Article pubs.acs.org/Langmuir
Novel Strategy Involving Surfactant−Polymer Combinations for Enhanced Stability of Aqueous Teflon Dispersions Mukesh Sharma,† Bhavesh Bharatiya,*,† Krupali Mehta,† Atindra Shukla,† and Dinesh O. Shah†,‡ †
Shah-Schulman Center for Surface Science and Nanotechnology, Dharmsinh Desai University, Nadiad-387001, Gujarat India Department of Chemical Engineering and Department of Anesthesiology, University of Florida, Gainesville 32611, Florida United States
‡
ABSTRACT: Among various polymers, the Teflon surface possesses extreme hydrophobicity (low surface energy), which is of great interest to both industry and academia. In this report, we discuss the stability of aqueous Teflon dispersions (particle size range of 100−3000 nm) formulated by a novel strategy that involves distinct combinations of surfactant and polymer mixtures for dispersion stabilization. As a first step, the hydrophobic Teflon particles were wetted using a range of surfactants (ionic, Triton, Brij, Tween, and Pluronic series) bearing different hydrophobic−lipophilic balance (HLB) and further characterized by contact angle and liquid penetration in packed powder measurements. The interaction between hydrophobic chains of surfactants and the Teflon particle surface is the driving force resulting in wetting of the Teflon particle surface. Further, these wetted particles in aqueous solutions were mixed with various polymers, for example, poly(vinyl alcohol) (PVA), polyvinylpyrrolidone (PVP), hydroxyethyl cellulose (HEC), and hydroxypropyl methyl cellulose (HPMC). The rate of sedimentation for the final dispersions was measured using a pan suspended into the dispersion from a transducer recording the increase in weight with time. A significant stability was noticed for Teflon particles suspended in surfactant + polymer mixtures, which was linearly proportional to the concentration of added polymer. The observed phenomenon can be possibly explained by molecular interactions between the hydrophobic chains of surfactant molecules and polar groups in the polymer architecture. Brij-O10 + HEC mixture was found to be the best surfactant− polymer combination for decreasing the sedimentation of the Teflon particles in the final dispersion. As measured by dynamic light scattering (DLS), the hydrodynamic volume of the Teflon particles increases up to ∼55% in the final formulation. These dispersions could be further explored for various technological applications such as paints, inks, protective coatings, and so forth.
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INTRODUCTION Teflon is a fluorocarbon polymer of tetrafluoroethylene (PTFE) and a registered trademark of the DuPont for its fluoropolymer resins. Important properties of Teflon include wear resistance, low friction, chemical inertness, large working temperature range, and dielectric strength. The resistive action of Teflon against thermal and chemical conditions is mainly governed by the higher value of bond energy for C−C (346 kJ/ mol) and C−F (485 kJ/mol) bonds, that also implies a very low polarity and cohesive energy. These extraordinary characteristics make it suitable for a wide range of products and applications involving pharmaceuticals, paints, inks, automobiles, medical instruments, semiconductors, filtration, petrochemicals, and nonstick cookware. These PTFE polymers are available in different forms like resins, films, coatings, and even powders. Among these are the colloidal dispersions in aqueous/nonaqueous media that involve the role of wetting agents to stabilize the inert submicrometer Teflon particles. Various Teflon based aqueous dispersions are formulated by DuPont, which exhibit low viscosity of about 20 cP. Large numbers of patents have reported the formation of stable © XXXX American Chemical Society
