@ chalmers.se. † Department of Chemical and Biological Engineering. ‡ Department of Applied Physics. (1) Somasundaran, P.; Krishnakumar, S. Colloids Surf., A 1997, 123-124, 491– 513. (2) Tonigold, K.; Varga, I.; Nylander, T.; Campbell, R. A. Langmuir 2009, 25, 4036–4046. (3) Sigal, G. B.; Mrksich, M.; Whitesides, G. M. Langmuir 1997, 13, 2749– 2755. (4) Oskarsson, H.; Holmberg, K. J. Colloid Interface Sci. 2006, 301, 360–369.

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(SPR), allowing temporally resolved measurements of surface interactions. More recently, the quartz crystal microbalance (QCM) technique has been applied to measure the kinetics of surfactant adsorption.6 In contrast to the optical techniques, which rely on changes in the interfacial refractive index, QCM relies on the fact that a change of the mass of the oscillating crystal, through, for example, adsorption events, induces a proportional change in the resonance frequency. Developed in the 1950s, this gravimetric method was originally employed to measure the adsorption of very small quantities of gas molecules.7 In the 1980s, based on the work of Stockbridge,8 Kanazawa et al. established means to operate QCM in the liquid environment, and successful detection of adsorbed mass was verified.9 With the introduction of combined frequency and dissipation measurements (∆f and ∆D), it was also shown that additional information on the structural properties of the adsorbed layer could be obtained.10 This extension of the technique, combined with the development of theoretical models to evaluate the viscoelastic properties of thin films, has contributed to the popularity of the technique to monitor the adsorption of, for example, soft biomacromolecules.11,12 More recently, the adsorption behavior of surfactants on different types of surfaces was investigated6,13-16 and compared with complementary techniques, such as ellipsometry. From these studies, it has been concluded (5) Sarkar, D.; Somasundaran, P. J. Colloid Interface Sci. 2003, 261, 197–205. (6) Tehrani-Bagha, A. R.; Holmberg, K. Langmuir 2008, 24, 6140–6145. (7) Urbakh, M.; Tsionsky, V.; Gileadi, E.; Daikhin, L. Probing the Solid/Liquid Interface with the Quartz Crystal Microbalance. In Piezoelectric Sensors; Springer Series on Chemical Sensors and Biosensors; Springer-Verlag: Berlin Heidelberg, Germany, 2007; Vol. 5, Part A, pp 111-149. (8) Stockbridge, C. D., Resonance Frequency versus Mass Added to Quartz Crystals. In Vacuum Microbalance Techniques; Behrndt, K. H., Ed.; Plenum: New York, 1966; Vol. 5, pp 193-205. (9) Keiji Kanazawa, K.; Gordon, J. G. Anal. Chim. Acta 1985, 175, 99–105. (10) Voinova, M. V.; Rodahl, M.; Jonson, M.; Kasemo, B. Phys. Scr. 1999, 59, 391. (11) Caruso, F.; Rodda, E.; Furlong, D. N.; Niikura, K.; Okahata, Y. Anal. Chem. 1997, 69, 2043–2049. (12) Larsson, C.; Rodahl, M.; Hook, F. Anal. Chem. 2003, 75, 5080–5087. (13) Karlsson, P. M.; Palmqvist, A. E. C.; Holmberg, K. Langmuir 2008, 24, 13414–13419. (14) Knag, M.; Sjo ¨blom, J.; Øye, G.; Gulbrandsen, E. Colloids Surf., A 2004, 250, 269–278. (15) Stålgren, J. J. R.; Eriksson, J.; Boschkova, K. J. Colloid Interface Sci. 2002, 253, 190–195. (16) Caruso, F.; Serizawa, T.; Furlong, D. N.; Okahata, Y. Langmuir 2002, 11, 1546–1552. 10.1021/ac1018149  2010 American Chemical Society Published on Web 10/13/2010

