Novel WS2-Based Nanofluids for Concentrating Solar Power: Performance Characterization and Molecular-Level Insights

Novel WS2-based nanofluids for concentrating solar power: performance characterization and molecular-level insights Article Accepted Version MartÍnez-Merino, P., Midgley, S. D., MartÍn, E. I., EstellÍ, P., Alcántara, R., Sánchez-Coronilla, A., Grau-Crespo, R. and Navas, J. (2020) Novel WS2-based nanofluids for concentrating solar power: performance characterization and molecular-level insights. ACS Applied Materials & Interfaces, 12 (5). pp. 5793-5804. ISSN 1944-8244 doi: https://doi.org/10.1021/acsami.9b18868 Available at http://centaur.reading.ac.uk/88909/ It is advisable to refer to the publisher’s version if you intend to cite from the work. See Guidance on citing . To link to this article DOI: http://dx.doi.org/10.1021/acsami.9b18868 Publisher: ACS Publications All outputs in CentAUR are protected by Intellectual Property Rights law, including copyright law. Copyright and IPR is retained by the creators or other copyright holders. Terms and conditions for use of this material are defined in the End User Agreement . www.reading.ac.uk/centaur CentAUR Central Archive at the University of Reading Reading’s research outputs online 1 Novel WS2-based nanofluids for concentrating solar power: performance characterization and molecular-level insights Paloma Martínez-Merino,1 Scott D. Midgley,2 Elisa I. Martín,3 Patrice Estellé,4 Rodrigo Alcántara,1 Antonio Sánchez-Coronilla,3 Ricardo Grau-Crespo,2 Javier Navas*,1 1 Departamento de Química Física, Facultad de Ciencias, Universidad de Cádiz, E-11510 Puerto Real (Cádiz), Spain. 2 Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, United Kingdom. 3 Universidad de Sevilla, Departamento de Ingeniería Química-Facultad de Química; Departamento de Química Física-Facultad de Farmacia, E-41012 Sevilla, Spain. 4 Univ Rennes, LGCGM, EA3913, F-35000 Rennes, France. Corresponding Author: *Javier Navas (javier.nava[email protected]). Keywords: WS2 nanosheets; Nanofluids; Dissociative adsorption; Ab-initio Molecular Dynamics; Surface chemistry; Concentrating Solar Power. 2 Abstract Nano-colloidal suspensions of nanomaterials in a fluid, nanofluids, are appealing because of their interesting properties related to heat transfer processes. Whilst nanomaterials based on transition metal chalcogenides (TMCs) have been widely studied in catalysis, sensing, and energy storage applications, there are few studies of nanofluids based on TMCs for heat transfer applications. In this study, the preparation and analysis of nanofluids based on 2DWS2 in a typical heat transfer fluid (HTF) used in concentrating solar power (CSP) plants is reported. Nanofluids prepared using an exfoliation process exhibited well-defined nanosheets and were highly stable. The nanofluids were characterized in terms of properties related to their application in CSP. The presence of WS2 nanosheets did not modify significantly the surface tension, the viscosity, or the isobaric specific heat, but the thermal conductivity was improved by up to 30%. The Ur factor, which characterizes the thermal efficiency of the fluid in the solar collector, shows an enhancement of up to 22% in the nanofluid, demonstrating great promise for CSP applications. The Reynolds number and friction factor of the fluid were not significantly modified by the addition of the nanomaterial to the HTF, which is also positive for practical applications in CSP plants. Ab initio molecular dynamics simulations of the nanoparticle/fluid interface showed an irreversible dissociative adsorption of diphenyl oxide molecules on the WS2 edge, with very low kinetic barrier. The resulting ‘decoration’ of the WS2 edge dramatically affects the nature of the interface interactions and is therefore expected to affect significantly the rheological and transport properties of the nanofluids. 3 1. Introduction Nano-colloids or nanofluids are colloidal suspensions of nanomaterials in a fluid. The addition of nanomaterials can modify properties such as the thermal conductivity, isobaric specific heat or heat transfer coefficient of the fluid. Therefore, nanofluids constitute an emerging technology for the improvement of heat transfer fluids (HTFs),1-4 and they are considered as promising alternatives to conventional HTFs in several applications, such as electronic cooling,5 nuclear reactors,6 thermal energy storage7 and biomedical applications.8 Nowadays, one of their most interesting applications is in solar energy systems. The presence of nanoparticles can lead to an increase in the absorption of incident solar radiation, which improves the global efficiency of the collectors.9-10 Nanofluids could enhance the features of the HTFs used in solar energy applications, more specifically