1446
Langmuir 1993,9, 1446-1448
Molecular Modeling for Oxidative Cross-Linking of Oleates Adsorbed on Surfaces of Minerals D.had,' M. Kaftory,' and A. B.Zolotoy Department of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel
N. P. Finkelstein and A. Weissman IMI-Institute for Research and Development, Haifa, Israel Received January 12,1993. In Final Form: March 16,1993
Oleic acid, chemicallyadsorbed on surfaces of certain minerals,undergoesoxidation of the double bonds in aqueous media at elevated temperatures. The oxidatively cross-linked molecules increase the hydrophobicity of the mineral surface dramatically. The reaction has been shown to take place on the surfaces of CaFz and MgCOs but not on CaC03. Molecular modeling of the resultant product (ratherthan the prereaction molecularorganization)on the surfacesof CaFzand CaCO3providesa rational explanation for the difference in behavior.
Introduction Fatty acids,prominent among them oleic acid, are used to change the surface properties of calcium- and magnesium-containing minerals and other solid phases in industrial practice, especially in flotation and in the treatment of fillers for plastics. Although such reagents can adsorbto the extent of severalmonolayers,the required effects are generally achieved by additions equivalent to a monolayer. Arecent review' showed that the adsorption of the first layer involves the formation on an oleate-tocalcium ionic bond and that, although the conformation of the adsorbed molecules is critical in determining the properties of the treated surface,experimentaldifficulties have precluded much study of this aspect. It has been known for several years that exposure of aqueous suspensions of many oleate-treated solids, e.g., calcium fluoride, magnesium carbonates, and apatite to elevated temperatures and aeration caused a dramatic increase in hydrophobicity of the surface. For CaFz2and MgCOs3 it has been shown that such treatment oxidizes the double bond with the formation of an ether linkage between adjacent adsorbed molecules. This oxidative cross-linking does not occur if the mineral is CaC03.l~~ The reason for this clear difference in behavior is not known. Since the cross-linking oxidation is independent of the chemical entities on the surface (the oxidation occurs on the surface of CaFz and MgCOs but not on the surface of CaCOs or MgFz) catalysis is not considered and we assume that the difference stems from differences in symmetry and geometry of tb lattice of the solid surface. We have used molecular modeling6 to explore differences in the cross-linkingtendency of oleate adsorbed on CaC03 (calcite form) and CaF2. The theoretical methodology that is usually applied in the study of reaction mechanismof small moleculescannot
* To whom correspondenceshould be addreseed.
(1) Finkeletein, N. P. Review of interactions in flotation of sparingly eolublecalciummineralewithanioniccollectors. Trane.Znet.Min.Metall., Sect C 1989,98, C167. (2) Hu, J. 9.; Misra, M.; Miller, J. D., Znt. J. Mineral Process 1986,18, 73. (3) Brandao, P. R. G.; Poling, G. V. In XVI Zntemtionul Mineral Pmceesing Congress;Forasberg, E., Ed.; Amsterdam, 1988; p 1463. (4) (a) Miller,J. D. Privata communicationof N. P. Finlreletain,1992. (b) Gieeekke, E. W.; Harris, P. J. In MintektiO; Council for Mineral Technolagy: Randburg, 19W, Vol. 2, p 269. (6) The molecularmodelingwaa performedby usingthe set of programs Quanta with Silicon Graphice hardware.
