Langmuir 1994,10, 4394-4396
4394
Automatic Surface Tension Measurements of Aqueous Surfactant Solutions by the Drop Volume Method
Personal computer NEC PC-9801RA
Hitoshi Matsuki* and Shoji Kaneshina
r
Department of Biological Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770, Japan
RS-232C adapter unit SIGMA KOKI if-BOX
I
Yuji Yamashita
Yamashita Technology System Limited Company, Kawauchi, Tokushima 771-01,Japan Kinsi Motomura
Department of Chemistry, Faculty of Science, Kyushu University 33, Fukuoka 812, Japan Received April 20, 1994. In Final Form: August 1, 1994
Introduction Amphiphile molecules cause a significant decrease in the surface tension of their aqueous solutions since they form the adsorbed film a t the water/air interface. When the surface tension of their aqueous solutions is measured as a function of temperature, pressure, and concentration and the experimental results are analyzed thermodynamically, useful information can be obtained with regard to the behavior of molecules in an oriented state a t interface and the formation of molecular aggregates in the solution. However, it is necessary for this purpose to measure the surface tension of the aqueous solution accurately. Such surface tension measurements of aqueous surfactant solutions have generally much difficulty which arises from the method and apparatus of measurements. The capillary rise, du Nouy ring, and Wilhelmy plate methods, which have been traditionally employed for surface tension measurements, seem to be inappropriate for accurate surface tension measurements because the surface tension values determined by these methods may be appreciably influenced by a state of the wetting and contact angles between surfactant solutions and ring or glass surface. In the present study, we designed a new apparatus adopting the drop volume m e t h ~ d , l -which ~ is capable of measuring the surface tension with satisfactory accuracy and reproducibility, for rapid and automatic surface tension measurements of aqueous surfactant solutions. .Further, some results obtained by use of the apparatus are demonstrated. Experimental Section Materials. Decylammonium chloride and lithium perfluorooctane sulfonate were synthesized and purified by the methods described p r e v i ~ u s l y . Lithium ~?~ dodecyl sulfate was purchased from Sigma Chemical Co. and its purification was made as described previ~usly.~ The purity of surfactants was checked by observing the surface tension vs concentration curves of their
* Author to whom correspondence should be addressed.
(1)Davies, J. T.;Rideal, E. K. In Interfacial Phenomena; 2nd ed.; Academic Press: New York, 1963;p 44. ( 2 )Padday, J. F. In Surface and Colloid Science; MatijeviC, E., Ed.; Wiley-Interscience: New York, 1969;Vol. 1, p 131. (3)Adamson, A. W. In Physical Chemistry of Surfaces, 4th ed.; John Wiley & Sons: New York, 1982;p 20. (4)Aratono, M.; Uryu, S.; Hayami, Y.; Motomura, K.; Matuura, R. J . Colloid Interface Sci. 1983,93,162. ( 5 ) Matsuki, H.; Ikeda, N.; Aratono, M.; Kaneshina, S.; Motomura, K.J . Colloid Interface Sci. 1992,150,331.
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Automatic micrometer controller SIGMA KOKl OMEC-2
Sensor signal amplifier YTS YDS-92
I1 , Electromotive micrometer head SIGMA KOKl OPT-MIKE-E SOM-25E
l u
rt-
Measurement
1 ~ 1 I
Photoelectric
Figure 1. Block diagram of the apparatus for the automatic surface tension measurement system. aqueous solutions. Water used to prepare the surfactant solutions was triply distilled from dilute alkaline permanganate solution after deionization. Principle of the Drop Volume Method. The drop volume method is known to measure the surface tension accurately.3 The surface tension y is related to the volume of drop formed at the end of the capillary tip of glass syringe V by the following equation6
where A@is the difference in density between aqueous solution and humid air, g the acceleration of gravity, r the radius of the capillary tip, and F the correction factor depending on the value ofr/P.6-8 Therefore, the exact calculation of the surface tension value requires determination of the volume of a falling drop from the capillary tip with great precision. So we have newly designed a n apparatus which can automatically obtain the volume of drop with sufficient accuracy and reproducibility by use of the electromotive micrometer and the photoelectric sensor. Apparatus of the Automatic Surface Tension Measurement System. "he block diagram of the apparatus for the automatic surface tension measurement system is shown in Figure 1. Figures 2 and 3 illustrate the schematic drawing of the measurement cell and the cross sectional view of the syringe holder, respectively. First the electromotive micrometer head (Sigma Koki Co., Ltd. OPT-MIKE-E SOM-25E)controlled by an automatic micrometer controller (Sigma Koki Co., Ltd. OMEC2) was attached on the syringe holder which was mounted in the measurement cell and this made it possible to push out the aqueous surfactant solution in glass syringe with constant speed of three steps (variable speed range 3-100 pm s-l). Second the (6)Lando, J. L.; Oakley, H. T. J.Colloid Interface Sci. 1967,25,526. ( 7 )Harkins, W. D.; Brown, F. E. J. Am. Chem. SOC.1919,41,499. (8)Wilkinson, M. C. J. Colloid Interface Sci. 1972,40,14.
0743-7463/94/2410-4394$04.50/00 1994 American Chemical Society
Langmuir, Vol. 10, No. 11,1994 4395
Notes
1 I
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Experimental constants
Calibration
0 Sample data Electromotivemicrometer head
0 Origin adjustment of the falling drop
0 Difference in density
0 Return to the zero
0 Acceleration of gravity
point of micrometer
0 Syringe calibration A
Main Menu r
Calibration Edit parameters Measurements
- v
Syringe holder Measurement parameters
Filter paper
4
Measurements
0 Number of measurements
1. Formation of a drop
0 Time interval between
2. Thermal equilibrium
measurements Glass cell
Sample solution
3. Adsorption equilibrium
0 Distance and speed of
4. Detection of the falling drop
syringe movement (constants of three steps)
0 Thermal and adsorption
5. Calculation of surface tension values
equilibrium time
I I m
End
Cell holder
r I
m 1
I
1I
J
I
Figure 2. Schematic drawing of the measurement cell.
