Anal. Chem. 1992, 64, 2502-2507
2502
Quartz Crystal Detector for Microrheological Study and Its Application to Phase Transition Phenomena of Langmuir-Blodgett Films Hiroshi Muramatsu' and Kazuhiko Kimura Technology Center, Seiko Instruments Inc., 563 Takatsuka-shinden, Matsudo-shi, Chiba 271,Japan
Thls paper shows that an AT-cut quartz crystal detector can be appiled to the rheological study of thin fllms. Measurement of the resonant frequency and the resonant reslstance of the quartz crystal was used to examine the mass change and the vlscoeiastlc change of the coated films. Thls method was utllized to characterize the phasetransition phenomena of the Langmuk-Biadgetl (LB) fRms of 1 , 8 d l o c t a d e c y I - r ~ c e r 2-01 (2CI80H) which has two phase-transition points. The resonant frequency and the resonant reslstance changes suggested that vlscoslty and mass increased at the first phasetransition polnl and eiastldty decreased and vlscoslty increased at the second phase-transitlon point. The transverse wave amplitude of the fluid film surface was estimated as a ratio against the shear vibrating amplitude of the quartz crystal surface by immersing the filmtoated quartz in water and 50 wt % sucrose solution to clarify the effect of these contact liquids. The ratio was determined to be 88% for the 50-layer 2CI80H LB fllms above the second phase-transition temperature.
INTRODUCTION
been published, and we cite several reviews which will provide a basic understanding of the above.6J5J8 Phase-transition phenomena of liquid crystal^,^^^^^ lipid multibilayer films,21 and Langmuir-Blodgett (LB) films22-24 have been studied using the quartz crystal and the surface acoustic wave device. In one of the LB films studies, the film thickness effect to the resonant frequency change was studied.22 It showed that the resonant frequency did not change with the LB films of 10 layers or less. However in the above studies, only the resonant frequency change was measured, but it is not enough to examine the detail of the viscoelastic phenomena. The importance of studying the viscoelastic phenomena on the coating films has been recommended, especially in the field of the electrochemical analysis to clarify the causes of the resonant frequency change in viscoelastic films.16 This can be done by measuring the resonant resistance (or the admittance) of the quartz crystal with lipid films when odorant adsorption take place25.26 and of the quartz crystal with electrochemically deposited films when swelled by water.l6827 In this work, viscoelastic changes on the phase transition of the 2ClsOH LB films were studied with the quartz crystal by the in situ measurement of the resonant frequency and the resonant resistance. The resonant frequency and the resonant resistance changeswere diagramed and used to study these viscoelastic phenomena.
