An Optical Method For Quantitatively Determining The Surface Free .

ArticleAnalytical Chemistrytion, 160 μL of the supernatant from each probing liquid wascarefully transferred from the centrifuge tubes to a 96-wellmicroplate. Optical densities (ODs) of these supernatants weremeasured using a microplate reader (Epoch, BioTek,Winooski, VT). A characteristic wavelength of 400 nm wasdetermined prior to the measurements. The OD400 was plottedagainst the surface tensions of the probing liquids. A maximumOD value was determined by peak fitting experimental pointswith data smoothing followed by third-order polynomial fittingusing OriginPro (Northampton, MA). Each measurement wasrepeated at least three times. Results were shown as mean standard deviation. RESULTSMorphology of the Micro- and Nanoparticles. Figure 2shows the electron micrographs of nine representative micro-Figure 1. Measurement principle of the maximum particle dispersion(MPD) method. The classical DLVO theory predicts that thecolloidal stability of a particle suspension is determined by the balancebetween the electrostatic repulsion and van der Waals attraction.Dispersing particles in a liquid of surface tension similar to the surfacefree energy (SFE) of the particles minimizes the van der Waalsattraction between particles across the suspending liquid, thusresulting in maximum particle dispersion. Hence, it is expected thatthe surface tension of the liquid (γlv) resulting in a maximum lightabsorbance should be close to the SFE of the particles (γpv). The lightabsorbance of the particle suspension in a series of liquids can beeasily measured with the optical density (OD) and compared using aregular microplate reader.Assuming the liquid media is of low dielectric constant andthus dispersion forces dominate, the geometrical meancombining rule, A12 A11·A 22 , derived from the Lifshitztheory can be applied. Hence:A 212 ( A11 A 22 )2(3)Relating the Hamaker constant to the surface tension and SFE(γ) using A 24πD2γ, where D is the minimum separationdistance between surfaces, we find:A 212 24πD2( γ1 γ2 )2(4)Figure 2. Scanning electron microscopy (SEM) micrographs showingthe morphology of the studied micro- and nanoparticles: (a)Triethoxycaprylylsilane-coated zinc oxide nanoparticles (TCS-ZnONPs); (b) multiwalled carbon nanotubes (MWCNTs); (c) graphenenanoplatelets (GNPs); (d) molybdenum(IV) sulfide flakes (MSFs);(e) neodymium(III) oxide (NO) NPs; (f) zeolite (large); (g) zeolite(small); (h) poly(vinylpolypyrrolidone) (PVPP); and (i) polystyrenemicroparticles (PS MPs).According to eq 4, when the surface tension of the suspendingliquid γ1 is equal to the SFE of the dispersed particles γ2, theinterparticle van der Waals attraction is minimized, thusresulting in the least agglomeration and the slowestprecipitation. The state of particle dispersion in a series ofsuspending liquids can be readily compared by measuring lightabsorbance. The surface tension of the liquid in which theparticles are maximumly dispersed, that is, with the highestoptical density, is expected to be equal to the SFE of thesuspending particles.Implementation of the MPD Method. Two sets ofprobing liquids were used in the measurement of particle SFE.One set was composed of 16 binary mixtures of ethanol andwater with surface tensions ranging from 22 mJ/m2 (for pureethanol) to 72 mJ/m2 (for pure water). Another set consistedof six pure alkanes ranging from C5 to C16, with a surfacetension range of 16 27 mJ/m2. Surface tensions of theseprobing liquids were determined at room temperature usingCDS.A trace amount of the particle stock solution was added tothe series of probing liquids, each at 0.5 mL, vortexed, and leftundisturbed for 10 30 min to allow natural sedimentation.When the natural sedimentation was too slow, centrifugationwas used to accelerate the process. After effective sedimenta-and nanoparticles that cover a range of chemistries, sizes,dimensions, and morphologies. These are (a) triethoxycaprylylsilane-coated