Langmuir 1999, 15, 8935-8936
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Reply to Comments on “The Binding of Monovalent Electrolyte Ions on r-Alumina. I. Electroacoustic Studies at High Electrolyte Concentrations”
The 1996 electrokinetic paper of Kosmulski and Rosenholm1 was the first study to demonstrate the use of an electroacoustic technique at high ionic strengths to investigate the adsorption of ions to metal oxide substrates. This fact is explicitly and clearly acknowledged in our recent electroacoustic publication examining the binding of monovalent electrolyte ions on R-alumina.2 Indeed, we were aware of the existence of ref 1 when we commenced our high salt electroacoustic work in 1997 and considered it a useful, interesting, and important publication. As is inherently the case for all new and useful scientific innovations, the field of high ionic strength electroacoustics has progressed rapidly since the publication of ref 1. In particular, the 1997 publication of Rowlands et al.3 identified and remedied a significant experimental deficiency which is encountered when the conventional electroacoustic calibration procedure4 is utilized at high ionic strengths. More specifically, Rowlands et al.3 found that at conductivities in excess of ca. 1 S m-1, the applied electric field is altered in both its distribution and magnitude. The latter factor is of particular importance to the AcoustoSizer instrument used in refs 1-3 as at high suspension conductivities, the amplifier driving the applied voltage becomes overloaded.3 As a result, the actual voltage applied across the suspension is significantly smaller than that specified by the instrument software, and so the calculated ζ potentials are unrealistically low. Under such circumstances, Rowlands et al.3 proposed an alternative calibration procedure, whereby the suspension sample is conductivity matched with, and “standardized” against a reference electrolyte solution possessing well-defined electrical and solution properties. The details of the high salt calibration were summarized by us in ref 2 but are discussed in considerably more depth in ref 3. The relative effects of utilizing the conventional electroacoustic calibration versus the improved procedure of Rowlands et al.3 are shown for two R-alumina/CsNO3 suspensions in Figure 1. For both 0.3 and 1.0 mol dm-3 CsNO3, Figure 1 clearly demonstrates that the ζ potential data measured using the conventional (low salt) electroacoustic calibration4 (open symbols) are of smaller magnitude than those obtained using the high salt calibration procedure of Rowlands et al.3 (filled symbols). Furthermore, the extent of the discrepancy increases markedly as the electrolyte concentration rises. The latter finding has obvious implications for cases in which the ζ potentials measured for suspensions containing different electrolyte species are being compared (see, for example, refs 1 and 2). For example, at a concentration of 1.0 mol dm-3, the specific conductivity of LiNO3 is approximately 25% below that of CsNO3. As the high salt calibration procedure of Rowlands et al.3 is based upon the suspension conductivity, the discrepancy between results obtained (1) Kosmulski, M.; Rosenholm, J. B. J. Phys. Chem. 1996, 100, 11681. (2) Johnson, S. B.; Scales, P. J.; Healy, T. W. Langmuir 1999, 15, 2836. (3) Rowlands, W. N.; O’Brien, R. W.; Hunter, R. J.; Patrick, V. J. Colloid Interface Sci. 1997, 188, 325. (4) O’Brien, R. W.; Cannon, D. W.; Rowlands, W. N. J. Colloid Interface Sci. 1995, 173, 406.
Figure 1. The ζ potential versus pH properties measured for two R-alumina/CsNO3 suspensions using the conventional (low salt) electroacoustic calibration4 (open symbols) and the high salt calibration of Rowlands et al.3 (filled symbols).
using the high salt and conventional (low salt) calibrations will be substantially greater for CsNO3 than for LiNO3based suspensions. It is important to note that as the calibration of Rowlands et al.3 is dependent only upon electrolyte-induced changes to the applied electric field, such findings are independent of the nature of the colloidal phase. In ref 2, we distinguished our recent electroacoustic study from that of Kosmulski and Rosenholm by asserting that “as their study preceded the high salt calibration procedure of Rowlands et al., so the data obtained at high suspension conductivities (>1 S m-1) are unrealistically low”. We believe that the above discussion substantiates that statement. Two further matters are worthy of consideration at this point. In their recent comments on ref 1 (which immediately precede this reply), Kosmulski and Rosenholm stated that both their1 and our2 interpretation of electroacoustic signals is based on “yet unverified assumptions that can potentially lead to more serious errors than the cell calibration itself.” We believe that it is doubtful that other as-yet unresolved experimental concerns (such as assumptions underlying the inter-related calculation of the particle size distribution) would lead to errors of the magnitude encountered when the conventional (low salt) calibration is used for very high ionic strength systems. Nevertheless, we do not dispute that other experimental concerns may have affected our results to a significant extent. The aim of ref 2 was to utilize the improved cell calibration in order to examine ion binding on R-alumina at high electrolyte concentrations. If, in a future study, other workers are able to demonstrate the use of an improved electroacoustic procedure to measure similar properties, we would consider the publication of their results (along with a statement distinguishing earlier work from their own) as entirely appropriate. Finally, in their comments on ref 2, Kosmulski and Rosenholm expressed “interest” that our results were “very similar” to their earlier data obtained on metal oxide
10.1021/la991060p CCC: $18.00 © 1999 American Chemical Society Published on Web 10/22/1999
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Langmuir, Vol. 15, No. 26, 1999
substrates. We do not dispute this fact. We also do not dispute that our results are similar to a large number of other electrokinetic studies of ion binding on metal oxide surfaces, all of which were carefully referenced in ref 2. Kosmulski and Rosenholm also expressed “surprise” that the interpretation of our results in ref 2 was similar to their own in ref 1. Again, we do not dispute this fact. The “hard-soft acid-base” analysis invoked by Kosmulski and Rosenholm is largely equivalent to the “structure makingstructure breaking” interpretation developed by Gierst et (5) Gierst, L.; Vandenberghen, L.; Nicolas, E.; Fraboni, A. J. Electrochem. Soc. 1966, 113, 1025. (6) Be´rube´, Y. G.; de Bruyn, P. L. J. Colloid Interface Sci. 1968, 28, 92. (7) Dumont, F.; Dang Van Tan; Watillon, A. J. Colloid Interface Sci. 1976, 55, 678.
Comments
al.5 and Be´rube´ and de Bruyn6 in the 1960s. It is the latter analysis, and its development with respect to colloidal surfaces by Dumont et al.7 in the 1970s, that was utilized (and fully referenced) by us in ref 2. Stephen B. Johnson, Peter J. Scales,* and Thomas W. Healy
Departments of Chemistry and Chemical Engineering, The University of Melbourne, Parkville, Victoria 3052, Australia Received July 23, 1999 In Final Form: July 23, 1999 LA991060P