The quartz crystal has been known as a sensitive mass detecting device1 and a liquid viscosity monitoring device.2-4 The equations for the resonant frequency change and the PRINCIPLE resonant resistance for the quartz crystal with a coating film and the quartz crystal in contact with liquid have been derived The equation of the resonant frequency change and the from theoretical models as shown in the next ~ e c t i o nA. ~ ~ ~ elastic ~ ~ film mass change on the AT-cut quartz crystal has large number of analytical applications in the area of gas been derived by Sauerbreyl as follows: sensing)t6 trace ion determination,7immunoassay,@gelation monitoring,'OJl and electrochemical examinationslz-17 have A F = -Amp"/jtpQ)1/2A (1) where A F is the resonant frequency shift, F is the resonant (1)Sauerbrey, G. 2.Phyzik 1959,155,206. frequency, Am is the surface mass change, jt is the shear (2)Kanazawa, K. K.; Gordon, J. G., 11. Anal. Chim.Acta 1985,175,99. modulus of the quartz crystal, pg is the density of the quartz (3)Haeer. H. E.Chem. Ene. Commun. 1986.43.25. (4)M&&atau, H.; TamiyGE.; Karube, I. A n d . Chem. 1988,60,2142. crystal, and A is the surface area of the quartz crystal. (5)King, W. H.,Jr. Anal. Chem. 1964,36,1735. The shear vibration of the quartz crystal in contact with (6)Hlavay, J.; Guilbault, G. G. Anal. Chem. 1977,49,1890. (7) Nomura, T.; Iijima, M. Anal. Chim.Acta 1981,131,97. liquid has been studied, and the equation for the resonant (8) Thompson, M.; Arthur, C. L.; Dhaliwal, G. K. Anal. Chem. 1986,
(9)Muramatau, H.; Dicks, J. M.; Tamiya, E.; Karube, I. Anal. Chem. 1987,59,2760. (10)Muramatau, H.; Suzuki, M.; Tamiya, E.; Karube, I. Anal. Chim. Acta 1988,215,91. (11)Muramatau, H.; Suzuki, M.; Tamiya, E.; Karube, I. Anal. Chim. Acta 1989,217,321. (12)Bruckenstein, S.;Swathirajan, S. Electroanal. Chem. 1985,30, 851. (13)Schumacher, R.;Borges, G.; Kanazawa, K. K. Surf. Sci. 1985,163, L621. (14)Bruckenstein, S.;Shay, M. J. Electroanal. Chem. Interfacial Electrochem. 1985,188,131. (15)Denkin, M. R.;Buttry, D. A. Anal. Chem. 1989,61,1147A. (16)Borjas, R.; Buttry, D. A. J. Electroanal. Chem. Interfacial Electrochem. 1998,280,73. (17)Bruckenstein, S.; Wilde, C. P.; Shay, M.; Hillman, A. R.; Loveday, D. C.J. Electroanal. Chem. Interfacial Electrochem. 1989,258, 457.
(18)Alder, J. F.;McCallum, J. J. Analyst 1983,108,1169. (19)Wohltjen, H.; Dessy, R. Anal. Chem. 1987,51, 1470. (20)Muramatau,H.;Suda,M.;Ataka,T.;Seki,A.;Tamiya,E.;Karube, I. Sens. Actuators 1990,A21-A23, 362. (21)Okahata, Y.;Ebato, H. Anal. Chem. 1989,61,2185. (22)Okahata, Y.;Kimura, K.; Ariga, K. J.Am. Chem. SOC.1989,111, 9190. (23)McCaffrey, R.R.;Bruckenstein, S.;Prasad, P. N.Langmuir 1986, 2,228. (24)Okahata, Y.;Ariga, K. J.Chem. Soc., Chem. Commun. 1987,132, 243. (25)Ye,X.;Muramatsu, H.; Kimura, K.; Sakuhara, T.; Ataka, T. J. Electroanal. Chem. Interfacial Electrochem. 1991,314,279. (26)Muramatsu, H.; Tamiya, E.; Karube, I. Anal. Chim.Acta 1991, 251, 135. (27)Muramatsu, H.; Ye, X.; Suda, M.; Sakuhara, T.; Ataka, T. J. Electroanal. Chem. Interfacial Electrochem. 1992,322,311.
0003-2700/92/0364-2502$03.00/0 0 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992
2503
P
I
7-
Flguro 1. Electrical equivalent circuit of the quartz crystal.