zinc oxide nanoparticles (TCS-ZnO NPs), (b)multiwalled carbon nanotubes (MWCNTs), (c) graphenenanoplatelets (GNPs), (d) molybdenum(IV) sulfide flakes(MSFs), (e) neodymium(III) oxide (NO) NPs, (f) largezeolite microparticles (MPs), (g) small zeolite MPs, (h)poly(vinylpolypyrrolidone) (PVPP) MPs, and (i) polystyrene(PS) MPs. The chemical formula, source, morphology, size,and literature SFE value, if available, of these particles aresummarized in Table 1.Proof of Feasibility of the MPD Method. We showedthe feasibility of the MPD method in determining the SFE ofTCS-ZnO NPs and MWCNTs using two sets of probingliquids, each with three repetitions. One was a polar liquid setcomposed of binary mixtures of water and ethanol, which12821DOI: 10.1021/acs.analchem.9b02507Anal. Chem. 2019, 91, 12819 12826

ArticleAnalytical ChemistryFigure 3. Determination of the surface free energy (SFE) of triethoxycaprylylsilane-coated zinc oxide nanoparticles (TCS-ZnO NPs) using themaximum particle dispersion (MPD) method. Each panel shows the optical density at 400 nm (OD400) as a function of the surface tension of theprobing liquids. Three runs of each measurement are presented to show reproducibility. (a) OD curves obtained with the polar liquid set, that is,water/ethanol mixtures. The OD curves show no local peak values but monotonically increase with reducing surface tension, indicating that theSFE of the TCS-ZnO NPs is lower than the minimum surface tension of the polar probing liquids. (b) OD curves obtained with the nonpolarliquid set, that is, single alkanes of varying carbon chains. The OD curves show a local peak value at 21.1 0.1 mJ/m2, indicating the SFE of theTCS-ZnO NPs. (c) Superimposed OD curves obtained with the polar and nonpolar liquid sets. A single peak appears in the large surface tensionrange from 16 to 72 mJ/m2, indicating uniqueness of the SFE measurement.Figure 4. Determination of the surface free energy (SFE) of multiwalled carbon nanotubes (MWCNTs) using the maximum particle dispersion(MPD) method. Each panel shows the optical density at 400 nm (OD400) as a function of the surface tension of the probing liquids. Three runs ofeach measurement are presented to show reproducibility. (a) OD curves obtained with the polar liquid set, that is, water/ethanol mixtures. The ODcurves show a local peak value at 25.3 0.5 mJ/m2, indicating the SFE of the MWCNTs. (b) OD curves obtained with the nonpolar liquid set, thatis, single alkanes of varying carbon chains. The OD curves show no local peak values but monotonically increase with increasing surface tension,indicating that the SFE of the MWCNTs is higher than the maximum surface tension of the nonpolar probing liquids. (c) Superimposed OD curvesobtained with the polar and nonpolar liquid sets. A single peak appears in the large surface tension range from 16 to 72 mJ/m2, indicatinguniqueness of the SFE measurement.provides a large surface tension range from 22 to 72 mJ/m2.The second one was a nonpolar liquid set comprised of sixpure alkanes of varying lengths, from C5 to C16, which covers asurface tension range from 16 to 27 mJ/m2. (Characterizationof the polar and nonpolar liquid sets can be found in FiguresS1 and S2.) The nonpolar liquid set overlaps the surfacetension range of the polar liquid set on the low surface tensionend, and extends the surface tension range of the polar liquidset by 6 mJ/m2 toward the lower end. Physicochemicalproperties of the probing liquids can be found in Table 2.As shown in Figure 3a, when measured in the polar liquidset, no peak in the optical density (OD) value can be found forthe TCS-ZnO NPs. Rather, the OD value quickly increasedwhen the surface tension is lower than 30 mJ/m2 andmaximizes in pure ethanol. When measured in the nonpolarliquid set, as shown in Figure 3b, an OD peak appears at 21.1mJ/m2, indicating the maximum particle dispersion. Figure 3cshows the superimposed measurements with the polar andnonpolar liquid sets. It is clear that the OD