Quartz Crystal Microbalance and Surface Plasmon Resonance Study

Orlando J. Rojas, Marie Ernstsson, Ronald D. Neuman, and Per M. Claesson ... Asoka Weerawardena, Calum J. Drummond, Frank Caruso, and Malcolm ...
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Langmuir 1995,11, 1546-1552

1546

Quartz Crystal Microbalance and Surface Plasmon Resonance Study of Surfactant Adsorption onto Gold and Chromium Oxide Surfaces Frank Caruso,? Takeshi Serizawa,$D, Neil Furlong,*lt and Yoshio Okahata$ CSIRO, Division of Chemicals and Polymers, Private Bag 10, Rosebank MDC, Clayton, Victoria 3169, Australia, and Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuda, Midori-ku, Yokohama 227, Japan Received November 21, 1994. In Final Form: February 7, 1995@ The adsorptionof two nonionic surfactants on hydrophobic gold and hydrophilic chromium oxide surfaces from aqueous solution has been investigated using a quartz crystal microbalance (QCM) and surface plasmon resonance (SPR). Adsorption isotherms for a nonyl phenol ethoxylate with average ethylene oxide chain length of 9 (N9) on gold and for octaethylene glycol monododecyl ether (C1zE8) on both gold and chromium oxide surfaces have been measured by QCM. Isotherms for N9 and C12E8 on the gold on the chromium oxide surface was surface can be described by Langmuir adsorption, while that of S-type. ComplementarySPR experiments of C12E8 adsorption onto gold suggest that the resonance frequency change of the QCM crystal in solution with adsorbed ClzEs (at saturation coverage) is 80% greater than that predicted by the Sauerbrey equation for air measurements. This implies a 2.08 Hzlng change for the QCM in contact with aqueous solution. This calibration factor was used to calculate the saturated surface coverage for the surfactants (and hence area per surfactant) at the solifliquid interface. These data are in good agreement with those reported in the literature for N9 and CnE8 adsorbed onto various hydrophobic and hydrophilic surfaces.

Introduction The adsorption of surfactants onto solid surfaces plays a major role in a variety of current technologies such as mineral flotation, lubrication, and detergency.l Surfactant adsorption is also of widespread interest because of potential applications in the fields of microelectronics, sensors, conductors, and thin insulators.2 Consequently, many studies have been undertaken in order to understand the uptake of various surfactants on solid s u r f a c e ~ . ~ - ~ Such studies have often employed techniques such as ellipsometry, calorimetry, and absorbance measurements for surfactants containing chromophores. The high sensitivity and simple relationship between mass and frequency of the quartz crystal microbalance (QCM)make it a n ideal in-situ tool for studying adsorption processes and as a chemical s e n ~ o r . ~The - ~ QCM technique is known to commonly provide mass-measuring sensitivity to the nanogram level, with the piezoelectric quartz crystal changing its fundamental oscillation frequency, F,, as mass is deposited onto (or depleted from) the crystal surface in accordance with the Sauerbrey expressionlo

* To whom correspondence should be addressed.

' CSIRO.

* Tokyo Institute of Technology.

Abstract published in Advance A C S Abstracts, April 15, 1995. (1)Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley@

Interscience: New York, 1978. (2) Pomerantz, M. Surfactants in Emerging Technologies; Marcel Dekker, Inc.: New York, 1987. (3) Clunie, J. S.; Ingram, B. T. In Adsorption from Solution at the SolidiLiquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: New York, 1983; pp 105-152. (4) von Rybinsky, W.; Schwuger, M. J. In Surfactant Science Series; Schick, M. J., Ed.; Marcel Dekker Inc.: New York, 1987; Vol. 23, pp 45-107. (5) Cases, J. M.; Villieras, F. Langmuir 1992,8, 1251. (6) Hlavay, J.; Guilbault, G. G. Anal. Chem. 1977,49,1890. (7) Schumacher, R. Angew. Chem. Int. Ed. Engl. 1990,29,329, and references cited therein. (8) Buttry, D. A. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker Inc.: New York, 1991; Vol. 17, p 1. (9) Krim. J.; Chiarello, R. J . Vac. Sci. Technol. 1991,A9, 2566.

