Response to comment on ''Point of zero charge of a corundum-water interface probed with optical second harmonic generation (SHG) and atomic force microscopy (AFM): new approaches to oxide surface charge''

The purpose of this response is to address several misrepre-sentations of Stack et al. (2001) and its intent in the precedingComment. At issue, primarily, is our observed point of zero salteffect (p.z.s.e.) of a single crystal face of corundum usingoptical second harmonic generation (SHG) at a pH ( 5) muchlower than the accepted value (in the 8–9 range) of the pristinep.z.c. of alumina powders. Dr. Kosmulski’s main claim is thatour anomalous observation is the result of surface contamination.Dr. Kosmulski asserts that “Stack et al. (2001) challengedthe well-established PZC and suggested the pristine point ofzero charge of aluminum oxide at pH 5 to 6. For an unawarereader, the message of the abstract and conclusions of theirpaper may be that the well-established value (pH 9) should beabandoned and the new value (pH 5 to 6) should be usedinstead.” This is an indiscriminate misrepresentation of Stack etal. (2001); we never claimed that our value should be taken as“deﬁnitive”; our paper clearly recognizes the difference be-tween our observation and the “accepted” pzc, and our paperdevotes a substantial amount of text to exploring possiblereasons for the difference. Stack et al. (2001) does not state inany manner that anyone should abandon the well-establishedvalue. Dr. Kosmulski’s “unaware reader” may, however, con-clude from the misrepresentations in the Comment that wemade such statements in Stack et al. (2001).The purpose of Stack et al. (2001) was not to assert a new,deﬁnitive pristine p.z.c. for alumina, but rather to show how theSHG technique could be used to obtain information aboutelectric ﬁelds at the surface of insulating minerals without theneed to interpret titration stoichiometry. Dr. Kosmulski’s pointis mainly that (a) silicate, sulfate, or some other contaminantcould adsorb to our surface and produce a signiﬁcant shift inthe pH of the p.z.c., or (b) the alumina itself is impure andcould exhibit a p.z.c. substantially different from that of a purealumina. We agree—and our manuscript does not say other-wise. Our abstract plainly states that the SHG (and AFM)results are different from expectations and points out that thereasons for the differences are unclear. In the last paragraph ofsection 4.1 of Stack et al. (2001), we speciﬁcally discuss thepossible role of contaminants in shifting the p.z.c. to lower pHvalues. The surface studied was not necessarily “pristine,” andour paper recognizes this fact repeatedly. We refer to our obser-vations as pertaining to the p.z.c. and never as an indication of thep.z.n.p.c. We have made a distinction between the sample westudied (which is nominally a basal surface of alumina but with a5° miscut and numerous polishing scratches) and the “clean alu-mina powder[s]” of the consensus (Stack et al., 2001).Dr. Kosmulski does a particular disservice with the statement “. . .the discrepancy between the results of Stack et al. (2001) andthose published by others indicates certain limitations of theSecond Harmonic Generation (SHG) method as the tool toestablish the pristine PZC of alumina (and other metal oxides).”In fact, Dr. Kosmulski’s primary scientiﬁc objections havenothing to do with the SHG technique per se but rather with thewet-chemical system to which SHG was applied. This is animportant distinction. The SHG is showing us that a point ofzero salt effect does indeed occur in the region of pH 5 for thesurface in question. It is a self-contradiction to impugn theSHG technique and then base one’s objections to the chemistryon the correctness of the SHG results.The discrepancy between our SHG results and the acceptedvalue for the pristine p.z.c. was indeed a source of concernduring our experimentation, and it prompted us to examine thesurface composition using Auger Electron Spectroscopy (AES)after the SHG experiments in an attempt to identify contami-nants that could have caused such a shift. As stated in section3.3 of our paper, we did not detect anything other than Al, O,and adventitious C (which is ubiquitous in surface analyses;see, e.g., Hochella, 1988). Any contaminants other than adven-titious carbon were therefore undetectable by AES. Could Sihave been present in sufﬁcient quantity to lower the p.z.c. to pH5 but remain undetectable? The most prominent Si LMM peak(i.e., that with the greatest sensitivity factor for Si) at 92 eVwould have an escape depth of 1.0 to 1.5 nm (Hochella, 1988;Bishop, 1989). For simplicity, let us assume 1.3 nm (one unitcell). Let us choose for comparison kaolinite, a material com-posed of silica and alumina with a p.z.c. near that which weobserved (pH 5.2; Schroth and Sposito, 1997). If our surfacewas kaolinite-like, with a half-monolayer of silica, we wouldhave roughly 3.33 atom percent Si within the analyzed surfacevolume. This amount of Si should be evident in the AES, givenan accepted sensitivity of 0.1 to 1 atom percent (Bishop, 1989).We detected no hint of an Si peak. If we assume a greaterescape depth (i.e., doubling it to 2.6 nm), the atom percentdecreases to 1.67%, but this amount should still be detectable.A reviewer has argued that the positive surface charge at pH5 on alumina (with a p.z.c. at 8 or 9) could be compensated bya much smaller surface concentration of anions than we haveassumed above if each compensating ion held one negativecharge. This argument is not compelling for silica, which has