The interfacial tension of immiscible liquids is an important thermophysical property that is useful in the behavior of liquids both in microgravity (Martinez et al. (1987) and Karri and Mathur (1988)) and in enhanced oil recovery processes under normal gravity (Slattery (1974)). Many techniques are available for its measurement, such as the ring method, drop weight method, spinning drop method, and capillary height method (Adamson (1960) and Miller and Neogi (1985)). Karri and Mathur mention that many of the techniques use equations that contain a density difference term and are inappropriate for equal density liquids. They reported a new method that is suitable for both equal and unequal density liquids. In their method, a capillary tube forms one of the legs of a U-tube. The interfacial tension is related to the heights of the liquids in the cups of the U-tube above the interface in the capillary. Our interest in this area arose from a need to measure small interfacial tension (around 1 mN/m) for a vegetable oil/silicon oil system that was used in a thermocapillary drop migration experiment (Rashidnia and Balasubramaniam (1991)). In our attempts to duplicate the method proposed by Karri and Mathur, we found it quite difficult to anchor the interface inside the capillary tube; small differences of the liquid heights in the cups drove the interface out of the capillary. We present an alternative method using a capillary tube to measure the interfacial tensions of liquids of equal or unequal density. The method is based on the combined capillary rises of both liquids in the tube

Balasubramaniam, R.

Delsignore, D.

Rashidnia, N.