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where A F is the change in resonant frequency, resulting from a change in mass Am; p, is the shear modulus of the quartz (2.947 x 1O1O kg m-l s-~); es is the density of the quartz (2648 kgm-3); andA is the electrode area (1.59 x m2). For our QCM operating in air, a frequency change of 1Hz corresponds to a mass increase of 0.87 ng (F, = 9 x lo6 Hz). Equation 1 holds for gas-phase measurements and is applicable only if one assumes that the adsorbed material is rigidly attached to the crystal surface and of negligible thickness in comparison to the crystal itself.1° Thiol films adsorbed on the crystal surface have been shown to satisfy these conditions and can be regarded as rigid films." Moreover, it has been demonstrated that LangmuirBlodgett multilayers as well as polymer films with a thickness up to 300 nm can also be considered as being rigidly attached to the crystal surface.12J3 In the past, QCMs have been used extensively as selective sensors in gas-phase s t ~ d i e s . l ~Detectors -~~ for CO, SOZ,NHs, hydrocarbons, and organophosphorous compounds have been deve10ped.l~Measurements have also been performed on biological components adsorbed from solution.21,22In these experiments, while the interfacial reactions are performed in solution, the initial (10) Sauerbrey, G. 2.Phys. 1969,155,206. (11)Yang, M.; Thompson, M.; Duncan-Hewitt, W. C. Langmuir 1993, 9 xn? -, (12) McCaffrey, R. R.; Bruckenstein, S.; Prassad, P. N. Langmuir 1986,2,228. (13) Orata, D.; Buttry, D. J. Am. Chem. SOC.1987,109,3584. (14) King, W., Jr. Res. lDeu. l969,20,28. (15) Karasek, F. W.; Gibbins, K. R. J. Chromatogr. Sci. 1971,9,535. (16) Kasemo, B.; Tomqvist, E. Surf Sci. 1978, 77, 209. (17) Mecea. V.: Bucur. R. V. J. Vac. Sci. Technol. 1980.17. 182. (18)Edmonds,'T. E.; West, T. S. Anal. Chim. Acta 1980,117, 147. (19) Alder, J. F.; McCallum, J. J. Analyst 1983, 108, 1169. (20) Guilbault, G. G. In Applications of PiezoeIectric Quartz Crystal Microbalances; Lu, C., Czanderna, A. W., Eds.; Elsevier: Amsterdam/ New York, 1984; p 251. (21) Guilbault, G. G. Anal. Chem. 1983,55, 1682.

D 1995 American Chemical Society

Quartz Crystal Microbalance and Surface Plasmon Resonance

Langmuir, Vol. 11, No. 5, 1995 1547

and final frequency measurements are made on the dry The chromophoric surfactant N9 was chosen for invescrystal before and after reaction. tigation in this study since there are reliable data in the literature (from U V absorbance measurements) on its Recently, the QCM technique has been extended to adsorption properties on different surface^.^^,^^ It can liquid-phase measurements and the study of phenomena therefore be used as a reference for comparison of the a t solifliquid interfaces, with particular attention being paid to the operation of the device in l i q ~ i d s . ~ Several , ~ ~ - ~ ~ values obtained in this work. ClZE8 was selected since there are large variations between the data obtained from applications have been reported including surfactant and calorimetric and ellipsometric The practical ion a d s o r p t i ~ n ,detection ~ ~ , ~ ~ of components eluted during interest in this surfactant also warrants better quantiliquid c h r o m a t ~ g r a p h y determination ,~~ of inorganic spefication of its adsorption to solid surfaces. c i e ~ , and ~ ~ immunochemical , ~ ~ In many cases in the liquid phase, the QCM does not behave as Experimental Section predicted by the Sauerbrey equation (eq 1).Anumber of factors such as interfacial liquid properties (i.e., density, Materials. Commercial grade nonyl phenol ethoxylate with viscosity, conductivity, and dielectric c o n ~ t a n t ) , ~ thin ~-~' an average ethylene oxide chain length of 9 (N9) was supplied film v i s c ~ e l a s t i c i t y , ~electrode ~ , ~ ~ m o r p h ~ l o g y , ~and ~ , ~ ~ , ~by~IC1 Australia. This surfactant contained less than 1.5 wt % mechanisms of acoustic ~ o u p l i n gimpact ~ ~ ~ ~on~ QCM polyethylene glycol impurity and was used as received. Monodisperse octaethylene glycol monododecyl ether (C12E8) was oscillation behavior. purchased from Nikko Chemicals, Japan, and was used without Despite numerous studies and recent work by Barnes further purification. The critical micelle concentrations (cmc)of et al.45on the theory of operation of the QCM in contact N9 and C&8 determined from surface tension data are 0.58 x with liquid and by Urbakh and on the theoretical and 0.92 x mol dm-3, r e s p e ~ t i v e l y . ~ ~ , ~ ~ description of the influence of surface roughness on the Nitric acid and propan-2-01 were AR grade and supplied by QCM response in liquids, QCM operation in liquids Rhone-Poulenc. Hydrogen peroxide (ARgrade) was purchased remains poorly u n d e r ~ t o o d . ~ Consequently, ~ most exfrom BDH. Spectroscopic grade chloroform was obtained from perimental results for QCM frequency shifts in liquids Merck. The water used in all experiments was obtained from deviate from theoretical prediction^,^^,^^ and studies of a three-stage Milli-Q purification system with a conductivity less than 1p S cm-'. All experiments were performed at 21 f adsorption of molecules a t QClWliquid interfaces give 1 "C. qualitative results.40 Quartz Crystal Microgravimetry. Quartz crystals with a In the present study, we have used the QCM to monitor fundamental resonance frequency, F,,, of 9 MHz were purchased the adsorption of the nonionic surfactants N9 and C12E8 from Kyushu Dentsu Co. (Omura-City,Nagasaki, Japan). The on gold and chromium oxide QCM surfaces from aqueous QCM electrodes had been prepared by thermal evaporation of solution. The technique of surface plasmon resonance gold to a thickness of 1000 A. The crystals were of the AT-cut (SPR) has been employed as a complementary surface type, coated on both sides with gold electrodes of 5 mm diameter. analytical tool to QCM. SPR has been demonstrated to The QCM is driven at 5 V DC, and the frequency is monitored with an Iwatsu frequency counter (Model SC7201) connected to be a powerful method for probing the properties and an EPSON microcomputer (Model PC-286). The quartz crystal interactions of various molecules at solid surfaces in real holder makes electrical contacts at the periphery of the crystal time.46 We demonstrate the potential for a combined on both sides. One side of the quartz crystal is sealed with a QCWSPR study for the examination of surfactant adrubber casing, maintaining it in an air environment, while the sorption, which eliminates the need for the theory other is exposed to aqueous solution. The advantage ofthis casing pertaining to the operation of the QCM in liquids in order has been demonstrated in a previous publication,22 which shows to obtain quantitative information on the adsorption of that the casing is essentialfor frequency stability when immersed surfactants a t QClWliquid interfaces. in liquids. The system is enclosed in an air thermostat, which is controlled to within 0.1 "C during measurements to reduce frequency fluctuations. The stability of the system during (22) Geddes, N. J.; Paschinger, E. M.; Furlong, D. N.; Ebara, Y.; measurement in air and water (after an initial stabilization Okahata, Y.; Than, K. A.; Edgar, J. A. Sens. Actuators B 1994,17, 125. period) is 1and 2 Hz, respectively. It should be noted that prior (23) Stockbridge, C. D. In VacuumMicrobalance Techniques;Behmdt, K. H., Ed.; Plenum Press: New York, 1966; Vol. 5, p 147. to the QCM being immersed in solution, the electrodes of the (24) Konash, P. L.; Bastiaans, G. L. Anal. Chem. 1980, 52, 1929. QCM were covered with a protective silastic film. This is to (25) Nomura, T.; Okuhura, M. Anal. Chim. Acta 1982, 142, 281. prevent degradation of the electrical contacts when submersed (26) Bruckenstein, S.; Shay, M. Electrochim. Acta 1985, 30, 1295. in solution. (27) Kanazawa, K. K.; Gordon, J . Anal. Chim. Acta 1985,175, 99. Substrate Preparation. The gold QCM surfaces were (28) Thompson, M.; Arthur, C. L.; Dhaliwal, G . K. Anal. Chem. 1986, 58, 1206. cleaned by exposure to piranha solution (one part HzOz in three (29) Muramatsu, H.; Tamiya, E.; Karube, I . Anal. Chem. 1988,60, parts HzS04)54for 2 min, followed by rinsing with pure water 2142. and drying with nitrogen. Caution: Piranha solution should be