frequency change in contact with liquid has been derived by Kanazawa et al.2 as follows:
where q is the viscosity of the liquid and PL is the density of the liquid. The resonant resistance of the quartz crystal is the resistance included in the electrical equivalent circuit of the quartz crystal (Figure 1). For the quartz crystal in contact with liquid, the resonant resistance has been derived as follows:4 R, = (2~Fp,q)'/~A/k~ (3) where k is the electro-mechano coupling factor. The resonant resistance (RI) reflects the mechanical resistance of the quartz crystal.4 Also the capacitance (C1) reflects the reflects the elasticity, and the inductance (LI) total mass of the quartz crystal and contact materials on it. Using the equivalent circuit, the resonant frequency of the quartz crystal is expressed as F = 1/(2~(L1C1)~/~). This equation means that the resonant frequency change includes the information of the mass change and the elasticity change on the quartz crystal. The resonant frequency is used for studying these changes because it can be measured with high sensitivity. The typical models on the shear vibrating quartz crystals, (a) coating with elastic films, (b) in contact with liquid, and (c) coating with viscoelastic films, are expressed in Figure 2, parta a, b, and c, respectively. Figure 2a is the model of eq 1. For this elastic film-coated quartz crystal, the resonant frequency changes with the mass change, and the resonant resistance has no change, since there is no energy loss in the films. The relation between the resonant frequency change and the resonant resistance can be expressed quantitatively in the resonant frequencyresonant resistance diagram (F-R diagram) as shown in Figure 3a. Figure 2b shows the model for in contact with liquid, where the resonant frequency change reflects the mass effect of the liquid which moves with the shear vibrating quartz plate. The image of Figure 2b is drawn according to the result of the transverse wave decrement profile in the liq~id.~l4928 Also this image has been shown by Glassford's liquid deposition study on the quartz crystal surface.2gThis liquid mass effect is defined as Am = alp^ where 1 is the viscous penetration depth expressed as 1 = (q/?rpf11/2.28Equation 2 is obtained by substituting this Am equation into eq 1. This means that the surface mass effect causes the resonant frequency change independent of the contact material types. As the factor (p~q)1/2appears in both eqs 2 and 3, the resonant frequency change and the resonant resistance have a linear relation as shown in Figure 3b. Figure 2c shows the quantitative explanation of the vibration model for the viscoelastic film-coatedquartz crystal. (28) Landau,L. D.;Lifshtz,E. M. Fluid Mechunics;Pergamon: Oxford, England, 1959, p 88. (29) Glassford, A. P. M. J. Vac. Sci. Technol. 1978,15,1839.
E
Figure 2. Schematic drawing on shear vibration of quartz crystal (a) with elastic film, (b) in contact with liquid, (c) with viscoelastlc film, (d) with viscoelastic film illustrating the viscosity or elastlcity increasing model, (e)with viscoelastic film iilustrattngthe film-thickness dependence to the Vibration amplitude of film surface, (f) with vlscoelastlc film and in contact with liquid, (9) with viscoelastic film and in contact with high viscosity ilquid, and (h) with thick viscoelastic film in contact with liquid.
rl+m
(a) Elastic filmdeposition
+
(b) Liquid with
viscosity change
Viacoaity increase
a
0 C m c
.-
increaae
v) v)
2 c
c
m
C
$ Q,
K
0
-
0
0
-
+m
(c) Viscoelastic film deposition
0
film
Resonant frequency change Flgurr 3. Quantltathrepatternson the resonantfrequencyand resonant resistance for (a) elastic film deposition, (b) viscosity changing of contactlng water, (c) viscoelastic film deposttion (dashed arrow shows the viscosity change of formed film), and (d) vlscosity change and B C 2 shows elasticity change in the viscoelastic fllm. A the viscosity increasing in the fluid film, A F 0 I shows the elasticity increasing in the viscous film, E I 2 shows the viscosity Increasing in the elastic film.
------
This image is made as the middle image between Figure 2a and Figure 2b. Also this image was suggested by Crane's theoretical study on the viscoelastic film-coated quartz crystal.30 In this image, the film thickness increase makes the resonant frequency decrease (e.g. the mass increase) and the resonant resistance increase (e.g. the viscosity increase in the films). This image can be expressed in the F-R diagram (30) Crane, R. A,; Fisher, G. J. Phys. D: Appl. Phys. 1979,12,2019.
