Langmuir 1986, 2, 228-229
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Novel Application of the Quartz Crystal Microbalance To Study Langmuir-Blodgett Films Robert R. McCaffrey,? Stanley Bruckenstein,* and Paras N. Prasad” D e p a r t m e n t of Chemistry, University at Buffalo, State University of N e w York, Buffalo, N e w York 14214 Received August 29, 1985. I n F i n a l Form: December 6, 1985
The quartz crystal microbalance has been used to follow t h e deposition of mono- a n d multilayer Langmuir-Blodgett films of calcium stearate on gold. T h e change in frequency of the oscillating quartz transducer is a linear function of the number of layers deposited. The weight associated with each deposited layer agrees within -25% of that calculated from t h e reported molecular surface area.
Introduction An oscillating quartz crystal can be used as a very sensitive mass measuring device because its resonance frequency changes upon the deposition of a given mass to the crystal surface. The application of t h i s phenomenon is widespread and has lead to terming this use of a q u a r t z crystal as the quartz crystal microba1ance.l Sensitivities on the order of 1 n g are readily obtained, and the change in frequency is a linear function of the mass a t t a c h e d to the surface of the crystal. The sensitivity characteristics of the quartz crystal microbalance m a k e i t ideally suited to s t u d y fractional monolayer and multilayer film^.^,^ Interest i n Langmuir-Blodgett films is widespread and, for example, one can form highly ordered ultrasubmicron thin films with interesting electronic and optical properties. In this paper we report o u r initial results applying the q u a r t z crystal microbalance technique to s t u d y calcium stearate monolayer and successively deposited multilayer Langmuir-Blodgett films. The determination of the mass deposited can provide useful information about molecular organization in s u c h films. Experimental Section Preparation. A glass dish trough, as described by B l ~ d g e t t , ~ was used to form calcium stearate monolayers on a 10-MHz AT-cut quartz oscillator crystal. It should be noted that the active weighing area of the crystal is coated with gold, and even though calcium stearate can deposit on both quartz and gold areas, only the material deposited normal to the gold surface is weighed. The reason is that the crystal only oscillates in the region it is subjected to an ac electric field. The ac field exists only between the two opposing gold electrodes. Mass attached to an oscillating part of the crystal lowers the frequency because of the additional work required to make the crystal oscillate. It is well-known that there is no response to mass attached to the crystal outside the gold electrodes.’ The sensitivity and reproducibility of the weighing step is 1 ng. The trough was coated with ferric stearate before each experiment.”,6 A cotton candle wicking thread soaked with ferric stearate was attached to the side edge of the trough with molten ferric stearate and was used as the floating compression barrier in the trough. Two rectangular Teflon-coated aluminum barriers were coated with ferric stearate and were used at each end on the trough. The dipping mechanism consisted of a stepping motor coupled to a vertical translational stage. Direction and speed were determined by the stepping motor controller. All solutions were prepared using Milli-Q water (Millipore Corporation). The pH of 0.1 mM solution of calcium carbonate solution was adjusted to 6.0 by bubbling carbon dioxide through it. This solution was used as the sublayer to form the calcium stearate monolayer.
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* T o whom all correspondence should be addressed. ‘Present address: E.G.&G., Idaho, Inc., Idaho Falls, ID 83415.