in concentrating solar power (CSP) plants based on parabolic trough collectors. Here, the typical HTF used is the eutectic mixture of diphenyl oxide and biphenyl.11 Improvements in the thermal properties of the HTFs can lead to an enhancement of the global efficiency of CSP plants.12-14 Since Choi first reported interesting enhancements of the thermal properties of fluids thanks to the incorporation of nanoparticles,15 many studies have analyzed this effect and reported significant improvements in thermal conductivity. For example, Chen et al showed an improvement of about 20% for nanofluids based on carbon nanotubes,16 while Xuan et al. found enhancements of about 60% for nanofluids obtained using Cu nanoparticles in water.17 However, an important issue surrounding nanofluids is that they should be stable, because nanofluids showing high stability have improved thermal properties over the time. Twodimensional nanomaterial may therefore be an interesting alternative to metallic or metal oxide nanoparticles because they present good physical stability, which can lead to a decrease 4 in agglomeration and sedimentation processes when they are included in colloidal suspensions, mainly thanks to their higher surface area. The present work shows the preparation of nanofluids based on 2D-WS2 and the typical synthetic oil used in CSP plants based on parabolic trough collector technology.11 The 2D nanostructures were prepared in situ in the fluid using a liquid phase exfoliation method. The nanofluids were characterized in terms of their stability and properties related to their efficiency in heat transfer processes, such as rheological and thermal properties. To understand the behavior of the nanofluids at the molecular level, theoretical calculations were performed based on molecular dynamics and ab-initio molecular dynamics. 2. Materials and methods 2.1. Preparation of nanofluids A nanofluid consists of three components: the fluid, the nanomaterial and the stabilizing agent or surfactant. For preparing stable nanofluids, the components are chosen according to the following considerations. The thermodynamic requirements for obtaining a stable colloidal suspension are reached when the tension at the solid-liquid interface, SL, is minimized. Fowkes18 and Owens-Wendt19 defined SL as: 𝛾SL=(𝛾Sp 𝛾Sd+1)𝛾Sd+(𝛾Lp 𝛾Ld+1)𝛾Ld−2(√𝛾Sp𝛾Lp 𝛾Sd𝛾Ld+1)√𝛾Sd𝛾Ld (1) where S and L mean solid and liquid, respectively, and p and d denote the polar and dispersive components of the surface tension. Therefore, to minimize the tension at the solid-liquid 5 interface the magnitudes of the 𝛾Sp𝛾Sd ⁄ and 𝛾Lp𝛾Ld ⁄ ratios should be similar, as should the values of 𝛾Sd and 𝛾Ld. The values of the polar and dispersive components were obtained using the WORK formula (Wendt, Owens, Rabel and Kaeble)20 following the procedure described in the literature,21 in which surface tension and contact angle are measured. Details of this calculation are shown in the Supporting Information. In this work, the fluid used was the eutectic mixture of diphenyl oxide (C12H10O, 73.5%) and biphenyl (C12H10, 26.5%), supplied by The Dow Company©. The nanomaterial used was WS2 (nanopowder, average size: 90 nm, purity > 99%, Sigma-Aldrich©) and the surfactant was cetyltrimethylammonium bromide (CTAB, purity > 99%, Sigma-Aldrich©). The amount of CTAB was chosen to obtain 𝛾Sp𝛾Sd ⁄≈𝛾Lp𝛾Ld ⁄. The ratio of the polar and dispersive component for bulk WS2 is 0.52.22 Several base fluids using different concentrations of CTAB were tested to calculate the ratio between the polar and dispersive components. Table 1 shows the CTAB concentration tested, the values of the surface tension components, and the ratios between them obtained in the experiments. The intermediate concentration tested has the best fit to the ratio of the surface tension components of the base fluid with that of the bulk WS2. After defining the base fluid, a liquid phase exfoliation (LPE) method was used to prepare nanofluids based on WS2 nanosheets. When an LPE method is developed using the appropriate base fluid, the 3D material is exfoliated and 2D nanostructures are obtained. The procedure followed here has been described previously,21 and can be summarized as: (i) 3.75 mg of 3D WS2 and 5 mL of the base fluid were added to four vials; (ii) these were sonicated for 8 h at 28-32 ºC using an Elma© Transsonic Tl-H-5 sonication bath (80 kHz, 150 W); (iii) the colloidal suspension was centrifuged twice. The supernatant obtained after the second centrifugation was the nanofluid. The present study considered four nanofluids, which were 6 prepared following the procedure described above, under the conditions shown in Table 2. Not all combinations of surfactant concentrations and sonification times are considered because conditions were being optimized after each preparation/characterization cycle. Table 1. Values of the surface tension and their components, and the ratio between them for the base fluid prepared using different concentrations of CTAB. CTAB / %wt. 