Teflon dispersions in water (up to 5 wt %) using nonionic surfactants as stabilizing agents.1−9 The stabilization of Teflon particles in a highly concentrated regime (about 60 wt %) is also reported, where a comparatively small amount of added surfactant provides exceptional stability.10−12 However, very little information is available in scientific literature about the mechanism for stabilization of such dispersions. The stabilization, wetting, and floatation of solid particles are strongly dependent on the adsorption of surfactant molecules at the particle surface. For many practical applications, surfactants are used as wetting agents.13,14 The most characteristic feature of the surfactants is their tendency to adsorb at the surface or interface mostly in an oriented manner. Surfactants are well-known to adsorb at the interface of various hydrophilic surfaces that include silica and different mineral oxides.15 The effects of polymer/surfactant adsorption on different particles surfaces like clay,16 silica,17 and bromide sols18 have been Received: April 2, 2014 Revised: May 20, 2014
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size distribution) was provided by Universal Medicap Ltd., Vadodara, India. The Milli-Q water obtained from a Millipore filter purifier was used for the preparation of all the dispersion formulations. Preparation of Dispersions. The Teflon particles were stirred in 1 wt % surfactant solution for 2 h using a magnetic stirrer. Unless specified, in an accordingly obtained mixture, a known quantity of polymer equivalent to 1 wt % was added and the final dispersion was vigorously stirred overnight using a magnetic stirrer. The concentration of Teflon powder was kept constant at 1 wt % for each measurement. Before each measurement, the dispersions were stirred for about 10 min to obtain a homogeneous mixture. Considering the practical applications of particle sizes below 200 nm, a strategy was used to eliminate the effects of large size particles. During sedimentation and DLS measurements, a waiting period of 3 min was allowed on the basis of quick sedimentation tendencies for heavy particles that may interfere during the measurements. Surface tensiometry and Contact Angle Measurement. The surface tension for 1 wt % aqueous surfactant solution was measured using an Attension Surface tensiometer (Biolin, model # Sigma 700) with a Wilhelmy plate as probe at 250C. The measurements were repeated five times to ascertain the reproducibility. Dynamic surface tension measurements were carried out using a bubble tensionmeter (Biolin, model # BPA-800P) at 25 °C. The contact angle measurements for 1 wt % aqueous surfactant solution were performed on PTFE (Teflon) surface using a KSV Attension contact angle goniometer at 25 °C. A solution drop was formed using a syringe capillary on freshly cleaned and dried surface, and further allowed to equilibrate for 1 min. The contact angles were measured using software operations based on Young’s equation. Similar measurements were repeated for 10 different drops, and average values were considered. Sedimentation Measurements. The sedimentation for aqueous Teflon dispersions was measured on an attension Surface tensiometer (Biolin, model # Sigma 700). The samples were stirred for 10 min before each measurement, and a waiting period of 3 min was applied. This strategy was useful to eliminate the effects of heavy particles that can interfere during the actual measurement. The rate of sedimentation for the final dispersions was measured, using a pan suspended into the dispersion from a transducer recording the increase in weight with time at 25 °C. The measurements were repeated five times, and average values were considered. Liquid Penetration Measurements. The wetting of powdered Teflon using surfactant solutions was achieved by a liquid penetration method using an Attension surface tensiometer (Biolin, model # Sigma 700). The powder Teflon was filled in metal cylinder, which was allowed to remain in surfactant solution for 3 min, and the resultant weight gain was noted. The measurements were repeated three times, and the results were highly reproducible. Dynamic Light Scattering (DLS). DLS experiments were performed to measure the average size of the Teflon particles in final formulation at a fixed scattering angle of 173° using a Malvern Zetasizer instrument (# Nano-ZS 4800, UK) equipped with a He−Ne laser operating at a wavelength of 633 nm at 25 °C. Each measurement system was repeated five times, and average values were considered. Dynamic light scattering measurement generates particle−particle correlation decay as a function of time (in microseconds) from scattered intensity fluctuation, which is called the normalized autocorrelation function of the light intensity, g(2)(t), which is related to the electric field normalized correlation function, g(1)(t), through the Siegert relation:25
