A Comparison of the Tribological Properties of Two Phosphonium Ionic Liquids

Round 1

Reviewer 1 Report

Authors presents original results on tribological properties of two phosphonium and one imidazolium ionic liquids (ILs) which can be of interest for wider community interested in tribology. I suggest minor revision of manuscript before it can be suggested for acceptation. Authors may find useful consider some comments and suggestions in revision their manuscript.

1)     Used ILs are hydrophobic, do not dissolve in water. On other hand they are hydroscopic and dissolve reasonable amount of water which can be gradually absorbed from air [1]. In case of [P6,6,6,14][phosphinate] as much as 15.6 mass% of water [2]. In [P6,6,6,14][DSC] it is much less. Viscosity of [P6,6,6,14][phosphinate] depends greatly on water concentration in IL as shown in following table [2].

               * IL saturated with water

Thus, it is of importance to know water concentration in studied ILs. One of conclusions in presented work is positive influence of higher viscosity on tribological properties of IL. Potential influence of IL water content should be mentioned. In future work, it will be important to study influence of water on tribological behaviour of ILs.

2)     Nomenclature: From point of uniformity of IL abbreviations used I suggest to use [P6,6,6,14][phosphinate] instead of [P6,6,6,14]phosphinate. Only once is for phosphinate anion used [phosphinate] on P. 2, L. 78.

[1]       Seddon, K.R., Stark, A., and Torres, M.J., Influence of chloride, water, and organic solvents on the physical properties of ionic liquids, Pure and Applied Chemistry, 72 (2000) 2275-2287; doi: 10.1351/pac200072122275.

[2]       Marták, J. and Schlosser, Š., Density, Viscosity, and Structure of Equilibrium Solvent Phases in Butyric Acid Extraction by Phosphonium Ionic Liquid, Journal of Chemical & Engineering Data, 62 (2017) 3025-3035; doi: 10.1021/acs.jced.7b00039.

Author Response

Reviewer's comments

Used ILs are hydrophobic, do not dissolve in water. On other hand they are hydroscopic and dissolve reasonable amount of water which can be gradually absorbed from air [1]. In case of [P6,6,6,14][phosphinate] as much as 15.6 mass% of water [2]. In [P6,6,6,14][DSC] it is much less. Viscosity of [P6,6,6,14][phosphinate] depends greatly on water concentration in IL as shown in following table [2].

Thus, it is of importance to know water concentration in studied ILs. One of conclusions in presented work is positive influence of higher viscosity on tribological properties of IL. Potential influence of IL water content should be mentioned. In future work, it will be important to study influence of water on tribological behaviour of ILs.

[1] Seddon, K.R., Stark, A., and Torres, M.J., Influence of chloride, water, and organic solvents on the physical properties of ionic liquids, Pure and Applied Chemistry, 72 (2000) 2275-2287; doi: 10.1351/pac200072122275.

[2] Marták, J. and Schlosser, Š., Density, Viscosity, and Structure of Equilibrium Solvent Phases in Butyric Acid Extraction by Phosphonium Ionic Liquid, Journal of Chemical & Engineering Data, 62 (2017) 3025-3035; doi: 10.1021/acs.jced.7b00039.

Author's response

The authors are thankful to the reviewer for the valuable advice. In the comment, we would like to mention that we are familiar with this phenomenon. However, in this study, we did not evaluate the influence of moisture. We used ILS as received and performed all the tribo tests in the environment of particular humidity. We agree with the reviewer that the possible influence of water on tribological properties should be mentioned in the manuscript. Therefore, we added the sentence to the discussion.

 Reviewer's comments

Nomenclature: From point of uniformity of IL abbreviations used I suggest to use [P6,6,6,14][phosphinate] instead of [P6,6,6,14]phosphinate. Only once is for phosphinate anion used [phosphinate] on P. 2, L. 78.

Author's response

We have changed the abbreviation throughout the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

The work by Gumbyte et al studies the tribological properties of three commercial ionic liquids (ILs) by means of two testing modes: reciprocation in a ball-on-plate tribometer and continuous sliding in a ball-on-disc tribometer. The work may be of interest to the readers of the journal although some parts need to be improved. 

The introduction on ionic liquids and their potential use as lubricants should be improved.

Concerning ILs definition and structure, please provide more recent literature. About the general properties of ILs and those of interest for the lubricant research area please provide literature (ie thermal stability https://doi.org/10.1021/ie5009597, surface activity https://doi.org/10.1016/j.molliq.2021.115988).

Concerning ILs as lubricant, lines 47-48 (Scant research systematically examines the differences between similar structures possessing ILs) should be rephrased. There are several studies which studied the tribological properties of structurally related ILs. However, this reviewer agrees that further studies are needed.

In the introduction, the authors claim that ILs were initially used as neat lubricants while over time the attention was devoted towards their utilization as lubricant additives. This reviewer agrees that ILs are nowadays no more considered as neat lubricants. Indeed, their cost would be too high. Therefore, the authors should test the solubility of the different ILs selected in some base oils so to better define the potential use of these compounds.

English level is satisfactory

Author Response

Reviewer's comments

The work by Gumbyte et al studies the tribological properties of three commercial ionic liquids (ILs) by means of two testing modes: reciprocation in a ball-on-plate tribometer and continuous sliding in a ball-on-disc tribometer. The work may be of interest to the readers of the journal although some parts need to be improved.

The introduction on ionic liquids and their potential use as lubricants should be improved.

Concerning ILs definition and structure, please provide more recent literature. About the general properties of ILs and those of interest for the lubricant research area please provide literature (ie thermal stability https://doi.org/10.1021/ie5009597, surface activity https://doi.org/10.1016/j.molliq.2021.115988).

Author's response

We have improved the introduction by adding a few newer references for particular properties of ILs.

 Reviewer's comments

Concerning ILs as lubricant, lines 47-48 (Scant research systematically examines the differences between similar structures possessing ILs) should be rephrased. There are several studies which studied the tribological properties of structurally related ILs. However, this reviewer agrees that further studies are needed.

Author's response

We refused the statement as the reviewer recommended.

Reviewer's comments

In the introduction, the authors claim that ILs were initially used as neat lubricants while over time the attention was devoted towards their utilization as lubricant additives. This reviewer agrees that ILs are nowadays no more considered as neat lubricants. Indeed, their cost would be too high. Therefore, the authors should test the solubility of the different ILs selected in some base oils so to better define the potential use of these compounds.

Author's response

We agree with the reviewer that solubility investigation in potential base oils could give valuable information. However, this study focused on the tribological properties, and the additional solubility study without tribological properties could be misunderstood. Therefore, we will leave this topic for further investigation.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Authors did adequate changes n manuscript. Some misprints in reference 48 need correction. Name of journal is "Pure and Applied Chemistry", pages 2275-2287.

I suggest acceptation of manuscript for publication after additins to the reference.

Reviewer 2 Report

The authors responded to this reviewer's remarks.

English level is satisfactory.