PURIFICATION OF TURKISH BENTONITES AND INVESTIGATION OF THE CONTACT ANGLE, SURFACE FREE ENERGY AND ZETA POTENTIAL PROFIL
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PURIFICATION OF TURKISH BENTONITES AND INVESTIGATION OF THE CONTACT ANGLE, SURFACE FREE ENERGY AND ZETA POTENTIAL PROFILES OF ORGANO-BENTONITES AS A FUNCTION OF CTAB CONCENTRATION H. ÇIFTÇI1 *, B. ERSOY1, AND A. EVCIN2 1
Mining Engineering Department, Afyon Kocatepe University, Afyonkarahisar, Turkey Material Sciences and Engineering Department, Afyon Kocatepe University, Afyonkarahisar, Turkey
2
Abstract—Purification of raw bentonites and organo-bentonite preparations is sometimes required for industrial use. Zeta (electrokinetic) potential (ζ), contact angle (wettability/hydrophobicity), and surface free energy (SFE) are important surface characteristics and vary significantly according to the applied surfactant concentration when preparing organo-bentonite. Changes in these characteristics determine the stability, behavior, and efficiency of organo-bentonites in various applications such as adsorption, composite materials, and drug-delivery systems. Knowing how much surfactant should be used to prepare organo-bentonite is, therefore, critical. The purpose of the present study was to determine the effect of concentration of the cationic surfactant cetyltrimethylammonium bromide (CTAB) adsorbed in organo-bentonite (prepared from two local and commercial raw bentonites with potential for use in adsorbent and composite materials) on the ζ potential, contact angle, and SFE profiles. The raw bentonites were purified using sedimentation and centrifugation techniques prior to preparation of the organo-bentonite. The purification results were evaluated in light of X-ray diffraction (XRD), cation exchange capacity (CEC), free swelling volume (FSV), X-ray fluorescence (XRF), and particlesize analysis data. Most of the gangue minerals (feldspar, calcite, clinoptilolite, opal, quartz, and mica) having particle size >5 μm were removed from the raw bentonites by using a one-stage sedimentation or a Falcon gravity separator (FGS). Higher yields (68.8% and 81.3% for two bentonites) were obtained with the FGS compared to sedimentation while purification levels were almost the same. ζ changed greatly from –35 mV (and –40 mV) toward 38 mV (and 40 mV) with increasing CTAB concentrations. Similar profiles were also obtained for wettability; maximum contact angles for organo-bentonites were measured as ~72–73o, while they were 12.65 and 14.1o for two purified and unmodified bentonites. SFEs were calculated using contact-angle data, and decreased to minimum values of 41.5–43.6 mJ/m2 from 78.6–78.2 mJ/m2 upon treatment of raw bentonites with CTAB. 100–130% CEC concentration was sufficient to prepare organo-bentonites with maximum hydrophobicity and positively charged surfaces. Keywords—–Bentonite . Contact angle . Montmorillonite . Organo-bentonite . Purification . Surface free energy . Zeta potential INTRODUCTION Bentonite, consisting predominantly of smectite minerals (montmorillonite, saponite, nontronite, etc.) with a layered structure, is a hydrous aluminum and magnesium silicate with a particle size of 150o. Materials are calle
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