investigated. The wetting and subsequent stabilization of these particles was achieved, which was controlled by surfactant interaction with the particle surface, though such feasibility is obvious for hydrophilic particles mixed with surfactants due to favorable interactions. Interestingly, surfactants are also applied for various water-repelling particle surfaces to make their surface hydrophilic. Dale et al.19 have successfully achieved the stabilization of hydrophobic silica particles using nonionic surfactants. Some industrially formulated hydrophobic silica dispersion are also reported.20 Wójcik et al.21 investigated the wetting characteristics of the PTFE surface by a mixture of anionic surfactants using contact angle measurements. The adsorption of nonionic and ionic surfactants on PTFE latex is reported by Bee et al.22 Dixit and Desai23 showed improved adsorption characteristics on PTFE particles for cationic + nonionic surfactant mixture by applying the most compact arrangement in the clusters formed at the solid−liquid interface. Thus, surfactants as wetting agent in pure and mixed states could prove effective for water incompatible surfaces like Teflon. Considering these advantageous properties, researchers in industry and academia have mostly concentrated on nonionic surfactants as wetting agents and subsequent stabilizers for Teflon dispersions. Even though numbers of industrially formulated Teflon dispersions are available on the market, a few academic studies are available describing their stability in the presence of external additives. Kratohvil and Matijevic24 investigated PTFE dispersions (using nonionic TX-100) formulated by DuPont in the presence of various external additives, namely, surfactants, electrolytes, and macromolecules. It is understandable that hydrophobic chains of surfactants have a significant role to play in the wetting of hydrophobic Teflon particles. It was revealed from our primary investigations that some surfactants are able to wet the PTFE surface effectively and a slow sedimentation was measured for dispersions of surfactant wetted Teflon particles in aqueous media. Surprisingly, the interaction of polymer with surfactant wetted Teflon particle showed significant improvement in the dispersion stability. We have introduced polar polymers with an idea that polymer molecules may coil around the wetted solid particle and provide more folds, loops, and tentacles at the interface, which should provide more steric hindrance and greater hydrodynamic volume and thus exhibit better dispersion stability. The improved dispersion stability could be achieved by preferential interactions in polymer + surfactant mixture at the Teflon−water interface. The possible changes in the hydrodynamic volume of the particle could also influence the dispersion stability. To the best of our knowledge, this is a novel approach for stable PTFE dispersions involving a surfactant + polymer mixture instead of a conventional nonionic surfactant based strategy. With the introduction of different surfactants and their mixtures with polymers, our results provide a better understanding of this unexplored methodology. The interactions of wetted Teflon particles with polymers provide exceptional stability, where the final formulations show better stability and longer shelf life for total phase separation of the particles, which increased to almost 5−6 times as compared to surfactant wetted unstable formulations.
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g(2)(t ) = 1 + β|g(1)(t )2 |
(1)
where β is the coherence factor (0 < β ≤ 1). g(1)(t) can be written as the Laplace transform of the distribution of the relaxation rates, G(Γ):
g(1)(t) =
MATERIAL AND METHODS
All the surfactants and polymers used for dispersion formulation were purchased from Sigma-Aldrich. The PTFE powder (100−3000 nm
∫0
∞
G(Γ) exp(−Γt ) dt
(2)
where Γ is the relaxation rate and the diffusion coefficient, B
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D = Γ/Q 2
