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Study of the Frequency Character of the Ringed-Electrode Piezoelectric Sensor in Liquid Phase and the Adsorption of CTMAB onto a Quartz Surface
Journal of Colloid and Interface Science 241, 386–391 (2001) doi:10.1006/jcis.2001.7749, available online at http://www.idealibrary.com on
Study of the Frequency Character of the Ringed-Electrode Piezoelectric Sensor in Liquid Phase and the Adsorption of CTMAB onto a Quartz Surface Fan Yin,∗, ‡ Youyu Zhang,† Yaohui Wu,† Yan Cai,† Qingji Xie,† and Shouzhuo Yao∗,1 ∗ Department of Chemistry, Hunan University, Changsha, 410082, P.R. China; †Chemical Research Institute, Hunan Normal University, Changsha, 410081, P.R. China; and ‡Department of Biology and Chemistry, Changshu College, Changshu, 215500, P.R. China Received January 12, 2001; accepted June 1, 2001; published online August 1, 2001
The resonance behavior and the electric equivalent circuit of the ringed-electrode piezoelectric sensor (REPS) operating in liquid phase have been studied. The results indicate that the REPS can be used to investigate a solution’s properties and mass loading on a quartz surface under different conditions. Using the change in the liquid’s conductivity, the critical micellar concentration (CMC) of cetyltrimetylammonium bromide (CTMAB) has been measured by the REPS. The result corresponds well with the reference data. Based on the mass effect, the adsorption process of CTMAB onto a quartz surface has been monitored in situ and the adsorption density has been investigated. The rate of CTMAB’s being adsorbed onto the quartz surface increases when CTMAB concentration increases. In addition, the adsorption density (adsorbed mass per unit area of CTMAB) is affected by the ion strength of solution. The higher the ion strength in the test solution, the greater is the adsorption density of CTMAB on the quartz surface. It has been shown that the REPS is a useful tool for studying quartz/solution interface interactions in real time. °C 2001 Academic Press Key Words: quartz surface adsorption; ringed-electrode piezoelectric sensor; frequency character; cetyltrimetylammonium bromide; critical micellar concentration.
INTRODUCTION
Piezoelectric devices have been widely used as liquid microsensors. Several papers have reviewed the applications of piezoelectric quartz crystals (PQCs) in the chemistry of solutions (1–5). In most instances, the PQC is used to measure the mass change at the electrode surface, using methods based on the work of Sauerbrey (6). Besides the mass effect, the frequency of the PQC is also affected by the properties of the liquid, such as density, viscosity, specific conductivity, and permittivity (7–11). The non-mass effect of PQCs has also been used as chemical and biochemical sensors (12–15). Sch¨on et al. used a PQC modified
1 To whom correspondence should be addressed. E-mail: [email protected]. edu.cn; fax: 861-731-8865515.
0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
with mesoporous titanium dioxide to measure the density of a liquid (12). He et al. studied the enzymatic hydrolysis of carboxymethyl cellulose with a PQC by measuring the changes in density and viscosity of the solution (13). Some applications of the non-mass effects of PQCs were reported from this laboratory (13, 16–18). Usually, a PQC is configured with two electrodes, one on each side of a thin disc of AT-cut quartz. A high-frequency alternating electric field is applied to the quartz crystal disc by the two electrodes. Recently, we have reported a new type of piezoelectric quartz crystal sensor named the ringedelectrode piezoelectric sensor (REPS) (19). In the REPS (shown in Fig. 1), the bare quartz surface is in direct contact with the liquid phase. The high-frequency alternating electric field is applied to the quartz crystal disc in two parts (Fig. 1c). The side part is applied to the quartz crystal disc directly by the ringed-electrode, while the center part is applied by the conductance of the liquid layer. This configuration allows the REPS to respond to the liquid properties. In addition, it is convenient for investigating the mass change on the quartz surface because the quartz surface contacts directly with the liquid. Using the REPS, we have studied the behavior of BSA adsorption onto the quartz surface (19). One characteristic of surfactants is their tendency to adsorb onto the interface in an orientated fashion. The adsorption of the surfactant is of interest as it is directly relevant to practical applications, such as detergency and flotation. The amount of surfactant adsorbed on an interface is an important parameter in the investigation of many interfacial phenomena. A carbon-14 technique (20) and a neutron reflection technique (21) have been employed for the determination of the adsorption density of surfactants on quartz surface. However, expensive instruments are needed. The critical micellar concentration (CMC) is also an important characteristic parameter of the surfactant. The physicochemistry properties of the surfactant solution, such as the decontamination capability, viscosity, density, specific conductivity, surface tension and osmotic pressure, can be changed when the concentration of the surfactant is equal to or higher
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Apparatus
FIG. 1. Schematic illustration of the REPS. (a) Bottom view. (b) Top view. (c) Side view.