(30) Thompson, M.; Kipling, A. L.; Duncan-Hewitt, W. C.; Rajakovi, L. V.; Cavic-Vlasak, B. A. Analyst (London) 1991, 116, 881. (31)Buttry, D. A,; Ward, M. D. Chem. Rev. 1992,92, 1355. (32) Yang, M.; Thompson, M. Anal. Chem. 1993,65, 1158. (33) Okahata, Y.; Ebato, H. Anal. Chem. 1991, 63, 203. (34)Abbott, A. P.; Loveday, D. C.; Hillman, R. A. J. Chem. Sac. Faraday Trans. 1994,90, 1533. (35) Nomura, T.; Watanabe, M.; West, T. S.Anal. Chim. Acta 1985, 175, 107. (36)Yao, S. Z.; Mo, Z. H. Anal. Chim. Acta 1987, 193, 97. (37) Roederer, J. E.; Bastiaans, G. J. Anal. Chem. 1983, 55, 2333. (38) Muramatsu, H.; Dicks, J. M.; Tamiya, E.; Karube, 1.Anal. Chem. 1987,59, 2760. (39) Thompson, M.; Dhaliwal, G. K.; Arthur, C. L.; Calabrese, G. S. IEEE Trans. Ultrason. Ferroelec. Freq. Control 1987, UFFC-34, 127. (40) Grabbe, E. S.; Buck, R. P.; Melroy, 0. R. J . Electround. Chem. 1987, 223, 67. (41) Ebersole, R. C.; Ward, M. D. J . Am. Chem. SOC.1988,110, 8623. (42) Hinsberg, W.; Wilson, C.; Kanazawa, K. K. J. Electrochem. Sac. 1986,133, 1448. (43) Yang, M.; Thompson, M. Langmuir 1993,9,1990. Urbakh, M.; Daikhin, L. Langmuir 1994, 10, 2836. (44) Duncan-Hewitt,W. C.; Thompson, M.Anal. Chem. 1992,64,94.

(45) Barnes, C.; DSilva, C.; Jones, J. P.; Lewis, T. J . Sens. Actuators A 1992,31, 159. (46) Geddes, N. J.; Martin, A. S.;Caruso, F.;Urquhart, R. S.;Furlong, D. N.: Sambles, J. R.: Than. K. A.:. Edear. J. A. J. Immunol. Methods 1994,' 175, 149.' (47) Furlong, D. N.; Aston, J. R. Colloids Surf: 1982,4, 121. (48) Aston, J . R.; Furlong, D. N.; Grieser, F.; Scales, P. J.;Warr, G. G. In Adsorption at the Gas-Solid and Liquid-Solid Interface; Rouquerol, J.; Sing, K. S. W., Eds.; Elsevier: Amsterdam, 1982; pp 97-102. (49) Tiberg, F.; Jonsson, B.; Tang, J.;Lindman, B. Langmuir 1994, 10, 2294. (50) Gellan, A,; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1985,81, 1503. (51) Gellan, A.; Rochester, C. H. J . Chem. SOC.Faraday Trans. 1 1985,81, 3109. (52) Gellan, A.; Rochester, C. H. J. Chem. Sac. Faraday Trans. 1 1985,81, 2235. (53) Partyka, S.; Zaini, S.; Lindheimer, M.; Brun, B. Colloids Surf: 1984, 12, 255. (54) Bain, C. D.; Evall, J.;Whitesides, G. M. J. Am. Chem. Sac. 1989, 211, 7155. I