𝜸𝐋 / mN m-1 𝜸𝐋 𝐩 / mN m-1 𝜸𝐋 𝐝 / mN m-1 𝜸𝐋 𝐩𝜸𝐋 𝐝 ⁄ 0.009 35.96 13.80 22.16 0.62 0.011 35.61 12.62 22.99 0.55 0.014 36.26 11.15 25.11 0.44 Table 2. Preparation conditions for the nanofluids analyzed in this study. Nanofluid CTAB / wt.% Sonication time / h #1 0.009 4 #2 0.014 4 #3 0.009 8 #4 0.011 8 2.2. Characterization of nanomaterial and nanofluids First, transmission electron microscopy (TEM) was used to analyze the shape and size of the nanostructure obtained from the LPE process. The TEM images were recorded using a JEM2100F microscope supplied by Jeol©. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques were also used to test whether the WS2 underwent significant changes during the exfoliation process. A Bruker® D8 Advance A25 diffractometer emitting Cu-Kα radiation (1.540 Å) and a Lynxeye detector were used. The measurement range was from 3º to 75º in the 2θ range, with an accuracy of 0.020º. The measurement conditions were 40.0 kV and 40.0 mA. Furthermore, XPS spectra were recorded using a Kratos Axis 7 UltraDLD spectrometer with monochromatized Al K radiation (1486.6 eV), a 20 eV pass energy and given with an accuracy of 0.1 eV. Stability is a key property of nanofluids because it can have a significant effect on the observed thermal properties. Stability was assessed using UV-Vis spectroscopy to analyze the sedimentation process, by evaluating the extinction coefficient and particle size measurements. UV-Vis spectra were recorded using a system consisting of a DH-2000-BAL halogen lamp and a USB2000+ spectrometer, both supplied by Ocean Optics®. Spectra were recorded between 300 and 900 nm and the extinction coefficient was extracted at  = 629 nm to analyze the evolution of nanomaterial in suspension in each nanofluid. Particle size measurements were also performed using the dynamic light scattering (DLS) technique by means of a Zetasizer Nano ZS system supplied by Malvern Instruments Ltd®. The extinction coefficient and particle size values were obtained every day in triplicate. Several measurements were performed to analyze whether the nanofluids prepared could replace the typical HTF used in CSP plants: density, surface tension, dynamic viscosity, isobaric specific heat and thermal conductivity. The fluid used in this study, which is the typical HTF used in CSP plants, was also characterized for comparison purposes. The density (  ) values were obtained using temperature-controlled pycnometry. Five values were registered to obtain statistical values. A KRÜSS GmBH DSA-30 Drop Shape Analyzer (Hamburg, Germany) was used to measure the surface tension of samples from the pendant drop technique. The comprehensive experimental procedure followed has been described previously.23 Surface tension values are determined from the drop shape analysis using the Young-Laplace equation. In this configuration, uncertainty was reported to be less than 0.1% at ambient condition with calibration gauges and less than 1.08% for distilled water in the 14 in terms of hydrodynamic diameter is shown in Figure 3. Nanofluids #1 and #2 are on average larger than 250 nm and show clearly particle sizes increasing with time. This is coherent with the values of extinction coefficient. The average particle size obtained for nanofluid #3 is lower than for nanofluids #1 and #2 (lower than 250 nm). Also, nanofluid #3 seems to be in constant change, and the particles are seen to increase in size over time, which suggests poor stability for this nanofluid. Finally, the average size for nanofluid #4 remained practically constant, at about 200 nm, after more than two weeks. These results agree with those obtained from UV-Vis spectroscopy, and confirm that #4 is the most stable of the four prepared nanofluids. Figure 3. Average particle size obtained by DLS for the nanofluids prepared. 3.3. Nanofluid performance Several thermophysical properties of the nanofluids were also analyzed in this study, to determine the nanofluid efficiency. Such properties are also involved in the heat transfer coefficient, h, as evidenced in the literature.45 Therefore, density, surface tension, dynamic 15 viscosity, isobaric specific heat and thermal conductivity were characterized in the present study. Numerous studies reported that heat transfer fluids are more efficient when their density increases, because of an increase in the load of nanoparticles.46 The density values measured at 298 K for each nanofluid are available in Table 3. Density values increase slightly for nanofluids, the highest increase of 0.45% corresponding to the nanofluid #4, which also showed the highest extinction coefficient values due to the higher load of nanomaterial in suspension. The volume fraction was calculated according to 𝜙=(𝜌nf−𝜌bf) (𝜌nm−𝜌bf)⁄ , where the subscripts “nf”, “bf” and “nm” refer to the nanofluid, base fluid, and nanomaterial, respectively. The density of 2D-WS2 is 7500 kg m-3.47 Again, the volume fraction was highest for nanofluid #4. Table 3. Values of density, the increase in density and the volume fraction. Nanofluid  / kg m-3 (𝝆𝐧𝐟−𝝆𝐛𝐟)𝝆𝐛𝐟 ⁄ / %  / vol.