On the Nature of Ionic Sign Preference in C. T. R. Wilson Cloud Chamber Condensation Experiments

Abstract

Experiments made to ascertain if the charge preference in the C. T. R. Wilson cloud method of condensation was due to different hygroscopic impurities brought into the embryonic droplets by capture of ions of different sign proved definitely negative. Since the electrical double layers at liquid‐gas surfaces are also incapable of explaining the phenomenon the following hypothesis was evolved and compared to existing experimental data together with results obtained for the first time on C₆H₅NO₂, C₆H₅Cl, C₄H₉Br, C₆H₅CH₃, C₆H₅NH₂ and new results on C₂H₆CO, C₆H₆ and C₂H₅I. It is assumed that the saturated vapor contains embryonic droplets of radius insufficient to bring surface tension forces into play. These droplets unlike the larger and more stable visible and invisible droplets are in a state neither crystalline nor liquid but in a pseudo‐crystalline state having definite space structure. In such pseudo‐crystalline or embryonic droplets the orientation of the molecules is determined by van der Waals forces which are directive. As in Thomson's theory at lower supersaturation the nascent surface tension forces preclude growth beyond a certain size. Capture of gaseous ion as in Thomson's theory facilitates condensation by adding dielectric attractive forces. If, the droplet and vapor consist of polar molecules whose dipole moment is oriented in such a direction as to bring approaching vapor molecules to the pseudo crystal surface in an orientation favorable to further structural growth the sign of charge favoring this process will produce visible droplets at a lower supersaturation than will the opposite sign. Once condensation has proceeded to a point where surface tension forces are fully developed the droplet goes over to the liquid state having an electrical double layer and neither sign of charge have any further influence on condensation. Using as the directive forces in condensation primarily hydrogen bond formation and dipole attraction the effects observed experimentally can be accounted for on plausible assumptions as to linkage in the embryonic drops. The reason that gaseous ions are not observed to grow to these droplets can be ascribed to the fact that these are already stable complex ions of very few molecules whose orientation is not that of the pseudo‐crystals. A capture of such an ion by a pseudo‐crystal of 10² to 10⁵ molecules will not, however, affect the pseudo‐crystal other than to facilitate condensation. It is believed possible that it is these pseudo‐crystalline droplets or groupings existing in liquids that account for the cybotatic state described by G. W. Stewart from x‐ray studies. In gases it is possible that they constitute the intermediate atmospheric ions when large enough to survive the acquisition of a charge. The larger or Langevin ions are then probably the nucleii once formed by supersaturation and condensation to visible size and which have subsequently re‐evaporated to invisible liquid droplets with surface tension forces active (10⁷ or more molecules) that have acquired a charge. The three groups of ions mentioned exist independently with charged carriers of sizes lying between them entirely absent except during the unstable conditions producing transition.