Langmuir 1995,11, 3275-3277 Reply to Comment on "Nature and Structure of Adsorption Layer on Apatite Contacted with Oleate Solution. 1. Adsorption and Fourier Transform Infrared Reflection Studies" In the comments to our work, the authors try to defend the proposed, in the 1960s assignment of the absorbance bands in infrared spectra ofthe adsorbed oleate molecules on calcium semisoluble minerals, i.e., that a singlet at about 1550 cm-l is due to the asymmetric stretching vibration of the carboxylate group and indicates a chemisorbed oleate monolayer, while a doublet with bands at about 1570 and 1540 cm-'is characteristic ofbulk calcium dioleate, observed at a higher adsorption density. This assignment was also used in our earlier ~ o r k l for - ~ an explanation of infrared results. However our recently obtained experimental results on the adsorption of oleate on a ~ a t i t eindicate ~ , ~ that the simple assignments ofsinglet as a chemisorbed layer and doublet as a surface precipitated product is not correct, and though it can be valid for some cases, it cannot be used as a general rule for all calcium-bearingsemisoluble minerals and alkyl carboxylates as the author of the comments would like to present it. Therefore,the observation of the singleUdoublet shape of the bands of the vas of the carboxylate group cannot be used as evidence for a particular chemical state of the adsorbed molecules. This is comprehensible if it is kept in mind that the position and shape of the infrared absorbance band are very sensitive to the nature and also to the structure (conformation)of the investigated molecules. Although the different alkyl carboxylatesadsorbed on calcium minerals are bonded to calcium atoms, the orientation of the carboxylategroup at monolayer coverage depends on the Ca-Ca distance on cleavage plane and conformation (lateral interaction) of the alkyl chain. In the case of a patchlike structure, especially at a submonolayer coverage, the size of the patches can also play an important role. In consequence,the position and shape of the absorbance bands of the carboxylate groups will represent all the possible conformations of this group at interface. Obviously for uniform surface structure a single narrow band is expected, two well-defined conformations will produce two separate bands, and different surface conformations will occur as a very broad band. In our work published in Langmuir (refs 4 and 5) in 1993 (part 1 pp 2370-2382 and part 2 pp 3357-3370) we present, on the basis of experimental results obtained by different in situ and ex situ spectroscopic methods (IR DRIFT, IR ATR, 13C CP/MAS NMR, X P S ) and hydrophobicity studies, the explanation of the mechanism of oleate adsorption on apatite at different adsorption conditions and the detailed description of the nature and structure of the produced different adsorption layers. One of the major conclusions presented in this work is that infrared spectra of the adsorbed layers are an excellent source of detailed information on the nature and also on the structure of the adsorbed layer. The real problem is a proper interpretation of the experimental results. One of the presented conclusions in this extended spectroscopic work is that the simple relation, monolayer of chemisorbed (1)Sivamohan, R.; de Donato, P.; Cases, J. M. Langmuir 1990,6, 637. (2)Hanumantha Rao; Cases, J. M.; Forssberg, K. S. E. J . Colloid Interface Sci. 1991,145,330. (3)Hanumantha, Rao; Cases, J. M.; de Donato, P.; Forssberg, K. S. E.J . Colloid Interface Sci. 1991,145, 314. (4) Mielczarski, J. A.; Cases, J. M.; Bouquet, E.; Banes, 0.;Delon, J. F.Langmuir 1993,9,2370-2382. ( 5 ) Mielczarski, J. A.; Cases, J. M.; Tekely, P.; Canet, D. Langmuir 1993,9,3357-3370.