% Base fluid 1056.00.9 -- -- #1 1056.61.2 0.06 0.009 #2 1056.51.0 0.05 0.008 #3 1057.91.8 0.18 0.029 #4 1060.81.4 0.45 0.075 The stability analysis and density measurements lead to the conclusion that nanofluid #4 is the most promising nanofluid. Nanofluids #1 and #2 show a very low load of nanomaterial, and nanofluid #3 shows poor stability as discussed previously. Therefore nanofluid #4 is the most promising, which may be rationalized by considering that it presents the highest load 16 of nanomaterial in suspension and it is also highly stable. Therefore, this nanofluid was characterized for its rheological and thermal properties. We now discuss the surface tension of the fluid, because of the major role this thermophysical property has in applications and processes involving heat transfer.48 The surface tension of the original HTF, the base fluid (HTF + surfactant), and nanofluid #4 are plotted against temperature in Figure 4. A good agreement is obtained between the measured surface tension of the fluid and available manufacturer data in the range 20-40°C with an average deviation of 1.21%. Figure 4 also shows that the surface tension of the base fluid and of nanofluid #4 are very close to that of the pure HTF at all temperatures. Figure 4. Surface tension of the HTF, the base fluid and nanofluid #4. Shear flow curves of the HTF, the base fluid and nanofluid #4 are reported in Figure 5a. All the fluids behave in a Newtonian manner, that is for the shear rate values the viscosity is constant. Comparison of viscosity values are also reported in Figure 5b, evidencing the decrease in viscosity with temperature and showing that the viscosity of the HTF is not 17 modified by the presence of surfactant and nanoparticles. The result is important because it means that the introduction of WS2 nanosheets does not modify the viscosity of the base fluid. Consequently, it will not induce any significant increase in pumping power, pressure drop or friction factor under application conditions, as demonstrated below. Figure 5. (a) Shear flow curves of the HTF, base fluid and nanofluid #4 at different temperatures; (b) Viscosity of the HTF, base fluid and nanofluid #4 against temperature. Isobaric specific heat is a key property in heat transport because it determines the energy storage capacity of the fluids. The isobaric specific heat values obtained for the base fluid and nanofluid #4 are shown in Figure 6a. The changes in isobaric specific heat between the HTF and the base fluid were found to be negligible, and therefore the values for the HTF are not shown for clarity of the figure. The values for the base fluid and for the nanofluid show a typical trend: the higher the temperature, the higher the isobaric specific heat. The isobaric specific heat for nanofluids is expected to decrease with respect to the base fluid because the isobaric specific heat of fluids is usually higher than that of solids, although some experimental results have shown the opposite behaviour.4, 49 In our case, CP decreased by about 1.6% with respect to the base fluid, but this variation is of the same order as the uncertainty of the measurements. 18 Figure 6. Temperature variation of (a) the isobaric specific heat, and (b) the thermal conductivity of the base fluid and the nanofluid #4; (c) the ratio of the thermal conductivity values for the nanofluid #4 and the base fluid. Dash lines joining the points are only a guide to the eye. Figure 6b shows the thermal conductivity values obtained for nanofluid #4 and for the base fluid versus temperature. The changes in thermal conductivity between the HTF and the base fluid were found to be negligible; therefore, the values for the HTF are not shown in the 19 figure for the sake of clarity. The thermal conductivity values for the nanofluid increased with temperature, which means they followed the opposite trend to the base fluid. This means that the heat conduction mechanism in the nanofluids may be different to that of the base fluid. This different trend with temperature leads to an important enhancement of thermal conductivity at temperatures get close to 100 ºC. Figure 6c shows the ratio of the thermal conductivity values for nanofluid #4 and the base fluid (knf / kbf), which gives the thermal conductivity enhancement for nanofluid #4. The