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Thesilica particle interactions in the presence of spermidinewere systematically investigated both from experimental and theoreticalpoints of view. The hydrodynamic radii and the corresponding polydispersityindices of the colloidal silica particles were determined by dynamiclight scattering (DLS) as a function of spermidine concentration.Whereas the effective size of the silica particles increases withincreasing spermidine concentration (pointing to the particle aggregation),the polydispersity index first increases reaches a maximum and thenfurther decreases with the increasing spermidine concentration. Fromthe mobility measurements it was concluded that the increase in spermidineconcentration causes less negative values of zeta potential, meaningthat the adsorption of spermidine leads to the less negative silicasurface. Moreover, Monte Carlo (MC) simulations also confirmed thatthe addition of spermidine reduces the repulsion between silica particles.The MC concentration profiles of spermidine close to the charged silicaparticle are in a very good agreement with the results obtained bytheory. An important motivation for our study is the effectivenessof multivalent ions to coagulate colloidal suspensions; e.g., themultivalent ions are exploited in the water purification process. [ABSTRACT FROM AUTHOR]

Many separation processes are relatedto the behavior of ions closeto charged surfaces. In this work, we examine uranyl ions, which canbe considered as rod-like molecular ions with a spatially distributedcharge, embedded in a system of like charged surfaces. The analysisof the system is based on an approximate field theory which is accuratefrom the weak to the strong electrostatic coupling regimes. The numericalresults show that close to the charged surface the ions are orientedparallel to the surface, whereas at distances greater than half ofthe ion length, they are randomly oriented. Due to the restrictionof the orientational degrees of freedom, the density of ions at thecharged surface decreases to zero. For large surface charge densities,the force between like charged surfaces becomes attractive, as a resultof charge correlations. The theoretical results are in good agreementwith Monte Carlo simulation results. [ABSTRACT FROM AUTHOR]