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ANALYTICAL CHEMISTRY, VOL. 84, NO. 21, NOVEMBER 1, 1992
as shown in Figure 3c. The relation in this F-R diagram is supported by the experimental results of the electrochemical deposition of polypyrrole where the ratio between the resonant frequency and the resonant resistance changes varied with the viscoelastic change of the films; it especially changed by swelling.27 Figure 3c gives additional information on the viscosity change of the coated viscoelastic films. In Figure 3c, the resonant resistance will vary with the viscosity change of the films, but in the case of the elasticity being dominant over the viscosity, the resonant frequency will show little or no change as shown by the dashed arrow in this figure. Figure 2d, which is a variety of Figure 2c, shows the model which has the viscosity or elasticity change in the films. By using this image and summarizing Figures 3a-c, Figure 3d can be drawn. In Figure 3d, the change of A B C shows the viscosity increase in liquid films. In this, the liquid films have no elasticity, and the viscous penetration depth in the films increases with the viscosity increase. This decay layer increase in the films causes the vibrating mass increase on the quartz crystal surface, and this makes the resonant frequency decrease; the viscosity increase also makes the resonant resistance increase. The slope for A- B C in this diagram is equal to that for Figure 3b. When the films have high viscosity, the transverse viscosity wave does not vanish within the thickness. This makes a limit for the resonant frequency and resonant resistance changes, and the point Z in Figure 3d is given as a limit point. And the resonant frequency change of the point Z corresponds to the resonant frequency change for the perfect elastic films (point E) because the resonant frequency change reflects the total vibrating mass on the quartz crystal surface. When the film elasticity increases in fluid films, the amplitude of the transverse viscosity wave will increase as shown in Figure 2d. And this makes the resonant frequency decrease shown as A F G I in Figure 3d accordingly become elastic dominant. In this case, the limit point I is given by the film thickness limitation. The change E I Z in Figure 3d shows the viscosity increase in the elastic films. This change includes the viscosity change of the viscoelastic films as shown by the dashed line in Figure 3c. The changes in Figure 3d suggest that the resonant resistance reflects the viscosity change and that the resonant frequency reflects the elasticity change in a defined small viscosity and elasticity region. And in this region, the film thickness is an important parameter in deciding the limit points of the resonant frequency and resonant resistance changes. Also, Figure 2e explains the image of this thickness dependence on the transverse viscosity wave. When the viscoelastic film-coated quartz crystal is in contact with liquid, the vibration model is shown in Figure 2f by combining parts b and c of Figure 2. Also Figure 2g shows a model with a higher-viscosity liquid. In these models, the resonant frequency change includes the total mass effect of the films and the liquids, and the resonant resistance includes the total energy loss in the films and the contact liquids. By using these models in Figure 3, an explanation of the rheological change on the coating films can be possible by diagraming the resonant frequency and the resonant resistance change as shown above. And in case two different phenomena occurred simultaneously, the changes in F-R diagram are expressed by a sum of F-R vectors.
CH3 - (CH2)17 - 0 - ~ H Z CH - O H I
CH3 - (CH2)17 - 0 - CHZ
Flgure 4. Molecular structureof 1,3dioctadecyl-ra~glycer-2-ol(2ClsOH).
