In the Laboratory
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An Inexpensive and Accurate Tensiometer Using an Electronic Balance Manuel Dolz,* Jesús Delegido, María-Jesús Hernández, and Julio Pellicer Departament de Termodinàmica, Facultats de Farmàcia i Fisica, Universitat de València, E-46100 Burjassot, València, Spain; *
[email protected] The importance of the experimental study of surface phenomena is not sufficiently reflected in the experiments performed in most teaching laboratories in physical chemistry, physics, and biophysics. This may be because experimental measurement of the contact angle, and even more so of surface tension, requires specialized and expensive equipment that is relatively difficult to use (1, 2). Alternative experiments tend to be rudimentary and unappealing. The possibility of carrying out quality experiments with cheaper material would be of considerable value to students in applied science and technology (chemistry, physics, pharmacy, medicine, engineering), where surface science is extremely important (3, 4 ). The study of interface properties facilitates understanding of phenomena ranging from capillarity, adhesion, surfactancy, wettability, and drug dosing to biomaterials compatibility, bacterial detection or separation analysis, flotation enrichment processes in the mining industry (5), and the use of surfactants in the chemical, food, and cosmetics industries (6 ). This study proposes a modification of the du Noüy tensiometer that in addition to affording excellent results in measuring surface tension also allows the performance of an interesting, elegant, very simple, and inexpensive experiment for undergraduate students. Background The surface separating two phases in contact, for example, liquid–gas, is considered to be of finite thickness and possesses thermodynamic properties distinct from those of each phase considered separately (3). In fact, the difference in structure and composition of the phases generates a force field at the interface, with a consequent increase in its free energy, internal energy, and entropy; in this context, the stability of the interface depends upon the increase in free energy of the system on incrementing its surface area. Such free energy per unit surface area is known as the surface tension coefficient (7). Equation 1 shows an increase in the interface surface area to require work that is stored as free energy; this work differs from one interface to another. γ = ∂G (1) ∂A p,T The surface tension coefficient of a liquid–air interface can be measured with sufficient accuracy and relative ease using any of three types of methods: static, semistatic, and dynamic (3, 5). A common semistatic technique is the detachment method, which measures the force required to detach or separate a ring, or the wet edge of a flat plate, from the liquid surface (Fig. 1). The detachment of a ring of radius rr occurs when the surface tension force is (8),
γ = Ff 4 π rr
(2)
where f is a dimensionless coefficient that depends upon the geometry of the ring (3, 8) and the curvature of the interface surfaces formed between the ring and the liquid (9, 10) (Fig. 2). Equation 2 can also be expressed as
γ= F L ef
(3)
where Lef is a parameter with length dimensions that can be regarded as the effective length of the liquid film drawn up between the ring and the liquid surface immediately prior to rupture. Measurement of surface tension by the detachment method is complicated in the practical teaching context. One option is to use highly sophisticated devices that register the detachment force of a plate or ring motor-driven upward from the surface of the liquid under computer control and involving high-precision sensors. An alternative is the system proposed by Sánchez-Rubio et al. (11), based on the use of a manual analytical balance, or the experimental device described below.
F
dynamometer
rr
Figure 1. Schematic representation of the detachment method to measure the surface tension (case of a ring).
rr
r
Figure 2. Visualization of the effective radius, r, of the liquid film formed when drawing up the ring (radius, rr) from the liquid surface.
JChemEd.chem.wisc.edu • Vol. 78 No. 9 September 2001 • Journal of Chemical Education
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In the Laboratory
Experimental Procedure
Procedure Three operational aspects must be taken into account to correctly measure surface tension with the detachment technique: the measuring system used and determination of its effective length; measurement of the detachment force; and the relative system–liquid surface displacement rate. The Measuring System A Mettler Toledo PB 303 balance with a sensitivity of 0.001 g was used, along with a Phywe aluminum ring with filed edges, a Nima platinum wire ring, and a Nima rectangular platinum plate, whose dimensions are shown in Table 1. Five measurements of the detachment force were made with each of the three systems, on an acetone surface presenting a surface tension at the temperature of the experiment (16 °C) of γ = 24.2 mN/m (value obtained by interpolating data given in ref 13). The mean values and their errors are shown in Table 1. Acetone was chosen as reference because of the great reproducibility of the m values obtained with it. Measurement of the Detachment Force The experiment was performed using the aluminum ring with non-negligible volume, placed in deionized water. The rate of descent of the liquid surface was about 0.037 mm/s. The net force values of the liquid upon the ring were plotted against time as shown in Figure 4. Force readings were obtained every 10 s for 150 s, and every 5 s thereafter. The tracing reflects three intervals. In the first interval the force exerted upon the ring can actually prove negative, 1258
0.000 g
Figure 3. Experimental setup for measuring surface tension of liquids. 25 20 15
F / mN
Apparatus The experimental setup is both very simple and inexpensive, and is thus well suited for practical class laboratories. It requires an electronic balance to measure hydrostatic loads from a lower hook, a metal ring or plate, and a broad funnel or cylindrical container to the base of which can be attached a plastic tube fitted with a stopcock (12) (Fig. 3). The minimum resolution of the balance may be 10᎑2 or ᎑3 10 g, depending on availability; the resulting absolute errors in measuring detachment force are of the order of 0.1 or 0.01 mN, respectively. In either case the sensitivity is equal or superior to that afforded by the traditional dynamometric techniques, including torsion. The proposed solution to the problem of possible interface movements caused by manipulation of the measurement apparatus is to detach the ring by evacuating the liquid contained in the funnel by opening the stopcock. This simple procedure allows relative surface–ring movement to be as slow as desired. The student conducting the experiment stops manipulating the apparatus as soon as the liquid begins to flow, and only needs to record the balance readout immediately prior to detachment of the ring. From then on, the balance will read zero if it was tared with the needle hanging from the lower hook before positioning on the liquid surface. Measurement of the detachment force from a liquid of known surface tension allows determination of the effective length of the ring used. After this has been obtained, and based on eq 3, we can calculate the surface tension of any other liquid if we know the corresponding detachment force for the same ring.
