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
Hakan Çiftçi, Bahri Ersoy, Atilla Evcin
Abstract
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 particle-size 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 ~72–73 o , while they were 12.65 and 14.1 o for two purified and unmodified bentonites. SFEs were calculated using contact-angle data, and decreased to minimum values of 41.5–43.6 mJ/m 2 from 78.6–78.2 mJ/m 2 upon treatment of raw bentonites with CTAB. 100–130% CEC concentration was sufficient to prepare organo-bentonites with maximum hydrophobicity and positively charged surfaces.