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1548 Langmuir, Vol. 11, No. 5, 1995

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Figure 1. Schematic diagram of the QCM apparatus used to monitor frequency changes due to surfactant adsorption. handled with extreme care, a n d only small volumes should be prepared at any time. This process was repeated further two

times. The QCM crystals were then immersed immediately in aqueous solution ready for the detection of surfactant adsorption. Chromium surfaces were prepared by sputtering chromium onto the gold QCM electrodes (or Pyrex microscope slides (20 x 20 mm) for X P S measurements) to a thickness of approximately 100 nm at a rate of 1 n d s using high vacuum sputtering (0.5 Pa atm of argon). The thickness was monitored with a quartz crystal oscillator (Maxtek Inc., CA, Model TM-100). The chromium surfaces were then exposed to air (formingchromium oxide) for a minimum of 50 h before being cleaned with piranha solution (as described above) and immediately immersed in aqueous solution ready for the adsorption experiments. Equilateral glass prisms (Groiss and Co., Australia) of refractive index 1.65 were used as the substrate for SPR measurements. The prisms were rigorously cleaned, first by immersion in nitric acid and then propan-2-01, followed by cleaningwith chloroform. Gold (99.99%pure) was then deposited directly onto one face of the prism using high vacuum sputtering (0.5 Pa atm of argon) at a rate of 1nm/s to yield a final thickness of approximately 35 ( f 5 ) nm (as measured by a quartz crystal microbalance). This thickness was chosen to optimize SPR coupling in the golaair system. The coated prisms were used immediately after preparation. QCM Measurements. The cleanedQCM (asdescribedabove) was immersed in 10 mL of stirred Milli-Q water. After stabilization of the fundamental resonancefrequencyof the QCM, an aliquot (between 3 and 800pL) ofthe stock surfactant solution (concentration lop2M)was injected (see Figure 1). The frequency change due to adsorption of the surfactant on the surface of the QCM was monitored as a functionof time. “In-solution”frequency changes were used to determine the degree of surfactant adsorption. “In-air”measurements (i.e., after surfactant adsorption, rinsing with water, and nitrogen drying) were found to be unreliable because of surfactant desorption during rinsing. Surface PlasmonResonance (SPR)Measurements. SPR data is commonly recorded in the form of reflectivity versus angle of incidence. Measurements of the SPR angle scan data were made using the Kretschmann arrangement.46 Prisms were placed onto a rotating table geared so that the detector moved at twice the angular speed of the prism. P-polarized light (TM) from a He-Ne laser, 1 = 632.8 nm, illuminated the prism. This incident beam was mechanically chopped at 1.7 kHz. A beam splitter reflected a small percentage of the incident beam onto a reference detector so that fluctuations in laser intensity could be monitored. Sample and reference signals were fed into phasesensitive detectors (PSD)which used the chopper signal as their phase input. The DC output from the sample and reference PSDs were stored on computer after analogue to digital conversion. The SPR experimental system has been previously described in detail.46 An unnormalized reflectivity was obtained by dividing the sample signal by the reference signal for each angular data step. External prism angles were converted to internal angles, and corrections in the reflectivity were made for reflection at the entrance and exit faces of the prism. The corrected reflectivity data was then fitted to Fresnel theory by assuming the ‘idealised layer model’. In this model the layers are isotropic and the substrate is perfectly flat. These fits provided estimates of the

