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Correlation of dynamic surface tension with sedimentation of PTFE particles and water penetration in powders Vidhi Shah, Bhavesh Bharatiya, Dinesh O. Shah, and Tulsi Mukherjee Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03725 • Publication Date (Web): 01 Dec 2015 Downloaded from http://pubs.acs.org on December 8, 2015

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Correlation of dynamic surface tension with sedimentation of PTFE particles and water penetration in powders Vidhi Shah1, Bhavesh Bharatiya1,#, Dinesh O. Shah1,2,3, Tulsi Mukherjee1

1

Shah-Schulman Center for Surface Science and Nanotechnology, Dharmsinh Desai University, Nadiad-387001, Gujarat, India 2

3

Center for Surface Science and Engineering, Chemical Engineering Department, University of Florida, Gainesville, FL 32611 USA

College of Earth and Environmental Sciences, Columbia University, New York, NY 10027, USA

Email Addresses: V. Shah ([email protected]) B. Bharatiya ([email protected]) D. O. Shah ([email protected]) T. Mukherjee ([email protected]) #

Corresponding author: Dr. Bhavesh Bharatiya, Assistant Professor, Shah Schulman Center for Surface Science and Nanotechnology, Dharmsinh Desai University, Nadiad-387001, Gujarat, India. Tel.: +91-268-2520504 Fax: +91268-2520501

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Abstract: The dynamic surface tension of aqueous poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) [(PEO-PPO-PEO)] type polymeric surfactant (P103, P105, F108, P123 and F127) solutions were correlated with water penetration in packed Teflon powders, sedimentation of Teflon suspensions in these solutions, foamability and contact angle measurements on Teflon surface. The DST trend with bubble life time indicated that overall slowdown in diffusion process in aqueous solutions is a function of higher polyethylene oxide (PEO) molecular weight for a given series of block copolymers containing equal PPO molecular weight, which favours slower diffusion kinetics to air-water interface caused by preferential partitioning in bulk water. The wettability of polytetrafluoroethylene (PTFE) powder illustrates better water penetration for polymers with low molecular weight and lower HLB values. The wettability for F127 solutions decrease with corresponding increase in concentration due to higher viscosity, which restrains the diffusion kinetics at the PTFEwater interface. The foamability decreases drastically with higher PEO molecular weight as attributed by slower diffusion kinetics leading to decrease in effective concentration of molecules at the foam interface. The contact angle on glass and PTFE surface are in good agreement with assumptions made by other analytical techniques showing lower value of contact angle with lower HLB of the Pluronic®, which relates to the higher adsorption of molecules at the interface. It is concluded that adsorption of molecules at PTFE-water interface decreases in aqueous Pluronic® solutions with corresponding increase of hydrophilic lipophilic balance (HLB), which is consistent with foaming, water penetration in packed powder of PTFE, rate of sedimentation and DST data. PTFE dispersion containing P123 showed maximum wettability and lowest sedimentation among series of block copolymers introduced, which is attributed to faster diffusion kinetics and higher PPO contribution fostering a faster adsorption at the PTFE surface. Dynamic surface tension of aqueous


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Pluronic® solutions seems to correlate well with the adsorption characteristics at air-water and PTFE-water interface. Key words: block copolymers, bubble lifetime, dynamic surface tension, foamability, sedimentation, water penetration in packed Teflon powder, contact angle, foam stability


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1. Introduction: The dynamics of surfactants at various interfaces remains a crucial parameter in many industrial applications involving inks, textile, paper, pharmaceuticals, cosmetics, perfumery and many others.1-7 Several authors have summarized the technological applications of surfactants in relation to dynamic surface properties e.g. wetting, emulsification, foaming, dispersion, among others.8-10 It is generally accepted that the surfactant molecules diffuse towards the air-water interface and reduce the surface tension due to adsorption process. This time dependent phenomenon is expressed as dynamic surface tension and its approach to equilibrium condition is a function of diffusivity, bulk concentration, interfacial distortion and charge distribution of the surface active species.11 This diffusion process continues for the fraction of time-scale (about 10ms-1000ms) depending on the type of system and solution conditions. The dynamic surface tension can be measured by various methods e.g. maximum bubble pressure, Langmuir trough and oscillating jet techniques.12 Different methods for surface tension measurements are discussed in detail based on the temperature regime, characteristic time and the application.13 Various reviews and reports discuss about surface activity of varieties of surfactants and their ability to alter the surface tension for different liquid media. 14-18 Unlike conventional low molecular weight non-ionic surfactants, Pluronics® are polymeric surface active structures, which also have solubility in water and surface activity due to EO and PO groups. Among different classes of the surfactants, the non-ionics demonstrate faster relaxation tendencies to the air-water interface.19 The relaxation time taken to achieve equilibrium surface tension (EST) is longer for high molecular weight surfactants, presumably due to slower diffusion kinetics.20 The temperature dependent kinetics of surfactants is also reported by Dalton and Eastoe

