Measuring Inner Layer Capacitance with the Colloidal Probe Technique

Round 1

Reviewer 1 Report

This manuscript describes properties of the inner layers in water/solid systems based on capacitance measurement using colloidal probe techniques. The experiments and analyses are systematic, and experimental results are well discussed. The overall quality of this manuscript is very high. Minor errors are found: line 288, p. 9, "p = 0.74" should be "p = 0.45", line 389, p. 11," becomes is" "are", line 405, p. 12," these these" "these".

Author Response

The reviewer comments are given in italic and our answers in normal font.

This manuscript describes properties of the inner layers in water/solid systems based on capacitance measurement using colloidal probe techniques. The experiments and analyses are systematic, and experimental results are well discussed. The overall quality of this manuscript is very high. Minor errors are found: line 288, p. 9, "p = 0.74" should be "p = 0.45", line 389, p. 11," becomes is" "are", line 405, p. 12," these these" "these".

We thank the reviewer for positive feedback and for pointing out the errors. We have corrected them in the revised version.

Reviewer 2 Report

This manuscript by Smith et al.  is a straight forward and well written paper dealing with direct force measurements between colloidal particles aimed to determine the Stern layer capacitance of the electric double layer.

I recommend its publications in Colloids and Interfaces provided some minor issues are addressed by the authors:

1) Authors are optimistic when considering that their results are in good agreement with those obtained by XPS techniques.  I agree that trends are similar, but the reported values disagree by a factor of 2 in some cases (specially for low salt-levels).  Could the author provide an explanation for that?

2) There is a typo in the chemical reaction shown in Eq. 12 (line 173) It was probably due to the pdf conversion of the file.  

4) According to other works [Phys. Rev. Lett. 104, 228301] [Phys. Rev. E 90, 012301], the regulation properties are dominant in the asymmetric interaction between charge and uncharged particles (interaction between dissimilar surfaces). Thus, in order to explore more accurately the regulation parameters, one should consider to investigate asymmetric interactions. Why the authors did not chose that scenario for in this work?

5) A similar study was carried by some of the authors using similar particles and exploring similar conditions than the ones used in this study [Phys. Rev. E 90, 012301]. In that case, they reported a decrease of the regulation parameter with salt concentration. However, according to the data shown in Figure 4, here they observed a slight increase upon addition of salt for silica and latex particles at pH 4. Authors should report this discrepancy between their own results. In fact, this discrepancy is also visible in this work between silica at pH 10 and silica at pH 4. Could the authors provide an explanation for this? Is this an evidence of a weakness of the regulation model?

5) In lines 233-240 author discuss the values of the measured Hamaker constant for sulfate latex and silica particles.  They mention that the values for latex is lower than the expected theoretically, while the value for silica is in good agreement with theory. They link the deviation for latex to roughness, but however they do not provide any roughness value of the particles to justify that silica is significantly smoother than latex.

6) Line 288: The regulation parameter value for CsCl is 0.45 instead of 0.74

7) In Figure 4, errors bars are only shown for the LiCl case. Authors report that the rest were omitted to gain visibity in the Figure. However, these error bars are missing in the inner layer capacitances plot for SL and silica particles at pH4. Does it mean that the error bars are lower than the mark sizes?

Author Response

The reviewer comments are given in italic and our answers in normal font.

This manuscript by Smith et al.  is a straight forward and well written paper dealing with direct force measurements between colloidal particles aimed to determine the Stern layer capacitance of the electric double layer.

I recommend its publications in Colloids and Interfaces provided some minor issues are addressed by the authors:



1) Authors are optimistic when considering that their results are in good agreement with those obtained by XPS techniques.  I agree that trends are similar, but the reported values disagree by a factor of 2 in some cases (specially for low salt-levels).  Could the author provide an explanation for that?

We agree that in some situations there are marked differences between the values obtained by the two techniques and we have therefore amended our manuscript accordingly on pages 4, 20, and 22. However, bearing in mind that these two techniques are based on a completely different underlying physics, the agreement is in our opinion still reasonably good. We do not have a definite explanation for the discrepancies at low salts, but we can hypothesize that these differences could be connected to the contribution of binding capacitance, which is more important at low concentration to determine the Stern layer capacitances. This explanation was added to page 22. We have also weaken the wording on pages 4 and 20.

2) There is a typo in the chemical reaction shown in Eq. 12 (line 173) It was probably due to the pdf conversion of the file.

Thank you for spotting the error, which is now corrected.

4) According to other works [Phys. Rev. Lett. 104, 228301] [Phys. Rev. E 90, 012301], the regulation properties are dominant in the asymmetric interaction between charge and uncharged particles (interaction between dissimilar surfaces). Thus, in order to explore more accurately the regulation parameters, one should consider to investigate asymmetric interactions. Why the authors did not chose that scenario for in this work?

The reviewer is correct that the regulation parameters can be probed more accurately in the asymmetric geometry. However, this is only true when the surface to be probed is weakly charged. In the present situation where monovalent electrolytes are being used, the surfaces are highly charged, and the asymmetric geometry provides no advantages over the asymmetric one. However, this point is not immediately obvious, and therefore an explanatory note was added to page 23.

5) A similar study was carried by some of the authors using similar particles and exploring similar conditions than the ones used in this study [Phys. Rev. E 90, 012301]. In that case, they reported a decrease of the regulation parameter with salt concentration. However, according to the data shown in Figure 4, here they observed a slight increase upon addition of salt for silica and latex particles at pH 4. Authors should report this discrepancy between their own results. In fact, this discrepancy is also visible in this work between silica at pH 10 and silica at pH 4. Could the authors provide an explanation for this? Is this an evidence of a weakness of the regulation model?

This is a valid point as we have not commented on the comparison between the results presented in [Phys. Rev. E 90, 012301]. It is true that in the [Phys. Rev. E 90, 012301] paper a decrease of the regulation parameter for carboxylate latex particles at pH 4, depicted in Fig. 3 of that paper. On the other hand, for sulfate latex particles the regulation parameter increases with increasing concentration at pH 4 as shown in Table II in the same paper. Therefore, the published results for sulfate latex particles are in perfect agreement with the present paper. As for the carboxylate latex particles, this is a surface with different chemical group and thus cannot be directly compared to sulfate or silica particles. Similarly, the different trends for silica at pH 4 and pH 10 presented in the present work, are in our opinion the consequence of shifting the surface reaction equilibrium and different specific adsorption at different pH, and this discrepancies are not the failure of the charge regulation model. This issue is now discussed at pages 18 and 19 in the revised manuscript.


5) In lines 233-240 author discuss the values of the measured Hamaker constant for sulfate latex and silica particles.  They mention that the values for latex is lower than the expected theoretically, while the value for silica is in good agreement with theory. They link the deviation for latex to roughness, but however they do not provide any roughness value of the particles to justify that silica is significantly smoother than latex.

In the original manuscript we have not cite any roughness values. We now add the roughness estimations from the earlier experiments of similar particles and discuss this issue on page 12.

6) Line 288: The regulation parameter value for CsCl is 0.45 instead of 0.74

We thank the reviewer for spotting the typo. We have corrected it.

7) In Figure 4, errors bars are only shown for the LiCl case. Authors report that the rest were omitted to gain visibity in the Figure. However, these error bars are missing in the inner layer capacitances plot for SL and silica particles at pH4. Does it mean that the error bars are lower than the mark sizes?

Good point. The error bars are now added for the two missing cases, see the revised Fig. 4.