Article pubs.acs.org/JPCC
Effect of Interfacial Microstructure of Adsorbed Poly(ethylene glycol) Monooleate on Steel Substrate on Sliding Friction in Oil in Water Emulsion Vathsala Kanagalasara,† Deepak Saxena,‡ and Sanjay K. Biswas*,† †
Nanotribology Lab, Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012, India Indian Oil Corporation Limited (R&D), Faridabad, India
‡
ABSTRACT: The concentration of a nonionic surfactant and water pH were varied in an oil-in-water emulsion to minimize the friction coefficient between a steel ball sliding on a steel flat. At a surfactant concentration near the CMC (critical micelle concentration) the oil droplet size was found to be minimum. In this paper we study the microstructure of the surfactant molecules self-assembled on the steel substrate in water to comment on the ability of the surfactant aggregate to attract and retain oil. We find that a near semicylindrical hemimiceller microstructure with hydrocarbon tails projecting into bulk water as obtained at CMC in near neutral water is best able to capture and retain oil in yielding a low coefficient of friction.
1. INTRODUCTION Colloidal stability is an important property that dictates the choice of surfactant for oil-in-water emulsions in metal-cutting and metal-forming process. Intermolecular processes in such systems have been studied extensively1,2 for different pH states of water,3 different types of surfactants,4 and different concentrations of surfactants.5 The structure and shape of the self-assembled liquid crystalline aggregates in the bulk liquid is well understood.6−9 The manufacturing processes however also demand that the emulsions lubricate the metal-to-metal contact efficiently. This necessitates an understanding of the solid/ liquid/aggregate interface. The specific energy of the solid surface sets up competing surfactant−surface and solvent− surface interactions that may change the bulk aggregate structures to surface-driven new structures such as hemimicelles.10−12 Given the ionic and hydrophilic nature of the substrate, parameters such as the pH of water, the ionic nature of the surfactant, and the surfactant concentration may need to be modulated accordingly to ensure the sliding interaction takes place when the frictional resistance is low. In principle the optimum interfacial microstructure should be such as to replace water by oil and wet the substrate accordingly by oil. This allows the shear plane at solid/solid contact to be transferred to the oil to generate a low friction hydrodynamic condition. Different microstructures of the adsorbed surfactants have been observed at solid/water interfaces; hemimicelles,13 reverse hemimicelles,14 admicelles,15 bilayers, and multilayers.16 Different experimental approaches have been used to explore adsorption mechanisms and the structure of adsorbed species.17 Kellar et al.,18 using Fourier transform infrared (FTIR)/internal reflection spectroscopy (IRS) techniques have shown that © 2012 American Chemical Society
adsorbed dodecylsulfate ions are physically adsorbed and have random orientation on fluorite surfaces. Koopal et al.19,20 have used self-consistent (mean) field lattice theory for adsorption and association (SCFA) to perform detailed adsorption measurements for both cationic and anionic surfactants on rutile. Somasundaran and co-workers21 have used fluorescence probes to study polarity and microviscosity of adsorbed dodecylsulfate layers on alumina. By use of time-resolved fluorescent studies they estimated the number of adsorbed surfactant ions which constitute surface aggregates. The data leads them to identify four adsorption regions,22 each region characterized by configuration.17 The Somasundaran map was validated and enhanced23 using luminescence probe, Raman,24 and electron spin resonance (ESR) spectroscopy.25 Fluorescent probes,26 ellipsometry,27 and neutron reflectivity28 have also been used to provide direct evidence for the existence of molecular aggregates on solid substrates. Atomic force microscopy (AFM) has been used for direct observation of surface aggregate structures. AFM has been used to study cationic surfactant aggregates29 on graphite,13 mica,30 and gold.31 The study showed that changing the hydrophobicity of the substrate and surfactant chemistry lead to a range of spherical, cylindrical, and planar geometries. AFM and surface force apparatus (SFA) are useful tools for tribological studies of emulsions as they provide information related to the physical state of the substrate. Many of such studies29−32 use double layer forces to image the modified Received: April 30, 2012 Revised: September 7, 2012 Published: September 10, 2012 20830