11

with a conclusion that the solubility of

non-ionic surfactants decreases at higher temperature resulting in faster relaxation kinetics 4 ACS Paragon Plus Environment

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and also accompanied by rapid reduction in dynamic surface tension due to presumably faster diffusion of surfactant molecules. Dynamic surface tension directly reflects the surfactant concentration at the interface at that stage, and accordingly the availability of free monomers and submicellar fragments to diffuse to and stabilize the freshly created interfaces of bubbles similar to those in the generation of foams and emulsions. Shah et al.21 found that the trends in dynamic surface tension correlate well to the foamability behavior for aqueous dodecyl sulfate solutions in presence of different counter ions. Similarly, the large surface active polymeric molecules can adsorb at the air-water interface and reduce the surface tension of water. The polymeric surfactants containing hydrophilic and lipophilic compartments show micellization in aqueous solutions.1-3, 22 Interestingly, not many water soluble polymers are known to affect the surface tension of water, except few polymeric surfactants, namely polyethylene oxide (PEO) and hydroxyethyl cellulose (HEC)23a and polyvinyl alcohol23b. The surface activity of these polymeric surfactants decreases at elevated temperatures due to higher viscosity and at higher bubble rate. The change in the dynamic surface tension for water-glycerine mixtures is investigated to understand the role of high viscosity in controlling the diffusion and related control of surface properties.24 Rebenfeld et al.25 investigated dynamic surface tension of aqueous solutions of cellulose based polymeric surfactant Methocel A15-LV and noticed significant increase in DST with higher bubble rate or increased viscosity of solutions.

Among the block copolymers possessing surface activity and aggregation in bulk, the aqueous solution behaviour of PEO-PPO-PEO type triblock copolymers is widely investigated in detail by several research groups with a special focus on micellar behaviour and possible structural transitions in presence of various stimuli, e.g. inorganic salts26, hydrotropes27, ionic surfactants28, alcohols29, hydrocarbons30, ionic liquids31 and many others. The aqueous spherical micelles of these commercially known Pluronic® block copolymers reveal temperature dependent structural transitions due to progressive dehydration of 5 ACS Paragon Plus Environment

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hydrophilic chains and accordingly enhanced hydrophobic forces.29 Appearance of sol-gel transitions in Pluronic® F127 aqueous solutions as a function of concentrations and temperatures is reported.32 Due to considerably high surface activity and commercial availability in large varieties of molecular weights, these PEO-PPO based block copolymers are suitable for applications involving drug delivery33, preparation of mesoporous silica34, microemulsions35, particle dispersion36, foaming37 and wetting38, among many others. We believe that the surface activity, dynamic surface tension, diffusivity and wettability of surfactants are closely interrelated. The ability of surfactants to adsorb at the interface primarily controls the subsequent interactions leading to particle stabilization in bulk dispersion. In our previous study39, we discussed the role of surfactant-polymer interactions to stabilize the Teflon® particles in aqueous dispersions in presence of conventionally used Brij, Tween, anionic surfactants and HEC type biopolymer. The role of surfactant dynamics and subsequent changes in the concentration of surface active molecules at the interface was discussed marginally. In current study, we report diffusion kinetics, wettability, particle sedimentation and foamability using series of industrially important Pluronic® block copolymeric surfactants. It is noticed that the applications involving these high molecular weight surface active molecules critically depends on their diffusion kinetics, which indeed varies with the structure, molecular weight and EO/PO ratio of Pluronic® block copolymers. Detailed investigations are attempted with an objective to establish understanding on diffusion kinetics of selected Pluronic® block copolymers (P103, P105, F108, P123, and F127), which clearly reveal the role of EO/PO molecular weight on dynamics of diffusion at the PTFE-water interface and bulk surfactant solution/air interface. A molecular mechanism is proposed on the basis of diffusion rates and affinity of Pluronic® molecules to adsorb at the PTFE surface. The results show a correlation between diffusion of Pluronic® molecules from bulk to interface and various technological applications, e.g. foaming, contact angle, sedimentation of particles and water penetration in porous media. 6 ACS Paragon Plus Environment