highest increase was about 30% at 90ºC. The large thermal conductivity enhancement is a promising feature for solar thermal applications. The performance of the nanofluids in CSP applications can be characterized by the useful energy production (Qu), which can be calculated as 𝑄u=𝑈r𝐴riΔ𝑇, where Ari is the internal surface of the receiver and T is the temperature difference between the receiver and the fluid. Ur is a factor which considers the typical enhancement of the heat transfer coefficient, h, and also the heat that can be moved from the fluid, typically defined from (𝜌𝐶P).12 When the values of e (𝜌𝐶P) are higher, the heat moved increases, and this leads to an enhancement in the thermal efficiency. Ur is defined as: 𝑈r=(1 ℎ+𝐴𝑟 2(𝜌𝐶P)𝑉)−1 (2) being V the flow rate. Higher values of Ur imply lower temperature values in the solar receiver, if Qu is assumed to be constant. This means the higher the Ur values, the lower the temperature in the receiver and the lower the thermal losses. If thermal losses are decreased, the solar collector will be more efficient. Thus, the thermal efficiency will be higher when Ur 20 values are high. A comparison of Ur values for the base fluid and nanofluid gives the performance enhancement of the nanofluid with respect to the typical HTF used in CSP plants. Details of the calculations of Ur are shown in the Supporting Information. Figure 7 shows the values of Ur for the base fluid and for the nanofluid #4 at several flow rates between 100 and 300 L min-1. In all cases, an increase in Ur is observed for the nanofluid. In the figure, the ratio between the Ur values for the nanofluid respect to the base fluid is included in the right side. We can observe an enhancement of up to 22%, which is promising for the use of this kind of nanofluid in CSP plants. Figure 7. Ur values and the ratio between the values of the nanofluid with respect to the base fluid at several flow rates. Dash lines joining the points are only a guide to the eye. 21 In addition, the friction factor (f) can also be evaluated, as a performance indicator of the nanofluid in comparison to the base fluid, from the thermophysical properties experimentally determined. This is done using the following equation:50 𝑓=0.25[𝑙𝑜𝑔(150.39 𝑅𝑒0.98865−152.66 𝑅𝑒 )]−2 (3) where 𝑅𝑒=𝜌𝑉av𝐷 𝜇 ⁄ is the Reynolds number. Vav and D are the average fluid velocity in the inner pipe, and the inner pipe diameter, respectively. Such an equation is valid for Re = 3000–108, turbulent flow and smooth pipes that generally compose CSP collector. An inner tube diameter D of 0.066m was considered12 and flow rates were varied between 100300 L/min,12 as done for the evaluation of Ur. It is observed in Figure 8 that, due to the slight change of both density and viscosity of the nanofluid in comparison to base fluid, that Re number and the friction factor of the nanofluid are not significantly modified compared to the base fluid, which is a positive feature for CSP applications. As expected, the Re number increases and the friction factor decreases when the flow rate increases or the temperature decreases. 22 Figure 8. Influence of temperature and flow rate in the friction factor and Re number for the base fluid and the nanofluid #4. Dash lines joining the points are only a guide to the eye. The performance of this nanofluid seems to be promising for CSP applications, which is a high temperature application (at about 380ºC in parabolic through collector technology). At these high temperatures, some undesirable processes may occur. Thus, nanofluid was tested in thermal cycles, reaching 573 K without stirring. The nanofluid was heated for 5 h, and the evaporation of the fluid was controlled. Figure 9a shows the results obtained for the 23 extinction coefficient at  = 629 nm before and after each cycle. After the first cycle, a slight increase of the extinction coefficient was observed. After three cycles, the changes in extinction coefficient are negligible, therefore the nanofluid reaches stability. In the inset of Figure 9a, the UV-vis spectra registered at the beginning and at the end of these tests are shown. We cannot observe significant changes in the spectra, suggesting that no chemical changes occur in the nanofluid after thermal cycles. Also, Figure 9b shows the values of the mean particle size. The values of particle size are stable for all cycles, confirming that the nanofluid remains stable after thermal cycles. Figure 9. Extinction coefficient values obtained at  = 629 nm (a), and particle size values (b) for the nanofluid #4 before and after thermal cycles. 3.4. Base fluid-WS2 reactivity In order to gain insights into the behavior of the experimental system from a molecular level viewpoint, classical and ab-initio molecular dynamics simulations were performed. 