Experimental Verification of "Electrodynamic" Interaction

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We have proposed "Electrodynamic Interaction" as a driving force of colloidal crystal formation. To clarify the mechanism of this novel force between ionic polymer particles, we have performed the systematic experiment by ultra-small-angle X-ray scattering (USAXS). The interparticle distance in the colloidal crystal was believed to decrease with increasing ionic strength. However, we found that it increased when added salt concentration is below 10^<-5>M. This fact means the effective interaction between colloidal particles should be an attractive one, which should based on difference mechanism from the theory already proposed, however. We thought that the counterions around particles have an important role for colloidal crystal formation. As the first experiment, we changed counterion species from proton to sodium ion. With exchanging from proton to sodium ion, the interpaticle distance changed. This fact certainly means that the interpaticle interaction is affected by counterion species. Secondaly, we used potassium, lithium, and tetramethylammonium (TMA) ions as counterions. The interparticle distance evaluated by USAXS again depended on the counterions species. These ions made the attractive interaction weaker than proton, and this effect is stronger in the order of potassium>sodium>TMA=lithium. This order is in good agreement in that for surface charge density of these cations, i.e. diffusion constant of counterions. Hence, we can conclude as following;(1) "Electrodynamic Interaction" is effective when ionic strength is below 10^<-5>M.(2) At this condition, the counterion atmosphere is highly overlapped.(3) The counterions in atmosphere can move anywhere in the colloidal crystal.(4) The interaction force depends on speed of counterion diffusion.(5) The strong attractive interaction is observed only when counterion is proton. Hence, the origin of attraction should be anomalously high electric conductivity of protons.