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2. Materials and methods: All the Pluronic® surfactants are obtained from BASF, Parsippany, NJ, USA as gift samples and were used without any further purification. Milli-Q water obtained from a Millipore® filter purifier was used for preparation of all aqueous solutions. PTFE powder was obtained as gift from Universal, Medicap Ltd., Vadodara, India. The PTFE and Glass surfaces were cleaned with Acetone (Analytical Grade, Sigma-Aldrich) and dried before measurements. The molecular characteristics of different Pluronic® surfactants used are listed in Table-1. 2.1 Surface Tensiometry: The surface tension for 1 wt % aqueous surfactant solution was measured using 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 and the standard deviation was ± 0.5 mN/m. Dynamic surface tension measurements were carried out using a bubble tensionmeter (Biolin, model # BPA-800P) at 25 °C using maximum bubble pressure method and as a function of bubble lifetime. 2.2 Water penetration/wettability of packed powder: The wetting of PTFE powder using surfactant solutions was achieved by a water penetration method using an Attension surface tensiometer (Biolin, model #Sigma 700). The PTFE powder was filled in metal cylinder hanged to a sensor that measures the change in weight with time. The end of the cylinder was allowed to dip in surfactant solution for 5 min. at a constant depth of 3 mm, and the resultant weight gain due to adsorption of water was noted. The measurements were repeated three times in order to ensure reproducibility.

2.3 Sedimentation measurement: The sedimentation for aqueous PTFE 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 7 ACS Paragon Plus Environment

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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. The standard deviation was ± 2 µg. 2.4 Contact Angle Measurement: The contact angle for 1 wt. % aqueous surfactant solutions were measured on PTFE (Teflon) and Glass surfaces 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 minute. 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. For dynamic contact angle measurements, the CA images were recorded at different tilt angles. The average standard deviation was ± 2o for each of these measurements. 2.5 Foamability and foam stability: Foamability and Foam-stability measurements were performed in the graduated glass cylinder attached to an aspirator. Air bubbles can be passed at a constant rate through a perforated disk attached at the bottom of the aspirator system for 30 seconds. The foamability was measured by calculating the height of the foam generated for different surfactant solutions. The foam stability in terms of the half-life of the foam was calculated by measuring the time required for the foam to collapse to half height from the starting point. The measurement was repeated at least five times for each surfactant solutions and average values are considered.


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Table-1: Molecular characteristics of Pluronic® block copolymers

#

*

Series-1

Series-2

Pluronic®

M.F.

P103 P105 F108 P123 F127

EO17PO60EO17 EO37PO56EO37 EO133PO50EO133 EO20PO69EO20 EO100PO65EO100

A.M.W. % HLB *CMC EST N (mN/m) (g.mol PEO (g.dm 1 3 ) ) 4950 30 9 0.740 14.3 32.4±0.12 6500 50 15 2.340 4.3 34.5±0.03 40 14600 80 27 45 0.2 37.5±0.23 5750 30 8 0.340 33.3 33.2±0.33 12600 70 22 740 1.5 36.8±0.29

#PPO ~3000 g/mol, *PPO ~3600 g/mol, M.F.=molecular formula, A.M.W.=average molecular weight, HLB=hydrophilic lipophilic balance, CMC=critical micelle concentration at 25oC, *= CMC reported literature, EST=Measured equilibrium surface tension for 1 wt.% aqueous solutions at 25oC. N= concentration multiple with respect to critical micelle concentration

3. Results and discussion: 3.1 DST of aqueous Pluronic® solutions by maximum bubble pressure technique:

Figure 1(a): Dynamic surface tension as a function of bubble lifetime for 1 wt.% aqueous Pluronic® solutions at 25oC. (■) P103, (○) P105, (●) F108, (□) P123, (▲) F127 The investigations on dynamic surface properties are important for various technological processes where fresh interface is being formed, which include foaming or film formation, the diffusion of surfactants at liquid-liquid interface, such as formation of microemulsions, or to a solid-liquid interface, e.g. wetting of fabric materials in textile applications.41 In DST method, the fresh interface is created with the formation of air bubble, where the surfactant monomers are expected to adsorb and subsequently reduce the interfacial tension. The existing micelles should then provide monomers through disintegration for the further adsorption. If the micelles are highly stable, the supply of monomers to the interface is not 9 ACS Paragon Plus Environment