m) values for 1 wt % surfactant solutions at 25 °C. The contact angle values were measured on cleaned a Teflon surface. Among surfactants employed, AOT showed maximum surface activity and reduced the surface tension (ST) of water up to 25 mN/m. The ethylene oxide based Pluronic surfactants and Tween surfactants were least surface active. Tween 20 and Tween 80 showed almost identical behavior for surface tension. An increase in contact angle was observed with corresponding increase in HLB, which is obvious due to increase of hydrophilicity for a solution drop on hydrophobic Teflon surface. The obtained results are useful in understanding the role of surfactants when it is added to water for wetting of Teflon particles. AOT and Brij-O10 exhibited the lowest contact angle which corresponded to their very low surface tension. The wetting performance of different surfactant solutions was measured using liquid penetration method in Teflon powder, which is added to a cylindrical metal probe attached to a transducer that measures the increase in weight. In this method, the probe filled with Teflon powder is dipped in surfactant solution for 3 min. The surfactant solution rises in the powder filled cylinder based on its possible interactions with the hydrophobic Teflon material. Figure 1A shows the wetting of Teflon particles in aqueous solutions of different surfactants. The surfactant solution rises in the powder filled cylinder as facilitated by interaction of hydrophobic surfactant chains with the hydrophobic Teflon surface. In other words, the Teflon particles are now covered by surfactant chains with polar headgroups facing the aqueous phase. Among various surfactants used for wetting, Brij-O10 showed maximum wetting performance. In a similar way, Brij-98 also showed better wettability compared to surfactants of the Tween, Triton, and Pluronic series. The surfactant solution rises due to wetting of hydrophobic Teflon particles and generation of hydrophilic channels that allow water to penetrate further. With a linear increase in the concentration of the surfactant, a relative increase in the wetting profile was observed (data not shown). The mechanism for such wetting and water penetration could be understood as shown in Scheme 1. The formation of waterliking channels facilitates adsorption of water due to polar interactions and capillary effect. The saturation is achieved once
(3)
The particle hydrodynamic diameter (Dh) was obtained using the Stokes−Einstein equation,
D=
kBT 3πηdh
(4)
Viscosity Measurements. The viscosity for aqueous solutions of surfactant + polymer mixture and final aqueous dispersions was measured using a Brookfield viscometer (model # LVDV IIIU) as a function of applied shear rate at 25 °C. The aqueous solutions containing 1 wt % each of surfactant and polymer were subjected to shear rate by using spindle #23. The average values were considered after repeating the measurements at least five times.
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RESULTS AND DISCUSSION Wetting of Hydrophobic Teflon Particles. A series of nonionic and anionic surfactants having different molecular characteristics and hydrophilic−lipophilic balance (HLB) was used to investigate their role in wetting of Teflon particles. Table 1 shows the contact angle (θ) and surface tension (mN/ Table 1. Characteristic HLB, Surface Tension, and Contact Angle (on Teflon) Values for 1 wt % Aqueous Solution of Different Surfactants at 25 °Ca MW(Hp) water SDS AOT Tween-20 Tween-80 Triton X-100 Brij-O10 Brij-98 Pluronic L-81 Pluronic F-68 Pluronic F-127
877.7 877.7 439.8 425.4 919.6 352.0 6686.0 8194.8
HLB
surface tension (mN/m)
14.3 13.4 14.0 12.1 15.9 02.6 15.9 13.2
71.59 31.37 25.22 34.00 36.10 30.78 30.56 31.64 32.12 39.55 37.75
± 0.027 ± 0.011 ± 0.027 ± 0.294 ± 0.358 ± 0.028 ± 0.021 ± 0.002 ± 0.028 ± 0.196 ± 0.203
contact angle (θ) 110.53 54.32 40.14 64.30 58.86 46.22 42.18 67.84 53.35 67.08 59.03
a
MW(Hp) = molecular weight of hydrophilic part; HLB = hydrophilic lipophilic balance.
Figure 1. (A) Wetting of Teflon powder in 1 wt % aqueous solution of surfactant at 25 °C as a function of time by using the liquid penetration method. AOT (■), TX-100 (○), Tween-20 (△), Brij-O10 (□), Tween-80 (●), Brij-98 (▲), SDS (◇), L-81 (★), F-68 (◆), and F-127 (☆). (B) Weight of water absorbed by surfactant treated Teflon particles in 3 min at 25°C. C
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Scheme 1. Schematic Diagram Showing Penetration of Surfactant Solution and Consequent Wetting of Packed Teflon Powder
Figure 2. Change in dynamic surface tension of surfactant solution as a function of bubble time at 25 °C.