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substrate. The imaging mode29 as well as the force curves has been used to map the geometry30 of the aggregates and the surface forces33 when a surfactant dispersed in a liquid, especially water, adsorb on the substrate. The interaction of anionic,34 cationic,22 and nonionic35 surfactants in a solvent with a variety of substrates of different hydrophobicities29−31 and ionic states22,34,35 have been studied to reveal not only the different geometries of the surface aggregate but also their potential for further adsorption of other matter suspended in the liquid. Adsorption of nonionic surfactants on solid substrates has however received less attention than the adsorption of ionic surfactants. Adsorption of nonionic surfactant on a number of solid surfaces has been studied by a variety of techniques. Forces between hydrophilic mica surfaces in solutions of pentakis(oxyethylene) dodecyl ether with added electrolyte were studied by Rutland and Christenson using the surface force apparatus.33 They conclude that that nonionic surfactants adsorb weakly on mica surfaces. Nonionic surfactants have been found to adsorb strongly on hydrophobic graphite.36 Adsorption increases with increasing concentration up to the critical micelle concentration (CMC), Patrick et al,37 have reported an AFM study of interfacial selfassembly structures of a series of nonionic surfactants, poly(oxyethylene) n-dodecyl ether (C12En) on graphite using soft contact imaging. Nonionic surfactants on graphite give force profiles which exhibit short-range repulsion that can be used for imaging in a manner similar to electrostatic imaging. C12E5C12E10 aggregates arrange in parallel stripes perpendicular to the underlying graphite symmetry axes. These may be interpreted as hemicylindrical micelles; the interpretation consistent with previous studies of ionic surfactants adsorbed on graphite.29 C12E23 shows a featureless layer, and C12E3 forms an anchored lamellar phase growing normal to the graphite surface. The interfacial structures are related to those formed in bulk solution and show that the initially adsorbed molecules template the interfacial aggregates, modifying their bulk selfassembly behavior.38 In this paper we are not only concerned with the adsorption of a nonionic surfactant on a hydrophilic steel surface but also with the wettability of the substrate modified, by hydrophobic oil droplets, in an oil-in-water emulsion. We commence our study by tracking the morphology of the surfactant aggregate in water on the substrate, as a function of water pH and surfactant concentration. This provides a qualitative understanding of the interaction potentials that can be used to rationalize the wettability of oil on the substrate. We believe that this information is important to understand the frictional behavior of steel on steel interaction lubricated by the emulsion. We investigate the morphology of adsorbed poly(ethylene glycol) monooleate molecules on steel using a liquid AFM technique. The AFM measurements are combined with measurement of contact angle, droplet size distribution and friction at different water pH and surfactant concentrations to obtain a mechanistic understanding of tribology of the oil-inwater emulsion system.
preparation. Deionized water, obtained by processing of distilled water through Millipore purification (Milli-Q, USA) system, was used to hydrolyze the substrate and emulsion preparation. Steel Sample Preparation. Steel substrates were polished mechanically (sequentially with a 1−3 μm and a 0.25 μm diamond paste), and the substrate was then sonicated with acetone and alcohol for 15 min to remove all polishing debris and then sonicated with Millipore water for 20 min. Diamond polishing of the substrate was done by rotating the substrate surface continuously, in forward and reverse directions alternatively against a diamond paste smeared rotating, polishing cloth, to eliminate any topographical directionality on the surface. The substrates were flushed with a stream of dry nitrogen gas and preserved in a desiccator. Before performing the experiments, substrates were kept in an ultraviolet (UV) cleaning chamber (Bioforce nanosciences, US) for 30 min to burn all carbonaceous contaminations which block the