A LABORATORY EXPERIMENT BASED ON THE FREUNDLICH SURFACE ACTIVITY EQUATION GEORGE W. BENNETT,GROVECITY COLLEGE. GROVE CITY, PENNSYLVANIA
The scarcity of simple pantitatiue experiments for students in colloid chemistry is noted, and an exfieriment of this character based on the Freundlich surface activity equation is devised. The constants in this equation are aaluated, their significance is found, and the limitations of this empirical expression are ascertained by the studat as he pqforms the experiment.
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Laboratory instruction in colloid chemistry suffers from a paucity of simple experiments involving quantitative measurements. Such experiments, moreover, are needed to supplement the recent colloid chemistry textbooks which include a number of mathematical expressions With this idea in mind a simple quantitative experiment based on the Freundlich surface activity equation has been devised, and is presented herewith in the hope that it may be of value to those who teach colloid chemistry. The Freundlich equation (1) states that the relative lowering of the surface tension of a liquid by a solute can be expressed by the empirical equation A =
Gaol.
-
olin. =
kCl'n
Wlol.
in which Cis the concentration of the solute in mols per liter, and k and n are constants. The left half of the equation is usually abbreviated by the use of the symbol A. Expressed logarithmically the equation takes the form logA = l o g k + l / n l o g C
It is commonly understood, and Freundlich so states, that this expression states the facts for only a limited range of concentrations, and fails at greater concentrations. This limitation and the tediousness of many surface tension measurements would seem to preclude the possibility of devising a suitable laboratory exercise based on this equation. Nevertheless, a t least some solutes give results that conform to this expression over a fairly wide range of concentrations; and the determination of the surface tension with the du Nuoy tensimeter is both rapid and simple. An expression that ranks in importance with the adsorption isotherm can thus be examined experimentally by the student. Experimental Solutions of methanol, ethanol, propanol, acetone, or acetic acid in water are made up, and their concentrations determined by any suitable methods. Aqueous solutions of methanol and propanol can be made by careful volumetric measurement if the assumption is made that there is no appreci517
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able volume change on mixing these materials; for aqueous solutions of ethyl alcohol such an assumption cannot be made, but the solutions must be analyzed-suitably by determination of the density and reference to standard tables. The liquids used were standard laboratory materials
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some of the organic liquids used are quite volatile a wide low-walled container cannot be used to hold the sample when the surface tension is determined. In place of the usual watch glass a small straight-walled jar, 3 cm. by 5 cm. is used to hold the sample, and the platinum loop of the instrument is lowered by means of a hook 3 cm. long attached to the arm of the tensimeter. The tension in the wire is then increased until zero reading is obtained. The surface tensions of pure liquids determined in such a small vessel do not differ noticeably from those determined in a wider vessel. The platinum ring of the instrument and the sample container must, of course, he kept perfectly grease-free. For purposes of calculating and graphing the results we have used the 1.5-
1.5
1.25
1.25
1.0 .9 .8
1.0
.7
.9 .8 .7
.6
.6
.5
.5
.4
.4
.3
.3
.2
.2
.15
.15
.1
.1
A
A
.3
.4
.5 .6 .7.8.91.0
1.5
2
3
4
5
6 7 8 910
15
Concentration. FIGURE 2
scale readings of the tensimeter itself rather than its corresponding value in dynes. The values of A can be calculated with d c i e n t accuracy by means of the slide-rule. These values may then be plotted against the concentrations in mols per liter on log log paper as in Figure 2, and if it is desired, the surface tension may be plotted against concentration as in Figure 1. Typical results of experiments of this nature are shown in the table and in the two figures. In the table the concentration in mols per liter of the constituent stated is given in the column marked C; in two cases the instrument readings are given in the columns marked "Scale Reading," but these values were omitted in the other cases for brevity; the relative lower-
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TABLE
ing of the surface tension is given in the columns marked A. Values of n and k are also given for each solute. The value for n in each case has been determined from the slope of the curves themselves rather than from the observed values of A, since values of n so calculated do not check well with each other. In Figure 1the surface tensions expressed as scale readings on our tensimeter are plotted against the concentration in mols per liter for the three alcohols, and for simplicity the curves for acetone and acetic acid are omitted. In Figure 2 the values for A are plotted against concentrations in mols per liter. In this figure the open circles mark the values for the alcohols; solid circles mark those for acetone, while the triangles indicate the values for acetic acid. Discussion The results illustrate very well that such divergent curves as those in Figure 1can all be expressed by a single equation, and it is impressive to the student to see how these curves straighten out to give the corresponding
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521
lines in Figure 2. In Figure 2 similarity of the slopes of all limes confirms the fact that n is a constant of little specific character, although it should be observed that the curves for members of the homologous series are nearly parallel. The increasing values for k on the other hand indicate that in general the surface activity in an homologous series increases with the number of carbon atoms. In the case of acetic acid there is a sharp break point in the curve as shown in Figure 2. It is at this point that the Freundlich equation begins to fail. Similar behavior is shown by the other liquids used when the concentrations are made too great, but this tendency has been indicated for acetic acid only. In like manner it is generally true that the first point in the table for each liquid does not fit the curve, and presumably we are dealing with the same phenomenon here. These disagreements with the equation serve admirably to emphasize its empirical nature. The amount of time to be spent on this experiment will be determined by the instructor. Excellent results can be had in a single afternoon if the student receives the solutions already analyzed, and if the results of several students, each workimg on a different system, are pooled.
Literature Cited "Colloid and Ca~illarv (1) . . FREUNDLICH. . . Chemistry." E. P. Dutton & Co.. New York City. 1922, p. 63. (2) LLOYDAND Swrnrn, Science, 64, 253 (1926).