decrease in the surface tension as a function of bubble lifetime is well-understood due to diffusion of surfactant molecules to the air−water interface with time from bulk solution. The values of surface tension for BrijO-10 aqueous solution increases considerably up to 72.7 mN/m in the presence of Teflon powder, which is identical to that of pure water. A very small increase in the surface tension for AOT solutions was observed with added Teflon powder. These results are in agreement with the wetting measurements. In other words, BrijO-10 has the highest adsorption capacity at the Teflon surface, which is reflected in the increase of surface tension at the air−water interface. With most of the Brij-O10 molecules involved in wetting the Teflon surface, their concentration at the air−water interface must decrease dramatically which is reflected by a higher dynamic surface tension. With AOT, the effect was less pronounced, and hence, the obtained wetting performance was inferior compared to that observed for BrijO10. In other words, the capillary action in polar channels is controlled by the surface tension of the liquid. The lower values of surface tension for AOT solution results in less significant capillary action and hence a low value of wetting compared to Brij-O10. Sedimentation Behavior of Surfactant Coated Teflon Particles in the Presence of Polymers. The wetting of Teflon particles by surfactants was expected to increase the stability of the formed dispersion, and the same should reflect a decrease of sedimentation rate. The sedimentation of Teflon particles in surfactant wetted conditions revealed that the wetted Teflon particles settle down fairly quickly. In other words, the wetting of Teflon powder surface by surfactant molecules inhibits its floating on the air−water interface, but is not strong enough to keep it suspended for a considerable period. The next idea was to investigate the effect of added water-soluble polymer. The rate of sedimentation for wetted AOT Teflon particles in the presence of PVP and PVA is shown in Figure 3. The amount of sedimented particles decreases markedly with addition of these water-soluble polymers. The result can be explained in terms of surfactant− polymer complexes. This is a clear indication of molecular interactions between the surfactant and polymer chains. Taking into consideration their significant performance as observed by wettability measurements, four surfactants, namely, Brij-O10, Brij-98, Triton X-100, and AOT, were selected for further investigations involving polymer interactions with surfactant coated Teflon particles. For each measurement, the specified amount of polymer was directly added to the surfactant wetted
all the hydrophilic sites are saturated with water for the surface of the packed powder coated with a certain surfactant. Due to packing of powder in the cylinder, a series of pores and channels is created bearing a specific volume. The amount of water absorbed by surfactant treated Teflon particles after the measurement period of 3 min is shown in Figure 1B. The surfactants belonging to the Brij series showed maximum wetting of Teflon particles, among the EO based nonionic surfactants. while all other surfactants including anionics (SDS and AOT) showed moderate wetting effect, the Pluronics exhibited the least wetting performance. It was concluded that the kinetic properties of surfactants could be a possible reason considering the individual affinity of a molecule for bulk and interface, and the rate of diffusion. Investigation on the kinetics of surfactants from bulk to interface was carried out with the aim to find the difference in the wetting efficiency for the given surfactant series. Scheme 1 shows the schematic diagram describing the penetration of surfactant solution in packed powder measurements. The wetting efficiency of surfactant solutions of Teflon powder (Figure 1) could be significantly influenced by the specific interactions of surfactant molecules at the Teflon surface. As the surfactant solution rises in the packed powder through channels between particles, the surfactant molecules may adsorb on Teflon particles as shown in Scheme 1. The hydrophobic chains of the surfactant molecules interact with the hydrophobic Teflon surface, thereby promoting wetting of particles and flow of water further. This phenomenon can be understood in detail by considering the rise of liquid in a capillary tube. As already known, the forces between the water and glass capillary enabled water to move upward, while cohesive forces among water molecules try to minimize the distance between them by pulling the bottom of the meniscus up against the force of gravity. The rise of water in the glass capillary is directly proportional to its surface tension.26−28 A powder filled metal cylinder consists of many such capillaries, where surfactant solution gradually rises due to deposition of surfactant molecules on the Teflon surface. Dynamic Surface Tension of Aqueous Solutions of Surfactants. The dynamic surface tension of the aqueous surfactant solutions is measured before and after addition of Teflon powder. Figure 2 shows the change in the dynamic surface tension of the surfactant solution as a function of bubble time. The results could be interpreted in terms of surfactant molecules to adsorb at the Teflon−water interface. The D
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Figure 3. Sedimentation rate of AOT wetted Teflon particles in the presence of polymers at 25 °C. Figure 5. Decrease in sedimentation of Brij-O10 wetted Teflon particles in the presence of different concentrations (wt %) of HEC.