0743-7463/86/2402-0228$01.50/0
The quartz crystal was cleaned by successive glow discharge treatments until the change in frequency between treatments became constant (approximately a 10 ng loss of gold). After each treatment, the crystal was washed with Milli-Q water and dried in flowing dry nitrogen. Frequency measurements were made with respect to a reference 10-MHz crystal by using an apparatus described elsewhere.‘ Procedure. The CaC03 solution was added to the trough until a positive meniscus was formed on the top edge. The top surface of the solution was then carefully cleaned by sweeping its surface with the Teflon-coated aluminum barriers 2 times with each barrier, after which the barriers were left at the ends of the trough. In two experiments, the quartz crystal was then lowered beneath the clean aqueous surface. A 0.1 mg/mL stearic acid benzene solution was added to the sample end of the trough in 25-mL increments by using a Gibson micropipet. A total of 150-170 pL of the stearic acid solution was generally sufficient to fill the sample end of the trough. After all benzene evaporated from the surface of the trough, ca. 5 pL of oleic acid was added to the compression end of the trough to exert a constant surface pressure of ca. 30 dyn/cm. A He-Ne laser aligned parallel to the film interface was used to monitor movement of the floating cotton barrier. The first layer of calcium stearate was deposited by raising the crystal a t a rate of ca. 0.5 mm/min through water interface. Subsequent layers were deposited by lowering and raising the crystal through the water interface at 2.0 mm/min. When the substrate was first lifted out of the film-water interface, one layer was deposited. Two layers were deposited on each successive cycle of lowering and raising the crystal. The frequency of the crystal was determined each time the crystal was raised out of the filmwater interface. This procedure produced an odd number of layers on the crystal for weight measurements. In one experiment, the quartz crystal was lowered through the aqueous surface after forming the calcium stearate layer a t the film-water interface, and the previous procedure was then followed. This technique produced an even number of layers on the crystal for weight measurements. Our results thus shows that a layer is deposited on the gold surface whether it is initially lowered into or it is initially raised out of the water subphase. The reason is that the gold surface can act hydrophobic or hydrophilic depending on the surface treatment. A plausible explanation for our result is that the clean gold surface acts in a hydrophobic manner when it is lowered into the water subphase and, therefore, a layer is deposited. On the other hand, when the gold substrate is immersed in the water subphase, because of possible surface oxidation, it becomes hydrophilic. Therefore, a layer is also deposited when the crystal oscillator is initially lifted out of the water subphase. (1) Lu, C.; Czanderna, A. W., Eds. Methods Phenom.: Their Appl. Sci. Technol. 1984, 7. (2) Bruckenstein,S.; Swathirajan,S. Electrochim. Acta 1985, 30, 851. (3) Bruckenstein, S.; Shay, M. J . Electroanal. Chem. 1985, 188, 131. (4) Blodgett, K. B. J. Am. Chem. SOC.1935, 57, 1007. (5) Langmuir, I.; Schaefer, V. J. J . Am. Chem. SOC.1937, 59, 2400. (6) Gaines, G. L., Jr. “Insoluble Monolayers at Liquid-Gas Interfaces”; Interscience Publishers: New York, 1966. (7) Bruckenstein, S.; Shay, M. Electrochim. A c t a , in press.
0 1986 American Chemical Society
Langmuir, Vol. 2, No. 2, 1986 229
Quartz Crystal Microbalance Application Frequency Change vs Coverage Plot 3000
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F i g u r e 1. Frequency change of oscillating quartz crystal vs. number of layers of calcium stearate that were deposited. Three experiments. Diamonds and squares, odd number of layers weighed; hexagons, even number of layers weighed. Table I. Linear Least-Square Summary of Frequency Dependence of Oscillating Quartz Crystal vs. Number of Calcium Stearate Layers Deposited slope (sd), expt no. no. of layers Hz/layer intercept (sd), Hz la 9 147.45 (6.82) -25.05 (39.18) 2b 19 152.52 (0.68) -90.88 (7.80) 3' 10 156.95 (4.78) -105.10 (31.71) l+2+3d 151.17 (1.31) -67.25 (12.14) a Corresponds to diamond data points in Figure 1. Corresponds to square data points in Figure 1. cCorresponds to hexagon data points in Figure 1. dData of experiments 1,2, and 3 combined and treated as one experiment.
