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Langmuir 1998,9, 363
Comment on "Adrorption of Oleate on Apatite Studied by Diffuee Reflectance Infrared Fourier Tranrform Spectrorcopy" In their paper, the authors Wen Qi Gong et ale1state that "High sensitivity using DRIFT technique was also obtainedby Sivamohanet aL Unfortunately,the influence of the stongly infrared absorbing atmospheric water was not eliminated by purging the instrument but by substracting, which reaulted in uncompensatedcuriouspeaks in the region of asymmetriccarboxylatevibrations (15001700~m-~)." We wouldliketomakeafew commentsabout this statement: (1)First of all, it is very regrettable that the paper of Sivamohan et al.2 hae focueed the attention of the authors only through a minor problem of water substraction. Sivamohanet aL2havedemonstratedthe existenceof mixed superficialphaees containing oleic acid. The origin of the band at 1651-1660 cm-l, attributed either to monocoordinated calcium oleate ions or to the adsorption of oleate ions through sodium ions, is also argued. (2) In our case2, the mineral was a natural fluorite (Sp = 1.14 m2/g) and not an apatite (Sp = 9.86 m2/g). (3) The IR profile of the spectra shown by Sivamohan et al.2 is not due to an absence of purge because the apparatus was always under constant dry air flow (d = 1600 L/h). The shape of the spectra is not "curious" at all. It is mainly due to small variations of the IR profile of the characteristic bands of water vapor as shown in Figure 1. These variations arise from the spectral measurements parameters, in particular the resolution (RES = 4 cm-1) and the zero fiing (ZFF = 4). In these conditions,our effective resolution is equal to 1 cm-l (Res/ !ZFF) (Figure 1). By subtracting the reddual water vapor from the initialspectrum,we obtain the IR profile obaerved in the spectra of Sivamohan et ala2 A schematic representation of such a profile is given in Figure 2. (4) Even if the instrument is purged with dry N2 for 3 h, it is impoeeible to remove totally the residual contri-
16'60
16k0 1620 YAVENJMBCR L M - I
Figure 1. Variation of the infrared spectrum of water vapor, taken at different time (Bruker IFS 88, MCT detector, RES = 4 cm-' and ZFF = 4). 3 0,6 -I
1550
1555
1560
1565
Wavenumber (arbitrary units) Figure 2. Schematic Infrared profile of eubtraction (11- 12) of two ban& (11and (2) having adifferenceof 1unit intheir position on the X axis.
bution of water vapor in the spectra. An illustration of this fact can be found, for instance,on Figure 2 of the paper by Wen Qi Gong et al.1 Although the intensity of characteristicbands of water vapor are m i n i m i d by using KM unite (minimization of weak IR bands), water still diaturbs the IR profile. (6) Finally, we recall that the band at 1650 cm-l has already been assigned in the literature by Hanamutha Rao et a l . 3 ~ 4to the formation of monocoordinated calcium oleate ion.
(1) Gong,Wen Qi;Parentich,A.; Little, L. H.; Warren,L. J. Adsorption of oleate on apatite studied by diffuw reflectance intrared Fourier transform spectroclcopy. Langmuir 1992,4118-124. (2) Sivamohan, R.; de Donato, P.;Caeea, J. M.Adsorption of oleate specier at the fluorite-aqueoua solution interface. Langmuir 1990, 6, 637444. (3) Hanumnntha Rao, K.;Caws,J. M.;de Donato, P.; Foreberg, K. 8. E. Mechaniiem of oleate interactionon dt-type mineral: IV Adsorption, electrokinetic and diffuee reflectance FT-IR studies of natural fluorite in the preaence of d i u m oleate. J. Colloid Interface Sci. 1991, 245,
P.de Donato, 0. Barrer, and J. M . Came' Laboratoire "Environnement et Min&ralurgie", INPL-ENSG de Nancy, UA 236 du CNRS, BP 40, 54501 Vandoeuvre Cedex, France Received March .ZS,1992 In Final Form:September 23,1992
314-329. (4) HanumanthaRao, K.;Casee, J. M.;Foreberg, K. 8. E. Mechanism of oleate interactionon salt-typemineral: V. Adsorptionand precipitation reactions in relation to the eolid/liquid ratio in the synthetic fuoritesodium oleate syntem. J. Colloid Interface Sci. 1991,145, 330-348.
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1993 American Chemical Society
