Surface Forces between Silica Surfaces in CnTACl Solutions and Surface Free Energy Characterization of Talc
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Direct surface force measurements were conducted between silica surfaces in octadecyltrimetylammonium chloride (C18TACl) solutions using an Atomic Force Microscope (AFM). The results showed that the hydrophobic force existed in both air-saturated and degassed C18TACl solutions. The attraction decreased with NaCl addition, and was the strongest at the point of charge neutralization (p.c.n.) of silica substrate.
The force measurement results obtained in CnTACl solutions showed that the attractions decayed exponentially and became the maximum at the p.c.n.'s. The decay lengths (D) increased with surfactant chain length. The measured forces were fitted to a charged-patch model of Miklavic et al. (1994) with rather large patch sizes. It was also found that the decay length decreased linearly with the effective concentration of the CH2/CH3 groups raised to the power of -1/2. This finding is in line with the model of Eriksson et al. (1989). It suggested that the long-range attractions are hydrophobic forces originating from the changes in water structure across a hydrophobic surface-solution interface.
For the TiO2/water/TiO2 system, the Hamaker constant was found to be 4x10-20 J. The force curves obtained in the TiO2/CnTACl system showed a repulsion-attraction-repulsion transition with increasing surfactant concentration. The long-range attraction observed between TiO2 surfaces in CnTACl solutions reached maximum at the p.c.n., and the decay length increased with chain length.
In present work, the thin-layer wicking technique was used to determine the surface free energy (gamma s) and its components of talc samples. The results showed that the basal surfaces of talc are weakly basic while the edge surfaces are acidic. The effect of chemicals on the surface free energies of talc was systemically studied. The results showed that CMC (carboxymethyl cellulose sodium salt) and EO/PO (ethylene oxide/propylene oxide) co-polymers made talc surface hydrophilic by increasing the surface free energies, especially gamma LW and gamma -. SOPA (sodium polyacrylate) increased greatly the zeta-potentials instead of the surface free energies.
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