English

NASA Technical Reports Server

.. Y 
I NASA Contractor Report 189133 
1 
Interfacial Tension Measurement of Immiscible 
Liquids Using a Capillary Tube 1 .  
I N. Rashidnia 
I Sverdrup Technology, Inc. 
Brook Park, Ohio 
R. Balasubramaniam 
Case Western Reserve University 
Cleveland, Ohio 
and 
D. Del Signore 
University of Toledo 
Toledo, Ohio 
March 1992 
Prepared for 
Lewis Research Center 
Under Contract NAS3-25266 
NASA 
National Aeronautics and 
Space Administration 
https://ntrs.nasa.gov/search.jsp?R=19920012019 2020-03-17T13:27:35+00:00Z
INTERFACIAL TENSION MEASUREMENT OF IMMISCIBLE LIQUIDS 
USING A CAPILLARY TUBE 
N. Rashidnia R. Balasubramaniam* D. Del Signore 
Sverdrup Technology, Inc. Case Western Reserve University University of Toledo 
Lewis Research Center Group Cleveland, Ohio 44106 Toledo, Ohio 43606 
Brook Park, Ohio 44142 
Introduction: 
The interfacial tension of immiscible liquids is an important thennophysical property that is 
useful in the behavior of liquids both in microgravity (Martinez et al (1987), Kani and Mathur 
(1988)) and in enhanced oil recovery processes under normal gravity (Slattery (1974)). Many 
techniques are available for its measurement, such as the ring method, drop weight method, 
spinning drop method, and capillary height method (Adamson (1960), Miller and Neogi 
(1985)). Kani and Mathur mention that many of the techniques use equations that contain a 
density difference term and are inappropriate for equal density liquids. They reported a new 
method that is suitable for both equal and unequal density liquids. In their method, a capillary 
tube forms one of the legs of a U-tube. The interfacial tension is related to the heights of the 
liquids in the cups of the U-tube above the interface in the capillary. Our interest in this area 
arose from a need to measure small interfacial tensions (around lmN/m) for a vegetable oil / 
silicone oil system that was used in a thermocapillary drop migration experiment (Rashidnia 
and Balasubramaniam (1991)). In our attempts to duplicate the method proposed by Kam and 
Mathur, we found it quite difficult to anchor the interface inside the capillary tube; small 
differences of the liquid heights in the cups drove the interface out of the capillary. 
We present an alternative method using a capillary tube to measure the interfacial tensions of 
*NASA Resident Research Associate at Lewis Research Center. 
1 
liquids of equal or unequal density. The method is based on the combined capillary rises of 
both liquids in the tube, with air being the medium above the liquids. This way of using the 
capillary tube is different from those used previously, such as by Clarkson (1984). Also, in 
this method, no difficulties are encountered in anchoring the interface inside the capillary tube 
as in our attempts to duplicate the method by Karri and Mathur. 
Principle: 
A simple sketch of the combined capillary rise method is shown in Figure la. A large 
container contains the two immiscible liquids, one on top of the other. A capillary tube is 
dipped into the upper liquid (liquid 1) to measure its surface tension with air (cia). Then, the 
capillary tube is pushed further down into the lower liquid (liquid 2) so that a liquid-liquid 
interface is present in it. This is best accomplished by using a pinch cork attached to a 
flexible tube connected to the top of the capillary before pushing it down into liquid 2. Upon 
releasing the pinch cork, the liquid-liquid interface naturally rises into the capillary (for 
wetting liquids).One may also establish l a  and 12 interfaces inside the capillary tube without 
having the two liquids in contact in the outer container. This is accomplished by dipping the 
capillary tube first into liquid 1, and subsequently, without allowing the column of liquid 1 to 
drain, dipping it into a container of liquid 2. This is in fact the preferred approach especially 
for equal density liquids and is used in the experiments reported here. For this case, the height 
L1 (Figure la) is zero. Assuming the interface 12 and l a  to be spherical, the following 
equation may easily be derived for the interfacial tension 
='la + gr @Ihl+P2h2-P1L1) (1) 
cose12 2cose12 O12 = -*la 
where CT is interfacial tension, 8 is contact angle, g is earth's gravitational acceleration, r is the 
capillary tube radius and p is liquid density. The various liquid heights are defined in Figure 
la. In deriving the above equation, the curvatures of both interfaces in the container are 
assumed to be zero. While this is easily realized for unequal density liquids, it is not 
necessarily the case for equal density liquids. For the data reported here, L is zero and so this 
situation does not arise. Specializing eq(1) for equal density liquids, the following equation 
1 
2 
may be obtained 
Equations 1 and 2 involve both the contact angles ela and O12. If the capillary tube is itself 
used to measure ala,  Bla drops out of eq( 1) because the capillary rise measured is 
which may be used directly in eq(1). 012 may be obtained from the geometry of the interface 
within the capillary (Figure lb). Assuming a spherical interface 
(4) 
612 = ~ 0 ~ - ~ ( 2 k / ( k  2 +1)) 
where k=y/r. If 612 is less than ten degrees, the error in assuming it to be zero in eq(1) or 
