Comment pubs.acs.org/Langmuir
Comment on “Influence of Microwaves on the Water Surface Tension”
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constant. This behavior was reasonable, but after switching off the heating, the surface tension remained constant in spite of a sharp decrease in temperature (about 20 °C) of the drop. This behavior cannot be explained in terms of surface tension. 2.3. Figure 1 shows that when the microwave intensity was 600 W, for a considerable period of heating time, a slight change in water temperature caused a sharp decrease in the surface tension. This is an abnormal behavior. 2.4. Once irradiation ceased, the surface tension gradually increased, but unlike the temperature, the values did not converge. If the method indicated the surface tension properly, then the values should have converged after 400 s. This phenomenon cannot be due to the memory of water as the trend in changes in surface tension against time seems to be stable for even a longer period of time. 3. Figure 6 of this paper shows the surface tension against temperature (solid symbols, during irradiation; unfilled symbols, after irradiation). Although this figure shows clearly that the surface tensions of various samples drop much further than the theoretical curves, the authors tried to acknowledge the measured values as the surface tension of water. 4. The results convinced the authors to comment that the surface tension reduction is not due solely to a thermal effect. They speculated that during microwave irradiation, the surface tension may be influenced by convection within the droplet and Marangoni surface flow. However, the surface tension value as a thermodynamic property γ = (∂G/∂A)T,P,n cannot be influenced by convection flow or the Marangoni effect. 5. The authors claim that the data in Figure 5 indicate that surface tension would return to its normal value eventually. However, neither the surface tension change trend nor the reported and recorded surface tension values return to their normal values. 6. The authors concluded that “there is certainly the potential to apply the methodology to industrial applications where the manipulation of surface tension is required without the use of chemical addition.” As a matter of fact, this is not a new finding because the manipulation of surface tension without using chemical addition has been a traditionally achievable feat in surface phenomena. 7. Although this experimental work should be appreciated, the measured values should be called pseudo surface tensions in the most optimistic case. Interfacial tension is a measure of the difference in molecular forces of two homogeneous phases. When there is a temperature gradient in a drop, the phase cannot be assumed to be homogeneous; therefore, it is not reasonable to call the measured value surface tension. 8. One of the respected reviewers of this comment called for new experiments to ensure and clarify the decrease in surface tension microscopically.
t is interesting to observe some novel data in the paper “Influence of Microwaves on the Water Surface Tension” by Parmer et al. in Langmuir in 2014.1 However, the validity of the presented discussion and conclusion is open to discussion. The authors tried to measure the surface tension of water using the regular pendant drop method as a function of time during and after microwave irradiation in order to characterize the influence of microwave heating on the air/water interface. But the main question is does the measured value indicate the surface tension of water? The main purpose of this comment is to draw attention to a misconception concerning the surface tension. Let us outline our concerns in the following items: 1. Although the edge profiles of a pendant drop can be analyzed by axisymmetric drop shape analysis (ADSA) to calculate the surface tension,2 it should be emphasized that first the results are calculated values and not a direct measurement of surface tension or based on measuring molecular forces. Second, in addition to certain limitations, the ADSA method is based primarily on a comparison with a pendant drop of pure water under equilibrium conditions. Stagnation (resting), homogeneity, and being free of any movement in the bulk of the droplet are the essence of any required equilibrium conditions to use the ADSA method. These principal features were significantly violated in this work. This is due to the fact that nonuniform surface evaporation (due to the shape of the droplet surface) and internal movement in the bulk of the droplet liquid (due to Marrangoni phenomenon) cannot be ignored at all here. In addition, the Marangoni effect due to the surface tension gradient within the droplet under nonisothermal conditions definitely changes its shape. Therefore, the ADSA method cannot be employed to calculate surface tension in the presence of convection or the Marangoni effect. 2. Although the data presented in this paper are valuable experimental data, the measured values are not really water surface tensions for the following reasons: 2.1. Figure 5 in this paper shows the surface tension and temperature of the water drop vs the microwave intensity and irradiation time. It clearly demonstrates that all samples have the same temperature of 10 °C after 420 s but that the surface tensions of various samples were from 57 up to 67 mN/m. All samples were pure water; therefore, the surface tensions of all samples should be 74.22 mN/m.3 Why does a pure liquid such as water have different values of its surface tension at a given temperature? Were the samples contaminated? The authors rejected this possibility as they cited that no contamination was reported for Teflon used in their microwave study even at higher temperature (>200 °C). The most convincing answer to this question is that the measured values can show a feature of water but not the property which is called surface tension. 2.2. It is interesting that when the microwave intensity was 150 W for a considerable period of heating time (more than 50 s), both the temperature and surface tension remained © XXXX American Chemical Society
Received: March 27, 2015 A
DOI: 10.1021/acs.langmuir.5b00780 Langmuir XXXX, XXX, XXX−XXX
Langmuir
Comment
M. T. Amiri† M. C. Amiri*,‡
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† Physics Department and ‡Chemical Engineering Department, Isfahan University of Technology, Isfahan, Iran
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
REFERENCES
(1) Parmar, H.; Asada, M.; Kanazawa, Y.; Asakuma, Y.; Phan, C. M.; Pareek, V.; Evans, G. M. Influence of Microwaves on the Water Surface Tension. Langmuir 2014, 30 (33), 9875−9879. (2) Hoorfar, M.; Neumann, A. Axisymmetric drop shape analysis (ADSA) for the determination of surface tension and contact angle. J. Adhes. 2004, 80 (8), 727−743. (3) (a) Vargaftik, N.; Volkov, B.; Voljak, L. International tables of the surface tension of water. J. Phys. Chem. Ref. Data 1983, 12 (3), 817− 820. (b) Cini, R.; Loglio, G.; Ficalbi, A. Temperature dependence of the surface tension of water by the equilibrium ring method. J. Colloid Interface Sci. 1972, 41 (2), 287−297. (c) Kayser, W. V. Temperature dependence of the surface tension of water in contact with its saturated vapor. J. Colloid Interface Sci. 1976, 56 (3), 622−627.
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DOI: 10.1021/acs.langmuir.5b00780 Langmuir XXXX, XXX, XXX−XXX