that the properties of the bulk solution, and particularly its viscosity, often have a significant influence on the measured ∆f and ∆D responses. Consequently, quantification of the adsorbed mass can be considered reliable only as long as the bulk properties remain small or unchanged. Although this must be considered a severe drawback, QCM presents several advantages compared with other techniques, particularly with respect to the type of surfaces that can be used. Since it is a gravimetric method, the type of surfaces is not constrained by the optical properties, which means that ellipsometry is mainly restricted to the study of the adsorption on silica or on modified silica. In the case of SPR, the method is restricted to adsorption studies on metallic surfaces (generally gold) or on self-assembled monolayers (SAM) formed on gold. QCM allows the use of a wide variety of materials from metals to minerals but also chemically modified surfaces, including SAM and polymers. Furthermore, in contrast to the optical methods, the mass determined from QCM data includes in addition to molecular mass both hydration mass and hydrodynamically coupled solvent. Therefore, the QCM-D technique is often combined with others techniques, enabling the amount of coupled solvent to be quantified and related to structural variations probed via ∆D.16-18 For surfactant adsorption, especially on surfaces with a hydrophobic character, the tendency to adsorb depends strongly on the critical micelle concentration (CMC). As a rule of thumb, there is typically a good correlation between the onset of adsorption and micelle formation.19 Hence, for surfactants with high CMCs, variations in the bulk properties of the solution are more likely to influence the measured response, irrespective of which surface analytical tool is used. While ellipsometry and SPR suffer from parasitic bulk effects due to changes in the bulk refractive index, QCM-D is sensitive to changes in the bulk viscosity.14 In fact, for surfactants with a high CMC, the measured response is generally a superposition of two phenomena, the bulk contribution and the contribution from the actual adsorption, which can very well be just a small fraction of the measured response. In recent years, the use of surfactants with shorter alkyl chains that allow improved biodegradability is being requested due to environmental concerns. For instance, the classical coconut cut based surfactants, i.e., with a C12-C14 alkyl chain, can in some cases be replaced by C10-C12 alkyl chain based surfactants, often prepared through the Ziegler process.20 Hence, these surfactants will have higher CMC, leading to more pronounced influence on the bulk properties at concentrations corresponding to the onset of adsorption. This thus stresses the need of efficient means to separate the actual adsorption process from bulk effects using surface analytical tools. To address this challenge, we investigate in this work the influence from changes in bulk viscosity on the QCM-D response and propose a simple means to separate the bulk response from (17) Hook, F.; Kasemo, B.; Nylander, T.; Fant, C.; Sott, K.; Elwing, H. Anal. Chem. 2001, 73, 5796–5804. (18) Reimhult, E.; Larsson, C.; Kasemo, B.; Hook, F. Anal. Chem. 2004, 76, 7211–7220. (19) Holmberg, K.; Jo¨nsson, B.; Kronberg, B.; Lindman, B., Surfactants and Polymers in Aqueous Solution. 2nd ed.; Wiley: Chichester, U.K., 2003; pp 39-66. (20) Hepworth, P., Non-Ionics Surfactants. In Chemistry and Technology of Surfactants; Farn, R. J., Ed. Wiley-Blackwell: Oxford, U.K., 2006; p 137.

Figure 1. Surfactants studied: Triton X100 (where n ) 9-10) and disodium lauroyl aspartate (C12AspNa2). Table 1. Critical Micelle Concentration Values Determined by Tensiometry surfactant

Triton X100

C12AspNa2

CMC (mM)