adsorption sites. Preparation of Emulsion. All the emulsions were prepared with paraffinic oil using the emulsifier. The paraffinic oil (0.2% v/v in water) was blended with the emulsifier using bath sonication for 2 min. The blend was diluted using Milli Q water. This concentrate was then probe sonicated using high intensity ultrasonic processor (Sonics 500 W, Connecticut, USA) for 5 min. To prepare the pH 10 emulsions, 1 M NaOH (requisite amount added to make pH 10 water) was mixed with water before the ultrasonic treatment. The variation of pH, obtained after sonication, was determined and is reported here as a function of surfactant concentration. Dynamic Light Scattering. The emulsion was subjected to particle size analysis using a 90 Plus particle size analyzer (Brookhaven Instruments Corp., Holtsville, NY). Nanotribometer. Tribological experiments were carried out under a 700-mN normal load using a ball on disk tribometer (Nanotribometer, CSM Instruments, Switzerland). The ball used is of DIN 100Cr6 steel and has a diameter of 2 mm. The steel ball was cleaned with acetone in an ultrasonic bath prior to the experiment. Two sensing mirrors attached to the cantilever head (which carries the ball at one end) perpendicular to each other (X and Z axis) measure displacements of the cantilever during sliding.39 During sliding, the friction coefficient is continuously estimated by measuring the X and Z displacements of the cantilever. All measurements were carried out under ambient conditions (relative humidity, 35%; temperature, 296 K) at 0.005 cm/s sliding speed. Goniometer. A contact angle analyzer apparatus (OCA 30 from DataPhysics Instruments GmbH, Germany) was used to measure surface/interface tension and contact angle. Droplets for analysis were placed on a substrate using an automatic injection pump. The automatic injection pump can delivers droplets of specific volumes from the syringe at a specified rate. The video system is a high-performance image processing unit with 132 MB/s data transfer rate (compatible to European standard CCIR and US standard RS-170), 50 images/s can be digitized using this unit. The microscope focus, aperture, and magnification are adjusted along with the tilt of the unit to establish the clearest resolution of the image. The 32-bit software SCA 20 was used to analyze the captured drop images. Both surface and interfacial tension were measured by pendant drop method.40,41 All the experiments were carried out at 296 K. Contact angle of oil with the surfactant dispersed water on steel (which was cleaned as above) was measured.
2. EXPERIMENTAL SECTION Sample Preparation. Bearing steel (DIN 100Cr6) is used as the substrate. Paraffinic light oil of viscosity 27.4 cP at 40 °C (SD fine chemicals limited, Laboratory Reagent, Mumbai, India) and PEG 860 monooleate (PEGMO) (Sigma−Aldrich, USA) are used as oil and surfactant, respectively, for emulsion 20831
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Figure 1. Volume ratio of emulsion droplets with varying concentration of PEGMO. Volume ratio, Vs/Vb, where Vs and Vb are the numbers of small and big droplets in a unit volume. (a) pH = 5.5, dmall droplet size 200 nm. (b) pH = 9.5, small droplet size 400 nm.
surfactant solution for 15−60 min before imaging. The surface aggregates were imaged in the deflection mode.
The receding contact angle was found to be only marginally different from the advancing angle. Images of the solid/liquid/ liquid (S/L/L) systems were captured and processed using the above software. For S/L/L systems a steel plate was immersed in the surfactant solution, and then oil droplets were placed on the metal surface from the syringe at a very slow rate and permitted to spread without restraint on the substrate. The oil/ water interfacial tension was plotted as a function of surfactant concentration to determine the CMC of the emulsifier. The interfacial tension (γOW) between oil and water and the contact angle at metal surface/oil/water (surfactant solution) junction (θOWS) were determined experimentally. Equilibrium of forces at the interface gives γSW = γOW cos θOWS + γSO
(1)
WA = γSW + γOW − γSO
(2)
3. RESULTS AND DISCUSSION The objective of this work is to create a contact condition where the oil from an oil-in-water emulsion may disjoin water
By use of eq 1, WA can be written as WA = γOW(cos θOWS + 1)
(3)
Figure 2. Friction coefficient of emulsion lubricated steel on steel sliding speed = 0.005 cm/s, contact pressure = 1.2 GPa.