\
THERMAL SENSOR
I
1
STIRRING BAR
--
-
----
EXPERIMENTAL SECTION Materials and Method. A 9-MHz, AT-cut quartz crystal with the dimension of 8 X 8 X 0.18 mm, which has a 3 Hzideg
~
1'1
/
ANALYZER^
'
VOLTMETER
THERMOSTATIC MODULE
Flgure 5. Schematic diagram of the measuring system. Celsiustemperature dependence at room temperature, was used. 1,3-Dioctadecyl-ruc-glycer-2-01(2ClgOH) was prepared using stearyl alcohol, 1,3-dichloro-2-propanol,and KOH. At first, stearyl alcohol was kept above the melting point, and KOH was added t o form the potassium alkoxide of stearyl alcohol. Then, 1,3-dichloro-2-propaolwas added to form 2ClgOH. 2ClsOHwas purified by the subsequent operation of (1) extraction using n-hexane and HCl solution, (2) dehydration using Na2S04, (3) evaporation, and (4)recrystallization in ethanol. The purity of 2ClgOH was confirmed using the thin-layer chromatography and the FT-IR spectrum. The molecular structure of 2ClsOH is shown in Figure 4. Preparation of LB Films on a Quartz Crystal. 2C180H was dissolved in benzene (2.65 mg mL-l), and the solution was spread on deionizedJdouble-distilledwater in the apparatus of LB films formation (Lauda, Model FW-1). The surface pressure of the spread monolayer was controlled to 40 mN m-l. The LB films were accumulated on the quartz crystal by moving in both directions to form Y-type films. The accumulation rate was 100 mm min-l. Apparatus. The schematic diagram of the measuring system is shown in Figure 5. An impedanceanalyzer (YokogawaHewlettPackard, Model 4192A) was used for monitoring the resonant frequency and the resonant resistance. An incubator (Advantec, Model PE-314) was used for a temperature sweep of 0.8 "C min-l. A P t thin layer sensor and avoltmeter (Advantest,ModelTR6840) was used for controlling the system and storing the measured data. Determination of Resonant Frequency and Resonant Resistance. As described in the previous paper: the resonant frequency and the resonant resistance were determined by the admittance measurement of the quartz crystal. The admittance of the quartz crystal is shown as follows: (4)
Equation 4 is converted by using the conductance ( G ) and the
ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992
2505
. t G
- 5000
53
EE -10000 - 2 -
- 2 -
8.72
8.74
8.76
8.78
FREQUENCY (MHz)
8.0
0
1
2
3
4
CONDUCTANCE (mS)
Figure 6. Admittance diagram on conductance (0) and susceptance (B) for the quartz crystal in contact with liquid.
susceptance ( B ) , which are real and imaginary parts of the admittance, as follows: The resonant resistance is obtained as the reciprocal expression of the maximum conductance value as shown in Figure 6. And the resonant frequency correspondsto the frequencyat this point. The measuring frequency range was set to step over the resonant frequency and to cover at least half of the admittance circle of the quartz crystal. About 100 measurements of admittance (GandB) were performed by the stepped frequencies in this range. The radius of the circle on the admittance diagram (G-B plot) was determined by adopting these data in eq 5. The least square method was used for this calculation. The resonant frequency was determined from the maximum conductance point on the admittance diagram. About 10 points around this maximum point were adopted to a polynomial equation, and the frequency value of this maximum point was determined from the equation. In this measuring system, the frequency range was set up automatically by using a resonant frequency value previous to the center frequency of the next measuring frequency range. The average measurement interval was 45 s at the continuous measurement. The reproducibility of this resonant frequency measurement is within fl Hz for the quartz crystal in air and f l O O Hz for the quartz crystal immersed in water at 30 "C. Phase-Transition Monitoring. The quartz crystal, coated with the 2ClsOHLB films in both sides,was dipped in the distilled water or 50 w t % sucrose solution. The resonant frequency and the resonant resistance were monitored during the temperature increasing and decreasing. Phase-transition phenomena were also determined by the differential scanning calorimeter (Seiko Instruments, Model DSC-200).
59
8W
Rheological Study on the Phase Transition of 2ClsOH LB Films. The resonant frequency and the resonant resistance were monitored for the quartz crystals coated with lo-, 30-, and 50-layers 2C180H LB films during the temperature increasing and decreasing in water (Figure 7). In Figure 7a, the resonant frequency change was obtained by subtracting the resonant frequency value of the naked quartz crystal in air at 30 "C from the measured resonant frequency value. In Figure 7, the resonant frequency and the resonant resistance changes show thickness dependence where the 50-layer films are thick enough to detect the phase-transition phenomena but the 10-layer films are not. In this section, the results from the 50-layer films were used to clarify the phasetransition phenomena of the 2C180H LB films. In Figure 7, the resonant frequency and the resonant resistance changes in the temperature increase process can be separated into five parts. In part a-b, there are a little frequency increase and a resistance decrease. This temperature region is below the phase-transition points. In part b-c, the resonant frequency decreased and the resonant resistance increased. And the temperature of the point c corresponded to the first phase-transition temperature mea-
20
30
50
40
60
TEMPERATURE I
2
1000
5 w
400
U
70
80
C
*
(b)
20
30
40
50
60
70
80
TEMPERATURE I C Figure 7. (a) Resonant frequency change on temperature lncreaslng and decreasing for the coated quam crystals with 2ClsOH LB films of (A) 10, (B) 30, and (C) 50 layers. (b) Resonant reslstance change on temperatue increasing and decreasingforthecoatedquartz crystals with 2C180HLB films of (A) 10, (B) 30, and (C) 50 layers. Dlrectbn of temperature scanning is shown by arrows.
