Exact Evaluation of the Salt Concentration Dependence of Interparticle Distance in Colloidal Crystals by Ultra-Small-Angle X-ray Scattering. 4. Effect of Counterion Species and the Possibility of New Factors for Colloidal Crystal Formation

Abstract

In our previous papers (Langmuir 1994, 10, 4423; 1996, 12, 5588; 1999, 15, 573), we have reported the salt concentration dependence of interparticle distance in colloidal crystals formed in highly charged colloidal dispersion by performing ultra-small-angle X-ray scattering (USAXS). We found that the interparticle distance−salt concentration relationship had a maximum at κa = 1.3 (κ-1, Debye length; a, particle radius). This result could not be explained by current theories proposed for colloidal stability, such as the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory. From the results of our previous USAXS study, we thought that the interpenetration of ionic clouds around particles and counterion sharing effect play an important role for the formation of colloidal crystal. In this study, we have investigated the added NaOH dependence, i.e., the degree of neutralization dependence of strong acid group at particle surface (a), on the nearest interparticle distance (2Dexp) in colloidal crystals by the USAXS te...