The experimental assembly employed is shown in Fig. 2. A 9-MHz AT-cut crystal with ringed-electrode on one face was used as a sensing element. An Iwatsu SC-7201 universal frequency counter (Japan) was employed to record the frequency. The REPS was fixed to a detection cell made of Teflon. The quartz surface outside the gold ring was covered with silicone resin, leaving bare quartz at the crystal center and the surrounding gold ring in contact with the sample solution. The crystal holder was directly connected to a homemade IC-TTL oscillating circuit. The circuit was supplied with a JWY-30B dc voltage regulator (Shijiazhuang Electronic Factory No. 4) and the working voltage was set at 5 V. Experiments were made at 20◦ C. A magnetic stirrer (Shanghai Electrocommunication Instrumentation Factory) was employed to stir the test solution. 5-l, 10-l and, 100-l microsyringes were used to add solution. Method of Measurement
than the CMC. Techniques being used for the study of the CMC include specific conductivity methods (22), osmotic pressure methods (23), dye adsorption methods (24), and dispersion methods (25). In this paper, the equivalent circuit of the REPS is depicted and discussed, and the resonance frequency of the REPS influenced by liquid properties is studied. Using the REPS, the adsorption of cetyltrimetylammonium bromide (CTMAB) onto the quartz surface is monitored in real time. The adsorption density of CTMAB on the quartz surface is measured by the REPS, and the CMC of CTMAB is also determined. The results show that the REPS can be used to study solution/quartz interfacial interactions and liquid properties. EXPERIMENTAL
Reagents Cetyltrimetylammonium bromide (CTMAB) was purchased from Guangzhou Chemical Reagents Factory (analyticalreagent grade). All chemicals were of analytical-reagent grade or better. Doubly distilled water was used throughout.
By changing the content of sodium chloride (NaCl) in pure water, NaCl solutions were used to examine the effect of solution conductivity on the resonance frequency of the REPS. Dioxane + water mixtures were chosen for determining the influence of the liquid permittivity on the resonance behavior of the REPS. The relative permittivity of dioxane + water mixture is very conveniently adjusted from 2 to 80 by changing the proportion of dioxane in liquid. Sucrose solutions were used to investigate the influence of density and viscosity of liquid exerted on the resonance frequency by changing the amount of sucrose in solution. With mild stirring, the adsorption process of CTMAB onto the quartz surface was monitored by adding CTMAB solution to a series of NaCl solutions with a microsyringe. The frequency shift of the REPS was recorded with a time interval of 30 s. Before the adsorption experiment, the REPS and the detection cell were pretreated with 0.001M CTMAB solution and rinsed thoroughly with distilled water to minimize extra adsorption of CTMAB onto the surface of the detection cell and the sensor holder. After being dried with clean air, the sensor’s quartz surface was treated with H2 SO4 + H2 O2 (3 : 1) for 10 min and then repeatedly rinsed with water. The bare quartz surface was made
Preparation of the REPS An AT-cut 9-MHz piezoelectric quartz crystal (12.5 mm in diameter) was used. One face of silver electrode (6.0 mm in diameter) of the PQC was dissolved in nitric acid and coated with a gold film (6.5 mm in diameter) by vacuum-evaporation. The central part of the gold electrode was dissolved in aqua regia with the bare quartz being exposed to the sample solution. The diameter of the bare quartz circle was 4.0 mm as shown in Fig. 1a. The other face of the silver electrode was sealed at the bottom of a glass tube to leave it in contact with air. All manufactured ringed-electrode piezoelectric sensors were stored in distilled water before use.
FIG. 2. Schematic diagram of the REPS system.
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fresh by dipping in a 10% solution of hydrofluoric acid for 30 s and rinsed thoroughly again with distilled water. A REPS was immersed in 20 ml of 0.1 M NaCl solution. With mild stirring, a stable oscillating frequency ( f 1 , frequency drift <1 Hz in 5 min) was obtained. A series of standard CTMAB solutions were injected into the detection cell. The corresponding frequency was recorded when a stable frequency ( f 2 ) was obtained. The shift of frequency for each solution was calculated as 1 f = f 2 − f 1 . The frequency shift, 1 f , was used for the calculation of the amount of adsorption according to the Sauerbrey equation (6). The principle of determining the CMC of CTMAB is based on measuring the change of the test solution’s conductivity using the REPS after adding CTMAB solution with a microsyringe. RESULTS AND DISCUSSION
Equivalent Circuits of the REPS For the REPS, the high-frequency alternating electric field being applied to the quartz crystal disc is divided into two parts (Fig. 1c). The side part is applied to the quartz crystal disc directly by ringed-electrode, whereas the center part is applied by the conductance of the liquid layer as mentioned above. In our experiment, the diameter of the electrode in the liquid, including the bare quartz circle, is larger than that in air. Thus, there is no fringing field on the liquid side between the two electrodes. The electrical equivalent circuit of the REPS near resonance frequency is depicted in Fig. 3. This equivalent circuit has been suggested by Shana et al. before (11, 26). The center capacitance is modeled by two independent capacitors, Cs and C2 , in series (Fig. 3). They correspond to the part of the center field that goes through the liquid on the liquid side of the REPS and that through the quartz, respectively (Fig. 1 and Fig. 3). Cs and C2 are therefore proportional respectively to the permittivity of the liquid and of the quartz. The resistive losses in the fluid are modeled by a resistor, Rs , which is inversely proportional to the liquid conductivity and connected in parallel with Cs . All three parameters are influenced of course by the exact geometry of the electrodes and the liquid proper-
FIG. 3. (a) Simple model to characterize the REPS in liquid. The resistance, Rs , is inversely proportional to the conductivity of the liquid. The capacitance, Cs , is proportional to the dielectric constant of the liquid. C2 is the capacitance formed between the Ag electrode and the liquid. C1 is the capacitance formed between the Ag electrode and the Au ringed-electrode. (b) Simple equivalent circuit of the REPS with ringed-electrode face contacting a liquid. Cm is the motional capacitance. Lm is the motional inductance. Rm is the motional resistance.