2t $
n
RESULTS AND DISCUSSION
-15000
K
0
AII
-
J[
'
II Cooling
1
--I/-
Heating
-31 ' ' 20 30
I
I
40
'
' 50
'
I
60
'
I
70
'
'-
0.4
80
TEMPERATURE I * C Figure 8. Chart of the differentialscanning calorimetryof 2CleOH with water on temperature increasing and decreasing. The upper arrow shows the first phase-transition point on the temperaturaincreaslng process.
sured by the differential scanning calorimetry (Figure 8).In part c-d,the resonant frequency increased and the resonant resistance decreased. Thistemperature range (part c-d) exists between the first and the second phase-transition points. In part d-e, the steepest resonant frequency and resonant resistance increases were observed. This temperature range corresponded to the second phase-transition point obtained by the differential scanning calorimetry. In part e-f, the resonant frequency increased, and the resonant resistance decreased again similarly to parts a-b and c-d. Figure 9a is the F-R diagram obtained by replotting the results of Figure 7a,b. In Figure 9a, the changes of part a-b
2506
ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992 1000
900 800
700
U
600
-12000
-10000
-8000
-6000
-4000
RESONANT FREQUENCY SHIFT / Hz (b)
RESONANT FREQUENCY SHIFT
(a)Diagramof the resonant frequency and resonant resistance changes on temperature lncreaslng and decreasing for 50-layer 2C18OH LB films coated quartz crystal. The direction of temperature scan Is shown by arrows. (b) Illustration of separated elements for parts b-c, c-d, and d-e. Flgwe0.
corresponds to the pattern of Figure 3b which is the F-R diagram for the liquid viscosity change on the quartz crystal. The slope of part a-b (25 Hz/deg Celsius) corresponded to the slope obtained from the naked quartz crystal with contact water viscosity change around 30 "C. This indicates that the changes of part a-b was caused by the viscosity change of water. In part b-c, the changes in the F-R diagram can be separated to two constituents,the resonant frequency decrease and the resonant resistance increase, as shown in Figure 9b. This resonant frequency decrease should be caused by the mass increase because the LB films have a sufficient elasticity to keep the transverse wave in the films below the phasetransition temperature, and the elasticity increase in the elastic films cannot decrease the resonant frequency. This mass increase can be caused by water swelling. The resonant resistance increase in this part indicates the viscosity increase in the films, and this viscosity increase can be explained by molecular arrangement change in the films, which can induced by the water swelling or the phase-transition phenomenon. In part c-d, the changes consist of the resonant frequency increase and the resonant resistance decrease as shown in Figure 9b. A decrease of the film elasticity can induce this resonant frequency increase. This elasticity decrease was probably the result of hydrophobic bond release, and the resonant resistance decrease indicates the film viscosity decrease. This viscosity decrease may be caused by the hydrophobic bond release or temperature-dependent viscosity change. In part d-e, the changes may be separated to the resonant frequency decrease and the resonant resistance increase as shown in Figure 9b. This resonant frequency decrease can be caused by the elasticity decrease because the hydrophilic
bonds of the films probably release in this second phasetransition temperature region, and this resonant resistance increase corresponds to the film viscosity increase. These results indicate that the hydrophilic bond release gives the film elasticity decrease and viscosity increase. In part e-f in Figure 7a, the resonant frequency change values, calculated by subtracting the resonant frequency value of the naked quartz crystal from that for the coated quartz crystals, for 30- and 50-layers LB films coated quartz crystal are almost equal. This result means that the total mass effects of the films and the contact water were almost the same in these two films and also that the transverse viscosity wave in the films remarkably decreased in the film thickness, i.e. the films became a liquid phase. Therefore, the