value transitssmoothly between the two sets of probing liquids, indicatingthat the MPD method is nonspecific to the probing liquidsused in measurements. Within the combined surface tensionrange from 16 to 72 mJ/m2, a single peak of the OD valueappears at 21.1 mJ/m2, indicating the SFE of the siloxanecoated ZnO NPs. This value is in good agreement with theliterature SFE value of similar silanes and siloxanes.31Figure 4a c shows the measurements of MWCNTs in bothpolar and nonpolar sets of the probing liquids. Opposite toTCS-ZnO NPs, the OD peak of MWCNTs appears at 25.3mJ/m2 when measured in the polar liquid set, while the ODvalue in the nonpolar liquid set monotonically increases withincreasing surface tension. Consequently, only a single ODpeak appears in the combined surface tension range of 16 72mJ/m2, indicating a unique SFE value at 25.3 mJ/m2.SFE of Micro- and Nanoparticles. Figure 5 shows theSFE measurements of all nine micro- and nanoparticles, eachwith three repetitions. Among these particles, TCS-ZnO NPs,MWCNTs, and GNPs were measured with both polar andnonpolar probing liquids, while the rest of the particles weremeasured with only the polar probing liquids that provide asufficiently large surface tension range to cover the SFEs ofthese particles. It is clear that the MPD method was able to12822DOI: 10.1021/acs.analchem.9b02507Anal. Chem. 2019, 91, 12819 12826

ArticleAnalytical ChemistryFigure 5. Determination of the surface free energy (SFE) of various micro- and nanoparticles using the maximum particle dispersion (MPD)method. Three runs of each measurement are presented to show reproducibility. (a) Triethoxycaprylylsilane-coated zinc oxide nanoparticles (TCSZnO NPs); (b) multiwalled carbon nanotubes (MWCNTs); (c) graphene nanoplatelets (GNPs); (d) molybdenum(IV) sulfide flakes (MSFs); (e)neodymium(III) oxide (NO) NPs; (f) zeolite (large); (g) zeolite (small); (h) poly(vinylpolypyrrolidone) (PVPP); and (i) polystyrenemicroparticles (PS MPs). Among these particles, TCS-ZnO NPs, MWCNTs, and GNPs (a c) were measured with both polar (hollow symbols)and nonpolar (solid symbols) liquid sets, while the rest of the particles (d i) were measured with only the polar probing liquids. The determinedSFE values are summarized in Table 1.tive particles studied here cover a large range of chemistries(silanes, carbon, rare-earth element, and polymers), sizes (from 50 nm to 5 μm), dimensions (0D, 1D, 2D, and 3D), andmorphologies (spheres, rods, fibers, plates, and cubes),demonstrating the versatility and robustness of this method.Understanding the SFE of these micro- and nanoparticlesprovides novel insights into many surface science and d ZnO NPs. Triethoxycaprylylsilane (TCS) is a silane/siloxane commonly found incosmetics and personal care products. It has also been used inhydrophobic coatings and Pickering emulsions. To the best ofour knowledge, the exact SFE of silane/siloxane materials hasnot been reported, although the SFE of one commonly useddetermine the SFEs of all tested particles as indicated byreproducible single OD peaks appearing in all measurements.The SFE of these micro- and nanoparticles are summarized inTable 1, and compared to the literature SFE values, if available. DISCUSSIONDespite its importance in many scientific and industrialapplications, literature values for the SFE of micro- andnanoparticles are not only scarce but also highly contentious,which highlights the urgency of developing an easy-to-usemethod in determining the SFE of particulate matters. Theinvention of the maximum particle dispersion (MPD) methodfor quantitatively determining the SFE of micro- andnanoparticles aids in accomplishing this task. Nine representa12823DOI: 10.1021/acs.analchem.9b02507Anal. Chem. 2019, 91, 12819 12826