Teflon powder in water and stirred for 2 h. The rate of sedimentation for surfactant wetted Teflon particles before and after the treatment of different polymers was measured. The sedimentation behavior for Teflon dispersions in the presence of different polymers, namely, poly(vinyl alcohol) (PVA), polyvinylpyrrolidone (PVP), hydroxy ethyl cellulose (HEC), and hydroxypropyl methyl cellulose (HPMC) were measured at 25 °C as shown in Figure 4. Among the investigated polymers, HEC (a general viscosity modifier) decreased the sedimentation to a considerable level. The effect of concentration of added HEC on Teflon particles wetted by Brij-O10 is shown in Figure 5. The
sedimentation of surfactant wetted Teflon particles is significantly decreased by increasing concentration of HEC. The time required for complete phase separation of the HEC added final dispersion containing Brij-O10 wetted Teflon particles was about 5 times more than its HEC-free solution. An increase in surfactant−polymer interactions provides better dispersion stability and decreases the rate of sedimentation. To understand these effects, two different mechanisms are proposed. (i) As per our first assumption, added polymer may replace some surfactant molecules from the surface of the
Figure 4. Sedimentation of Teflon particles in 1 wt % surfactant solution mixed with 1 wt % polymer at 25 °C. PVA (○), PVP (□), HEC (●), and HPMC (■). E
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Figure 6. (A) Change in viscosity of different surfactant + polymer aqueous mixtures as a function of applied shear rate at 25 °C. HEC (■), Brij-O10 + PVA (★), Brij-O10 + PVP (◆), TX-100 + HEC (□), Brij-O10 + HEC (○), AOT + HEC (Δ), and Brij-98 + HEC (▼). (B) Viscosity for Teflon dispersions containing surfactant + polymer mixtures measured with applied shear rate of 50 rpm at 25 °C.
Teflon powder due to competitive adsorption. The resultant adsorption provides exceptional stability to the dispersion due to flocculation of polar groups outside the hydrophobic region. The hydrogen bonding of −OH groups and subsequently formed loops and tentacles allows the Teflon particles to disperse and provides stability to the resultant formulation. (ii) To describe it further, a second assumption is made based on the possibilities involving formation of H-bonding between the polar region of the surfactant and the polymer chains. The polymer molecule may coil around the wetted Teflon particle and provide more folds, loops, and tentacles at the interface, which should provide increased steric hindrance and thus better dispersion stability. The hydrogen bonding between the ethereal oxygen in the polymer with polar groups of a nonionic surfactant is a governing factor for introducing an increase in the overall hydrodynamic volume of the particles. Viscosity Measurements. The viscosity measurements on aqueous surfactant + polymer mixtures were carried out at 25 °C to investigate the possible formation of surfactant−polymer complexes leading to a mechanism for significant improvement on the Teflon stability in aqueous media. As shown in Figure 6A, the viscosity for aqueous solutions of Brij-O10 mixed with PVA/PVP is around 20 cP, while the measured viscosity for aqueous mixture comprising Brij-O10 and HEC is >500 cP. These results are in agreement with sedimentation data showing less sedimentation tendency for HEC added dispersion compared to PVA/PVP added cases. It must be noted that viscosity for pure HEC is around 170 cP, which is a clear indication that the solution