10 5 0
50
100
−5
150
200
250
Time / s
−10
Figure 4. Detachment force exerted by the liquid (deionized water/ air) on the aluminum ring versus time (relative ring–liquid surface displacement rate v = 0.037 mm/s).
Table 1. Mass Corresponding to Detachment Force, Effective Length Dimensiona/mm
m/g
Lef /m
Aluminum ring
D = 52
2.178 ± 0.024
0.298 ± 0.003
Platinum ring
D = 20
0.306 ± 0.001
0.124 ± 0.001
Rectangular plate
L = 20
0.097 ± 0.001 0.0393 ± 0.0006
System
aD
is the diameter; L is the shorter length.
since the force upon its submerged portion may exceed the surface tension force. This interval is apparent when the ring is sunk sufficiently deep in liquid. It should be taken into account that the balance was tared with the ring suspended in air; consequently, the weight of the ring exerts no influence upon the system. When the force exerted upon the submerged portion of the ring equals the surface tension force, the balance readout equals zero (graph intersection on the x axis). The second interval is represented by the segment between the previous interval and the maximum force value (Fmax). In this case, the ring has not yet separated from the liquid surface. The third interval corresponds to the descending portion of the graph and is attributable to the deformation (narrowing) of the fluid film producing a progressive decrease in the balance readout up to the point where the ring finally detaches from the surface. The true detachment force value is reflected by the balance immediately before the readout drops to zero (9); its value is usually about 3.3% less than the maximum force recorded.
Journal of Chemical Education • Vol. 78 No. 9 September 2001 • JChemEd.chem.wisc.edu
In the Laboratory Table 2. Mass Corresponding to Detachment Force and Surface Tension for Platinum Ring in Acetone v/mm s ᎑1
m/g
γ/mN m᎑1
0.013
0.315
24.9
0.025
0.310
24.5
0.050
0.311
24.6
0.075
0.310
24.5
0.100
0.308
24.4
Table 3. Surface Tension of Liquid/Air Interfaces Substance
Experimental
Literature (13)
T/°C
γ /mN m᎑1
Acetonitrile
22.0
29.7 ± 0.3
20
29.3
Benzyl alcohol
23.3
38.3 ± 0.4
20
39.0
Butyl alcohol
23.5
23.6 ± 0.3
23.5
24.3
Ethyl acetate
20.5
23.4 ± 0.4
20.5
23.8
Glycol
23.3
47.4 ± 0.4
20
47.7
Methyl alcohol
22.0
22.4 ± 0.2
22
22.4
Pyridine
24.0
38.9 ± 0.4
20
38.0
Toluene
22.3
29.2 ± 0.3
20
28.5
T/°C
γ /mN m᎑1
Because torsion dynamometers are unable to record the last portion of the graph, the true detachment force cannot be determined with such systems. Even if extreme care is taken, the required manipulation of these instruments can induce movement capable of rupturing the liquid film for any F value in an unacceptably large and indeterminate range from the maximum force. Relative System–Liquid Surface Displacement Rate The possibility of regulating liquid flow through the lower tube of the funnel by means of the stopcock allows relative system–liquid surface displacement rates ranging from very low values (drop by drop) to rates sufficiently high to ensure conveniently short measurement times—provided the balance recordings are easy to read. In this study we obtained detachment force values for the platinum ring placed on the acetone surface, for 5 displacement rates of between 0.013 and 0.10 mm/s, as determined from volume output. To measure such small detachment surface velocities in the absence of vibrations, any other experimental design would require the use of costly, highprecision equipment. The detachment force values obtained and the corresponding surface tension values are shown in Table 2; there are no significant differences in the last four values. The only abnormally large detachment force corresponded to the lowest relative system–liquid surface displacement rate (drop-by-drop flow). This may be because at the instant each drop was released, movement induced at the bottom of the liquid volume propagated upward and triggered premature detachment of the ring (last segment in Fig. 4) making the detachment force readout slightly greater than in the other four cases. Thus, any measurement in the velocity interval 0.025– 0.100 mm/s would provide γ values that may be considered correct. Higher flow rates could cause a vortex effect that would likewise affect the liquid surface and thus alter the readings.
Hazards The chemicals used in this experiment must be handled with care. They may be highly flammable, irritating to the eyes and respiratory system, or toxic by inhalation. Consult the relevant MSDS before selecting a substance for your laboratory. Results and Conclusions The method described here was used to determine the surface tension of different liquids. Table 3 shows the experimental results and the values reported in the literature (13). Even in the worst situation, the experimental value is within 2% of the literature value (14). In conclusion, our experimental setup is at least as economical as the commercial torsion dynamometers, yet it affords equal or even superior resolution, the total absence of vibrations, and very simple handling characteristics. Added advantages are the possibility of varying the relative ring (or plate)–liquid surface displacement rate and of keeping this rate practically constant throughout the measurement process. This procedure familiarizes students in applied science and technology with the experimental study of surface tension by means of a simple, accurate, rapid, and highly sensitive method that offers the same advantages as other much more costly and sophisticated systems. Acknowledgments We thank Marina Herráez and Alicia López (Department of Pharmacy and Pharmaceutical Technology, Valencia University) for their kind contribution. Supplemental Material Notes for the instructor and detailed laboratory documentation are available in this issue of JCE Online. W
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