FIG. 4. The double logarithmic form of the relationship of the resonance frequency (in Hz) of the REPS to the concentration of NaCl solution (in M).
ties. As for the REPS with given geometry of the electrodes, the resonance frequency is affected only by the properties of the liquid. The Effect of the Liquid Properties on the Resonance Frequency The change in the resonance frequency as the conductivity of the liquid increased by the addition of NaCl solution is shown in Fig. 4. The frequency shift, 1 f , for pure water was set to zero. Over the whole concentration range of NaCl solution, 1 f decreases by as much as 8 kHz. When NaCl concentration is lower than 1 × 10−5 M, no frequency shift is observed. The frequency shift decreases linearly with NaCl concentration, which ranges from 1 × 10−5 M to 0.012 M. The relationship between 1 f and NaCl concentration can be expressed as log (−1 f ) = 1.2424 × log C + 6.5738 (r = 0.9993). The resonance frequency is not affected by the change of NaCl concentration when it is higher than 0.04 M. This can be explained as follows. (1) In low electrolyte concentration (lower than 1 × 10−5 M for NaCl), the field applied to bare quartz is mainly through Cs , for Rs is very large and can be thought of as an open circuit. Thus, the resonance frequency is unaffected by the conductivity of the liquid (2). In high electrolyte concentration (higher than 0.04 M for NaCl), the field applied to bare quartz is mainly through Rs , for Rs is very small and the circuit can be thought of as a short circuit (Fig. 5). The resonance
FIG. 5. Schematic illustration of the REPS seen from the side. The dotted lines are a qualitative representation of the electric field lines between two electrodes when the REPS is dipped in (a) a nonconducting liquid and (b) a perfectly conducting liquid.
RINGED-ELECTRODE PIEZOELECTRIC SENSOR BEHAVIOR
FIG. 6. The dependence of the resonance frequency of the REPS on the permittivity of Dioxane + water mixture.
frequency is thus unaffected by the conductivity in high ion strength liquids. Dioxane + water mixtures were chosen for the investigation of the influence of the solutions’ permittivity on the frequency behavior of the REPS. The permittivity of the mixture increases when the water content increases, whereas the density and viscosity of the mixture vary only slightly with its composition (27). The resonance frequency decreases when the relative permittivity of the solutions increase (shown in Fig. 6). The resonance frequency of the REPS in pure dioxane solution was set to reference value. In high ionic strength solutions, the resonance frequency is unaffected by the change of liquid’s permittivity because the circuit can be thought of as a short circuit (Fig. 5b), as discussed above. Sucrose solutions were used to investigate the influence of the density ρ and viscosity η of the solutions on the resonance frequency. According to Kanazawa et al. (8), the frequency shift of the PQC as a function of density and viscosity can be expressed as 1 f = − f s3/2 (ρη/πρq µq )1/2 ,
[1]
where 1 f is the resonant frequency shift of the crystal, f s is the resonant frequency, η is the viscosity of the liquid, ρ is the density of the liquid, µq is the shear modulus of the quartz crystal, and ρq is the density of the quartz crystal. According to Eq. [1], a REPS with higher f s is more sensitive to changes of viscosity and density than a PQC. Figure 7 shows the frequency shifts of a REPS and a PQC in sucrose solutions relative to pure water as a reference. The results of the experiment show that the resonant frequency of the REPS in high ionic strength solution decreases more rapidly than that of the PQC. In a nonelectrolyte solution, the slope of the REPS is the same as that for the PQC. This phenomenon can be explained by the slight decrease in the permittivity of the liquid caused by adding sucrose to the solution (27), which results in an increase of resonance frequency. The co-effect causes a minor shift of resonance frequency in pure water compared with that in higher ion strength solution. A linear relationship between (−1 f ) and (ρ × η)1/2 is found. The linear regression equations are as
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FIG. 7. The relationship between the resonance frequency of the REPS and the value of (ρη)1/2 in sucrose solutions. The REPS in pure water (m); the REPS in 0.1 M NaCl solution (d); the PQC in pure water (j).
follows: 1 f = 1776.77 − 1824.26 × (ρ × η)1/2 (r = 0.9996, REPS in pure water), 1 f = 1863.42 − 1874.68 × (ρ × η)1/2 (r = 0.9998, PQC in pure water), 1 f = 1918.33 − 2063.86 × (ρ × η)1/2 (r = 0.9988, REPS in NaCl solution). In conclusion, the REPS can act as a liquid permittivity sensor, a conductivity sensor in low ionic strength solutions and a normal PQC in perfectly conducting liquids. The CMC Measurement of CTMAB Generally, the ionizability of an ionic surfactant, such as CTMAB and sodium dodecylsulfonate (SDS), will decrease when the concentration of these surfactants is equal to or higher than their CMC. Based on its sensitivity to the conductivity of the liquid, the REPS was used to measure the CMC of CTMAB. Figure 8 shows that the resonance frequency decreases linearly with the increase of CTMAB concentration when the
FIG. 8. Dependence of frequency shifts of the REPS after changing the concentration of CTMAB solution.