L
Micrometer head adapter
Syringe guide Plunger Spring
Figure 4. Block diagram of the measurement procedure for the automatic surface tension measurement. micrometer to stop pushing the plunger of the syringe. Thus, by the travel in the micrometer, one obtains the volume of the drop. The signal from the sensor was amplifiedby a sensor signal amplifier (YTSYDS-92) and transmitted to the personal computer (NEC PC-9801RA) by a RS232C serial interface, and a series of operations was completely automated by the computer control. The measurement was carried out under the initial conditions (experimental constants and measurement parameters) input from the computer and further the volume value of drop obtained was converted to the surface tension value simultaneously. The formation of drop under the capillary tip was made by the method described previo~sly.~The measurement procedure for the automatic surface tension measurement is shown in Figure 4. Temperature of the measurement cell was kept constant by placing the cell in a thermostated water bath controlled to fO.O1 K by a thermistor digital controller (Techno1 Seven Co., Ltd. (3511-2).
Figure 3. Cross sectional view of the syringe holder.
Results and Discussion The surface tension y of the aqueous ionic surfactant solutions was measured by use of the newly designed apparatus a t 298.15 K and 0.1 MPa. In Figure 5, the y values of aqueous decylammonium chloride (DeAC) solution measured at constant molality ml are plotted against the time t for which the drop formed under the tip of glass capillary was allowed to stand. It is found that the y values decrease slightly within a few minutes due to the adsorption process of DeAC and remain constant with sufficient reproducibility a t more than 10 min from a low concentration in the neighborhood of the phase transition point of adsorbed film (6.85mmol kg-l and 66.80 mNm-l)l0 to a high concentration above the critical micelle concentration (cmc, 60.87 mmol kg-l and 31.98 mN m-l).ll Taking into account that the y value is not seen to be time
photoelectric sensor using a n optical fiber (Keyence FS2-62) attached at the front of the syringe holder enabled us to detect the falling transparent drop from the capillary tip. Once the drop is detected, a feedback mechanism (RS232C adapter unit (Sigma Koki Co., Ltd. if-BOX)) instructs the electromotive
(9) Motomura, K.; Iwanaga, S.; Hayami, Y.;Uryu, S.; Matuura, R. J. Colloid Interface Sci. 1981,80, 32. (10) Aratono, M.; Uryu, S.; Hayami, Y.;Motomura, K.; Matuura, R. J . Colloid Interface Sci. 1984,98, 33. (11) Matsuki, H.;Ando, N.; Aratono, M.;Motomura, K. Bull. Chem. SOC.Jpn. 1989,62,2507.
Sensor cord Glass syringe Sample solution
Sensor holder Drop of the solution
wsmmmmL
Photoelectricsensor
4396 Langmuir, Vol. 10, No. 11, 1994 69
c L
Notes
1 c
z
0
Figure 5. Surface tension vs time curves of DeAC at 298.15 KandO.1MPa: ( l ) m l =4.687mmolkg-1,(2)29.878,(3)59.252, (4)70.201.
dependent in this time region and in the whole concentration range, the constant y value obtained by the apparatus can be regarded as an equilibrium value of the adsorption of DeAC. Next, the y vs ml curves oflithium dodecyl sulfate (LiDS) and lithium perfluorooctane sulfonate (LiFOS) are illustrated in Figure 6. In the case of anionic surfactant solutions, the water-repellent glass capillary coated with dimethylpolysiloxane was used in order to avoid the influence of the wetting between surfactant solution and glass surface on the equilibrium value of surface tension.12 The results are drawn by the closed circle in the figure; the surface tension values are seen to be reproducible enough to determine the respective cmc values of LiDS and LiFOS. Further, they are compared with those obtained by using the previous apparatus having a micrometer operated manually (open c i r ~ l e ) .Both ~ , ~ the values are in complete agreement with each other within the experimental error of 0.05mN m-l. This factjustifies that the present apparatus gives the exact y values of LiDS and LiFOS solutions. Therefore, we may say that (12) Ikeda, N.; Matsuki, H.; Aratono, M.; Motomura, K. Rep. Cent. Adv. Instrum. Anal., Kyushu Univ. 1991,8, 9.
5
10
15
0
m, I mmol kg.'
Figure 6. Surface tension vs molality curves at 298.15 K and 0.1 MPa: (1)LiDS, (2) LiFOS (0)the results obtained by the present study, ( 0 )the results from ref 5.
the newly designed apparatus can provide the accurate and reproducible y values of aqueous surfactant solutions rapidly and automatically and be helpful to investigate the behavior ofmolecular aggregates such as the adsorbed film and micelle. summary
Anew apparatus based on the principle of drop volume method was designed to measure the accurate and reproducible surface tension values of aqueous surfactant solutions rapidly and automatically. The surface tension of some aqueous surfactant solutions was measured by use of the apparatus at 298.15 K and 0.1 MPa. The equilibrium surface tension values of surfactants were able to be determined within the experimental error of 0.05 mN m-l in the whole concentration range. Acknowledgment. The present study was supported in part by a Grant-in-Aid for Encouragement of Young Scientists (No.04740257)from the Ministry of Education, Science and Culture of the Japanese Government.