be adopted here. The number of parameters to be examined is too large. We wggest here an alternative methodology for the study of the possible product formation from long chain adsorbed molecules. The idea is to optimize a d compare the energy and geometry of the product in different molecular arrangements instead of optimizing molecular packing as the first step and then examining the possibility of cross linking. The main advantage of the alternative method is that, by imposing restraints before starting the optimization,the number of parameters to be refiied is reduced. The restraints are based on the following assumptions: (a) the adsorption takes place on surfaces produced by breakage and, therefore,characterizedby cleavageplanes; (b)the cleavage planesof CaCOsandCaF2are(014)and (111),respectivelx (c) the planes are ideally cleaved (neglecting poesible roughness). A comprehensive review and analysis of the adsorption regarding physicochemicalconditionsthat are affecting the oxidation were recently published.6 The modeling does not take into consideration some of these conditions,such as the effects of solvent (water),molecular packing prior to the oxidation, and the oxidation mechanism. Our assumptions and the philosophy behind the modeling are that the presence of the product (or its absence) is a proven fact and the modeling will assist in rationalization of the experimental results. We are currently studying the effects of packing and oxidation mechanism on the formation of the cross-linkingproduct. Although the nature of the chemical bonding formed between the oleate molecules and the surface is not fully characterized,we rely on the understandingthat the nature of the bonds of all the moleculesin the first layer is identical in all sites and we believe that the symmetry and the geometry of the surface control the structure of the adsorbed molecules at the binding sites. We therefore did the modeling by converting bonds between the Ca2+ ions of the surface and the oleate by replacing cos2- (in CaCOs) by RC02- or by replacing P by RC02- (in CaFz).' The cross-linkingwas introduced by convertingthe double bonds of neighboring oleate molecules to an ether bond. (6) Young,C. A.; Miller, J. D., AIME 122nd Annual Meeting, Reno,
NV,1993. (7) The COS%moiety is replaced by the carboxylichead group. The
original pceitions of the two oxygen atoms which were located in the bulk
of the crystalline mineral are replaced by the new oxygen atom of the oleate. We did not consider charge differencea thoughneutralitymay be achieved by OH- coordination to Ca*; thew important ampecta will be investigated by both experimental and computational methode.
0743-1463/93/2409-1446$04.00/0Q 1993 American Chemical Society
Langmuir, Vol. 9, No. 6, 1993 1447
Letters
+4.99i+
+ 3.86i+
Figure 1. (a, top) Structure of the 014 plane of CaC03 (calcite form). (b, bottom) Optimized structure of three cross-linked oleate moleculesbonded to the 014 plane of CaC03. (Open circles are hydrogen and carbon atoms; filled ones are oxygen atoms.)
The structure (polymer) was refined by molecular mechanics calculations8with two constraints: (a) distances between identical oxygen atoms, which form the bonds with the surface, were fixed at the values of the intercationic distances of the two-dimensional structure of the solid surface; (b) the C-0 bond length at the ether bond was initially fixed at 1.4 A and the resulting strain was relieved at a later stage of the refinement. Results and Discussion CaC03. Three planes have been suggestedas cleavage planes, i.e (OOl), ( O l l ) , and (014), according to the hexagonal crystal system. We have chosen the (014)plane for reasons put forward by Addadi.g The two-dimensional lattice of the (014) plane in CaC03 (calcite) is shown in Figure la. The structure may best be described as a repeating twodimensional parallelogram block with side distances of 4.99 A (within rows, 1-2 in Figure la) and 2 X 4.05 A (between rows, 1-4.45 in Figure la). These are the distances between equivalent Ca2+cations (also between equivalent atoms of the C03%groups). This two-dimensional structure provides minimal parking area of 20.2 A2 for an oleate moleculelo (in case that each calcium cation is bonded to an oleate);this value is too small for an oleate and a full coverage cannot be expected. It should be noted that, whereas adjacent C03%moieties along the row (see 1and 2 in Figure la) are equivalent(relatedby translation), (8) Using Charm force field program. (9) (a) Addadi, L.; Weiner, S. R o c . Natl. Acad. Sci. U.S.A. 1985,82, 4110-4114. (b) Addadi, L.;Moradian, J.; S h y , E.; Maroudas, N. G.; Weiner, S. R o c . Natl. Acad. Sci. U.S.A. 1987,84, 2732-2736.
Figure 2. (a, top) Structhe of the 111 plane of CaF2. (b, middle) Optimized structure of three cross-linked oleate molecules in a raw bonded to the 111 plane of CaF2. (Open circles are hydrogen and carbon atoms; filled ones are oxygen atoms.) (c, bottom) Optimizedstructureof three cross-linkedoleate molecules in the “random walk” mechanism, bonded to the 111 plane of CaF2. (Open circles are hydrogen and carbon atoms; filled ones are oxygen atoms.)
