In the Laboratory
From Polarimeter to Contact Angle Goniometer—Inexpensive Conversion of Laboratory Equipment Konrad G. Kabza* and Kevin Cochran Department of Chemistry, SUNY at Fredonia, Fredonia, NY 14063 Recent decades have brought about dramatic changes in the interests of chemical researchers. Interdisciplinary areas of chemistry such as environmental chemistry, polymer science, and material science have become important parts of the undergraduate curriculum. However, introduction of these multidisciplinary areas into classical chemistry curricula often requires acquisition of specialized analytical equipment. At the same time, more traditional characterization techniques such as polarimetry have been replaced by the routine use of FT-IR and GC-MS techniques in undergraduate organic chemistry laboratories. While such classical instruments remain serviceable, they have in many cases fallen into disuse. Here we describe a simple method whereby a polarimeter is converted into a contact angle goniometer. Contact angle goniometry is an example of a traditional analytical tool that would be an asset to any physical chemistry training and that would still be of relevance to many interdisciplinary areas. Such instrumentation provides a simple and inexpensive way to introduce and measure fundamental properties such as the surface tension of a variety of materials including metals and polymers. Contact angle measurements are indispensable in the study of adhesion, wettability, and liquid thermodynamics (1–3). Many experiments using such measurements can be devised for traditional physical chemistry laboratory sequences. The concept of surface energy is important in materials science, chemistry, and physics. Thus it could not only enhance students’ learning but also provide them with new perspective on the fascinating field of surface chemistry and physics. This paper describes a relatively simple conversion of a polarimeter into a contact angle goniometer. We utilized a Kern Full-Circle Polarimeter to do the conversion. An important goal of our work was to make the conversion at minimum expense. This forced us to make as few changes to the original instrument as possible. It also meant that we had to utilize as many of the polarimeter’s original components as possible. To illustrate, let us examine basic components of a contact angle goniometer. The main components of every goniometer are a small magnification telescope equipped with a movable cross-hair and a circular protractor calibrated in degrees to read the tilt of the crosshair from horizontal. The telescope is usually mounted on an optical bench and is aligned with a movable stage on the same optical bench. The stage should be able to traverse from left to right, forward and backward, and up and down with respect to the optical line of the telescope. Most goniometers are also equipped with a microsyringe that can reproducibly place liquid drops on the analyzed surfaces. This syringe must be attached to the movable stage and travel along with it during the adjustments, but it should also be adjustable (height and side-to-side) with respect to the stage. The actual contact angle measurements are usually performed inside an environmental chamber. This is a small container partially filled with the liquid of interest and *Corresponding author.
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equipped with optically clear windows. The windows permit illumination of the sample during the measurement. Finally, contact angle goniometry requires a source of illumination that is adjustable in intensity and placement. The conversion described here used a cross-hair equipped telescope mounted on the protractor of the polarimeter. This was a constant focus telescope and thus the stage for sample placement required a traversing movement to place the liquid drop in the focal point of the telescope. If the telescope were an adjustable focus variety the sample stage would not require the traversing (back and front) movement. To achieve fine adjustment of the stage movement we purchased two rack and pinion strips, which, when put together, allowed x–y movements of the stage. The vertical positioning of the sample stage was achieved by mounting an adjustable rod holder onto the traversing rack and pinion strips. (A list of all purchased parts is attached at the end of this note.) Because the moving mechanism of the stage raised the level at which the sample is placed, we had to raise the position of the protractor and the telescope. This can be clearly seen on the picture of the goniometer (Fig. 1.). For focusing of our goniometer we utilizing a screwadjusted movable stage (see Fig. 1.) because we had a spare one on hand. A standard rack and pinion mechanism—or better yet, focusable telescope—would have worked just as well. We have also utilized some unusual components in our design. For example, instead of machining a true environmental chamber, we simply used a plastic square dome. This dome is actually a cover of scrap box made of extruded poly-
syringe position adjustment screw wing nut (coarse vertical adjustment) environmental chamber pendant drop
microsyringe
light source vertical adjustment
to and from stage adjustment
telescope
sideway adjustment
protractor-dial from original polarimeter
stand (from polarimeter)
Figure 1. Side view of the goniometer constructed from a KernFull-Circle Polarimeter.