ArticleAnalytical Chemistryphase.16 The optimal performance was found in a liquid phasewhose surface tension matches the SFE of the exfoliated 2Dnanomaterials.18,19 This is not unexpected as the exfoliated 2Dnanomaterials would be maximumly dispersed in the liquidwith matching surface energies.Neodymium(III) Oxide NPs. Neodymium(III) oxide is arare-earth oxide that shows increasing applications in catalysisand additive manufacturing of ceramics and magnets. Understanding its SFE allows more accurate and efficient tuning ofmanufacturing processes such as the sintering dynamics ofpowder into solid ceramics and glass. To the best of ourknowledge, the SFE of neodymium(III) oxide NPs has not yetbeen reported. Here, we have determined their SFE to be 30.4 1.6 mJ/m2 using the MPD method.Zeolites: 3D (Cubic) Porous MPs. Aluminosilicates andclays are another important category of materials often used atthe micro- and nanoscale. In particular, zeolites, which arehydrated aluminosilicates with microporous structures, areoften used as molecular sieves for purification in the form ofmicroparticles as well as for the fluid catalytic cracking of highmolecular weight hydrocarbons. In their use as adsorbents andcatalysts, their efficiencies are largely dependent on their SFE.However, the SFE of zeolites are very difficult to measure dueto their inherent porosity, which prohibits the use of thetraditional contact angle method. In the present work, wedetermined the SFEs of two zeolite-A cubic MPs with differentsizes (4 vs 1 μm). Because both particles are of micrometersize, their SFEs do not differ significantly, with 30.8 0.1 mJ/m2 for the larger zeolite MPs and 31.7 1.3 mJ/m2 for thesmaller zeolite MPs. Our measurements are slightly lower thanthe SFE of zeolites determined with the capillary penetrationmethod, that is, 34.49 mJ/m2.42PVPP and Polystyrene: Polymeric MPs. PVPP is acommon polymeric material used in pharmaceutical excipients,as well as filtration/binding agents used in the production ofalcoholic beverages. Polystyrene is commonly used as modelparticles for studying nanotoxicology, drug delivery, and selfassembly. Because of their importance, there are many studiesthat report the SFE of polymeric particles and bulk polymers.The SFE of these polymeric materials is commonly reported inthe range of 30 43 mJ/m2.43 45Here, the SFEs of PVPP and polystyrene MPs aredetermined to be 34.2 1.5 and 35.8 0.4 mJ/m2,respectively. These measurements fall into the lower end ofthe literature values. Understanding the SFE of polymericparticles plays an important role not only in their applicationsbut also in their syntheses. It has been long known thatselecting a correct synthesis liquid is paramount for controllingthe polydispersity of polystyrene nano- and microspheres.46 Inthis case, it is most likely that the surface tension of thesynthesis liquid and the SFE of the polystyrene particles have asynergistic effect in determining the final size distribution ofthe nano- and microspheres.Advantages and Limitations of the MPD Method. TheMPD method has three key advantages that make it superb toexisting methods in determining the SFE of particles. First, theMPD method is versatile and applicable to various particles. Asdemonstrated by the SFE measurements of nine representativemicro- and nanoparticles, the MPD method is not limited bythe chemistry, size, dimension, or morphology of the particles.This method is successful in determining the SFE of particlesencompassing 3 orders of magnitude from 50 nm to 5 μm insize. Second, the MPD method is simple in principle and thuspolymeric siloxane, polydimethylsiloxane (PDMS), is typicallyestimated to be around 20 23 mJ/m2.31 Here, the SFE ofTCS-ZnO NPs was determined at 21.1 0.1 mJ/m2, which isconsistent with previous estimations.Multiwalled Carbon Nanotubes (MWCNTs): 1D (Fibrous) Nanomaterials. The SFE of CNTs plays an essentialrole in determining their dispersibility and aggregation states incomposite materials, as well as their surface interactions withpolymer matrix.32,33 MWCNT is one of the most studiedparticulate matters in term of its SFE. However, availableliterature values showed large variations from as low as 4 mJ/m2,34 to