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enough and is reflected in higher dynamic surface tension value. If micelles disintegrate faster (less stable), the rapid supply of monomers at the interface ensures lower values of dynamic surface tension. We introduce similar approach for the wetting of PTFE particles in water using polymeric surfactants. The dynamic surface tension for 1 wt.% aqueous Pluronic® solutions is measured by maximum bubble pressure method at 25 oC (Figure-1(a)). The changes in DST are plotted against the bubble lifetime. For series-1 of Pluronic® block copolymers under analysis (equal molecular weight PPO ~3000 g.mol-1), aqueous solution of P103 depicts maximum reduction in surface tension and reaches equilibrium much faster compared to P105 and F108. This suggests that the rate of adsorption must be fastest for P103. For block copolymer series-2 with same molecular weight of PPO ~3600 g.mol-1, P123 reduces the surface tension faster than F127. The faster reduction in DST for P123 and P103 is attributed to higher surface activity and diffusivity to the air-water interface presumably by fast disintegration of micelles, lower cmc and lower molecular weight (see table-1). The surface tension decreases with time due to diffusion of molecules at the interface and subsequent adsorption. The tendency of reduction in surface tension follows the order as: P103>P105>P123>F127>F108. The solubility and entanglement of molecules increases in bulk solution with higher PEO molecular weight, which is accordingly expected to reduce the preferential adsorption affinities at the interface. It must be noted that the overall molecular weight and hence the mass of molecule increases with PEO addition. The bulkiness of molecules also facilitate in slower diffusion kinetics of F108 as compared to P103 and P105. For P103 and P123 with same amount of hydrophilic group contribution (~30 % PEO), higher DST is measured for P123 (40.5 mN/m), which could be attributed to higher molecular weight. Though, it is clear from the plots that equilibrium is achieved much faster for P123 compared to all other copolymers suggesting fastest adsorption kinetics, formation of less stable micelles and supply of the monomers to the interface from bulk at much faster rate attributed by higher PPO molecular weight. Earlier, Buckton and Machiste42 investigated 10 ACS Paragon Plus Environment

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on series of hydrophilic Pluronic® block copolymers (F127, F87 and F108) and concluded that DST of these polymeric solutions was related to the PEO content and/or total molecular weight of the polymer.

Figure 1(b): Dynamic surface tension for aqueous Pluronic® P123 and F127 solutions at different concentrations and as a function of bubble lifetime (s) at 25oC. (■) 0.05s (●) 0.1s, (▲) 0.5s, (∆) 1.0s As dynamic surface tension is manifested by the diffusivity of the molecules towards the interface, the same is a function of decrease in bubble lifetime and accordingly created fresh interfacial area for the molecules to adsorb. Figure-1(b) shows the dynamic surface tension of aqueous P123 and F127 solutions as a function of different bubble lifetime (s) and concentrations at 25oC. An increase in the DST is observed with a corresponding increase in the bubble rate (lower bubble lifetime). It is attributed to less time per bubble for surfactant molecules to diffuse to the surface. As the lifetime of bubble increases, the diffusion process from bulk to interface for these polymers continues for longer time and hence it increases the surface concentration at the bubble surface causing a decrease in surface tension. In each case, as the lifetime of bubble increases, the dynamic surface tension decreases. At lower bubble lifetime, DST is higher due to less concentration of molecules involved at these interfacial boundaries. For 2 wt.% P123 aqueous solution, the DST is decreased to 34.1 mN/m at bubble lifetime of 1s from initial value of 38.9 mN/m at 0.05s bubble lifetime. 5% F127 aqueous solution showed 50.4 mN/m DST at bubble lifetime of 0.05s, which decreases 11 ACS Paragon Plus Environment

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to 44.4 mN/m at bubble lifetime of 1s. The higher surface activity of P123 against F127 is again confirmed by lower DST values at each bubble lifetime data. With increase in bulk concentration the surface can be saturated much quickly. For higher polymer concentrations, the difference in DST at different bubble lifetime gradually becomes less. Considerably more difference in DST for F127 compared to P123 can be explained in terms of higher bulk partitioning and low surface activity. The PEO induced retardation in surface activity (in other words reduction in preferential partitioning to interface) and accordingly favoured slower diffusion kinetics is the probable reason for higher surface tension of F127 molecules under experimental conditions. As clearly observed from these results, the DST is independent of the concentrations at higher values. The same can be accepted as the critical aggregation concentration for these block copolymers under dynamic conditions, which increases with corresponding decrease in bubble lifetime (data not shown). This peculiar solution behaviour is expected to control the adsorption characteristics at various interfaces including PTFE-water type, which is a driving force for the dispersion of powders in various aqueous media.