0.3

73.0

the response of adsorbed surfactants. To evaluate the approach, the surfactant systems were chosen according to their CMC. Nonionic surfactants usually have a CMC much lower than ionic surfactants,21 and one can therefore anticipate that with such systems the bulk contribution is expected to be low. On the other hand, diionic surfactants have significantly higher CMCs, and thus a substantial contribution to the measured surface response is expected. For that reason, the disodium N-lauroyl aspartate, C12AspNa2, which has two negative charges, was investigated and compared with the Triton X100, a nonionic surfactant. Their structures and CMCs are presented in Figure 1 and Table 1, respectively. Note that the CMC values differ by 2 orders of magnitude. The study was carried out by measuring ∆f and ∆D at multiple harmonics (3rd, 5th and 7th) upon addition of surfactant solutions at subsequently increased concentrations on gold (being a stable model surface for surfactant adsorption). The reference measurements were made using a SiO2-coated QCM-D sensor crystal. MATERIALS AND METHODS Materials. Triton X100 (Aldrich, lab. Grade) was used as purchased. Milli-Q water (resistivity > 18 MΩ) was used for the preparation of aqueous solution. The synthesis of the disodium lauroyl aspartate (C12AspNa2) via the Schotten-Baumann reaction is described in detail elsewhere.22 Determination of the Critical Micelle Concentration. The critical micelle concentrations of the surfactant studied were determined by tensiometric measurements. Surface tension measurements were carried out on a Sigma 70 tensiometer (KSV) using the du Nou¨y ring method. The temperature was kept at 20 °C (±0.01 °C) by a cryostat Neslab RTE-200. The glassware was cleaned with chromosulfuric acid, and the ring was burned prior to use. The CMC values were taken at the point on the surface tension vs concentration curve at which the surface tension reaches a constant value. Contact Angle Measurements. The contact angle values were determined with a DAT1100 device from Fibro AB (Stockholm, Sweden). The drops of 5 µL were applied on the gold crystal after cleaning, and the stable values were obtained after a few seconds. The measurements were repeated five times on each crystal. (21) Holmberg, K.; Jo¨nsson, B.; Kronberg, B.; Lindman, B., Surfactants and Polymers in Aqueous Solution. 2nd ed.; Wiley: Chichester, U.K., 2003; p 44. (22) Bordes, R.; Tropsch, J.; Holmberg, K. J. Colloid Interface Sci. 2009, 338, 529–536.

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Figure 2. QCM-D measurements of adsorption at 20 °C of Triton X100 (a, left) and C12AspNa2 (b, right) at a gold surface using successively higher surfactant concentrations followed by rinsing with water. The change in frequency (∆f) is monitored at the 3rd overtone of the fundamental frequency of oscillation. The replacement of one solution of surfactant by another solution of higher concentration was chosen instead of rinsing between each surfactant solution, to avoid the possible buildup of surfactant layers. The data were not normalized to the overtone number.

Prior to the measurements and after a thorough oxidative cleaning of the surface with UV-ozone and wet oxidative treatment (H2O2/NH3), a contact angle of 88° was determined for the gold surfaces. It is generally admitted that the nature of gold depends on its history, and despite a rather hydrophilic surface,23 explained by the presence of freshly formed gold oxide,24 relatively high values of the contact angle can be found.25 In the present case, our focus was on the reliability of the measurements to ensure the stability of the surface. Quartz Crystal Microbalance with Monitoring of the Dissipation (QCM-D). A QCM-D instrument (model D300) from Q-sense AB (Gothenburg, Sweden) was used. The measurements were done under nonflowing conditions, to avoid perturbations during the shear oscillation of the crystals. The AT-cut crystals coated with a 100 nm thin gold layer were also from Q-sense AB (Go¨teborg, Sweden). Prior to use, the cleaning procedure was done as follow: 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 crystals coated with silica from Q-sense AB (Go ¨teborg, Sweden) were prepared prior to use by the following cleaning: UV-ozone treatment for 10 min, rinsing with ethanol, rinsing with Milli-Q water, drying with N2, and finally 10 min of UV-ozone treatment. The measurements were done at 20 °C with a baseline corresponding to the load of the crystal by Milli-Q water. The solutions of surfactant were injected in the crystal chamber using a homemade addition device, going from the lowest to the highest concentration without rinsing the surface between the additions. The crystal was finally rinsed with Milli-Q water to remove poorly adsorbed surfactants. The automatic addition device uses electric valves that control several flow channels connected to a collector at the inlet of the QCM-D cell. The valves were controlled via an electronic interface using remote control by the COM port of a computer. The (23) Smith, T. J. Colloid Interface Sci. 1980, 75, 51–55. (24) King, D. E. J. Vac. Sci. Technol. A 1995, 13, 1247-1253. (25) Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116–1121.