Journal of Chemical Education • Vol. 74 No. 3 March 1997
In the Laboratory Table 1. Comparison Data for Contact Angle Measurements Using Various Liquids and Surfaces Probing Liquid
Analyzed Surface
Contact Angle (literature)b (measured)a
water
polyethylene
102.5 ± 1.4
103
water
teflon
111.1 ± 3.3
112
water
paraffin
99.4 ± 2.3
110
n -propanol
paraffin
22.1 ± 0.8
22
CH2I2
paraffin
60.5 ± 0.9
61
CH2I2
teflon
86.1 ± 0.9
85.9
CH2I2
polyethylene
48.7 ± 0.9
Table 2. Price and Source of Materials Used in Conversion Part
Price ($)
Source
Vertical rack/pinion
49.00
Edmund
—
Horizontal rack/pinion
90.00
Edmund
—
Telescope
200.00
Edmund
We built one from parts available in the department.
Fisher cat. # DS32246A
We used a 1.00 mL syringe and attached a micropipet to it.
Microsyringe
10.00
TOTAL
359.00
46; 51.9c
a
Each value is the average of at least 8 measurements. These data were compiled by Arthur W. Adamson (4). c Two values were reported.
Comments
b
styrene. The flat surface of the stage itself was just a glass pane cut to size with polished edges glued to the vertical adjustment rack and pinion mechanism (see Fig. 1). The biggest challenge was to insure that the converted goniometer would provide accurate measurements. For this reason, the moving parts of the setup had to be very well machined and installed. Secondly the telescope had to be mounted accurately. Finally, once assembled the instrument had to be calibrated. The calibration was accomplished by using a series of measurements that could be compared to abundant literature data (4–6). Table 1 includes an example of a set of advancing contact angles (Θa) measured with variety of probing liquids on selected surfaces. A more reliable calibration could be performed using comparative studies with an ASTM contact angle goniometer. We compared the values for the wettability of polyethylene by water with the marketed model of Ramè-Hart instrument. Measured values were within ± 2° for the same user and ± 3° for different users. Construction of the goniometer from a polarimeter was inexpensive. The cost, including a cross-hair-equipped telescope, microsyringe, and two rack and pinion stage adjustments was less than $500.00 (Table 2). One reason for this
low cost is that we utilized the polarimeter’s stand as our optical bench. The total expense (excluding the cost of polarimeter) was approximately 1/30 of the cost of a commercially available instrument. We believe that this sort of inexpensive conversion of a classical but less used analog polarimeter is a solution for educators interested in introducing surface science experiments at smaller campuses. Literature Cited 1. Andrade, J. D. In Polymer Surface Dynamics; Andrade, J. D., Ed.; Plenum: New York, 1988; pp 9–24. 2. Ferguson, G. S.; Whitesides, G. M. Modern Approaches to Wettability: Theory and Applications; Shoder, M.; Loeb, G.; Eds.; Plenum: New York, 1991; pp 1–15. 3. Zisman, W. A. In Contact Angle: Wettability and Adhesion; Gould, R. F. Ed.; Advances in Chemistry 43; American Chemical Society: Washington, DC, 1964; pp 1–52. 4. Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990; pp 379–421. 5. Good R. J.; Paschek, J. K. Wetting, Spreading, and Adhesion; Padday, J. F. Ed.; Academic: New York, 1978; pp 3–80. 6. Neumann, A. W.; Good, R. J. In Surface and Colloid Science, Vol. 11, Experimental Methods; Good, R. J.; Stromberg, R. R., Eds.; Plenum: New York, 1979; pp 31–91
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