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concentration of CTMAB is lower than its CMC. When the concentration is higher than the CMC, the resonance frequency decreases slowly because the ionizability of CTMAB is insignificant. According to the point of inflection on the curve, the CMC has been measured as 0.92 mM at 20◦ C. The experimental result agrees well with that in Ref. (28). The Adsorption of CTMAB onto Quartz Surface Sauerbrey (6) presented an equation relating the change of oscillating frequency (1 f ) to the mass loading (1m) of the crystal surface, ¢ ¡ 1 f = − f 02 1m/NAρ ,
[2]
where N is the frequency constant, ρ is the density of quartz, A is the piezoelectric active area, and f 0 is the fundamental oscillation frequency of the crystal. When the PQC operates in liquid, the resonance frequency may be influenced by a number of factors. These factors include the interfacial liquid properties (i.e., density, viscosity, conductivity, and dielectric constant) (9, 29) and the rigidity of the thin film deposited on the electrode (30). To reduce the influence of the test solution’s properties, NaCl solutions, with a concentration of NaCl higher than 0.04 M were used as background solution. The changes of the test solution’s viscosity, density, and permittivity could be reasonably ignored after adding CTMAB solution because the amount of CTMAB being added was very small. We supposed that a rigid film of CTMAB formed as the molecules were adsorbed onto the quartz surface, for the CTMAB molecule is very small and the film is only one or two molecular layers thick. According to the work of Sauerbrey (6), the adsorption density (adsorbed mass per unit area) of CTMAB on the quartz surface can be calculated by 0 = −1 f /2.26 × 10−6 f 02 ,
[3]
where 0 is the adsorption density in g cm−2 , 1 f is the frequency shift in Hz, and f 0 is the fundamental frequency of the quartz crystal in Hz. According to Fuerstenau (31), the quartz surface is charged negatively and can therefore adsorb cationic surfactants. Figure 9 shows the decrease of the REPS oscillation frequency after adding CTMAB solution. The decrease of the frequency is mainly due to the adsorption of CTMAB on the quartz crystal, as the influence of the liquid properties could be neglected for the conditions of the experiment. The adsorption process finished within 12 min. In such a slow adsorption process, hydrophobic forces between alkyls of CTMAB molecular may interact and lead to the formation of double molecular layers of CTMAB on the quartz surface. The rate of adsorption increases with increasing CTMAB concentration. The larger the concentration of CTMAB is, the larger is the adsorption amount.
FIG. 9. Frequency shifts of the REPS after adding CTMAB. Background solution: NaCl (0.1 M). CTMAB concentration (10−5 M): 2.0 (d); 4.0 (j); 8.0 (m).
The adsorption isotherm of CTMAB on the quartz surface is shown in Fig. 10. The adsorption density of CTMAB depends not only on the concentration of CTMAB but also on the ionic strength of the background solution. By increasing the CTMAB concentration, the adsorption density increases rapidly in low concentration solutions of CTMAB. When the concentration of CTMAB is higher than 3 × 10−5 M, the adsorption density increases slowly. These results can be explained by the electrostatic interaction between the quartz surface and the CTMAB positive ion. When the concentration of CTMAB is higher than 3 × 10−5 M, most of the surface charge on the quartz surface is neutralized by CTMAB positive ions, and the tendency of adsorption decreases with the weakening of electrostatic interaction. Moreover, when the quartz surface has been neutralized, the extra adsorption of CTMAB caused by hydrophobic adsorption gives the surface a positive charge, and the tendency of adsorption decreases with increasing electrostatic repulsion. The adsorption density of CTMAB onto the quartz surface increases as the NaCl concentration of the test solution is increased. The phenomena can be explained by the double layer of the quartz/CTMAB-solution interface being compressed with the increase of electrolyte concentration, which might increase the interaction between the quartz surface and the CTMAB molecule.
FIG. 10. The dependence of adsorption density on concentration of CTMAB. NaCl background solutions: 0.05 (m); 0.1 (d); 0.2 (j) M NaCl.
RINGED-ELECTRODE PIEZOELECTRIC SENSOR BEHAVIOR
CONCLUSION
The resonance behavior of the ringed-electrode piezoelectric sensor operating in liquid phase has been studied. The experiment indicates that the influence of liquid properties is the same as that for the normal PQC when the concentration of electrolyte solution is higher than 0.04 M (for NaCl). The resonance frequency shift is linearly related to the change of NaCl concentration in the case that the concentration of NaCl solution is between 0.01 mM and 0.012 M. In nonelectrolyte solutions, the resonance frequency decreases as the solution’s permittivity increases. The resonance frequency decreases with increases in density and viscosity of the liquid. The electrical equivalent circuit of the REPS near resonance frequency has been depicted and helps explain the phenomenon observed in the experiment. Under certain conditions, according to both the mass effect and to the non-mass effect, the REPS can be used both to study the adsorption of adsorbate onto quartz surface and the changes of a solution’s properties, such as conductivity, permittivity, density, and viscosity. Based on the non-mass effect, the CMC of CTMAB has been determined by the REPS. The result agrees well with that obtained by other methods. The mass loading of CTMAB on the quartz surface can be expressed by the Sauerbrey equation according to the frequency shift. The adsorption process of CTMAB on a quartz surface from solution was monitored in situ by the REPS. It is shown that adsorption density and adsorption rate of CTMAB are related to ionic strength. As an economical, convenient, and sensitive analytical tool, the REPS is expected to find wider applications for research on liquid properties and on interface phenomenon on quartz surfaces. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China and the Teaching and Research Award program for Outstanding Young Teachers in Higher Education Institution of MOE, P.R.C.