viscosity increases as a consequence of polymer−surfactant complexation. Under application of applied shear, an almost linear decrease in the viscosity is observed. For such non-Newtonian mixtures, this shear thinning behavior is attributed to breaking of hydrogen bonds due to the effect of applied shear forces.29 As a consequence of a high number of H-bond junctions formed by surfactant−polymer complexes, the system exhibits even higher viscosity. It is understood that these interactions enable Teflon particles to uniformly disperse in final dispersions. The measured viscosity values for Teflon dispersions containing different surfactant coated particles mixed with polymer at applied shear rate of 50 rpm are shown in Figure 6B. These
values are marginally higher compared to those of Teflon-free surfactant−polymer aqueous mixtures. The extent of primary adsorption by surfactant at Teflon surface and complexation arising of surfactant−polymer interactions at the interface have their typical contribution to the overall stability. The probable origin of surfactant−polymer complexation might be the bridging of two or more polymer units by means of H-bonding between water molecules and hydrophilic junctions belonging to two distinct units. The coated hydrophobic surfactant tail on the Teflon surface is expected to reinforce this bridging mechanism. The restriction imposed by the confinement of the tail at the surface of Teflon forces the hydrophilic groups in the polymer to be closer to each other and to assume a more extended conformation by interacting with water-loving surfactant chains. Such highly hydrated bridged units provide more loops and tentacles, and hence, in such conditions, a better exposure of the hydrophilic groups to the external water is favored. For a single cellulose unit, the numbers of H-bonding sites are more compared to those of PVA and PVP, which allows stronger interactions with polar groups of adsorbed Brij-O10 molecules. The same can be understood by the slope of the viscosity curve for HEC added systems showing a significant decline compared to mixtures containing PVA/PVP. Hence, the breaking of hydrogen bonds and subsequent decrease in the viscosity are strongly evident. For surfactants containing TX-100 and Brij-98, the viscosity values were in a similar range, though the highest viscosity was observed for the TX-100 + HEC mixture. In the case of TX100, Brij-O10, and Brij-98, the hydrogen bond formation between oxygen in ethoxylated chains and polar −OH groups of HEC is the dominating phenomenon. Poly(vinyl alcohol) can interact with ethereal oxygen in Brij-O10 via H-bond formation. The carbonyl group in PVP can interact with −CH2 unit in Brij-O10, but these interactions are limited due to less numbers of H-bonding sites. For anionic AOT, favorable interactions with HEC are expected due to polar headgroups interacting with −OH groups in the polymer. It is understood that, due to these interactions, the Teflon particles are uniformly distributed in the final dispersion. Surfactant wetted particles in the absence of a polymer were showing faster sedimentation in about 8−9 min, while their dispersions containing Brij-O10, Brij-98, and TX-100 and AOT mixed with F
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Figure 7. (A) Hydrodynamic radius of Teflon particles in the presence of Brij-O10 and Brij-O10 + HEC mixture at 25 °C. (B) Hydrodynamic radius of Teflon particles in the presence of Brij-O10 and Brij-O10 + HEC mixture in different weight ratios at 25 °C.