real (er)and imaginary ( ~ i components ) ofthe relative permittivity of the film and the film thickness ( d ) . The prism is sealed in the side of a small polytetrafluoroethylene vessel (of total volume 3.5 mL) using a rubber 1 cm diameter O-ring. A plasmon resonance curve for the golawater system was first measured. Variations in the gold film due to water-induced changes were monitored using a fixed-angle measurement (i.e., as described below, the detector was set to an external angle of incidence that was 2”off the SPR resonance minimum). Any drift in the reflectivity at this angle with time indicated that the optical parameters of the gold film were changing due to deteriorating gold adhesion. Films showingdrift were discarded. When no drift occurred, the full SPR curve was recorded and then fitted to Fresnel theory to extract the thickness and optical parameters of the gold film in contact with water. These parameters were required for the subsequent determination of the adsorbed surfactant thicknesses. The kinetics of surfactant adsorption were followed by fixing the detector at an external angle of incidence 2” off the SPR resonance minimum on the critical angle side of the minimum and monitoring the reflectivity as a function of time.46 At this point, the rate of change of reflectivity (AR) with angle is approximately linear. The fixed-angle reflectivity data were divided by the beam intensity in the absence of surfactant to give the absolute reflectance. An aliquot (between1.5and 60pL)ofthe aqueous stock solution of surfactant (concentration lop2M) was then injected into the vessel with stirring. Adsorption of the surfactants was monitored via changes in the reflectivity with time. The plasmon angle increased during surfactant adsorption. Afull plasmon resonance curve was recorded when the reflectivity was constant or after 30 min. From this full plasmon resonance curve and using the calculated gold parameters in water, the effective thickness of the surfactant layer could be determined from fitting to Fresnel theory. X-ray Photoelectron Spectroscopy ( X P S ) Measurements. XPS measurements were conducted using a Vacuum Generators Escalab V spectrometer with a non-monochromatic 200 W (10kV, 20 mA) Mg Ka source and a hemispherical analyzer operating in fixed analyzer transmission mode. The total pressure in the main vacuum chamber during analysis was Pa. Spectra were collected normal and at typically 2 x grazing angle (75”)to the sample surface. The chemical elements present were identified from survey spectra. High resolution spectra were recorded from individual peaks for every element detected. The elemental composition of the surface was determined by a first principles approach.55 The random error associated with quantitativeelemental analysis was determined to be 5 1 0 % (usually 7-8%). Details of the XPS system and analysis procedure are described elsewhere.56 XPS examination of the piranha-cleaned chromium-coated microscope slides confirmed the presence of chromium oxide at the surface. At normal emission, the fraction of chromium in oxide form (Cr203or CrO2)was found to be 75%. (TheXPS spectra of Cr203 and CrO2 are indistinguishable.) At grazing angle emission, the fraction of chromium in oxideform was determined to be slightly higher at 85%. This result is expected as the XPS probe depth at grazing angle emissionto the surface for chromium metal is between 1and 2 nm, whereas at normal angle emission the probe depth is of the order of 5 nm.57 The percentage of organic contaminants (of all the elements detected) at normal emission and grazing angle emission were found to be 27% and 37%,respectively. This suggests that there is no discrete layer of organic contaminants at the surface. The presence of an organic layer at the surface would result in a significant increase in the percentage of organic material observedgoing from normal to grazing angle emission. Rather, the organic material is embedded in the chromium and chromium oxide as well as being present at the surface. For the purposes of this study, the chromium surfaces are treated essentially as chromium oxide surfaces. (55) Grant, J. T. Surf Interface Anal. 1989, 14, 271.

(56) Gengenbach, T. R.; Vasic, Z. R.; Chatelier, R. C.; Griesser, H. J. J. Polym. Sci., Polym. Chem. Ed. 1994, 32, 1399. (57) Tanuma, S.; Powell, C. J.;Penn, D. R. Surf Interface Anal. 1988, 11, 577.

Quartz Crystal Microbalance and Surface Plasmon Resonance 20-1

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Time (mins) Figure 2. Frequency changes of the gold QCM responding t o additions of an aqueous solution of N9 to water: (a)0.15 x M N9 and (b) 3.0 x M N9. The arrow indicates the time at which the N9 solution was injected into the water. Water Contact Angle Measurements. Sessile drop contact angle measurements were determined as described by Lamb and Furlong.58Before any cleaning procedure, contact angles on the QCM electrodes (gold surface) were close to In the case of piranha-treated QCM electrodes,sessile contact angles of 85" have been measured.59 These contact angles indicate the presence of hydrocarbon contamination on the surface. Sessile contact angles of 18.0 f 1.0"were measured for the piranhacleaned chromium oxide covered glass slides, clearly suggesting that the chromium oxide surface is relatively hydrophilic. Atomic Force Microscopy (AFM). AFM imaging of samples was performed in air or water using a Digital Instruments Nanoscope I11 AFM in constant deflection mode. Silicon nitride cantilevers (Digital Instruments Inc.) were used to obtain all images. Further details can be found in a previous publication.60 Surface coverage (and hence area per molecule) data rely upon the actual surface area of the QCM being the same as its geometrical area. AFMmeasurementson the gold and chromium oxide QCM surfaces show that the surface area and geometrical area are in difference by no more than 5 and lo%, respectively.

Results and Discussion Quartz Crystal Microgravimetry. Figure 2 shows typical frequency changes of the gold QCM for N9 adsorption from 0.15 x and 3.0 x M N9 aqueous solutions. The frequency immediately decreases upon the addition of N9, indicating that there is a n uptake of N9 at the electrode surface. Adsorption of N9 is very rapid, and equilibrium is attained within 1 min. The adsorption isotherm for N9 on hydrophobic gold QCM surfaces is shown in Figure 3. (The equilibrium surfactant concentration equals the surfactant solution concentration since adsorption removes a negligible amount of surfactant from solution.) The open circles represent the experimental data points. To describe the measured frequency changes (AF) for surfactant adsorption, we used a Langmuir expression of the form61,62

(58) Lamb, R. N.; Furlong, D. N. J. Chem. Sac. Faraday Trans. 1 1982,78,61. (59) Geddes, N. J.; Paschinger, E. M.; Furlong, D. N.; Caruso, F.; Hoffmann, C. L.; Rabolt, J. F. Thin Solid Films, in press. (60)Mansur, H. S.; Grieser, F.; Marychurch, M. S.; Biggs, S.; Urquhart, R. S.;Furlong, D. N. J. Chem. SOC.,Faraday Trans.,in press. (61) Langmuir, 1. J. Am. Chem. Sac. 1917,39, 1848. (62) Heimenz. P. C. Princides o f Colloid and Surface Chemistm: ' Marcel Dekker Inc.: New Yoik, 1986; pp 427-441.