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Study of the Frequency Character of the Ringed-Electrode Piezoelectric Sensor in Liquid Phase and the Adsorption of CTMAB onto a Quartz Surface Fan Yin,∗, ‡ Youyu Zhang,† Yaohui Wu,† Yan Cai,† Qingji Xie,† and Shouzhuo Yao∗,1 ∗ Department of Chemistry, Hunan University, Changsha, 410082, P.R. China; †Chemical Research Institute, Hunan Normal University, Changsha, 410081, P.R. China; and ‡Department of Biology and Chemistry, Changshu College, Changshu, 215500, P.R. China Received January 12, 2001; accepted June 1, 2001; published online August 1, 2001
The resonance behavior and the electric equivalent circuit of the ringed-electrode piezoelectric sensor (REPS) operating in liquid phase have been studied. The results indicate that the REPS can be used to investigate a solution’s properties and mass loading on a quartz surface under different conditions. Using the change in the liquid’s conductivity, the critical micellar concentration (CMC) of cetyltrimetylammonium bromide (CTMAB) has been measured by the REPS. The result corresponds well with the reference data. Based on the mass effect, the adsorption process of CTMAB onto a quartz surface has been monitored in situ and the adsorption density has been investigated. The rate of CTMAB’s being adsorbed onto the quartz surface increases when CTMAB concentration increases. In addition, the adsorption density (adsorbed mass per unit area of CTMAB) is affected by the ion strength of solution. The higher the ion strength in the test solution, the greater is the adsorption density of CTMAB on the quartz surface. It has been shown that the REPS is a useful tool for studying quartz/solution interface interactions in real time. °C 2001 Academic Press Key Words: quartz surface adsorption; ringed-electrode piezoelectric sensor; frequency character; cetyltrimetylammonium bromide; critical micellar concentration.
INTRODUCTION
Piezoelectric devices have been widely used as liquid microsensors. Several papers have reviewed the applications of piezoelectric quartz crystals (PQCs) in the chemistry of solutions (1–5). In most instances, the PQC is used to measure the mass change at the electrode surface, using methods based on the work of Sauerbrey (6). Besides the mass effect, the frequency of the PQC is also affected by the properties of the liquid, such as density, viscosity, specific conductivity, and permittivity (7–11). The non-mass effect of PQCs has also been used as chemical and biochemical sensors (12–15). Sch¨on et al. used a PQC modified
1 To whom correspondence should be addressed. E-mail: [email protected]. edu.cn; fax: 861-731-8865515.
0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
with mesoporous titanium dioxide to measure the density of a liquid (12). He et al. studied the enzymatic hydrolysis of carboxymethyl cellulose with a PQC by measuring the changes in density and viscosity of the solution (13). Some applications of the non-mass effects of PQCs were reported from this laboratory (13, 16–18). Usually, a PQC is configured with two electrodes, one on each side of a thin disc of AT-cut quartz. A high-frequency alternating electric field is applied to the quartz crystal disc by the two electrodes. Recently, we have reported a new type of piezoelectric quartz crystal sensor named the ringedelectrode piezoelectric sensor (REPS) (19). In the REPS (shown in Fig. 1), the bare quartz surface is in direct contact with the liquid phase. The high-frequency alternating electric field is applied to the quartz crystal disc in two parts (Fig. 1c). The side part is applied to the quartz crystal disc directly by the ringed-electrode, while the center part is applied by the conductance of the liquid layer. This configuration allows the REPS to respond to the liquid properties. In addition, it is convenient for investigating the mass change on the quartz surface because the quartz surface contacts directly with the liquid. Using the REPS, we have studied the behavior of BSA adsorption onto the quartz surface (19). One characteristic of surfactants is their tendency to adsorb onto the interface in an orientated fashion. The adsorption of the surfactant is of interest as it is directly relevant to practical applications, such as detergency and flotation. The amount of surfactant adsorbed on an interface is an important parameter in the investigation of many interfacial phenomena. A carbon-14 technique (20) and a neutron reflection technique (21) have been employed for the determination of the adsorption density of surfactants on quartz surface. However, expensive instruments are needed. The critical micellar concentration (CMC) is also an important characteristic parameter of the surfactant. The physicochemistry properties of the surfactant solution, such as the decontamination capability, viscosity, density, specific conductivity, surface tension and osmotic pressure, can be changed when the concentration of the surfactant is equal to or higher
386
RINGED-ELECTRODE PIEZOELECTRIC SENSOR BEHAVIOR
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Apparatus
FIG. 1. Schematic illustration of the REPS. (a) Bottom view. (b) Top view. (c) Side view.
