Influence of protonation level on the specific surface area of polyaniline and its application to surface processes

Abstract

The variation In the specific surface areas of polyaniline (PANI) and polyaniline/silica (PANI/Si02) substrates was monitored at different protonation levels of the system. Aniline was polymerized chemically from an acidic solution using an oxidant, ammonium peroxydisulfate. Polyaniline thus obtained was treated with double distilled water (pH = 6.95), aqueous hydrochloric acid (pH = 1.09) and aqueous ammonia (pH = 10.15) solutions in order to make the PANI matrix neutral and charged, respectively. Preparation of PANI/Si02 was performed by the chemical polymerization of aniline in the presence of fine colloidal silica particles in aqueous media. A stable PANI/Si02 composite was' found to be formed in the reaction mixture under experimental conditions. These substrates were characterized by a wide range of experimental techniques including Infrared (IR) spectroscopy, X-ray diffraction and scanning electron microscopy (SEM) measurements. The treated PANI and PANI/Si02 matrices were found to be correlated with the idealized forms of PANI protonated states proposed by A. G. MacDiarmid. The acid treated matrix seemed to be positively charged while the other two matrices seemed to be either neutral or negatively charged. Infrared spectroscopy studies yielded qualitative information. on the treated PANI and PANI/Si02 substrates. From the spectra, the band characteristics confirmed differences between treated PANI matrices. Also, the spectra exhibited absorption bands attributable to both polymer and silica components. The diffused X-ray scattering patterns of the treated PANI and PANI/Si02 indicated the amorphous nature of the substrates. The scattering pattern of the PANI and that ofPANI/Si02 seemed to be indifferent. This finding suggested that incorporation of silica particles did not have any influence on the structure of the substrates. Surface morphology obtained by SEM provided very interesting results. The acid treated PANI is consisted of agglomerates and stacked over the surface to make a big deposit. On the other hand, double distilled water treated matrices showed granular morphology in which grain 1 Abstract aggregated to a stone like body. The base treated PANI matrices also showed granular morphology but the grains were collected to a body like coral with sharp edges. For the PANI/Si02 substrate the morphology was found different compared to PANI. In this case most of the portionSwere powdery and other portionswere granular with a shape like small stone. The specific surface areas of PANI and PANI/Si02 substrates were determined from an aqueous solution of methylene blue (MB) dyestuff by allowing the dye to adsorb onto the surface of the substrates. This adsorption phenomenon produced the monolayer coverage on the surface of the adsorbent. The PANI and PANI/Si02 substrates were used as adsorbents and found to be adsorbed the dyestuff, MB from its aqueous solutions at various pH. The adsorption was monitored spectroscopically by measuring the change of absorbance at Amax 664 nrn. The cationic dye, MB showed a significant adsorption on the base treated PANI (negatively charged) and PANI/Si02. The adsorption isotherms were found to follow Langmuir isotherm. The specific surface area was calculated using the value of monolayer capacity, which was estimated from the plot of amoUlit of MB, adsorbed against equilibrium concentration of the MB solution. For two different molecular cross-sectional areas ofMB (130 N, BET with N2 and 78 A2, BET with Ar) the specific surface area of acidic PANI were found as 15 and 9 m2 g'!, for neutral PANI they were 11 and 7 m2 g'l, for basic PAN! 29 and 17m2 g' 1, for PANI/Si02 in acidic condition 21and 13 m2 g'!, PANI/Si02 in neutral condition 18 and 11m2 g'l, and PANI/Si02 in basic condition 33 and '20 m2 g'l, respectively. The surface area measurements showed higher area for basic PANI than the acidic and neutral PANI substrates and PANI/Si02 composite showed more surface area than that ofPANI. The incorporation of fine silica particles to the polymer might lead to this increased surface area.