Differences in surface tensions of non-ionic surfactant solutions as measured by the drop-volume and Wilhelmy-plate techniques

Abstract

The surface tensions of aqueous solutions of hexaethylene oxide n-dodecyl ether (C₁₂E₆) have been measured in the region of the critical micelle concentration (c.m.c.) using both the drop-volume and the Wilhelmy-plate techniques. The data obtained gave similar values for the c.m.c. (8.5 × 10^–5 mol dm^–3). However, significant surface-tension differences occurred in the decade of concentration below the c.m.c., the differences being manifested in apparent values for the limiting surface areas per molecule of 63 Ų molecule^–1(drop-volume) and 73 Ų molecule^–1(plate), or else as a surface tension difference of ca. 3 mN m^–1 for 10^–5 mol dm^–3 solutions. The discrepancy is discussed in terms of Marangoni effects operative in the drop detachment process which modify the Harkins and Brown correction factor. The effect is expected to be important in those systems which show a high surface dilational modulus, such as non-ionics of low c.m.c. and protein solutions. The use of the drop-volume technique in such systems is unsound.