Frequency Behavior of a Quartz Crystal Microbalance (Qcm) in Contact with Selected Solutions

A device was constructed to monitor viscosity of solutions using fundamental frequency of 9 MHz and 10 MHz quartz crystal. Piezoelectric quartz crystals with gold electrodes were mounted by O-ring in between liquid flow cell. Only one side of the crystal was exposed to the solutions which were pumped through silicon tube by a peristaltic pump. The measured frequency shift was observed in order to investigate the interfacial behavior of some selected solution in contact with one surface of Quartz Crystal Microbalance (QCM). An analysis of the interaction between an AT-cut quartz crystal microbalance and various liquid system of analytical interest is presented. The analysis which includes piezoelectric effects and other influences; liquid properties, experimental conditions and the characteristic of the solution are reported. Oscillation in distilled water is taken as a reference. The frequency change caused by the density ( , gcm ) and viscosity ( , gcm s ) were found to be proportional to the square root of the product, ( ). The result suggested that analysis of small frequency shifts during EQCM studies needs to account for changes in and of the solution. Generally, all the liquid tested showed an increment of the frequency shift with increasing content of solutes. For each solution, the frequency was recorded as the concentration increases from distilled water to a very concentrated solution. The frequency measurements carried out for saccharide solution produces the maximum changes of frequency shift compared with other solutions.


INTRODUCTION
Quartz crystal microbalance (QCM) is a wellestablished method for the measurement of small changes in mass, based on the relationship between changes in mass of material attached to the crystal and the oscillation frequency of the crystal. It has also been found that they also play an important role in probing interfacial processes or acoustic properties of liquids. The operation of a QCM relies on the excitation into mechanical resonance induced by an electrical field across the quartz crystal with two metal electrodes on opposite sides of the quartz crystal plate.
The AT-cut of quartz has long been the most commonly used cut for quartz crystal oscillator applications. New advances in QCM methodology has enable the use of the QCM principle while the quartz crystal is immersed in liquid media [1,2] . Konash and Bastiaans [3] reported that a crystal with one electrode in contact with an organic solvent also oscillates. They found that the frequency depends on the density and the arrangement may be used as a detector for liquid chromatography. Kanazawa and Gordon [4] presented a quantitative description of the influence of the liquid properties on the oscillation frequency. They determined the behavior of the crystal/fluid system by examining the coupling of the elastic shear waves in the crystal to the viscous shear wave in the liquid. Kurosawa et al. [5] examined the oscillating frequencies in various solutions by comparing the oscillating frequencies in distilled water. They observed that f ∆ or W f ∆ in solutions is a linear function of ρη , except for salt and high polymer solutions. We have expanded the same experiment but with different flow cell configuration and used it on some solutions of analytical interest. The frequency behavior of the QCM immerse in those solutions is reported in this paper.

MATERIALS AND METHODS
Selected solutions with various concentrations were prepared using analytical-reagent grade chemical and distilled water. The specific gravity and viscosities were measured by pycnometer and Ubblohde viscometer. We used AT-cut quartz discs with gold electrodes and fundamental frequencies of 9 MHz and 10 MHz (ICM Co., Oklahoma City, US) in all experiment. A flow cell was constructed in which one face of the crystal is in contact with the liquid. The crystals were clamped between two O-rings as shown in Fig. 1. Solutions are propelled to the flow cell by a micro-tubing pump, (MP-3, Eyela, Tokyo Rika) through 0.5mm diameter silicon tube. The flow-rate was between 1.3-1.6 ml min -1 and no dependence of the flow-rate on the frequency change was found. The surface of the working electrode was cleaned in-situ by alcohol solution repeatedly after every experiment. The frequency change was monitored by a universal counter (Model 5313A, Agilent) which was monitored by a microcomputer with a GPIB interface programmed by LabVIEW.
After stabilization of the frequency with water, test solutions were applied stepwise from low to high concentrations. At the end of each experimental run, water was applied to check the reversibility of the frequency. The frequency of the crystal was measured every 5 seconds for all concentration of solutions. Owing to the low densities of pure ethanol and methanol, they were applied first, followed by test solutions mixed with water. Finally, water was applied. For water-immiscible organic solvent such as hexane, benzene and toluene, the oscillating frequency in water was measured and then acetone was applied to the crystal, followed by the application of a large volume of solvent in question.
To understand the relationship between the frequency change of the QCM immersed in solution and the properties of the liquid (density, ρ and viscosity, η ), results obtained by Kanazawa and Gordan [3,4] was referred. However, in this experiment, oscillation in distilled water was taken as a reference and a modified version the Kanazawa and Gordon equation was obtained and is given below [5] : where w and w represent the density and viscosity of water, respectively. Under the present condition, Equation (1) were examined by changing n, f b , ρ and η . Figure 2 shows the frequency shift of the quartz for gold-sucrose, gold-glucose and gold-maltose interfaces at concentration of 5-25 wt%. We have observed that the experimental value of W f ∆ − for the aqueous saccharide increased linearly with the increasing of sample concentration. Similar trends were also observed by other scientist [2,[5][6][7] for sucrose and glucose solution. We also found that sucrose gave a higher frequency change compared to glucose and maltose for both 9 and 10 MHz QCM. The increase in frequency shift is greatly due to the increment of the molecule adsorbed to the gold surface. The binding process changes the refractive index of the medium in contact with the metal surface. The different responses may due to the different formula structure of saccharide. Sucrose has the highest sensitivity because it has a longer diffusion path compare to other saccharide. In the other words, the more molecules are on the surface, the greater the frequency shift, (1). It was found that the oscillation frequency of the crystal depends on the physical parameter (density and viscosity) of the liquid properties. Figure 3 shows a plot of the frequency change against W W ρη ρ η − using aqueous sucrose, maltose and glucose for the 9 MHz and 10 MHz crystals where only one side of the crystal was expose to the solutions. The slope of the straight line that reflect the sensitivity of the detection of sucrose, glucose and maltose are 7.14 x 10 2 (10 MHz), 6.10 x 10 2 (9 MHz), 7.14 x 10 2 (10 MHz), 6.10 x 10 2 (9 MHz) and 7.13 x 10 2 (10 MHz) , 6.09 x 10 2 (9 MHz) respectively. According to the theory, the plot of

RESULT AND DISCUSSION
should give a line with a slope of unity. This is verified by the data on  w w with these crystals in various solution are illustrated in Table 1. It was found thatf w increased with increasing f o (fundamental frequency). The 10 MHz crystals are more sensitive to the mass changes but were found to be less stable compared to 9 MHz crystal.

CONCLUSION
We have carried out our QCM measurements with some aqueous solutions. An uncoated quartz crystal was used for monitoring changes of viscosity and density of the liquid simultaneously. Our results implies that the frequency change is proportional to w w . With the QCM principle used in liquids, it is possible to detect properties of liquids up to the ppm range. From the experimental results, we also can determine a solute in a selected solution. The relations between the frequency shift, f w and are reported. The above results are very helpful for understanding the frequency behavior in contact with liquid phase and offer new potential routes for studying liquid properties in the future.