Results and Discussion Figure 1presents the results of three independent experiments in which up to 19 layers of calcium stearate were deposited. Table I summarizes the linear least-squares constants obtained from analysis of the data in Figure 1. All experiments were analyzed by using a linear model and produce a good correlation. Within the precision of the data, there is no justification for using a polynomial model. However, the existence of a negative intercept suggests that the first layer(s) deposited pack more loosely than the subsequent layers. In view of the fact that gold has a very high surface energy (yoAu)is 1300-1700 dyn/cm, see ref 8) compared to water, it appears reasonable that the gold surface may pull the stearate ions flat to make them pack more loosely in the first layer. It has been reported that stearic acid on mercury occupies 120-140 A2/molecule below 40 dyn/cm? This surface area is considerably larger than that occupied on the water surface, again consistent with loose packing on mercury which has high surface energy (yo Hg is 484 dyn/cm) compared to water. The least-square slope obtained by combining the data of the three experiments is 151.17 (sd = 1.31) Hz/layer which yields a surface mass density of 322 ng cm-*. This conversion of frequency change to surface mass density uses the experimental area of the active gold region on the (8) Alexander, B. H.; Dawson, M. H.; Kling, H. P. J.Appl. Phys. 1951, 22, 439. (9) Ellison, A. H. J. Phys. Chem. 1962, 66, 1867.
crystal and the conversion factor between frequency change and change in mass. I t has been shown that the calcium ion content of a stearic acid film spread on an aqueous buffer solution containing a calcium solute depends on its pH ~ a l u e . ~ J ~ l ~ No calcium stearate in the film is reported for pH 9 when sodium (bi)carbonate is used as buffer. I t has also been reported that water is included in the transferred monolayer film as calcium stearate monohydrate.'O However, the surface pH of solutions containing carbon dioxide is higher than the bulk pH because of the concentration gradient of carbon dioxide caused by its escape from the surface.13 Thus, under our experimental conditions, the condensed film is composed of calcium stearate monohydrate. The reported value of calcium stearate's molecular surface area in the plane of the film at the water-air interface compressed at 30 dyn cm-l is ca. 19.5 X cm2 per stearate ion.'l By use of the molecular weight of calcium stearate monohydrate (312 g mol-' for [CH3(CH,)16COOCao.5.0.5H20]), the calculated value of the surface mass densty is 266 ng cm-2. The difference between the experimental and calculated values is greater than that which can be attributed to experimental error. Two likely explanations exist. First, ions and solvent may be incorporated in the multilayer film. This hypothesis is consistent with the observed change in weight increment between the first and subsequent layers. Second, the surface roughness of gold on the quartz crystal may increase the effective surface area for the deposition of the Langmuir-Blodgett film. The crystals used were finished with a 5-wm abrasive so that the rugosity of the surface would involve structures large compared to the size of the molecule. At this time we cannot determine whether one or both possibilities have caused the observed results.
Conclusion The quartz crystal microbalance has the sensitivity and reproducibilitynecessary to determine the mass of material associated with the deposition of Langmuir-Blodgett monolayer-thick films. In the case of the deposition of calcium stearate films deposited on gold, we found a linear relationship between the change in mass attached to a quartz oscillator crystal and the number of layers deposited. Our experimental results suggest that some water may be incorporated between layers of multilayer-thick films and/or the surface topography of substrate may lead to a slightly larger effective area for film formation. Acknowledgment. We thank Professor D. A. Cadenhead for helpful discussions. This work was supported by the Airforce Office of Scientific Research through AFOSR Contract 349620855COO52 and Grant 83-0004. Registry No. Au, 7440-57-5; quartz, 14808-60-7; calcium stearate, 1592-23-0. (10) Bagg, J.; Abramson, M. B.; Fichman, M.; Haber, M. D.; Gregor,
H.P. J. Am. Chem. SOC.1964,86, 2759.
(11)Deamer, D. W.; Meek, D. W.; Cornwell, D. G. J. Lipid Res. 1967, 8, 255. (12) Sasaki, T.; Matuura, R. Bull. Chem. SOC.J p n . 1951, 24, 2741. (13) Thomas, J. G. N.; Schulman, J. H. Trans. Faraday SOC.1954,50,
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