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where c is the equilibrium surfactant concentration, AFma is the maximum frequency change, and Kads is the adsorption constant. The inset of Figure 3 is a Langmuir plot for the N9 adsorption data. From this plot, AFma and Kads can be calculated from the slope and intercept, respectively (eq 2). Kadsfor N9 on gold was calculated to be 5.58 x lo4M-l, and AF,,, was determined to be 64 Hz. This value of AF,, was then used to generate a theoretical Langmuir isotherm for the data (shown as the solid curve in Figure 3). Adsorption for N9 remains low below the cmc (0.58 x mol dm-3; indicated by a n arrow in Figure 3). Increasing the concentration above the cmc results in increased adsorption followed by a plateau. It is clear from the adsorption isotherm (and Langmuir plot) that N9 adsorption onto gold can be adequately described by Langmuirian behavior. This is consistent with previous studies of N9 adsorption onto hydrophobic surface^.^^,*^ Figure 4 shows typical frequency changes of gold and chromium oxide QCM crystals for the adsorption of ClzEB from 1.58x M ClzEB aqueous solutions. In each case, the frequency is observed to decrease upon the addition of C&8, and adsorption equilibrium is achieved within 1min. The frequency decrease for C12& on gold, however, is observed to be twice that for ClzE8 on chromium oxide.

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66

Internal angle (degrees) Figure 6. Full plasmon ,resonance curves for plain gold in water and C12E8 on gold in solution at saturation coverage (C,,Es concentration of 1.58 x M). The solid lines represent the fitted theory, and the symbols are experimental data points. The inset shows the values derived from the Fresnel fits for the gold and at saturation coverage.

be independently determined since fitting to the full We have not as yet studied the adsorption kinetics in detail. plasmon resonance curve allows the degeneracy in the fit Interestingly, we might expect differences in kinetics parameters to be resolved. For a nonabsorbing film such between polydisperse N9 and monodisperse C I ~ E ~ . ~ ~as C12Ee on the gold surface however, it is not possible to The adsorption isotherms for ClzEs on hydrophobic gold obtain a unique solution to the three parameters E,, ci, and hydrophilic chromium oxide QCM surfaces are shown and d as the presence of a n overlayer on the surface in Figure 5 . The open circles and squares represent the perturbs the resonance observed for the bare gold subexperimental data points for CIZE8 adsorption on gold and strate. The simultaneous determination of the refractive chromium oxide, respectively. The arrow indicates the index and film thickness for nonabsorbing overlayer films cmc of C12E8. Each isotherm shows a characteristic is not possible using SPR (or likewise ellipsometry). maximum in frequency change a t surfactant concentraTherefore, it is necessary to choose avalue for the dielectric tions above the cmc. The solid curve for the ClzE8 permittivity crof the overlayer. For the fitting of the ClZE8 adsorption data on gold has been generated using a AFm,, SPR curves, cr was fixed a t 1.96 (refractive index, n, = of 56 Hz, which was calculated from the corresponding 1.40), and d and ci varied to obtain minimization of the Langmuir plot. Kadsfor Cl2E8 on gold was determined to least squares errors. (Setting ci to zero and allowing only be 6.39 x lo4 M-I. The adsorption data obtained for the d to vary gave the same results as varying both ci and d.) gold surface is well fitted by a Langmuir isotherm (see This value of er was chosen from previous work on the Figure 5). In contrast, the adsorption isotherm for ClzE8 adsorption of ethylene glycol monododecyl ethers on on chromium oxide QCM surfaces is characterized by an s i l i ~ a Although . ~ ~ ~ ~ the ~ calculated layer thickness ( d )is S-shape a t the beginning, followed by a horizontal plateau. sensitive to small uncertainties in the refractive index Adsorption does not commence until a surfactant con(n),66the surface coverage (r)data is essentially indecentration of about 0.2 x low4M. At this point, adsorption pendent of the value of refractive index chosen.65 This is strongly increases before finally achieving a plateau after because n and d are covariant in such a way that they to the cmc. This is consistent with previous work on C E E ~ a large extent cancel out when r is calculated (see eq 3). adsorption onto silica.49 I t should be noted that, in the fitting for the ClzE8 layers The frequency changes observed for N9 and C1zE8 cannot adsorbed on gold, it is assumed that the parameters be directly transposed to surface coverages using the previously deduced for the gold remain unchanged. Figure Sauerbrey equation (Am = -0.87AF) because the QCM 6 shows the full plasmon resonance curves of experimental responds to any interfacial mass change, not only the reflectivity data and Fresnel fits (solid lines) for gold in surfactant mass uptake. We therefore employed SPR as water and C12E8 adsorbed onto gold in aqueous solution a complementary technique in order to determine the a t saturation coverage (C12Ea concentration of 1.5 x surface coverage of these surfactants a t the interfaces M). The values of E,, ~ i and , d for gold and C12E8 derived studied in the QCM experiments and to use this result to from the Fresnel fits are shown in the inset. obtain a relationship between the frequency change of Normalized reflectivity changes with time for various the QCM with mass uptake in liquids. concentrations of C1zE8 are shown in Figure 7. Figure 7 Surface Plasmon Resonance Measurements. Fitshows that increasing the concentration of ClzEs results ting the Fresnel equations to the experimental full angle in a more rapid reflectivity change. For concentrations plasmon data (which includes the critical angle) enables below the cmc (curves a , b and c ) , the equilibrium the optical parameters of the gold film and subsequently reflectivity changes represent partial coverage of the the surfactant layers to be evaluated.64 For the gold film, surface. For concentrations of ClzEs above the cmc (curves the real (er) and imaginary (ci) components of the relative d and e), the reflectivity comes to equilibrium a t 1.165 f permittivity of the gold as well as the thickness ( d ) can 0.005 (i.e., a change in reflectivity of 0.165 f 0.005) and ( 6 3 )Furlong, D. N.; Drummond, C. J.; Young, B., manuscript in preparation. (64) Cowen, S.; Sambles, J. R. Opt. Commun. 1990, 79, 427.