resonant frequency and the resonant resistance changes in part e-f are explained as the temperature-dependent viscosity decrease of this liquid films. These rheological changes in Figure 9a would be summarized as follows. The first phase transition occurred around point c, and the viscosity increase and a small mass increase also occurred at this point. This viscosity increase was explained by loosening of the hydrophobic bonds in the 2CuOH LB films, and the water molecules were allowed to penetrate. At the second phase-transition point (part d-e), the elasticity decreased and the viscosity increased. These results indicate that the hydrophilic bonds in the 2C180H LB films disconnected. In the temperature decreaseprocess, the resonant frequency and the resonant resistance changes were simpler than the changes in the temperature increase process. These changes in the temperature decrease process can be separated into three parts. Part f-g showed the opposite change of part e-f but had the same tendency. The temperature decrease might induce this viscosity increase. In part g-h, the resonant resistance decreased and the resonant frequency decreased. The resonant frequency and resonant resistance values of point h have the same value of point b. This indicates that the phase-transition phenomena progressed only by one step in the temperature decrease process, and this change probably includes the two phasetransition steps which occurred in the temperature increase process. The correspondence of parts a-b and h-i indicates that the films were not removed from the quartz crystal surface. This result is different from the result in air where the films melt and run down from the crystal surface above the second phase-transition point. Estimation of the Transverse Viscosity Wave Decrease in the Films. The film-thickness dependence of the resonant frequency change at the second phase transition points has been show in our results (Figure 7) and in a previous report.22 This film thickness dependence was explained by the transverse wave decrease in the fluid films above. This wave-decrease amplitude can be estimated by subtracting the mass effect of the contact liquid from the total resonant frequency change because the resonant frequency change contains the mass effect of the films and the contact liquids on the quartz crystal surface. This mass effect of the liquids can be determined by varying the liquid viscosity. This principle is explained by parts f and g of Figure 2 which show the model of the viscoelastic film-coated quartz crystal in contact with liquids with different viscosities. In Figure 2f,g, the quartz crystals have the same surface vibration amplitudes, but the mass effects by the contact liquids are different with the liquid viscosities. Figure 10a shows the resonant frequency response of the naked quartz crystal in air, in water, and in 50 wt % sucrose solution in the range of 30-70 "C. Figure 10b also shows the resonant frequency response of the coated quartz crystal with
ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992
. t
\A
i
4 -loooo
v)
:
31
In air
D'
amplitudes of the films have the same value for both in contact with water and 50 wt % sucrose solution, the equation for the 50 wt % sucrose solution can be presented as follows:
In water
E' F\
AF(J-L)/ M(G-I)= aAF(D-F)/AF(A-C)
aT
1
t -30000
K
20
M(J-K) = M(D-K) 30
40
50
60
TEMPERATURE I
P .
70 '
C
0
CY
4 -loooo -20000
5a
$w -30000 20
30
40
50
60
- M(D-J) - M(D-J)
(8) (9)
By substituting eq 8 for eq 6 to eliminate A(J-K) and eq 9 for eq 7 to eliminate A(J-L), and eliminating A(D-J) from these equations, the next equation is obtained:
v)
K
M(J-L) = hF(D-L)
80
tz
31
(7)
As the resonant frequency change value of point J could not be measured, this point should be eliminated using the next equations:
solution
-20000
5
2607
70
80
TEMPERATURE I C Flgura 10. (a) Resonant frequency change on temperature Increasing for the naked quartz crystals In alr, In water, and in 50 wt % sucrose solution. (b) Resonant frequency on temperature lncreaslng for 50layer 2Cl8OH LB flims coated quartz crystals In air, in water, and in 50 wt % sucrose solution.
