Concentration-dependence of faradaic currents and conductivity in a polystyrene sulfonic latex suspension Chengming Wang, Toshiyuki Nagahashi, Koichi Aoki *, Jingyuan Chen Department of Applied Physics, Fukui University, 3-9-1, Bunkyo, Fukui-shi 910-8507, Japan Received 26 March 2002; received in revised form 15 May 2002; accepted 9 June 2002 Abstract The conductivity, k , in a suspension of polystyrene sulfonic latex without supporting electrolyte showed a linear dependence on the volume fraction, v f , of the latex for v f B/0.03 with a finite intercept. In contrast, this deviated upward from the linear line for v f / 0.03. These variations were qualitatively consistent with the dependence of the voltammetric reduction current of H  on v f without supporting electrolyte. The current values were only a few percent of the theoretical diffusion-controlled current that could be observed in the suspension with supporting electrolyte. This fact indicates the electrostatic immobilization of the hydrogen ions by sulfonic latex particles. A plot of the current against k at common values of v f showed that the current for v f / 0.07 was smaller than the value predicted from the conductivity. This can be explained in terms of a combination of the increase in electrostatically unbounded H  estimated by conductance measurements and electric migration in which the electrochemical depletion of [H  ] also causes the depletion of the latex. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polystyrene latex; Colloidal suspensions; Electrostatic immobilization; Conductance; Voltammetry; Reduction of hydrogen ion 1. Introduction A physicochemical field of interest for polyelectro- lytes is the electrostatic interactions [1,2] between polymer and polymer as well as those between polymer and counterion. This is conspicuous especially for suspensions of mono-dispersed microparticles when the foreign electrolyte is removed sufficiently from the suspensions. Then, the particles are well-ordered even in the solution phase [3  /8], and are known as colloidal crystals. The ordering is provided by electrostatic interaction between neighboring charged particles. They exhibit a striking iridescence similar to precious opals [9] when the inter-distance between closest neigh- boring particles is of the order of the wavelength of visible light [10,11]. The iridescence is caused by optical interference of the diffraction from the ordered lattice, and has been analyzed quantitatively by diffraction theory [12], similarly to the method used with X-rays for molecular crystals. The formation of the crystalline structure indicates that counterions are immobilized around latex particles [13,14]. The immobilization of counterions obviously leads to a decrease in ionic mobility. This decrease has been observed as a decrease in the conductivity of the latex suspensions. The decrease is more remarkable with a decrease in concen- tration of supporting electrolyte, because the local ionic atmosphere near the latex particle is more structured against a uniform ionic distribution caused by support- ing electrolyte [15,16]. The conductivity of the suspen- sions shows a linear relation to the molar fraction of the microparticles, and its slope, corresponding to the molar conductivity, decreases with the concentration of foreign electrolyte [17,18]. The faradaic current of a counterion is also a good measure of ion mobility [16,23] if the counterion is electroactive. The current is often measured at micro- electrodes [19 /26] because the microelectrode technique provides a well-defined current  /potential curve without the use of deliberately added supporting electrolyte [27]. The reduction current of hydrogen ion in a polystyrene latex suspension decreased with a decrease in concentra- tion of supporting electrolyte [23  /25]. This observation * Corresponding author. Tel.:  /81-776-27-8665; fax:  /81-776-27- 8494 E-mail address: d930099@icpc00.icpc.fukui-u.ac.jp (K. Aoki). Journal of Electroanalytical Chemistry 530 (2002) 47  /52 www.elsevier.com/locate/jelechem 0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0022-0728(02)01004-5