IN contrast to ionic detergents, the non-ionic type provides an opportunity to alter the hydrophilic portion of the molecule by varying the number of ethylene oxide units in the polyoxyethylene chain ; lengthening this chain increases the hydrophilic properties of the molecules. A series of compounds based on hexadecyl alcohol, of general formula :Me(CH2)15(0CH2CH2)nOH where n varied from six to twenty-one, has been synthesised (Elworthy and Macfarlane, 1962). A study of their micellar shape and hydration has been made. In the present communication the critical micelle concentrations (CMC) and areas/molecule at the air/water interface are reported.APPARATUS AND METHOD Surface tension measurements were made using the Wilhelmy plate method (Harkins and Anderson, 1937). A depolished platinum plate, perimeter, 1 = 6.045 cm., was suspended from the arm of a torsion balance, sensitivity 40.2 mg. The flask containing the solution was lifted by a rack and pinion device until the plate just touched the surface of the solution. The weight to lift the plate from the interior of the liquid to this point in the surface was determined. To ensure zero contact angle, all measurements were made by lifting the plate in this manner, and a saturated atmosphere was maintained above the solution. The difference between the weight of the plate in air and in the surface, W, is related to the surface tension, y , byMeasurements were made at 25" f 0.01". All glassware and the plate were cleaned with chromic acid, followed by thorough rinsing in distilled water. All flasks containing solutions were set aside for 3 hr., the flasks thoroughly drained, and fresh solutions made up in the same flasks. This procedure compensates for adsorption of solute by the glassware.
RESULTS AND DISCUSSIONA check of the apparatus was made by measuring the surface tension of water, y = 72.0, 72.1 dynes/cm. (Harkins, 1959, gives 72.0 dynes/cm.), and of methanol, y = 22.3 (Timmermans, 1950, gives 22.2 dynes/cm.). Ageing effects were noted for all but the most concentrated solutions 100 T