30 Figure 13. X-ray photoelectron spectra of (a) S 2p; (b) W 4f; (c) C 1s; and (d) O 1s obtained for the solid extracted from the nanofluid. 4. Conclusions We have presented a comprehensive study of nanofluids based on 2D-WS2 and have demonstrated their remarkable thermophysical properties for applications as heat transfer fluid in CSP plants. We achieved highly stable nanofluids using the liquid phase exfoliation method, as evidenced by extinction coefficient and particle size measurements. We found that the surface tension of the nanofluid did not change significantly with respect to the typical HTF used in CSP plants. From rheological measurements, we could conclude that the nanofluid behaves in Newtonian manner, and the introduction of WS2 nanosheets did not 31 modify significantly the viscosity of the typical HTF used in CSP plants. Consequently, this will not induce any significant increase in pumping power, pressure drop or friction factor under real conditions. In addition, the thermal conductivity of the nanofluid was improved by up to 30% with respect to the HTF, which is promising for solar thermal applications. We observed an enhancement of up to 22% in the parameter Ur characterizing the thermal efficiency of the nanofluid in the solar collector, thus confirming that this kind of nanofluid could be very useful in CSP plants. Also, the Reynolds number and the friction factor of the nanofluid were not significantly modified compared to HTF, which is also reassuring for practical applications. Finally, thermal heating/cooling cycles were performed. We observed the nanofluid was stable in these cycles, which is also imperative for application in CSP plants. Molecular level simulations have given us very useful insights about the structure and behavior of these WS2-based nanofluids. AIMD simulations revealed a rapid dissociative adsorption of diphenyl oxide molecules at the W-terminated WS2 (10-10) edge, occurring with a very low kinetic barrier, and a very negative adsorption energy. The dissociation fragments will remain adsorbed, decorating the edge and forming new W-O and W-C bonds, as a large energy would be required for desorption. XPS measurements confirmed the dissociative adsorption, providing clear evidence of the formation of permanent W-O and WC bonds in the nanofluid. The decoration of the WS2 edge could have significant impact on the rheological and transport properties of the nanofluids, and might explain some of the appealing features reported here for this kind of system, although the link between the two observations can only be speculated at the moment. This is a topic that calls for further theoretical and experimental investigation. 32 Acknowledgements We thank the Ministerio de Ciencia, Innovación y Universidades of the Spanish Government for funding under Grant No. RTI2018-096393-B-I00, and for the financial support related to measurements of thermal properties, which were carried out using devices acquired under Grant No. UNCA15-CE-2945. We also thank Andalusian Government for funding under Grant No. sol-201800107510-tra. PE acknowledges the European Union through the European Regional Development Fund (ERDF), the Ministry of Higher Education and Research, the French region of Brittany and Rennes Métropole for the financial support of surface tension device. This work made use of the UK national supercomputer facility ARCHER, via RGC’s membership of the UK HPC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202). Molecular Dynamics calculations were made through CICA - Centro Informático Científico de Andalucía (Spain). Supporting Information Calculations of the ratio of the surface tension components. UV-vis spectra registered for the nanofluids prepared. Nanofluid performance. Base fluid-surfactant-WS2 interactions. XPS survey spectrum for WS2 exfoliated. Authors contributions P.M.-M. contributed to the preparation and characterization of nanofluids, and writing the article; S.D.M. contributed to the theoretical calculations of the kinetic barriers; E.I.M. contributed to the classical molecular dynamics calculations; P.E. contributed to the rheological and surface tension measurements, discussion of the results, and writing the article; R.A. contributed to the characterization of the nanofluid; A.S.C. contributed to the classical molecular dynamics calculations, and writing the article; R.G.-C. contributed to the AIMD and the kinetic barriers calculations, discussion of the results obtained, and writing the article, J.N. contributed to the direction of the work, discussion of all the results and writing the article. Conflicts of interest The authors declare no competing financial interests. 33 References (1) Chen, M. J.; He, Y. R.; Zhu, J. Q.; Wen, D. S. 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