(65) Tiberg, F.; Landgren, M. Langmuir 1993,9, 927. (66)Reiter, H.; Motschmann, H.; Orendi, H.; Nemetz, A,; Knoll, W. Langmuir 1992,8, 1784.

Quartz Crystal Microbalance and Surface Plasmon Resonance

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[C,,Esl (lo4M) Figure 8. Adsorbed thickness of ClzEs on gold as a function of Cl2Ea equilibrium concentration. The cmc is indicated by the arrow. The solid curve represents a theoretical Langmuir isotherm in which the thickness of ClzEs on gold at saturation coverage is 37 A and &s = 6.09 x lo4 M-l. is representative of saturation coverage. In all cases, adsorption equilibrium is attained within 30 min. This is considerably slower than the equilibrium times observed for C12E8 adsorption onto gold in the QCM experiments. It should be pointed out that SPR measures changes in optical thickness whereas the QCM detects mass changes. Figure 8 shows the thickness of C&s adsorbed on gold as a function of C&8 equilibrium concentration. The open circles represent the experimental thicknesses for C & & on gold, determined using a value of 1.40 for the refractive index of the C& layer (see earlier). The solid curve is a theoretical Langmuir fit to the experimental data, with the thickness of on gold a t saturation coverage calculated to be 37 A from the Langmuir plot. Kadsfor ClzEs on gold was determined to be 6.09 x lo4M-l, which is in close agreement to t h a t calculated from the QCM data. The SPR experimental data is, within experimental error, well fitted by a Langmuir adsorption curve. The maximum thickness of C12E8 is obtained a t concentrations near and above the cmc. A conventional method of calculating surfactant surface coverage (r)from surface plasmon data is that of de Feijter,67which employs the relation

(3)

where d is the thickness of the adsorbed surfactant and n b is the refractive index of the surfactant solution. If the surfactant layer is nonabsorbing, nf, the refractive index of the surfactant film, has no imaginary part. The value of the refractive index increment, dn/dC, of a surfactant solution Of C12E8 is 0.142 cm3/g.49The surface coverages were calculated using the adsorbed thicknesses of C12E8 that were determined from the fitted SPR experimental data. Figure 9 shows the surface coverage of C12E8 on gold as a function of C12E8 concentration. The solid curve is a theoretical Langmuir isotherm in which rmax equals 1.72 x g/m2. The QCM calibration factor required to obtain the same surface coverage from both the SPR and QCM data for ClzE8 adsorbed on gold from aqueous solution is 2.08 Hz/ ng(i.e., Am = -0.48AF), not the 1.15 Hzhgexpectedfrom the Sauerbrey equation for air measurements. Using this new calibration factor, the solution frequency changes for adsorption onto gold (from Figure 5) have been converted to surface coverages. The results, plotted in Figure 9, show the remarkable agreement between the SPR and QCM data over the whole concentration range studied. Recently, Grabbe et studied the adsorption of immunoglobulin G (IgG)from aqueous solution onto silver QCM electrodes electrogravimetrically. They concluded that 55% of the frequency change was due to adsorption, while 45% was due to a change of the "solution properties". This agrees with our SPR and QCM work: the calibration factor of 2.08 suggesting that 55% ofthe frequency change in solution is due to C12E8 adsorption. Our recent studies on IgG adsorption onto gold QCM surfaces have also yielded a solution calibration factor of 2 Hz/ng.68 I t is interesting that the calibration factor is the same for a small nonionic surfactant (C12E8) or a large globular protein (IgG). Application of the 2.08 calibration factor to the QCM N9 data on gold yields a saturation coverage of (1.8 f0.1) x g/m2, which is equivalent to a n average surface area per molecule of 0.56 i 0.04 nm2. This is close to the 0.66 nm2reported by Furlonget aL4I for N9 on methylated silica (hydrophobic), and somewhat lower than the 0.75 nm2 determined for N9 on other hydrophobic s ~ r f a c e s . ~ ~ A value of 0.54 nm2 is reported for maximum packing of N9 at the airlwater interface.4a Such concordancebetween this and previous studies adds credence to the value of 2.08 Hz/ng for the solution calibration factor. The saturation surface coverage of ClzE8 on gold is (1.6 f 0.1) x g/m2 (Figure 9), which corresponds to a n area per molecule of 0.56 f 0.04 nm2. This is slightly lower than the 0.77 f 0.03 nm2 measured by Gellan and Rochestersoin a calorimetric study of C&8 adsorption on hydrophobic graphitized carbon. In their it was suggested that each alkyl chain is only partly involved in direct interaction with the carbon surface and that the area occupied by each molecule is dependent on the stereochemical requirements of the ethylene chains protruding into the aqueous phase. The area value obtained in this current work may be rationalized in the same way given that there are numerous possible oxyethylene chain configuration^,^^^^^ all of which are influenced by the degree of hydration of the chain.70 The area (67) de Feijter, J. A,; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759. (68) Caruso, F.; Rodda, E.; Furlong, D. N., manuscript in preparation. (69) Tanford, C.; Nozaki, Y.; Rohde, M. F. J. Phys. Chem. 1977,81, 1555. (70) Heusch, R. Ber. Bunsenges. Phys. Chem. 1979,83, 834.