eq(2) is less than 1.5%. The working equation used to calculate o12 for the case with L1=O is 
Experimental procedure: 
Pyrex glass capillary tubes with an ID of 1.15 mm and length of lOcm are used. The liquids 
are contained in quartz cells of inner dimensions 1 x 1 x 4.5 cm and a thickness of 1.25 mm. 
All the measurements reported are at room temperature (around 21 deg C k 2 deg C; the room 
temperature was steady during the course of every experiment to * 0.5 deg C). The top end of 
the capillary is connected via a rubber tubing to a suction device. The apparatus is mounted 
on an optical table; no vibration isolation is provided. 
Before each experiment, the capillary tubes and the quartz cells are cleaned as follows. They 
are immersed in a micro cleaning solution mixed with water and allowed to stand overnight. 
They are then cleaned and rinsed with distilled water. This is followed by rinsing with 
acetone, methanol and distilled water, in that order. Finally they are dried in a vacuum oven 
at 120 deg C for an hour. 
The capillary tube is frrst dipped into liquid 1 and then into liquid 2 to establish liquidl/air and 
liquidl/liquid2 interfaces within it. To ensure that the interfaces are in equilibrium locations 
3 
and the capillary tube is wetted by the liquids, a small suction followed by a release to 
atmospheric pressure is applied a few times to the top of the capillary. The equilibrium 
locations are stable and repeatable. The interfaces are established quite rapidly (a few minutes 
maximum) using the present method. The various heights needed for the measurement of ola 
and o12 are obtained using a cathetometer. Typical heights are on the order of 3 to 15 mm 
and the cathetometer resolution is 0.01 mm. A stereo microscope is used to measure the 
diameter of the capillary tube (diameter is 1.15 f 0.015 mm) and the lengths required to 
calculate e12. 
Distilled water is used as liquid 2 in all the cases except for the anisaldehyde / ethylene glycol 
pair, for which liquid 2 is ethylene glycol. For many cases, there is density inversion within 
the capillary; however, no instability is observed and the l a  and 12 interfaces are stable in all 
the experiments reported. 
Results and Discussion: 
Interfacial tensions of nine liquid pairs (Table 1) have been measured using the combined 
capillary rise method proposed here (with L1=O). They cover a fairly wide range of values of 
the interfacial tension and the density difference between the liquids. Figure 2a and 2b show 
the interfaces obtained in the quartz cell and in the capillary tube for cyclohexane/water and 
benzaldehyde/water systems respectively (in these pictures Llfo). Representative 
measurements of the contact angle (612) for all the systems in Table 1 revealed that 612 is less 
than ten degrees; hence e12 has been assumed to be zero in Eq(5) for computing the interfacial 
tensions. 
Table 1 presents the results for the interfacial tensions. The sample standard deviations are 
less than 0.5 mN/m (typically around 2% of the mean); thus the data is very reproducible. The 
accuracy of the cathetometer used is 0.01 mm. However, the interfaces appear thicker in the 
cathetometer field of view due to refraction effects; hence placement of the cross-hairs has an 
. 
4 
inherent error. If we assume a worst case error of 0.lmm in measuring liquid heights in the 
capillary (typical heights are 3 to 15 mm), this would indicate a 2-3% cumulative error in the 
interfacial tension. Comparing the measured interfacial tensions with those in the literature, it 
is seen that for non-equal density liquids the agreement is good, with a maximum deviation of 
9.3% for octanoVwater and benzaldehyde/water interfaces. The experiments did indicate that 
benzaldehyde was very prone to coat the glassware with crystals; it is not clear whether this 
plays a role. The interfacial tensions for cyclohexane, carbontetrachloride and octanol with 
water are slightly lower than those in the literature (Donahue and Bartell (1952) and Girifalco 
and Good (1957)). For equal density liquids the agreement of our data with the results of 
Karri and Mathur is much worse. The maximum deviation is 32% for benzonitrile/water 
interface. Kani and Mathur report the interfacial tension of 3-phenyl-1-propanol with water 
to be zero, while the measurements herein indicate the value to be around 7.9 mN/m. We 
found that the cleanliness of the tubes and experiment cells are crucial in order to measure the 
interfacial tensions with repeatability; hence we strictly followed the cleaning procedure 
mentioned earlier The liquids used are of ordinary purity and not high purity. Other than 
general laboratory precautions, no additional steps have been taken to ensure the punty of the 
liquids and the interfaces. Thus, as in all surface tension measurements, cleanliness remains 
an issue. This perhaps plays a role in the deviations between the current measurements and 
the literature values. In view of the large deviations, independent measurements are needed to 
confirm the accuracy of the interfacial tensions of the equal density liquids in Table 1. 
While the lowest interfacial tension measured with the combined capillary rise technique is 
reported to be around 3.4 mN/m in Table 1, lower values around 1 mN/m have also been 
measured for the neutral density liquid pair comprising of silicone oil and vegetable oil, in 