as high as 82.6 mJ/m2.35 Two methods have beendeveloped to specifically measure the SFE of CNTs, bothtaking advantage of the fibrous shape of the CNTs. Barber etal. modified a single MWCNT to simulate the Wilhelmy platetechnique.36 A single MWCNT was attached to an atomicforce microscopy (AFM) tip and dipped into various testliquids. The SFE of the MWCNT was calculated from differentcapillary forces measured during the advancing and recedingprocesses. Using this method, these workers estimated the SFEof MWCNTs to be 27.8 mJ/m2.36 Nuriel et al. calculated theSFE of fibrous nanomaterials by using various polymer melts asprobing materials and measuring their contact angles onMWCNTs via scanning electron microscopy (SEM).37 Thismethod estimated the SFE of MWCNTs to be 45.3 mJ/m2.37It should be noted that both of these methods are based on themeasurement of single MWCNTs. Hence, both methods sufferfrom errors due to variations among individual MWCNTs. Incontrast, our MPD method determines the SFE of MWCNTsas an averaged thermodynamic property. Here, we report theSFE of MWCNTs to be 25.3 0.5 mJ/m2, which is near theSFE value reported by Barber et al.36Graphene Nanoplatelets and MoS 2 Flakes: 2D(Planar) Nanomaterials. Graphene nanoplatelets and MoS2flakes are two widely used 2D nanomaterials, especially in theenergy and semiconductor industry. The plane shape of thesenanomaterials facilitates their fabrication into relatively smoothsurfaces that permit SFE measurements using the traditionalcontact angle method. Available literature values of SFE forthese two 2D nanomaterials, obtained with the contact anglemethod, are almost identical, 46.7 mJ/m2 for graphene38 and46.5 mJ/m2 for MoS2.39Here, we determined the SFEs of graphene and MoS2 to be30.3 0.9 and 28.6 0.6 mJ/m2, respectively, significantlylower than previously expected. These differences in SFE maybe explained by the so-called wetting transparency effect.40When determining the SFE of 2D nanomaterials using thecontact angle method, one needs to immobilize a thin film, ifnot a single layer, of these nanomaterials onto a macroscopicsubstrate, usually made of hydrophilic materials such as micaor glass. Consequently, the contact angle phenomenon of the2D nanomaterials, as well as the resultant SFE, would beinfluenced by that of the hydrophilic substrate, thus resultingin a relatively higher SFE estimation.The SFE of 2D nanomaterials determined here could beuseful in a range of applications. For example, one recent andincreasingly popular method of synthesizing 2D nanomaterialsis their exfoliation in the liquid phase.41 The solid precursor forthe 2D nanomaterial, for example, graphite for graphene, isadded into a selected liquid and ultrasonically agitated, whichexfoliates the monolayered nanomaterial from its bulkprecursor. Recent studies showed that the efficiency ofexfoliation was largely affected by the selection of the liquid12824DOI: 10.1021/acs.analchem.9b02507Anal. Chem. 2019, 91, 12819 12826

ArticleAnalytical Chemistryrequires no complicated theoretical interpretations. This isparticularly advantageous over the classical contact anglemethod in which theoretical interpretations are a necessity forthe SFE measurements. Third, the MPD method is easy-to-use,fast, and inexpensive. The only specialized facility needed forthe measurement is a microplate reader that is low-cost andreadily available in many research laboratories. Once theprobing liquids are prepared, calibrated, and stored, the entireSFE measurements, including particle dispersion, sedimentation, and optical analysis, can be completed usually within anhour. In addition, because t

Principles of the MPD Method. Principles of the maximum particle dispersion (MPD) method stem from the DLVO theory of colloidal stability. As shown in Figure 1,inan ideal situation with a single type of particles suspended in a liquid, the particles can interact either with each other or with the suspending liquid. Given such a system of liquid