The experimental assembly employed is shown in Fig. 2. A 9-MHz AT-cut crystal with ringed-electrode on one face was used as a sensing element. An Iwatsu SC-7201 universal frequency counter (Japan) was employed to record the frequency. The REPS was fixed to a detection cell made of Teflon. The quartz surface outside the gold ring was covered with silicone resin, leaving bare quartz at the crystal center and the surrounding gold ring in contact with the sample solution. The crystal holder was directly connected to a homemade IC-TTL oscillating circuit. The circuit was supplied with a JWY-30B dc voltage regulator (Shijiazhuang Electronic Factory No. 4) and the working voltage was set at 5 V. Experiments were made at 20◦ C. A magnetic stirrer (Shanghai Electrocommunication Instrumentation Factory) was employed to stir the test solution. 5-l, 10-l and, 100-l microsyringes were used to add solution. Method of Measurement
than the CMC. Techniques being used for the study of the CMC include specific conductivity methods (22), osmotic pressure methods (23), dye adsorption methods (24), and dispersion methods (25). In this paper, the equivalent circuit of the REPS is depicted and discussed, and the resonance frequency of the REPS influenced by liquid properties is studied. Using the REPS, the adsorption of cetyltrimetylammonium bromide (CTMAB) onto the quartz surface is monitored in real time. The adsorption density of CTMAB on the quartz surface is measured by the REPS, and the CMC of CTMAB is also determined. The results show that the REPS can be used to study solution/quartz interfacial interactions and liquid properties. EXPERIMENTAL
Reagents Cetyltrimetylammonium bromide (CTMAB) was purchased from Guangzhou Chemical Reagents Factory (analyticalreagent grade). All chemicals were of analytical-reagent grade or better. Doubly distilled water was used throughout.
By changing the content of sodium chloride (NaCl) in pure water, NaCl solutions were used to examine the effect of solution conductivity on the resonance frequency of the REPS. Dioxane + water mixtures were chosen for determining the influence of the liquid permittivity on the resonance behavior of the REPS. The relative permittivity of dioxane + water mixture is very conveniently adjusted from 2 to 80 by changing the proportion of dioxane in liquid. Sucrose solutions were used to investigate the influence of density and viscosity of liquid exerted on the resonance frequency by changing the amount of sucrose in solution. With mild stirring, the adsorption process of CTMAB onto the quartz surface was monitored by adding CTMAB solution to a series of NaCl solutions with a microsyringe. The frequency shift of the REPS was recorded with a time interval of 30 s. Before the adsorption experiment, the REPS and the detection cell were pretreated with 0.001M CTMAB solution and rinsed thoroughly with distilled water to minimize extra adsorption of CTMAB onto the surface of the detection cell and the sensor holder. After being dried with clean air, the sensor’s quartz surface was treated with H2 SO4 + H2 O2 (3 : 1) for 10 min and then repeatedly rinsed with water. The bare quartz surface was made
Preparation of the REPS An AT-cut 9-MHz piezoelectric quartz crystal (12.5 mm in diameter) was used. One face of silver electrode (6.0 mm in diameter) of the PQC was dissolved in nitric acid and coated with a gold film (6.5 mm in diameter) by vacuum-evaporation. The central part of the gold electrode was dissolved in aqua regia with the bare quartz being exposed to the sample solution. The diameter of the bare quartz circle was 4.0 mm as shown in Fig. 1a. The other face of the silver electrode was sealed at the bottom of a glass tube to leave it in contact with air. All manufactured ringed-electrode piezoelectric sensors were stored in distilled water before use.
FIG. 2. Schematic diagram of the REPS system.
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fresh by dipping in a 10% solution of hydrofluoric acid for 30 s and rinsed thoroughly again with distilled water. A REPS was immersed in 20 ml of 0.1 M NaCl solution. With mild stirring, a stable oscillating frequency ( f 1 , frequency drift <1 Hz in 5 min) was obtained. A series of standard CTMAB solutions were injected into the detection cell. The corresponding frequency was recorded when a stable frequency ( f 2 ) was obtained. The shift of frequency for each solution was calculated as 1 f = f 2 − f 1 . The frequency shift, 1 f , was used for the calculation of the amount of adsorption according to the Sauerbrey equation (6). The principle of determining the CMC of CTMAB is based on measuring the change of the test solution’s conductivity using the REPS after adding CTMAB solution with a microsyringe. RESULTS AND DISCUSSION
Equivalent Circuits of the REPS For the REPS, the high-frequency alternating electric field being applied to the quartz crystal disc is divided into two parts (Fig. 1c). The side part is applied to the quartz crystal disc directly by ringed-electrode, whereas the center part is applied by the conductance of the liquid layer as mentioned above. In our experiment, the diameter of the electrode in the liquid, including the bare quartz circle, is larger than that in air. Thus, there is no fringing field on the liquid side between the two electrodes. The electrical equivalent circuit of the REPS near resonance frequency is depicted in Fig. 3. This equivalent circuit has been suggested by Shana et al. before (11, 26). The center capacitance is modeled by two independent capacitors, Cs and C2 , in series (Fig. 3). They correspond to the part of the center field that goes through the liquid on the liquid side of the REPS and that through the quartz, respectively (Fig. 1 and Fig. 3). Cs and C2 are therefore proportional respectively to the permittivity of the liquid and of the quartz. The resistive losses in the fluid are modeled by a resistor, Rs , which is inversely proportional to the liquid conductivity and connected in parallel with Cs . All three parameters are influenced of course by the exact geometry of the electrodes and the liquid proper-
FIG. 3. (a) Simple model to characterize the REPS in liquid. The resistance, Rs , is inversely proportional to the conductivity of the liquid. The capacitance, Cs , is proportional to the dielectric constant of the liquid. C2 is the capacitance formed between the Ag electrode and the liquid. C1 is the capacitance formed between the Ag electrode and the Au ringed-electrode. (b) Simple equivalent circuit of the REPS with ringed-electrode face contacting a liquid. Cm is the motional capacitance. Lm is the motional inductance. Rm is the motional resistance.