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software was developed in Python (vers. 2.4). Prior to use, all the flow channels were rinsed with ethanol and water. RESULTS AND DISCUSSION Figure 2a shows the temporal variations in frequency, ∆f, and energy dissipation, ∆D, observed upon addition of Triton X100 (CMC ) 0.3 mM) at increasing concentrations (from 0.03 to 0.45 mM) in pure water. Figure 2b shows the same type of data for the adsorption of C12AspNa2 (from 7.3 to 110 mM, CMC ) 73 mM). The peaks in the curves are due to slight variations of the temperature and the pressure in the measurement cell upon sample injection and were disregarded from the analysis. The last stage in each experiment are rinsing steps, leading to partial desorption. Concerning Triton X100, Figure 2a shows a situation where the measurements are essentially not influenced by the bulk contribution. This is supported by the fact that when the CMC is reached, after the fourth addition, the change in frequency saturates, while ∆D remains low. The small change in ∆D also suggests adsorption of a nondissipative film. Hence, the adsorbed mass of Triton X100 can be directly obtained using the linear relation between ∆f and ∆m (see below). In contrast, the situation is significantly more complicated in the case of C12AspNa2. Upon injections of increasing C12AspNa2 concentrations, the increase ∆D is significantly larger than for Triton X100 (Figure 2b), suggesting that in this case, there is either adsorption of a dissipative (nonrigid) film or an appreciable influence from changes in the bulk viscosity. This is also supported by a larger change in ∆f (-110 Hz) for C12AspNa2 compared with Triton X100 (-60 Hz). Furthermore, the absence of a plateau, once the CMC is reached, is unexpected and most likely the result of changes in the bulk solution. ∆f recorded at the 3rd, 5th, and 7th overtones versus concentration upon adsorption of Triton X100 and C12AspNa2 on gold is shown in Figure 3. Each point in these graphs corresponds to the mean of the stabilized response after the injection of the surfactant solution. Triton X100 initially displays a linear decrease in ∆f and reaches a plateau value at its CMC, in

Figure 3. (a) Variation of the frequency vs concentration normalized to CMC for Triton X100 (in blue) and C12AspNa2 (in red) at 20 °C at the 3rd (O), 5th (∆), and 7th (0) overtone. (b) Variation of the dissipation vs concentration normalized to CMC for Triton X100 (in blue) and C12AspNa2 (in red) at 20 °C at the 3rd (O), 5th (∆), and 7th (0) overtone.

where n () 1, 3, 5, . . .) is the overtone number, and C ) 17.7 ng Hz-1 cm-2 is a constant valid for the 5 MHz AT-cut crystals used in this work and converts to a mass of around 350 ng cm-2. The values observed for Triton X100 are in fact consistent with the deposition of a bilayer from vesicles of phospholipids.28,29 In contrast, normalization of the response for

C12AspNa2 (Figure 3a) does not converge to a single value (Figure 4). In addition, no plateau value is in this case reached at the CMC. This behavior signals, in turn, that the amount of C12AspNa2 being adsorbed cannot be directly determined. Similar effects have been previously observed and attributed to the bulk contribution,14 which in the case of QCM-D measurements is reflected by significant changes in ∆D. For Triton X100, the maximum ∆D value observed at 1.5 × CMC is lower than 0.5 × 10-6 (Figure 3b) and does not vary with the overtone number. In the case of C12AspNa2, the increase in ∆D is more than 1 order of magnitude high and also displays a strong overtone dependence. As already suggested, this reflects either a significant bulk effect14 or the formation of a viscoelastic film17 or possibly a combination of these two effects. Before evaluating different means to quantify the contribution to the response from the bulk properties, we stress that this set of experiments allow us to determine some key criteria that should be fulfilled in order for the adsorption of surfactants to be easily interpreted. First, a saturation of the interface at the CMC should be observed. Second, the response at different overtones normalized to the overtone number should converge to a common value. Third, the dissipation should be relatively low (