Langmuir zyxwvu 1995,11, zyxwvu 3491-3499 3491 zyxwvutsrq Wetting Characteristics and Stability of Langmuir-Blodgett Carboxylate Monolayers at the Surfaces of Calcite and Fluorite Woo-Hyuk zyxwvut Jang, Jaroslaw Drelich, and Jan D. Miller* 412 WBB, Department zyxwvuts of Metallurgical Engineering, University zyxw of Utah, Salt Lake City, Utah 84112 Received November 28, 1994. In Final Form: May 26, 1995@ Although surface chemistry fundamentals of semisolublemineralshave been studiedby many researchers, detailed understanding of these systems is still incomplete. In situ Fourier transform infrared internal reflection spectroscopy (FT-IWIRS) techniques have recently been usedto successfullydescribe the adsorption of carboxylates at semisoluble mineral surfaces. The wetting characteristics of these adsorbed films, however, require further consideration. In this regard, the hydrophobicity and stability of transferred Langmuir-Blodgett (LB) monolayers of fatty acids at fluorite and calcite surfaces have been studied by contact angle measurements with water and diiodomethane. Generally, it was found that the transferred LB monolayers of fatty acids at a calcite surface are unstable whereas such monolayers transferred onto a fluorite surface are stable, as revealed from advancingand receding contact angle measurements. These results are believed to be due to incomplete reaction of the fatty acid monolayer at the calcite surface. In addition it was found that a closely packed well-ordered stearate monolayer similar to that of a transferred LB monolayer can be formed at fluoritesurfacesby spontaneous adsorption and self-assemblyfrom aqueous solutions. Introduction Semisoluble salt minerals such as fluorite (CaF21,calcite (CaC03), apatite (Calo(P04)a2 (X zyxwvuts = F, OH, or Cl)),schee- lite (CaWOJ, gypsum (CaSO4*2H20), magnesite (MgC03), and barite (BaS04) are commonly used by industry as raw materials for fillers, fluxes, fertilizers, cements, chem- icals, etc. These semisoluble salt minerals are naturally hydrophilic, and fatty acids are frequently used zyxwvu to establish a hydrophobic state and thus modify their surface-wetting properties. Without such modification the separation of semisoluble salt minerals by flotation cannot be achieved. Also, such treatment facilitates the use of these semi- soluble salt minerals as fillers for plastics in the production ofpolymer-mineral composites. In this regard, an under- standing of the surface chemistry of fatty acid adsorption at semisoluble salt mineral surfaces is important from both a fundamental and a practical point of view., It has been recognized from infrared studies that a fatty acid adsorbs at the surfaces of semisoluble salt minerals through chemisorption of the carboxylate group with a lattice cation at low collectorconcentrations, typically less than 1.0 x M.1-5 At higher concentrations of the collector andor the cation, and at higher pH values, surface precipitation or collector colloid adsorption occurs by reaction of the carboxylate collector with cations released from the lattice.2-8 Because the fatty acid interacts not only with the lattice cation sites of the semisoluble salt mineral but also with the dissolved cations in solution, the solution chemistry of these minerals and the pH of @ Abstract Dublished inAdvanceACSAbstracts. August 1,1995. (1) Peck, A.*S.;Wadsworth, M. E. Prepr. Pap.-& Mrner. Process. (2) Finkelstein, N. P. Trans.-Inst. Min. Metall. 1989,98(C), 157- Congr., 7th 1966, 259-267. 176. -. .. (3) Gong, Q. W.; Parentich, P.; Little, L. H.; Warren, L. J. Langmuir (4) Kellar, J. J.; Young, C. A.; Knutson, K.; Miller, J. D. J. Colloid zyxwvuts (5) Young, C. A.; Miller, J. D. To be submitted for publication. (6) Laskowski, J. S.; Vurdela, R. M.; Liu, Q In Prepr. Pup.-Int. Miner. (7) Bunge, R. C.; Fuerstenau, D. W. SME Annual Meeting, Albu- (8) Free, M. L.; Miller, J. D. Int. J. Miner. Process., submitted. 1992, 8, 118-124. Interface Sci. 1991,144,381-389. Process. Congr., 16th 703-715. querque, NM February 14-17, 1994; Preprint. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0743-7463195f2411-3491$09.00/0 00 0 0 0 0 Calcite (In-situ FT-NIMRS) '10-6 10.5 10'~ 10-2 Equilibrium Oleate Concentration (M) Figure 1. Oleate adsorption isotherms for fluorite (pH 9.5)s and calcite (pH 9.2)12 as determined by in situ FT-IMRS experiments (the original isotherms have been reconstructed using correction factors of 2.3 for fluorite and 1.2 for calcite, based upon sample area con~iderationsl~). the system play an important role in determining the nature of the adsorbed species. Oleate adsorption isotherms for semisoluble salt min- erals have been established by several resear~hers.~-l~ Figure 1 presents the adsorption isotherms for oleate at fluoriteg and calcite12 surfaces (pH = 9) as determined using in situ FT-IMRS (note that the original isotherms from refs 9 and 12 have been reconstructed usingcorrection factors of 2.3 for fluorite and 1.2 for calcite, based upon sampling area consideration^'^). Although the shape of the oleate adsorption isotherm is similar at both fluorite (9) Kellar, J. J.; Cross, W. M.; Miller, J. D. Appl. Spectrosc. 1989,43, (10) Hu, J. S.; Misra, M.; Miller, J. D. Int. J. Miner. Process. 1988, 1456-1459. 18. 57-72. ,-- -_ (11) Rao, K. H.; Cases, J. M.; Forssberg, K. S. E. J. Colloid Interface (12) Young, C. A.; Miller, J. D. SME Annual Meeting, Reno, NV, (13) Mielczarski, J. A.; Cases, J. M.; Bouquet, E.; Barres, 0.; Delon, -. J. D. Colloids Surf A 1994,93, Sci. 1991, 145,330-348. February 15-18, 1993; Preprint. J. F. J. F. Langmuir 199% 9, 2370-2382. (14) Free, M : L.; Jang, W. H.; Mille] 127-135. 0 1995 American Chemical Society