3.2 Water penetration through packed PTFE powder in presence of aqueous Pluronic® solutions: The low surface energy of PTFE favours higher contact angle of water droplet, which accordingly defines difficulties to disperse PTFE powders in aqueous dispersions.39 Figure-2 shows the aqueous phase penetration wettability data for PTFE powders (size range: ~ 0.10.3µ) in presence of 1 wt.% aqueous solutions of different Pluronic® block copolymers.


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Figure 2: Water penetration mass2 (g2) vs time profile for packed PTFE powder in presence of 1 wt.% aqueous Pluronic® solutions at 25oC. (∆) pure water, (■) P103, (○) P105, (●) F108, (□) P123, (▲) F127 The least wetting observed in presence of pure water is obvious due to higher contact angle and accordingly hindered interactions with hydrophobic PTFE surface. In presence of aqueous Pluronic® solutions, a significant water penetration is noticed. An initial increase in the penetration slows down gradually with time and almost reaches plateau region. The following mechanism is proposed based on the penetration of water through porous micro channels of PTFE powder filled inside the metal cylinder. These aqueous Pluronic® solutions are all above CMC (critical micelle concentration) except F108 (see table-1), which defines an important role of free and aggregated polymeric molecules as controlled by the micellar kinetics. Carter et al.43 investigated on the residual moisture content in the fabric by comparing the dynamic surface tension with adsorption of surfactant monomers at the fabric surface, and concluded that decrease in free monomer concentration increases the DST and hence the amount of water penetration in fabric capillaries. As the aqueous solutions interact with PTFE powder, the free molecules adsorb at the interface of porous channels initiating formation of hydrophilic pathway for water penetration. It is presumed that hydrophobic PPO units of Pluronics® adsorb at PTFE surface allowing formation of polar channels with hydrophilic PEO units extending into water, and further allow water penetration through a mechanism similar to capillary-rise effect. A plausible graphical representation describing 13 ACS Paragon Plus Environment

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this wettability pattern is shown in Scheme-1. The amount of water penetrated in porous PTFE channels decreases with increase in the PEO molecular weight of copolymer for a given series of Pluronic® solutions. In presence of P123 and P103, higher wetting is evident, highlighting the importance of faster molecular diffusion (see Figure-1a) at the PTFE surface for the formation of porous hydrophilic channels. The trend in wettability follows the order as: P123>P103>P105>F108>F127. Among the polymeric surfactants under observation, P123 favours faster diffusion and adsorption affinity at the hydrophobic PTFE surface as influenced by highest PPO block contribution and lowest HLB. It is concluded that the wettability follows a pattern representing HLB dependency for given series of Pluronic® (constant PPO molecular weight), but the same also changes for different PPO contribution and same % PEO contribution (e.g. P123 and P103). F127 shows least wetting presumably due to higher PEO contribution and subsequent higher partitioning in aqueous solutions. As shown in Figure-2, for the experimental period of 300s, the amount of penetrated water almost reaches a constant plateau region, which correlated with total wetting of the PTFE powder. Figure-3 shows the change in wettability as a function of added F127 concentrations.

Figure-3: Water penetration mass2 (g2) vs time profile for packed PTFE powder in presence of different wt.% aqueous F127 solutions at 25oC. (■) 0.1, (●) 0.5, (▲) 2.0, (∆) 5.0 With increase in the concentration of F127 in aqueous solution, a corresponding decrease in the water penetration is evident. F127 is hydrophilic copolymer with higher PEO molecular weight and preferential partitioning in aqueous bulk phase. As proposed, the adsorption of 14 ACS Paragon Plus Environment

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free polymer molecules drives the formation of porous channels with hydrophilic sites. At lower concentrations around CMC of F127 ~0.1wt.%

