A Simple Apparatus for Measuring Dynamic Surface
Tension E. V. Srisankar, J. P. Shah, and K. S. Narayan Research Centre, Hindustan Lever Limited, Chakala, Andheri East, Bombay 400 099, India Surface tension lowering, a fundamental property of surfactants, is brought ahour by the adsorption of amphiphilic molecules nt interfaces. Surface tension (ST) is oerhaos the most commonly measured property of ~ " r f a c t a i tsol&ons, be it for research or for routine laboratorv Two . exoeriments. . facets uf s~irfacetension lou,ering, thc equilibrium value and the rate at which this \ d u e is reached, are i m ~ o r t a n t h. u i librium surface tension (EST) is the &antit; usually &easured, from which, bv ~ u l- v i n zthe Gibbs adsor~tioniso- a .. therm, the area per molecule of the surfactant a t t h e interface can be calculated. However, adsorption is a kinetic process and the EST gives no information of this rate process. For this purpose, the rate of ST lowering or dynamic surface tension (DST) has to be measured. The kinetics of surfactant adsorption is important in a variety of phemomena such as lathering, emulsification, etc. Such measurements would also be of value in illustrating the effect of structure (chain leneth. branchinz head-zrouo size). concentrations, e~ectrolytes,etc., on &usionUto b e interface and related aspects like micellar breakdown. When a liquid is forced out through an elliptical orifice surface tension forces cause the stream to oscillate between two ellipses, the major axes of which are perpendicular to each other1, and stationary waves are produced as shown in Figure 1. The wavelength increases progressively with decreasing S T as a result of increasing adsorption of the surfactant. Ultimately the wavelength corresponds to the EST value (see Fig. 2). T h e apparatus required to follow this process can he conveniently made in a classroom experiment.
Method Thevibrating jet method for the measurement of DST is a well-established technique2. Its theory was worked out by Lord Rayleigh as early as 1879 and has been widely used. Essentially the method involves the measurement of the wavelength of an oscillatory jet of liquid emerging from an elliptical orifice as it progresses in time and space. The orifice is the most critical3part of the apparatus. Apart from this one would require a camera and a halogen lamp (or any other suitable light source) for photographing4the jet for the measurement of wavelength. Alternatively, the wavelength can he measured using a cathetometer, although i t is somewhat cumbersome. Construction Figure 3 shows the assembled setup. Beaker A (1 L)contains the necessary solution which drips into beaker B (1L) at a steady rate. B contains an averflow tubeso that thelevel ofliquid in the beaker can be maintained constant. The solution flows from B through acoiled tube immersed in a constant-temperature bath to a glass tube that ends in a capillary with an elliptical orifice. It should optimallybe of l-mm i.d., and the easiest method is to draw out a thick capillary
' Vandegrift, A. E. J. Colioid interface Sci. 1967, 23, 43.
Adamson, A. W. A Textbook of Physical Chemistry, Academic: New York, 1973. Owens. 0. K. J. Colloid Interface Sci. 1969. 29. 496. be. ~ : ; ~ a t s " m u r a .Tenside ~. Det. 1983,~20,218. When the iengtn ol the capillary portion increases the numoer of clear Naves n the jet aecreases.
378
Journal of Chemical Education
Figure 1. Photograph of a typical jet of sodium dodecyi benzene sulfonate Figure 3.
(SDBS) obtained wiU? the capillary in
limo :ms
-
Figure 2. Dynamic surface tension plot lor some illustrative samples. A, 3.4 mM SDBS solution containing 1 mM sodium oleate; B, 4.3 mM SOBS solution; and C. 4.3 mM SDBS solution containing 0.1 mM potasslum palmitate.
from aglass tube of 2--5-mm i.d. It is then cut at the neck such that only a very short length (less than 1mm) of the capillary portion is left on the glass tube. After heating, the capillary tip is gently pressed to give an elliptical orifice.The tip is then ground to obtain a flat surface. It is essential that only aminimnmlength of the orifice5 be retained in order to get a horizontal jet of liquid of reasonable length (-7-10 cm). A few such capillaries are prepared, and a suitable one is selected by observing the number of discernible nodes formed. One should obtain over 10 waves in order to make measurements up to 50 ms. For the setup in Figure 3, around 500-800 mL of the surfactant solution would be required. However, the amount can be reduced to 300-400 mL by suitahle modifications.
Measurement For measurement the liquid in the beaker is allowed to emerge from the orifice through a pinchcock arrangement at a selected constant flow rate that is fixed hy the height of the liquid level in the beaker B above the orifice. The emerging wavy jet is photographed with the helpof a35-mrn camera. Sharp photographsareobtained if a oawerful lamo source (500-W haloeen lamo) havine a rectaneular " slit 113cm X .3 rm in vur srrtrp) to cover the number r,f waves in the jet is used.'l'he lampand slit arc placed h~,rlro~~t~llgal~mg rhe lmeof the jet just behind it.
Figure 4. Effect of elecbolytes on DST. A. 2.4 mM SDBS solution; B. 2.4 mM SDBS in pressure of common detergent ingredients. 7.4 mM NatS04. 3.8 mM Na6P.0,., and 0.6 mM Na&O..
F@M 3. The dynamic surtace tension setup. Inset shows mica1 capillary dlmenalona (a = 0.075 cm, b = 0.045 cm).
Surface tension values are calculated using the equation
where? issurfare tension in mNm-', k is the capillary constant, o is the density of the rolutiun, V is the flow rate in milliliters per second, and A is the wavelength of the wave in question. 'l'hc millisecond value for each wave can he calculated from the distanced of the midpoint of the wave from the orifice and the velocity of the jet 9
where a is the cross-sectional area of the ellintical orificee. The constant k is determined from an experiment performed with dratilled water using the value of -,tilo = 72.R mNm-' at 23 T. In a typical experiment with 0.08% SDBS, the following values were ~
~
~~
~
~
~
~~~
~~~~
~
~
~
obtained: p = 0.998 g mL-I, V = 2.0 mL s-', 9 = 188 cm s-' corresponding to 5.3 ms cm-', and k = 15.07 cm-'. In Figure 1 a photograph of a typical jet is shown, and in Figure 2 plots of y vs. time (ms) for some typical surfactant materials is shown. I t can he seen that even small amounts of the saturated fatty acid soap, potassium palmitate affects the rate of surface tension lowering of the syndet dodeeyl benzene sulfonate (SDBS) substantially while even large amounts of the unsaturated soap, oleate does not affect the S T lowering rate appreciably.
Suggested Experiments Some typical experiments for classroom demonstration can include t h e effect of electrolyres in increasing t h e rate of reduction of surface tension of ionic surfactants (a typical example is shown i n Fig. 4 where the presence of electrolytes brings down t h e ST spontaneously) a n d synergistic interactions i n mixtures of surfactants of either similar or dissimilar types. The area of the ellipse Tab, where aand bare the semi-major and semi-minor axes, respectively, can be calculated by measuring aand b microscooicallv. or the area of an unknown orifice can be directlv determined'by c&nparlng the velocity of the jet for a known capillary at the same pressure.
Volume 84
Number 4
April 1987
379