FIG. 4. The double logarithmic form of the relationship of the resonance frequency (in Hz) of the REPS to the concentration of NaCl solution (in M).
ties. As for the REPS with given geometry of the electrodes, the resonance frequency is affected only by the properties of the liquid. The Effect of the Liquid Properties on the Resonance Frequency The change in the resonance frequency as the conductivity of the liquid increased by the addition of NaCl solution is shown in Fig. 4. The frequency shift, 1 f , for pure water was set to zero. Over the whole concentration range of NaCl solution, 1 f decreases by as much as 8 kHz. When NaCl concentration is lower than 1 × 10−5 M, no frequency shift is observed. The frequency shift decreases linearly with NaCl concentration, which ranges from 1 × 10−5 M to 0.012 M. The relationship between 1 f and NaCl concentration can be expressed as log (−1 f ) = 1.2424 × log C + 6.5738 (r = 0.9993). The resonance frequency is not affected by the change of NaCl concentration when it is higher than 0.04 M. This can be explained as follows. (1) In low electrolyte concentration (lower than 1 × 10−5 M for NaCl), the field applied to bare quartz is mainly through Cs , for Rs is very large and can be thought of as an open circuit. Thus, the resonance frequency is unaffected by the conductivity of the liquid (2). In high electrolyte concentration (higher than 0.04 M for NaCl), the field applied to bare quartz is mainly through Rs , for Rs is very small and the circuit can be thought of as a short circuit (Fig. 5). The resonance
FIG. 5. Schematic illustration of the REPS seen from the side. The dotted lines are a qualitative representation of the electric field lines between two electrodes when the REPS is dipped in (a) a nonconducting liquid and (b) a perfectly conducting liquid.
RINGED-ELECTRODE PIEZOELECTRIC SENSOR BEHAVIOR
FIG. 6. The dependence of the resonance frequency of the REPS on the permittivity of Dioxane + water mixture.
frequency is thus unaffected by the conductivity in high ion strength liquids. Dioxane + water mixtures were chosen for the investigation of the influence of the solutions’ permittivity on the frequency behavior of the REPS. The permittivity of the mixture increases when the water content increases, whereas the density and viscosity of the mixture vary only slightly with its composition (27). The resonance frequency decreases when the relative permittivity of the solutions increase (shown in Fig. 6). The resonance frequency of the REPS in pure dioxane solution was set to reference value. In high ionic strength solutions, the resonance frequency is unaffected by the change of liquid’s permittivity because the circuit can be thought of as a short circuit (Fig. 5b), as discussed above. Sucrose solutions were used to investigate the influence of the density ρ and viscosity η of the solutions on the resonance frequency. According to Kanazawa et al. (8), the frequency shift of the PQC as a function of density and viscosity can be expressed as 1 f = − f s3/2 (ρη/πρq µq )1/2 ,
[1]
where 1 f is the resonant frequency shift of the crystal, f s is the resonant frequency, η is the viscosity of the liquid, ρ is the density of the liquid, µq is the shear modulus of the quartz crystal, and ρq is the density of the quartz crystal. According to Eq. [1], a REPS with higher f s is more sensitive to changes of viscosity and density than a PQC. Figure 7 shows the frequency shifts of a REPS and a PQC in sucrose solutions relative to pure water as a reference. The results of the experiment show that the resonant frequency of the REPS in high ionic strength solution decreases more rapidly than that of the PQC. In a nonelectrolyte solution, the slope of the REPS is the same as that for the PQC. This phenomenon can be explained by the slight decrease in the permittivity of the liquid caused by adding sucrose to the solution (27), which results in an increase of resonance frequency. The co-effect causes a minor shift of resonance frequency in pure water compared with that in higher ion strength solution. A linear relationship between (−1 f ) and (ρ × η)1/2 is found. The linear regression equations are as
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FIG. 7. The relationship between the resonance frequency of the REPS and the value of (ρη)1/2 in sucrose solutions. The REPS in pure water (m); the REPS in 0.1 M NaCl solution (d); the PQC in pure water (j).
follows: 1 f = 1776.77 − 1824.26 × (ρ × η)1/2 (r = 0.9996, REPS in pure water), 1 f = 1863.42 − 1874.68 × (ρ × η)1/2 (r = 0.9998, PQC in pure water), 1 f = 1918.33 − 2063.86 × (ρ × η)1/2 (r = 0.9988, REPS in NaCl solution). In conclusion, the REPS can act as a liquid permittivity sensor, a conductivity sensor in low ionic strength solutions and a normal PQC in perfectly conducting liquids. The CMC Measurement of CTMAB Generally, the ionizability of an ionic surfactant, such as CTMAB and sodium dodecylsulfonate (SDS), will decrease when the concentration of these surfactants is equal to or higher than their CMC. Based on its sensitivity to the conductivity of the liquid, the REPS was used to measure the CMC of CTMAB. Figure 8 shows that the resonance frequency decreases linearly with the increase of CTMAB concentration when the
FIG. 8. Dependence of frequency shifts of the REPS after changing the concentration of CTMAB solution.