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oleate, is characterized by a singlet at 1550 cm-', and surface calcium oleate precipitation by a doublet at about 1570 and 1540 cm-l is not valid for the apatite-oleate system. First of all it is important to understand why a calcium oleate precipitate shows a well-resolved doublet for the Uaa of the carboxylate group in contrary, for example, to cadmium oleate which shows a singlet, as a majority of others mono- anddivalent oleate salts. Hitherto, this issue has been not discussed in detail in the literature. We presented the explanation that the doublet components at 1573 and 1537 cm-l are characteristic for two structural conformations of the carboxylate groups, with different distances between oxygen and calcium atoms. These two types of carboxylate groups have well-defined positions in a crystalline structure. This was shown by the ATR studies with the use ofpolarized light and X-ray diffraction studies. It should be noted here that the studies of the relationship between the structural and spectral features were made parallel by IR and X-ray methods for all samples, also during (in situ monitoring) different conditioning (at controlled atmosphere and temperature). These studies show that any perturbation of the wellcrystalline structure results in dramatic changes in shape of the carboxylatedoublet. The changes in infrared spectra are very specific and indicate the formation of intermediate states which are limited by the two well-distinguished states (doublet). The narrow bands of doublet (fwhm of each band is below 20 cm-l) are transformed to a very broad band (fwhm about 50 cm-l) with a position at maximum resulting from population of intermediate states. These changes are reversible. It was shown that the perturbation of the crystalline structure of calcium oleate can be easily achieved by the removal of water (by vacuum treatment) from a well-crystalline sample, by heating the sample, or by incorporation of other molecules in the calcium oleate structure. The latter perturbation is common by the precipitation of calcium oleate in an acidic solution, where oleic acid is a coprecipitated product, or by the use of oleate contains impurities (homologies, isomers). Thus, the precipitated calcium dioleate can show infrared spectra with a well-separate doublet or a very broad band with a single maximum positioned between the doublet components. These studies, supported by 13C CPflMAS NMR spectroscopic data, clearly indicate that the shape, position, and intensity ofthe bands in the region ofthe vasofthe carboxylate group in calcium oleate depend strongly on an environment and organization of carboxylate groups in the investigated structure. Moreover, they show that any structural changes in the orientation (conformation)of the carboxylate group versus calcium atoms can be easily monitored by the use of infrared spectroscopy. For adsorbed multilayer of calcium oleate on mineral the infrared spectra will be the same if the adsorbed layer will form a surface structure identical to that characteristic for bulk precipitate salt. At close to monolayer coverage the influence of interface is strongest, hence, the changes in the region of the vas of the carboxylate group for the adsorbed oleate layer on apatite are expected strongest because of interaction of oleate with calcium atoms present within the surface structure of mineral. If one assumed that the surface is homogeneous and the uniformly oriented carboxylate groups with well ordered hydrocarbon chains are formed, one has to expect a single narrow band (the more uniform structure, the narrower band) characteristic of the vas of the carboxylate group. In fact we observed a doublet at close to monolayer coverage and a broad band with a single maximum at higher
0743-746319512411-3275$09.00/0 0 1995 American Chemical Society
3276 Langmuir, Vol. 11, No. 8, 1995 adsorption density. This is opposite to the general assignment proposed by the authors of the comments. The singlet at higher adsorption density was explained by us as the formation of nonuniform (partly amorphous) crystalline structure of surface precipitated calcium oleate. The doublet at close to monolayer coverage was explained by the formation of two types of ordered (oriented and uniform) carboxylate groups in the chemisorbed layer on apatite surface. At multilayer coverage the doublet was also observed but at the adsorption conditions which ensure the formation of well-organized structure. These results indicate the close relationship between the structure of the adsorption layer and the related modifications of the absorbance bands in the infrared spectra. The authors of the comments do not agree that the adsorbed oleate can form two different well-defined conformations of carboxyl group at monolayer coverage showing a well-resolved doublet. They strongly suggest that the presence of the doublet in infrared spectra indicates a surface precipitation of calcium dioleate but not chemisorption. It is difficult to distinguish a twodimensional condensed chemisorbed monolayer (monocoordinated with a lattice calcium) from a two-dimensional layer formed by the surface precipitation only on the basis of infrared spectra, unless a prior assumption (this which the authors of the comments want to defend) is made. For this reason we applied 13C CP/MAS NMR spectroscopy which shows that the mobility of the carboxylate groups of the adsorbed oleate molecules on apatite is much lower than that found for a precipitated calcium dioleate. This study provides the first direct experimental evidence that the carboxylate groups of the adsorbed molecules are strongly anchored to the apatite surface. Therefore, we concluded, that the observed doublet in the infrared spectra of the same sample recorded by both in situ and ex situ techniques does not exclude the chemisorbed character of a two-dimensional layer. We found that the carboxylate doublet is more pronounced if the oleate used in these experiments has high purity (minimum isomers, homoloques), the adsorption is performed at pH 10 (no coadsorption of oleic acid), the solidsolution ratio is very low, and the initial concentration of oleate solution is not too high. These conditions have to be fulfilled for the formation of a well-organized adsorption layer. It was also shown that the chemisorbed oleate monolayer contains water that most probably surrounds calcium atoms which, similarly as in the case of precipitated crystalline calcium dioleate, ensure the organization of the adsorbed layer. The removal of the water from the adsorbed layer (by soft vacuum treatment) results in the formation of a singlet at about 1557 cm-l. It was shown that this process is reversible. All these observations indicate that the organization (conformation) of the carboxylate groups can be easily modified by the changes in an environment of these groups also in the case of the adsorbed molecules. On the contrary the changes in an orientation of hydrocarbon chains are more difficult to occur because they undergo a strong lateral interaction. Hence, in contrast to the possible structural changes of the carboxylate groups, the hydrocarbon chains can appear as uniform and ordered structures. Therefore, a relationship between the ordered structure of hydrocarbon chains and the organization of the carboxylate groups is not as simple as the authors of the comments would like to suggest. The above discussion is based on the relevant experimental results and their descriptions are presented in our work published in Langmuir as was mentioned above. According to specific remarks to our work raised by the authors ofthe comments the followinganswers are offered.