where γ is the tension and O, W, and S are oil, water, and steel substrate, respectively. The work of adhesion (WA) may be estimated using eq 3.1 We have reported elsewhere42 the effect of time on the geometry of oil droplets in water dispersed with sodium oleate and oleic acid surfactants. The contact angle did not change in the time interval of 1 to 1000 s after the droplet was deposited. As the present contact angle measurements were done within 600 to 900 s of deposition, the static contact angle data we present here may be assumed to be valid. AFM. Surfactant aggregates at the solid−liquid (surfactant aqueous solution) interface were imaged using an AFM (‘‘Innova’’, Veeco, Santa Barbara, USA). Si3N4 cantilevers (Thermo Microscope, CA, USA) associated with a pyramidal tip were used for imaging. A V-shaped cantilever of stiffness 0.15 N/m was used in all the experiments. The cantilever normal stiffness was found using thermal vibration technique, built into the software. All the tips were cleaned in a UV chamber for 30 min before experiments. An aqueous surfactant solution was introduced into the AFM fluid cell. Aggregate topography was imaged by flying the tip above the plane of hard contact (steel substrate) using a force set point in the precontact region.29 To avoid or to reach acceptable thermal drift, the fluid cell was immersed in
at the substrate. If this is possible the shear plane in sliding tribology is in the oil, and the condition approaches that of hydrodynamic lubrication. It has been shown43 that it is possible to achieve this condition when an anionic (sodium oleate − HLB=18) surfactant is used at near CMC concentration in the emulsion. It was shown that a combination of DLVO and capillary forces creates a regime where nanometric size oil droplets are strongly attracted to a steel substrate. This concentration of surfactant was also found to be optimal in minimizing friction. The present work is concerned with an industrially popular polyethylene glycol based nonionic surfactant PEGMO, and the work is focused on interfacial microstructure which enables the oil to wet the steel surface. The pendant drop method gave an estimate40,41 of the CMC of nonionic PEGMO in high purity (pH ≈ 6.3) water as 0.625 mM. The CMC increases to 0.73 mM when the pH is increased to 10 from 6.3. As in the case of sodium oleate,43 Figure 1 shows that a ratio of the numbers (per volume) of small oil droplets (diameter < 120 nm, pH 5.5; diameter < 250 nm, pH 9.5) and big oil droplets (diameter > 200 nm, pH 5.5; diameter > 400 nm, pH 9.5) is highest at the CMC, at both pH 5.5 and 20832
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Figure 5. The pH of oil-in-water emulsions after sonication with varying surfactant concentration: (red solid circles) emulsion prepared using pH = 10 water, (black solid squares) emulsion prepared using pH 6.3 water (designated neutral), (yellow/red shaded background) isoelectric band of iron.
Figure 3. AFM image (200 × 200 nm, tip normal load 5 nN) of surfactant aggregates on steel. pH = 5.5, surfactant concentration: (a) 0.15 mM, (b) 0.625 mM, (c) 2.5 mM (arrows indicate spots where there may be projections of hydrophilic head into water). pH 9.5, surfactant concentration: (d) 0.625 mM, (e) steel in water (pH 6.3) with no surfactant.
Figure 4. 2D line profile of AFM image, Figure 3b. The profile line is 90° to the stripes. Figure 6. Oil drop shape with varying surfactant concentration.
pH 9.5. Figure 2 shows that the coefficient friction of the emulsion lubricated steel/steel sliding interaction is also the least at roughly the same surfactant concentration. These observations confirm our previous model44 but beg further questions as to why and how is it possible for oil in water to wet a steel surface so comprehensively, so as to achieve a low (0.06) coefficient of friction. To address the issue, the surfactant is first dispersed in water alone and allowed to settle on the steel surface. AFM image of the substrate (Figure 3b) shows that the surfactant molecules at CMC (also for comparison an image of steel substrate in the absence of surfactant is shown in Figure 3e) are arranged as a collection of wavy but parallel stripes. Two dimensional profiles were taken along straight lines transverse to the stripe length. Figure 4 shows one such profile; the profile is periodic with an
average spacing of 4.47 ± σ (standard deviation σ = 0.235 nm), and the rms (root-mean-square) maximum height of the surface is 0.2 ± 0.035 nm (the standard deviation refers to all the data from 10−15 frames). Such striped morphology of adsorbed surfactants has been observed by others.13,29,37,45−50 Geometry of the microstructure in relation to the estimated molecular length of the surfactant molecule indicates periodic structures: bilayers, spheroids, hemispherical, cylindrical miceller, semicylindrical hemimiceller. There is still much debate on the interpretation of such (Figure 4) surface topographical data, derived using different techniques. For the present PEGMO molecule the alkyl chain length is known to be 1.82 nm,51 and the diameter of the polymerized polyethylene oxide headgroup is 0.93 nm,52 an estimate of the head 20833
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Figure 7. Oil/water/substrate: (a) contact angle θOWS and (b) work of adhesion (WA) with surfactant concentration.