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concentration of CTMAB is lower than its CMC. When the concentration is higher than the CMC, the resonance frequency decreases slowly because the ionizability of CTMAB is insignificant. According to the point of inflection on the curve, the CMC has been measured as 0.92 mM at 20◦ C. The experimental result agrees well with that in Ref. (28). The Adsorption of CTMAB onto Quartz Surface Sauerbrey (6) presented an equation relating the change of oscillating frequency (1 f ) to the mass loading (1m) of the crystal surface, ¢ ¡ 1 f = − f 02 1m/NAρ ,
[2]
where N is the frequency constant, ρ is the density of quartz, A is the piezoelectric active area, and f 0 is the fundamental oscillation frequency of the crystal. When the PQC operates in liquid, the resonance frequency may be influenced by a number of factors. These factors include the interfacial liquid properties (i.e., density, viscosity, conductivity, and dielectric constant) (9, 29) and the rigidity of the thin film deposited on the electrode (30). To reduce the influence of the test solution’s properties, NaCl solutions, with a concentration of NaCl higher than 0.04 M were used as background solution. The changes of the test solution’s viscosity, density, and permittivity could be reasonably ignored after adding CTMAB solution because the amount of CTMAB being added was very small. We supposed that a rigid film of CTMAB formed as the molecules were adsorbed onto the quartz surface, for the CTMAB molecule is very small and the film is only one or two molecular layers thick. According to the work of Sauerbrey (6), the adsorption density (adsorbed mass per unit area) of CTMAB on the quartz surface can be calculated by 0 = −1 f /2.26 × 10−6 f 02 ,
[3]
where 0 is the adsorption density in g cm−2 , 1 f is the frequency shift in Hz, and f 0 is the fundamental frequency of the quartz crystal in Hz. According to Fuerstenau (31), the quartz surface is charged negatively and can therefore adsorb cationic surfactants. Figure 9 shows the decrease of the REPS oscillation frequency after adding CTMAB solution. The decrease of the frequency is mainly due to the adsorption of CTMAB on the quartz crystal, as the influence of the liquid properties could be neglected for the conditions of the experiment. The adsorption process finished within 12 min. In such a slow adsorption process, hydrophobic forces between alkyls of CTMAB molecular may interact and lead to the formation of double molecular layers of CTMAB on the quartz surface. The rate of adsorption increases with increasing CTMAB concentration. The larger the concentration of CTMAB is, the larger is the adsorption amount.
FIG. 9. Frequency shifts of the REPS after adding CTMAB. Background solution: NaCl (0.1 M). CTMAB concentration (10−5 M): 2.0 (d); 4.0 (j); 8.0 (m).
The adsorption isotherm of CTMAB on the quartz surface is shown in Fig. 10. The adsorption density of CTMAB depends not only on the concentration of CTMAB but also on the ionic strength of the background solution. By increasing the CTMAB concentration, the adsorption density increases rapidly in low concentration solutions of CTMAB. When the concentration of CTMAB is higher than 3 × 10−5 M, the adsorption density increases slowly. These results can be explained by the electrostatic interaction between the quartz surface and the CTMAB positive ion. When the concentration of CTMAB is higher than 3 × 10−5 M, most of the surface charge on the quartz surface is neutralized by CTMAB positive ions, and the tendency of adsorption decreases with the weakening of electrostatic interaction. Moreover, when the quartz surface has been neutralized, the extra adsorption of CTMAB caused by hydrophobic adsorption gives the surface a positive charge, and the tendency of adsorption decreases with increasing electrostatic repulsion. The adsorption density of CTMAB onto the quartz surface increases as the NaCl concentration of the test solution is increased. The phenomena can be explained by the double layer of the quartz/CTMAB-solution interface being compressed with the increase of electrolyte concentration, which might increase the interaction between the quartz surface and the CTMAB molecule.
FIG. 10. The dependence of adsorption density on concentration of CTMAB. NaCl background solutions: 0.05 (m); 0.1 (d); 0.2 (j) M NaCl.
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CONCLUSION
The resonance behavior of the ringed-electrode piezoelectric sensor operating in liquid phase has been studied. The experiment indicates that the influence of liquid properties is the same as that for the normal PQC when the concentration of electrolyte solution is higher than 0.04 M (for NaCl). The resonance frequency shift is linearly related to the change of NaCl concentration in the case that the concentration of NaCl solution is between 0.01 mM and 0.012 M. In nonelectrolyte solutions, the resonance frequency decreases as the solution’s permittivity increases. The resonance frequency decreases with increases in density and viscosity of the liquid. The electrical equivalent circuit of the REPS near resonance frequency has been depicted and helps explain the phenomenon observed in the experiment. Under certain conditions, according to both the mass effect and to the non-mass effect, the REPS can be used both to study the adsorption of adsorbate onto quartz surface and the changes of a solution’s properties, such as conductivity, permittivity, density, and viscosity. Based on the non-mass effect, the CMC of CTMAB has been determined by the REPS. The result agrees well with that obtained by other methods. The mass loading of CTMAB on the quartz surface can be expressed by the Sauerbrey equation according to the frequency shift. The adsorption process of CTMAB on a quartz surface from solution was monitored in situ by the REPS. It is shown that adsorption density and adsorption rate of CTMAB are related to ionic strength. As an economical, convenient, and sensitive analytical tool, the REPS is expected to find wider applications for research on liquid properties and on interface phenomenon on quartz surfaces. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China and the Teaching and Research Award program for Outstanding Young Teachers in Higher Education Institution of MOE, P.R.C.
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