HEC were found to be stable for 58, 53, 49, and 51 min, respectively. Earlier, the sedimentation profile for these dispersions showed almost a plateau after few minutes, indicating no further accumulation of particles in the sedimentation pan. This trend is attributed to highly stable particles in the nano regime (submicrometer) which destabilized much slowly and is not clearly quantified due to a very low proportion in added powder containing a large size range of 100−3000 nm. At the noted destabilization point, the medium becomes optically transparent and the whole solid mass settles to the bottom. Thus, it can be concluded that particles are now stabilized almost 5−6 times compared to only surfactant wetted conditions. It would be interesting for a future study to investigate the effect of higher concentration of these added surfactants/polymers and of controlling their viscosity and dispersion stability for particles in narrow size range. Hydrodynamic Volume of Teflon Particle in Final Formulation. The hydrodynamic values of Teflon particles in the presence of added Brij-O10 and HEC + Brij-O10 mixture is shown in Figure 7A. Due to higher the viscosity of dispersions, DLS measurements were carried out in 10 times diluted concentration of all the components. The DLS measurements were carried out to measure the hydrodynamic volume of the Teflon particles in the presence of 0.1 wt % surfactant Brij-O10 and added polymer HEC. The distribution peak around 10 nm is evident for micelles of BrijO10. The appearance of a distribution peak at around ∼135 nm must be due to surfactant wetted Teflon particles. With the addition of HEC, the intensity of this peak increases and the peak is shifted to about 157 nm. As the measurements were done after a waiting period of 3 min, the larger size particles (above 1 μm) are expected to settle down and hence were not recorded during the experiments. The intensity of the original peak observed for Brij-O10 aggregates decreases with addition of HEC due to the formation of mixed surfactants + polymer aggregates. If we consider average particles as spherical in geometry, this observed increase in size for the final formulation adds to about ∼55% volume to the initial hydrodynamic volume of the particles. This is in clear
agreement to our assumption that surfactant−polymer interactions with Teflon particles significantly increase the hydrodynamic volume and provide greater stability. The hydrodynamic size of Teflon particles in different weight proportions of HEC is shown in Figure 7B. With gradual addition of polymer, the peak intensity corresponding to the hydrodynamic size of the pure Brij-O10 solution decreases and another peak around 100 nm appears, which is an indication of dispersed Teflon particles. In the presence of equal weight fraction of the added Brij-O10 and HEC, the peak corresponding to Brij-O10 aggregates shows a gradual decrease in intensity and broadening. The broad peak is mainly due to excess aggregates formed by surfactant−polymer complexation. Overall, the addition of HEC provides stability to the Brij-O10 wetted Teflon particles and is able to disperse them, which otherwise were showing faster sedimentation. These results support our primary assumption that the polymer loops around the surfactant wetted Teflon particles and increases the hydrodynamic volume of Teflon + adsorbed film, which ultimately stabilizes Teflon dispersions. The schematic diagram shown in Scheme 2 describes possible interactions between Scheme 2. Plausible Surfactant−Polymer Interactions Responsible for Stabilization of Teflon Powder by Increasing Hydrodynamic Volume of Teflon Particles
G
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ionic surfactant and polymer molecules at the Teflon surface. For ionic surfactants, polar headgroup interacts strongly with −OH groups in HEC, while for nonionic surfactants polar ethoxylated chains provide the H-bonding site.
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CONCLUSION A series of surfactants was employed to investigate their wetting performance on Teflon powder (particle size range of 100− 3000 nm). The wetting performance and dynamic properties of the surfactants were found to be inter-related, and the surfactants from Brij series showed best wetting performance among others, including Triton X-100, Tween, and Pluronics. The wetted Teflon particles were further investigated in the presence of different polymers with an idea to delineate the effect of surfactant−polymer interactions at the Teflon surface and the resultant effect on the dispersion stability. The replacement of few surfactant molecules with high molecular weight polymers could be a possible reason for the increase in the overall size and subsequent stability. It is also assumed that polymers increased the hydrodynamic volume of the wetted Teflon particles by interacting with the surfactant chains at the particle surface. The proposed H-bonding formation for the ethereal oxygen in the surfactant chain and the oxygen of polar group in polymer may drive the formation of H-bonding and hence more loops and tentacles. The hydrodynamic size of the Teflon particles in the final formulation increases up to ∼55% with addition of polymer and subsequent formation of surfactant−polymer complexes. The observed dispersions were highly stable compared to the surfactant only wetted Teflon dispersion. We report on a novel route for tuning the Teflon dispersion stability with an introduction of surfactant− polymer interactions, which can be useful for applications involving paints, inks, automobiles, and many others.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +918141717887. Fax: +91-0268-2520501. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Universal Medicap, Baroda for providing Teflon powder as a gift sample.
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REFERENCES
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dx.doi.org/10.1021/la5012605 | Langmuir XXXX, XXX, XXX−XXX