the 50-layer films in water and in 50 wt % sucrose solution in the range of 30-70 "C. These results are plotted by subtracting the resonant frequency value of the naked quartz . crystal at 30 "C in air. In Figure loa, the resonant frequency changes of the naked quartz crystal by the mass effect of water are AF(A-B) a t 30 "C and AF(D-E) at 70 "C. Also the resonant frequency changes in 50 wt % sucrose are AF(A-C) at 30 "C and AF(D-F) at 70 "C. These changes correlate to ( p ~ ) l / zof the liquids. In Figure lob, point G showsthe resonant frequency change for the 50-layer films in air at 30 "C. By using point G, the resonant frequency change of the coated quartz crystal in water a t this temperature (AF(A-H)) can be separated to the film mass effect (AF(A-G)) and the mass effect of water (AF(GH)). The effect of 50 wt 5% sucrose solution in the resonant frequency change can also be defined as AF(G1). The resonant frequency change of the 50-layer film-coated quartz crystal in air at 70 "C could not be measured because the melt film run down from the quartz crystal surface in air a t this temperature. So that nonmeasured point (point J) was supposed for this resonant frequency change, and then AF(J-K) and AF(J-L) can be defined as the mass effect of water and 50 wt % sucrose solution, respectively. These mass effects of liquids depend on the surface shear vibration amplitude of the coating films. As the transverse viscosity wave decreases in the fluid films, A(J-K)/A(GH) is smaller than A(D-E)/A(A-B). When a vibrating amplitude ratio a (0ICY I1)is supposed for the film surface vibration amplitude against the quartz crystal surface vibration amplitude, the next equation can be presented for the quartz crystal in contact with water: AF(J-K)/AF(GH) = aAF(D-E)/AF(A-B) (6) When making an assumption that the surface vibration
= (AF(D-K) - AF(D-L))/(AF(D-E)AF(GH)/AF(A-B) AF(D-F) AF(GI)/AFCA-C)] (10)
By using the resonant frequency change values in Figure 10, CY was determined to 0.88. This value indicates that the surface vibrating amplitude of the 50-layer 2ClsOH films is 88% against the surface vibrating amplitude of the quartz crystal. In this, a great deal hinges on the value of AF(D-J), so numbers of experiments on different sucrose concentrations will give a better statistical value of a.
CONCLUSION In this work, the quartz crystal was applied to study the rheological phenomena of the films on the quartz crystal surface. The resonant frequency and the resonant resistance of the quartz crystal coated with the 2ClsOH LB films were diagramed and used to study the rheological phenomena. At the first phase-transition point of the films, the resonant frequency decreased and the resonant resistance increased. This indicates that the film had a small mass increase and viscosity increase. This viscosity change was explained by the disappearance of the hydrophobic bonds in the films. At the second phase-transition point, the resonant frequency increased and the resonant resistance increased. This indicates that the films had an elasticity decrease and a viscosity increase. These changeswere explained by the disappearance of the hydrophilic bond in the films. To study the shear vibration decrease amplitude in the films, a ratio (CY)was given to estimate this amplitude as follow: a = (film surface vibration amplitude)/
(quartz crystal surface vibration amplitude) As the transverse vibration amplitude of the coated film surface defines the mass effect of the contact liquids, this value a was estimated by immersing the coated quartz crystal in the different viscosity liquids. By using the water and 50 wt % sucrose solution, a was estimated to 0.88 for the 50layer 2ClsOH LB films a t 70 "C (above the second phasetransition point). The results of this paper showed the quartz crystal can be used for the rheological study of the coating films. This analytical technique can be widely applied to characterize various coating films.
RECEIVED for review December 2, 1991. Accepted July 27, 1992. Registry No. 2C180H, 18794-74-6.