C03%moieties in adjacent rows (4.05 A apart, 1and 4 in Figure la) are related by a rotation of 60’ (causingthem to be in staggered conformation). As a result, when the model of oleates bonded to the surface is produced by the procedure outlined above, it is seen that oleate molecules
1448 Langmuir, Vol. 9, No. 6, 1993
in neighboringrowsare not parallel and their double bonds are too far to be linked by ether bonds. The oleate molecules d o n g the rows are parallel to one another and can be linked by ether bond. However, the results of the refinement of the modeled “polymer” show that, as successiveoleate molecules (ions)are added, an increasing strain is imposed on the ether bond. This result is readily understood the interoleate distances (at the adsorption sites) are fixed to 4.99 A while at the cross-linkingsite the C - 4 distance (in C-O-C fragment of the ether bond) is 2.4 A, the chains have to be distorted to allow the bond to be made. With each succeeding oleate, the degree of distortion increases (see Figure lb). Thus, whereas for three molecules the distance between equivalent atoms of the first and third molecules at the surface is ca. 10 A while at the ether bond it is ca. 4.8 A, for 11 molecules the distances are ca. 50 and 24 A, respectively). Clearly,molecular strain will prevent crosslinking of an array of more than a few adsorbed oleates. CaF2. The two-dimensional lattice of the (111)plane in CaFz is shown in Figure 2a. It has a hexagonal array of CaZ+ with equal interatomic distances of 3.86 A. This structure provides an area of 12.9 A2for each calcium ion. This value is too small fo an oleate molecule whose projected area at closest packing should be greater than ca. 22 A2.10 Clearly,it is not anticipated that every calcium cation will form a bond. If every second Ca2+were to bond an oleate molecule, the available area would be 25.8 A2 per molecule. Two possible cross-linking product arrangements have been tested: (a) a straight line ar(10) Parkingarea of stearate (Langmuirmonolayer)is 20.5A1;’hparking areas obtained from solid state oleic acid are 22.5, and 21.7 A z l l b (two modifications). The value taken from the crystal structureof oleic acid may be taken as a lower limit to the parking area of oleate. Other values such as 26.8, 33, v d 34.3 A2 are mentioned in Cases, J. M.; Pourier, J. E.; Canet, D., Sohd-liquid Interactions in Poroua Media; Cases, J. M., Ed.;Tecnip: Paris, 1985; p 335. (11) (a) Fox, H. W. J. Phys. Chem. 1957,61,1058. (b) Abrahamson, S.; Rideretedt-Nahringbayer,I. Acta Crystallogr. 1962,15,1261.
Letters rangement (1--2--3 in Figure 2a) and (b)“random walk” or zigzag arrangement (1-.2-4 in Figure 2a). Refinements show a significant advantage to possibility b (see parts b and c of Figure 2, reepectively). The straight line arrangement suffers from the effect of cumulative strain noted for CaCOs (although to a lesser extent). The alternative “random walk” arrangement providesa means of avoiding the buildup of strain through the formation of a bond with a molecule that is at 120” to the straight line direction (see Figure 2b). The cumulative strain is reduced because 1-94 distance (6.68 A) is shorter than 1-93 (7.72 A). Also, at every “tu”’ (by 120”)a renewal of the polymer is formed (as if this is the first molecule in a new polymer). When there is no full coverage, the availability of an alternative bonding arrangement is an important advantage. Therefore the surface of CaFz is a better template for the formation of cross-linked oleates than CaCOs. From the above we conclude that the two major factors that determine the ability of the adsorbed oleate molecule to undergo oxidative cross-linking are (1)the interchain distances, which are determined by the repeating intercationic distances on the surface plane, and (2) the availabilityof alternative routes (suchasthe random walk) to overcomeobstaclessuch as strain and disorder in surface coverage.
Conclusion We believe that the methodology outlined above will be useful in allowing the effect of the conformation of adsorbed species on the properties of the surface to be gauged. Acknowledgment. The project was supported by Israel Chemical,Ltd.,and, in parts, by the Wolfson Family CharitableTrust Program for the Absorption of Immigrant Scientists (A.B. Zolotoy) and by the Technion V.P.R. Fund-Alexander Goldberg Memorial Research.
