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Langmuir 1999, 15, 8934
Comments Comments on “The Binding of Monovalent Electrolyte Ions on r-Alumina. I. Electroacoustic Studies at High Electrolyte Concentrations”
In 1996 a pioneering paper1 was published on measurements of electrokinetic mobilities at very high ionic strengths. A prototype version of the apparatus was used and numerous bugs in the manufacturer’s software were fought, but finally the principal rules governing electrokinetics of oxides at high salt concentrations were discovered. So far these rules have not been challenged. In addition to ref 1, the results were shown at a few scientific conferences shortly before and after the publication. More recently the results were substantiated using the correlation between the zeta potential and rheological properties.2 In 1999 Johnson et al.3 conducted similar experiments using alumina (titania and zirconia were used in ref 1) employing an improved calibration method. It appears from the paper that no credit other than to establish an erroneous experimental procedure should be given to ref 1: “To the authors’ knowledge, this is the most comprehensive19 study of ion binding on colloidal materials undertaken at both nondilute solids and high electrolyte concentrations.” Footnote 19 reads as follows: “ (19) Kosmulski and Rosenholm (Kosmulski, M.; Rosenholm, J. B. J. Phys. Chem. 1996, 100, 11681.) have electroacoustically studied monovalent ion adsorption on titania and zirconia under nondilute electrolyte and solid concentrations. However, the majority of their investigations were undertaken only at high pH values. In addition, their study preceded the high salt calibration procedure of (1) Kosmulski, M.; Rosenholm, J. B. J. Phys. Chem. 1996, 100, 11681. (2) Kosmulski, M.; Gustafsson, J.; Rosenholm, J. B. J. Colloid Interface Sci. 1999, 209, 200. (3) Johnson, S. B.; Scales, P. J.; Healy, T. W. Langmuir 1999, 15, 2836.
Rowlands et al.,18 so the data obtained at high suspension conductivities (>1 S m-1) are unrealistically low.” No other reference to ref 1 has been made in ref 3. Interestingly enough, the results shown in ref 3 are very similar to those reported in ref 1: shifts of the iep toward more basic pH values, the absence of any negative zeta potentials when the critical concentration of certain salts is exceeded, common intersection points of electrokinetic curves obtained at different ionic strengths at ζ * 0, and finally the salt specificity, even the differentiating effect of small cations and large anions has been confirmed. Also the interpretation of the results in ref 3 (hard-soft acid base rule) is surprisingly similar to that published in ref 1. This consistence suggests that in fact Johnson et al.3 rather substantiated the conclusions of ref 1. Probably not only titania, zirconia, and alumina but also other metal oxides (but not silica)4 behave in a similar way. It should be admitted, however, that in both refs 1 and 3 the interpretation of electroacoustic signals is based on yet unverified assumptions (which are addressed in ref 1) that can potentially lead to more serious errors than the cell calibration itself. Marek Kosmulski*
Department of Electrochemistry, Technical University of Lublin, 20618 Lublin, Poland Jarl B. Rosenholm
Department of Physical Chemistry, Abo Akademi, 20500 Turku, Finland Received April 27, 1999 In Final Form: July 19, 1999 LA990509B (4) Kosmulski, M. J. Colloid Interface Sci. 1998, 208, 543.
10.1021/la990509b CCC: $18.00 © 1999 American Chemical Society Published on Web 10/07/1999