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[C,,E,I (lo4M) Figure 9. Surface coverage of Cl& on gold versus the Cl& concentration obtained from SPR and QCM experiments. The cmc is indicated by the arrow. The solid and broken curves represent theoretical Langmuir isotherms for the SPR and QCM data, respectively, and were generated using rmax = 1.72 x g/m2for the QCM g/m2for the SPR data and 1.70 x data. of 0.56 f 0.04 nm2 per &E8 molecule on gold compares well with the area of 0.64 nm2 for C1& at the aidwater interfa~e.~~ The saturation surface coverage for C12E8 on chromium oxide in aqueous solution (determined using the 2.08 calibration factor) is (0.97 f 0.12) x g/m2, corresponding to an area per molecule of 0.93 f0.12 nm2.These values are the same, within experimental error, as those recently reported by Tiberg et al. ,49 who investigated the adsorption of C12E.3on silica by means of null ellipsometry. These workers concluded that Cl2E8 adsorbs as small, possibly close to spherical micelles on silica. Earlier reports of C&8 adsorption on silica had yielded varied results with areas per molecule of ranging from 0.55 to 0.83 nm2.51-53A number of different orientations of C&B on the silicdwater interface are postulated from these studies, including bidimensional micelles and bilayer structures. Our investigations involving atomic force microscopy (AFM) are currently underway in an attempt to unravel the adsorption orientation of CIZEBat the gold liquid and chromium oxideAiquid interfaces. The factor of 2.08 H z h g determined for ClzEs adsorbed onto gold in aqueous solution represents an 80% enhancement in the QCM response compared with the 1.15 H z h g predicted by the Sauerbrey equation for air measurements. The larger frequency change detected by the QCM is probably due to entrapped water within the film. Previous studies72have shown that the adsorption (71) Drummond, C. J.;Warr, G. G.; Grieser, F.;Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1985,89, 2103.

of relatively large and hydrophilic proteins such as /3-globulin (Mw lo5)adsorbed onto lipid-coated QCMs a t the aidwater interface produce frequency changes 1.5 times larger than those obtained in air. In these experiments, the discrepancy between the frequency measurements was also attributed to the protein molecules vibrating on the QCM with surrounding water when in contact with solution. In contrast, the adsorption of small (Mw < 5000)hydrophobic proteins from solution onto lipid coated QCMs a t the aidwater interface obey the Sauerbrey equation,72as do various lipid Langmuir-Blodgett films deposited onto QCM crystals immersed in water.73 In these films it is thought that the films are rigid and have negligible interaction with water. In general, the solution calibration factor can be connected with shape, size, and hydrophobicity of the adsorbed amphiphilic molecule^.^^^^^ Investigations concerning the adsorption of various molecules from liquids onto QCM surfaces are in progress in order to gain a further understanding of the parameters that define the relationship between QCM frequency oscillation and adsorbed layer masses in liquids.

Conclusions The application of the QCM technique to study the adsorption of surfactants a t solidfliquid interfaces has been demonstrated. Complementary SPR experiments allowed determination of the QCM calibration factor when in contact with aqueous solution. This factor was calculated to be 2.08 Hzhg, compared with the 1.15 H z h g expected from the Sauerbrey equation for air measurements. The combined QCWSPR study for the examination of surfactant adsorption eliminates the need for the theory pertaining to the operation of the QCM in liquids in order to quantitatively follow the adsorption processes. In addition, the QCM technique has submonolayer sensitivity and is equally effective in the study of adsorption of chromophoric and nonchromophoric adsorbates. Application of the above calibration factor to the adsorption data for N9 on gold QCM electrodes and C12E8 on both gold and chromium oxide QCM electrodes yielded area per molecule values for each surfactant a t saturation coverage which compare favorably with those reported in the literature.

Acknowledgment. We thank Thomas Gengenbach for the XPS measurements. This work was performed as part of an AustraliadJapan collaboration, funded in part through the Department of Industry, Technology and Commerce, Commonwealth Government of Australia. LA9409236 (72) Ebara, Y.; Okahata, Y. Langmuir l993,9, 574. (73) Okahata,Y.;Kimura, K.; Ariga, K. J.Am. Chem. SOC.1989,111,

9190.