Comments Definition of “in situ” measurement refers to all measurements performed under the conditions which could have a critical influence for investigated properties of samples. Hence, the infrared studies of the influence of the water content on the structure of the adsorbed layer were really carried out at controlled water vapor pressure conditions (ref 5, Figure 11, p 3369). Similarly the spectroscopic “in situ” studies of the adsorption layer (ref 5, Figure 9, p 3367) were performed when mineral samples were in contact with oleate solution. Detail description of these “in situ” experiments is available in the published work. In light of this it is difficult to understand the basis of all the statements on this subject presented in the comments. We do not think that the authors of the comments are correct suggesting large systematic errors in our quantitative estimation. At first, a large surface area of the apatite sample, about 66 m2/g excludes the “as much as 50 to 100% errors” suggested by the author of the comments. Secondly, other characteristic phenomena, observed with an increase of adsorption density, the dramatic change in adsorbance intensity in reflection spectra, shift in the position of the vasof the CH2 band, all indicate the transition and changes in hydrophobi~ity,4?~ from monolayer to multilayer coverage, which are in good correlation with our quantitative estimation. Moreover, in very recently published work we reported6,’ detailed quantitative evaluation of the orientation of the adsorbed oleate molecules on the basis of reflection spectra which agree very well with the observations made p r e v i ~ u s l y . ~ , ~ In the recent work a detailed description of the surface structure of adsorbed oleate molecules on apatite is proposed. The authors of the comments found support for their assignment from the infrared analysis of transferred Langmuir-Blodgett (LB) layers of oleic acid and stearic acid on calcium fluoride. The recorded reflection spectra show a singlet band at about 1550 cm-l indicating the formation of calcium carboxylates for both reagents. The author of the comments assumed that the deposited LB layer is a good model for self-assembled monolayers. In our opinion substrate has a significant influence on adsorption of oleate; therefore the results obtained on fluorite are not expected to be the same as in the case of apatite. A strong influence of cations on the formed structure of LB flms of carboxylic acid was already d e m o n ~ t r a t e d .For ~ ~ example, ~ on the silicon(ll1)plane the transferred LB monolayers of cadmium arachidate show the hydrocarbon chains oriented perpendicular to surface, for monolayers of cadmium arachidate the chains were tilted 33”;however, arachidic acid monolayer was not ordered at all.9 Moreover, LB monolayers of the same species could show domains differently oriented as was recently reportedlo for LB barium arachidate monolayers, which contain three types of regions with different organization. Nevertheless, in the comments the reported results are interesting and we would like to make some remarks. The reported reflection spectra (ref 11, or reference paper 14 in the comments) show identical position and shapeofabsorbance bands for oleic and stearic acid deposited on fluorite (ref 11, Figures 4 and 5). If there is not another reason for that, this striking similarity indicates that substrate has the major influence on the (6)Mielczarski, J. A.;Mielczarski, E.; Cases, J. M. CoZEoids Surf. A 1994,93,97. (7) Mielczarski, J. A.; Mielczarski, E. J.Phys. Chem. 1995,99,3206. (8)Yazdanian, M.; Yu, H.; Zografi, G. Langmuir 1990,6, 1093. ( 9 ) Outka, D.A.; StShr, J.;Rabe, J. P.; Swalen, J. D.; Rotermund, H. H.Phys. Rev. Lett. 1987,59, 1321, (10) Schwartz, D. K, Viswanathan, R.; Zasadzinski, J.A. N. Langmuir 1993,9,1384. (11)Jang, W. H.; Miller, J. D. Langmuir 1993,9,3159.
Comments
recorded spectra. Hence, although initial densities of the transferred LB films (oleic and stearic) were different, the carboxylate groups undergone rearrangement to the positions governed by the position of the surface calcium atoms forming the observed identical structures. Moreover, this striking similarity allows the assumption that the very broad band (fwhm about 50 cm-l) at about 1550 cm-’ seems to consist of three components at about 1570, 1550, and 1540 cm-l. Each ofthe components is probably characteristic of different calcium surface sites. The surface plane of the fluorite reflection element used in those studies is not known. For one type of surface calcium sites a narrow singlet would be observed. In the comments the authors reported that the transferred LB film of calcium distearate shows a singlet. This obviously indicates one conformation of carboxylate group in this film structure but not chemisorption as could be misinterpreted using the proposed assignment by the authors of the comments. In light of the definition of chemisorption and surface precipitation presented in the comments, the LB film of calcium distearate is rather similar to surface precipitated product and, hence, should show doublet, which is contrary to their experimental observation.
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In summary, we do not see any experimental data which can clearly support the general assignments ofthe singlet/ doublet issue as suggested by the authors ofthe comments. The variation observed in the position and shape of the v,, of the carboxylate group of oleate on apatite is interpreted by us as a result of the structural changes in the orientation and conformation of the carboxyl group in the adsorption layers. Moreover, we do not agree with another suggestion presented in the comments about the similarity of adsorption of oleate on apatite, with that on calcite and fluorite. Our experimental results indicate substantial differences between the adsorption of oleate on these calcium minerals. The latter subject will be discussed in our future paper. Jerzy A. Mielczarski and Jean M. Cases* Laboratoire “Enuironnement et Minkralurgie”, UA 235 CNRS, INPL-ENSG, B.P. 40, 54501 Vandoeuure-12s-NancyCedex, France Received September 14, 1994 LA940754P
