Adsorption of Ions to the Surface of Dilute Electrolyte Solutions: The Jones-Ray Effect Revisited Poul B. Petersen and Richard J. Saykally* Contribution from the Department of Chemistry, UniVersity of California, Berkeley, California 94720 Received May 17, 2005; E-mail: Saykally@berkeley.edu Abstract: The controversial observation of a minimum in the surface tension of dilute aqueous electrolyte solutions by Jones and Ray in the 1930s is confirmed by new resonance-enhanced second harmonic generation (SHG) experiments demonstrating surface enhancement of simple inorganic anions in the same concentration range. New experiments show that the quadruply charged ferrocyanide, Fe(CN)6 4- , anion is not surface active at high concentrations, as expected, but at dilute concentrations, the anion is strongly attracted to the interface with a Gibbs free energy of adsorption of -6.8 kcal/mol. Using this value, the original Jones and Ray data are fit to a simple model of the surface tension with qualitative agreement, although better agreement is found for all 13 Jones and Ray salts with an even stronger surface adsorption. 1. Introduction In the period 1935-42, Jones and Ray published five controversial papers in this journal concerning the surface tension of aqueous electrolyte solutions at dilute concentrations. 1-5 Using the capillary rise method, they measured a minimum in the surface tension near 1 mM for 13 different inorganic salts. A decreasing surface tension implies a net surface excess of the ions, contrary to the accepted theories, which hold that electrolytes are repelled from the interface and the outermost surface layer of water is completely devoid of ions. The original papers have been followed by others both supporting and refuting the finding, but the “Jones-Ray effect” remains today an unresolved controversy. In 1934, Onsager and Samaras had just published their model of the surface tension effects, based on a continuum dielectric media and describing the ions as point charges. 6 The ions were repelled from the surface by image charge forces, as first proposed by Wagner, 7 leaving the outermost surface layer virtually free of ions. Moreover, Langmuir had already proposed a model in 1917 for the surface of electrolyte solutions with the outermost layer of the interface (∼4 Å) being pure water atop of a uniform solution. 8 He was thus quick to dismiss the Jones-Ray findings as an artifact due to the experimental technique, viz. adding ions to water changes the thickness of the wetting layer inside the capillary by several hundred Å, effectively altering the capillary radius. 9,10 Others were more receptive. Bikerman proposed a model for the surface tension involving three contributions that could account for the surface tension minimum. 11 Dole first presented his model for the surface tension 12,13 and then, with Swartout, reproduced the experimental minimum in the surface tension for KCl using an advanced version of the ring method that is unaffected by the artifacts proposed by Langmuir. 14 The theoretical model of Dole is close to the one we present here. An alternative approach to measuring the surface tension is the maximum bubble pressure method, wherein gas bubbles of an inert gas are formed at the end of a capillary tube at a given rate. The method is thus a dynamic measurement, with bubble lifetimes ranging from one to hundreds of seconds. The first bubble pressure experiment on dilute electrolyte solutions failed to observed a minimum for all bubble lifetimes (5-120 s). 15 A second study reproduced the surface tension minimum at long bubble lifetimes (120 s) but not at short (15 s) times and attributed the Jones-Ray effect to organic contaminations building up at the surface, although identical results were obtained for samples prepared from both a powdered and single- crystal salt. 16 A third study first dismissed the Jones-Ray effect on a thermodynamic basis 17 but later observed a minimum in the surface tension with the bubble pressure method, for all bubble lifetimes. 18 In this case, the surface tension minimum was greatest at short times (12 s) but still observable at longer times (250-500 s), and the authors attributed the observation to nonequilibrium electrification dynamics of the surface. (1) Jones, G.; Ray, W. A. J. Am. Chem. Soc. 1935, 57, 957-958. (2) Jones, G.; Ray, W. A. J. Am. Chem. Soc. 1937, 59, 187-198. (3) Jones, G.; Ray, W. A. J. Am. Chem. Soc. 1941, 63, 288-294. (4) Jones, G.; Ray, W. A. J. Am. Chem. Soc. 1941, 63, 3262-3263. (5) Jones, G.; Ray, W. A. J. Am. Chem. Soc. 1942, 64, 2744-2745. (6) Onsager, L.; Samaras, N. N. T. J. Phys. Chem. 1934, 2, 528-536. (7) Wagner, V. C. Phys. Z. 1924, 15, 474-477. (8) Langmuir, I. J. Am. Chem. Soc. 1917, 39, 1848-1906. (9) Langmuir, I. Science 1938, 88, 430-432. (10) Langmuir, I. J. Chem. Phys. 1938, 6, 873-896. (11) Bikerman, J. J. Trans. Faraday. Soc. 1938, 34, 1268-1274. (12) Dole, M. Nature 1937, 140, 464-465. (13) Dole, M. J. Am. Chem. Soc. 1938, 60, 904-911. (14) Dole, M.; Swartout, J. A. J. Am. Chem. Soc. 1940, 62, 3039-3045. (15) Long, F. A.; Nutting, G. C. J. Am. Chem. Soc. 1942, 64, 2476-2482. (16) Passoth, G. Z. Phys. Chem. 1959, 211, 129-147. (17) Rusanov, A. I.; Faktor, E. A. Russ. Chem. ReV. 1974, 43, 933-950. (18) Kochurova, N. N.; Rusanov, A. I.; Myrzakhmetova, N. O. Dokl. Phys. Chem. 1991, 316, 176-178. Published on Web 10/18/2005 15446 9 J. AM. CHEM. SOC. 2005, 127, 15446-15452 10.1021/ja053224w CCC: $30.25 © 2005 American Chemical Society