9, 10, 40

, a significantly higher water

adsorption is noted due to lower viscosity under the same driving force of capillary phenomenon. A gradual increase in the bulk viscosity for F127 with increase in concentration is attributed to entanglement of PEO chains and accordingly increased internal friction. With subsequent concentrations of F127, a gradual decrease in the wetting profile is observed. If the assumption of hydrophilic channel formation is considered, the reason behind these observations can be understood. According to the hypothesis describing the formation of hydrophilic channels and resultant water penetration, the adsorption is a direct function of available free polymer molecules reaching to the PTFE surface and adsorbing in a sequential pattern. It is well documented that the formation of micelles also increases with an increase in the concentration of F127, which is also accompanied by increase in the bulk viscosity.9, 44 Accordingly formed micelles are presumably more stable as the concentration of F127 is gradually increased. These two factors may slow down the diffusion kinetics and may result in less concentration of available free polymer molecules for the adsorption at the PTFE surface. The multiple layer adsorptions involving PEO chain interactions can not be ruled out. The primary adsorption layer formed on the PTFE surface at lower concentrations may be followed by another layer of polymers interacting in reverse pattern through entanglement of hydrophilic PEO chains. Though, the adsorption at higher concentration seems to be mainly governed by the bulk viscosity. The increase in viscosity favours slow diffusion kinetics at the interface as reported earlier24, 25, which clearly defines the less concentration of polymers at the PTFE-water interface when bulk viscosity is higher. Waghmare et al.45 reported on under-water super-hydrophobic glass surface by change in the surfactant concentration in solutions instead of tailoring the entire surface. In this approach, formation of ‘pillar’ type morphology for Tween-60 molecules was noticed. An extremely hydrophobic glass surface is obtained at critically higher surfactant concentrations with 180o contact angle 15 ACS Paragon Plus Environment

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for an oil drop. This change in wettability and super-hydrophobicity to the glass substrate was attributed to the entropic steric stabilisation and van der Walls forces at higher surfactant concentrations. The oil droplet inside the surfactant solution experiences a stable CassieBaxter (CB) state without transforming to Wenzel (Wn) state, which was due to the influence of steric interactions that outweigh the fluidic drive due to Laplace pressure. For the steric stabilization, the conformation of adsorbed Pluronic® type macromolecules at the PTFEwater interface must be critical as it will control the distance dependence of the steric repulsion for a stable system.46 Various experimental investigations for sterically stabilized surfaces confirm that the block copolymeric macromolecules are in extended conformation normal to the interface.47,48 Accordingly, the PPO molecules interacting with the PTFE surface and extension of hydrated PEO chains normal to the PTFE surface can be accepted. The pictorial representation of proposed hypothesis is shown in scheme-1.

Scheme-1: Plausible molecular mechanism showing the water penetration though hydrophilic micro channels at lower viscosity and retardation in water penetration at higher concentration due to higher viscosity


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Figure 4: Weight of sediments (g) vs time profile for PTFE dispersion in presence of 1 wt.% aqueous Pluronic® solutions at 25oC. (■) P103, (○) P105, (●) F108, (□) P123, (▲) F127 The stability of aqueous PTFE dispersions wetted by polymeric surfactants from series-1 and series-2 is investigated using sedimentation measurements as shown in Figure-4. The observed trend is in good agreement to the wettability results illustrating significance of higher surface activity. P123 shows lowest sedimentation rate and hence highest dispersion stability. The higher value of sediments obtained for hydrophilic polymers (F127 and F108) defines less tendencies of adsorption at the PTFE surface, which results in faster aggregation of PTFE particles due to hydrophobic interactions. In addition, the effect of gravity and viscosity of continuous medium controls the sedimentation rate of particles. If long PEO chains of hydrophilic copolymers interact with bulk water, the hydrophobic interactions between PTFE particles must be reduced significantly and hence provide better stability compared to low HLB Pluronic® block copolymers. However, it is now clear that the tendency of adsorption for low molecular weight Pluronic® block copolymers is a governing factor for the adsorption at the PTFE surface, which also control the effective concentration of molecules at the interface. The tendency of surfactants in reducing the surface tension in static and dynamic conditions match well with their adsorption tendencies at the PTFE surface, which presumably also decides their effective concentrations at the hydrophobic exterior and hence the amount of absorbed water. We conclude that wettability and 17 ACS Paragon Plus Environment

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sedimentation profile follow the HLB order for a respective series, but faster diffusion kinetics and adsorption is preferred for a given Pluronic® with higher PPO contribution and same % PEO in the molecular structure. To investigate further on these observations, detailed experiments on contact angle and foamability were attempted, which is expected to give information about the diffusion characteristics of the polymers at the PTFE-water and airwater interface, respectively.