celles.29,31 To resolve the issue it is necessary to know precisely the variation in height across an aggregate. AFM is unfortunately not a very definitive technique to determine the shape of the aggregates.34 Nonionic surfactants are known17 to adsorb on hydrophilic surfaces by hydrogen bonding of surfactant polar groups with the hydroxyl groups present on the solid surface. It has been suggested53 that the molecules adsorb on the polar metal surface with their hydrophilic heads attached to the surface and the hydrophobic tails oriented to the aqueous phase. PEGMO consists of a hydrophobic long chain olefin and a more hydrophilic polymerized polyethylene oxide (PEO) headgroup. The oxygen atoms54 periodically present in the PEO chain are very electronegative and thus strongly polarize the O−C bonds, resulting in relatively strong electric dipoles54 where O has a partial negative charge (Oδ2−) and the two carbon neighbors have partial positive charges (2Cδ+). Since the O−H bonds in water are also strongly polarized, the partially positive water hydrogen can strongly interact with the partially negative PEO oxygen (strong hydrogen bonding). The overall effect is to produce a weak cationic surfactant.17 Depending on the quantity and nature of oxides present on a steel surface the isoelectric point of steel has been estimated55,56 to be between 5.2 and 6.7. Figure 5 shows that at 0.625 mM (CMC) surfactant concentration (apriori water pH = 6.3) the emulsion pH is ∼5.5, at 0.73 mM (CMC) surfactant concentration (apriori water pH = 10) the emulsion pH is 9.5. The pH of the emulsion (corresponding to an apriori 6.3 water pH) is on the lower bound of the isoelectric line of steel, especially at low PEGMO concentration. This suggests that the steel under this condition is charged negatively but weakly. The mildly cationic surfactant is therefore likely57 to set up an electrostatic interaction with steel with the polar head adsorbing on the substrate and the hydrocarbon tail extending into water. Figure 6 gives the geometry of an oil drop deposited on the steel surface decorated with the PEGMO surfactant in water as a function of surfactant concentration and water pH. Figure 7 gives the corresponding contact angle data. Figure 8 gives a schematic of a possible adsorption scheme for different surfactant concentration when the emulsion pH ≈ 5.5. Figure 3a is an AFM image of the substrate at 0.15 mM PEGMO concentration in water. The image shows patches of stripes (top and bottom of the figure) but otherwise a heterogeneous and disordered surface. The stripes have an average spacing of 2.86 ± 0.86 nm; the contact angle of an oil
Figure 8. Schematic of surfactant aggregates on steel (a) below CMC at pH 5.5, (b) and (c) at CMC for pH 5.5 without and with oil on steel, respectively, (d) at CMC for pH 9.5.
Figure 9. Schematic of PEGMO ester hydrolysis under alkaline conditions (at pH 10 adjusted using NaOH).
to tail molecular length is 2.75 nm. The observed spacing of the stripes is therefore somewhat less than twice the ideal molecular chain length, and the observed rms (root-mean-square) roughness of the adsorbed stripes is about one order smaller than half the stripe spacing. The periodic striped microstructure may be accounted for by a self-assembled monolayer of parallel stripes,22 cylindrical micelles, or semicylindrical hemimi20834
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Figure 10. Droplet size distributions of emulsions with varying concentration of PEGMO at (a) pH = 5.5 and (b) pH =9.5.
drop on this surface is 120°. This suggests that some of the molecular head groups though adsorbed on the surface are unshielded and provide hydrophilicity while some of the alkene chains may be extended on the surface providing hydrophobicity. The surface stripes (Figure 3a) may be clusters of 3 or 4 PEGMO molecules that stand normal to the substrate in a plane perpendicular to the stripes. Figure 8a is a schematic of the possible electrostatic events58,10 at a low surfactant concentration. The fact that with increasing surfactant concentration (less than CMC) the contact angle of the oil drop decreases steadily (Figure 7) suggests that the hydrophilic groups are increasingly shielded. If this interaction between the surfactant molecules and substrate is realistic at low surfactant concentration, with increasing concentration the molecules will form aggregates at the surface. The architecture may either be semicylindrical hemimicelle with two opposedly radiating hydrocarbon tails at the surface pinned by dispersive forces to the substrate or the aggregate is a cylindrical micelle with limited interaction of the cylindrical surface with the substrate and the polar head groups projecting into water. This is clarified by injecting an oil drop into the water and allowing it to settle on the substrate. Figure 6 and Figure 7a show that with increasing surfactant concentration the oil drop flattens, to increasingly wet the substrate, registering a contact angle of 42° at the CMC. This can happen only when the oil is adsorbed by the hydrocarbon tails of a semicylindrical hemimicelle. Any miceller formation would lead to expulsion of oil and large contact angles of the order of 130° and above. The work of adhesion of oil also increases with concentration until about CMC (Figure 7) indicating that in this range (