order to measure da/dT in the temperature range 20 to 50 deg C, (Rashidnia and 
Balasubramaniam). Thus it appears that the combined capillary rise technique can be used for 
a broad range of interfacial tensions. Additional testing is required to establish the accuracy of 
the method for a larger class of immiscible liquid pairs especially of equal density. 
5 
References: 
A. W. Adamson (1960),"Physical Chemistry of Surfaces," Interscience Publishers Inc. 
M. T. Clarkson (1984),"Measurements on the Liquid Rise in Glass Capillaries at Micro 
emulsion/Oil/Brine Interfaces," Journal of Colloid and Interface Science, Vol 102, No 2, pp 
563-5 66. 
. 
D. J. Donahue and F. E. Bartell (1952),"The Boundary Tension at Water-Organic Liquid 
Interfaces," J. Phys. Chem., 56, 480. 
L. A. Girifalco and R. J. Good( 1957),"A Theory for the Estimation of Surface and Interfacial 
Energies I. Derivation and Application to Interfacial Tension,"J. Phys. Chem., 61, 904. 
S. B. Reddy Kani and V. K. Mathur (1988),"Measurement of Interfacial Tension of 
Immiscible Liquids of Equal Density,"AIChE J., Vol 34, NO 1, pp 155-157. 
I. Martinez, J. M. Haynes and D. Langbein(l987),"Fluid Statics and Capillarity," in Fluid 
Sciences and Material Sciences in Space, H. U. Walter (ed), pp 53-81, Springer-Verlag, Berlin. 
C. A. Miller and P. Neogi (1985),"Interfacial Phenomena," Marcel Dekker Inc., New York. 
N. Rashidnia and R. Balasubramaniam (1991),"Thermocapillary Migration of Liquid Droplets 
in a Temperature Gradient in a Density Matched System," Experiments in Fluids, Vol 11, pp 
. 
167- 174. e 
J. C. Slattery,"Interfacial Effects in the Entrapment and Displacement of Residual Oil," 
AIChE J., V0120 NO 6, pp 1145-1 154. 
Liquid Pair 
Anisole/ 
Distilled Water 
Diethyl Adipate/ 
Distilled Water 
Benzonitrilel 
Distilled Water 
p-Anisaldehyde/ 
Ethylencglycol 
3-phcnyl-l-propanol/ 
Distilled Water 
2-Octanol/ 
Distilled Water 
Benzaldehyde/ 
Distillcd Water 
Cyclohexane/ 
Distilled Water 
Carbon1 et rachloride/ 
Distilled Water 
Density 
k g b  
996 
998 
1009 
998 
1010 
998 
1119 
1114 
998 
998 
819 
998 
1044 
998 
n g  
998 
1583 
998 
Table 1. 
Measured Interfacial 
Tension mN/m 
(Mean) (Std. Dev.) 
25.2 
12.9 
19.0 
3.4 
7.9 
8.71 
13.4 
48.0 
43.5 
034 
0.18 
0.31 
0.28 
0.17 
0.2 
0.27 
0.65 
0.38 
Literature Value of 
Interfacial Tension mN/m 
, 
('I Karri and Mathur (1988) 
('I Girifalco and Good (1957) 
7 
Figure 1. -Principle of the combined 
capillary rise method. 
i 
(a) Cyclohexane/water system. 
ORIGINAL PAC& 
BLACK AND WHITE PHOTOGRAPH 
(b) Benzaldehydelwater system. 
1' 
h 
Figure 2. -The combined capillary rise method. 
8 
REPORT DOCUMENTATION PAGE 
I March 1992 I F 
Form Approved 
OMB NO. 0704-0188 
1. TITLE AND SUBTITLE 
Interfacial Tension Measurement of Immiscible Liquids Using a 
Capillary Tube 
12a. DISTRIBUTION/AVAILABlLlTY STATEMENT 
i. AUTHOR(S) 
N. Rashidnia, R. Balasubramaniam, and D. Del Signore 
126. DISTRIBUTION CODE 
r. PERFOR~~~G~~RGANIZATION NAME(S) AND ADDRESS(ES) 
Interfacial tension; Capillary tube; Equal density liquids; Immiscible 
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Lewis Research Center Group 
Brook Park, Ohio 44142 
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ial Contractor Report 
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11. SUPPLEMENTARY NOTES 
Unclassified -Unlimited 
Subject Categories 34 and 35 
13. ABSTRACT (Maximum 200 words) 
The interfacial tension of immiscible liquids is an important thermophysical property that is useful in the behavior of 
liquids both in microgravity (Martinez et al. (1987), Karri and Mathur (1988)) and in enhanced oil recovery processes 
under normal gravity (Slattery (1974)). Many techniques are available for its measurement, such as the ring method, 
drop weight method, spinning drop method, and capillary height method (Adamson (1 960), Miller and Neogi (1985)). 
Karri and Mathur mention that many of the techniques use equations that contain a density difference term and are 
inappropriate for equal density liquids. They reported a new method that is suitable for both equal and unequal 
density liquids. In their method, a capillary tube forms one of the legs of a U-tube. The interfacial tension is related 
to the heights of the liquids in the cups of the U-tube above the interface in the capillary. Our interest in this area 
arose from a need to measure small interfacial tensions (around 1 mN/m) for a vegetable oil/ silicone oil system that 
was used in a thennocapillary drop migration experiment (Rashidnia and Balasubramaniam (1 991)). In our attempts 
to duplicate the method proposed by Karri and Mathur, we found it quite difficult to anchor the interface inside the 
capillary tube; small differences of the liquid heights in the cups drove the interface out of the capillary. We present 
an alternative method using a capillary tube to measure the interfacial tensions of liquids of equal or unequal density. 
The method is based on the combined capillary rises of both liquids in the tube. 
115. NUMBER OF PAGES 14. SUBJECT TERMS 
I I I 
Standard Form 298 (Rev. 2-89) NSN 7540-01 -280-5500 
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Interfacial tension measurement of immiscible liq uids using a capillary tube

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