3.3 Contact angle for aqueous Pluronic® solutions on PTFE and glass :

Figure-5A: Contact angle (θ) on PTFE and glass surface for 1 wt.% aqueous Pluronic® solutions at 25oC. Figure-5B: (left) equilibrium contact angle images for 1 wt.% Pluronic® solutions on PTFE at 25oC. (right) dynamic contact angle images for 1 wt.% aqueous P105 solution showing change in advancing (θA) and receding (θR) contact angle for different tilt angles on PTFE at 25oC. The interfacial tension at the PTFE-water interface should be reduced if regular surfactant adsorption continues, which is also expected to alter the contact angle of water on the hydrophobic PTFE surface. Figure-5A shows change in the contact angle (θ) for 1 wt.% aqueous Pluronic® solutions on flat glass and PTFE surface. On PTFE surface, the contact angle (CA) for pure water is ~110o, which is reduced significantly in presence of aqueous Pluronic® solutions. CA is reduced to 55o in presence of P103 and P105, while F108 reduces the contact angle up to 76o. Higher CA for F108 is a manifestation of higher DST and 18 ACS Paragon Plus Environment

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subsequently less favoured diffusion kinetics at the PTFE-water interface or less adsorption on PTFE-water interface. Almost identical tendency for reduction in contact angle were noticed for P123 and F127, where CA is reduced to ~40o. The higher CA reduction in presence of polymers from series-2 can be explained in terms of higher preferential adsorption due to strong interactions with low energy PTFE surface. Figure-5B shows the contact angle images in equilibrium and dynamic conditions. A significant decrease in the CA value in presence of different block copolymeric solutions is clearly visible for equilibrium CA images. For 1 wt.% aqueous P105 solutions, the advancing and receding contact angle reveal a gradual increase in the advancing contact angle values with corresponding increase in the tilt angle. The dynamic contact angle data for other Pluronic block copolymers showed identical wettability behavior and are shown in electronic material section. The aqueous Pluronic® solutions from series-1 decrease the CA for hydrophilic glass surface (CA~42o) to almost identical values around ~10o. However, the observed reduction in CA up to 22o in case of P123 and F127 reveal that the PPO molecular weight for respective Pluronic® type is governing the adsorption at the solid-liquid interface and resultant reduction in contact angle. Due to higher PPO contribution for series-2, the adsorption tendencies at hydrophilic glass surface must decrease and hence the contact angle is high. It is thus concluded that PPO molecular weight and HLB for Pluronic® in a given series is critical for their diffusion and adsorption tendencies at solid-liquid interface. Brandani and Stroeve49 investigated on the adsorption-desorption kinetics of PEO-PPO-PEO block copolymers on gold surface modified by self-assembled monolayer of methyl terminated long chain alkanethiol. The AFM images revealed adsorbed micelles for copolymers with relatively high PPO content, while monolayer type morphology was observed for copolymers with higher PEO molecular weight. Our observations are in good agreement illustrating decrease in CA for polymers with higher PPO content, which correlate with higher concentration of surface active molecules at the solid-liquid interface. 19 ACS Paragon Plus Environment

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3.4 Comparison of Foamability and foam stability for aqueous Pluronic® solutions For technological applications involving formation of air-water interface and subsequent adsorption of surface active molecules is dependent on the micellar stability40, 50 and hence to the concentration of free molecules and submicellar aggregates. The diffusivity and concentration of free surfactant monomers also influence foamability and foam stability. Shah et al.51 reported highest foamability for surfactants forming less stable micelles, but the observed foam stability was least. If micelles are highly stable, the supply of monomers to the film surface will not be sufficient due to higher relaxation time and hence foamability will also decrease.52 As shown in Figure-5, the foamability and foam stability varies with type of Pluronic® block copolymer. H1 represents the height of foam generated, while T1/2 is the time taken in seconds for the generated foam to collapse upto half of the initial height. For series1, P103 showed highest foamability and foam stability, while P105 and F108 generated considerably low volume of foam. For series-2, the foamability was almost identical, but the foam stability was better for P123.

Figure-6: The foam height (H1, cm) and foam stability (t1/2, s) for 1 wt.% aqueous Pluronic® solutions at 25oC. According to an investigation by Patist et al.41, the foamability also varies with the type of method for the foam generation. If the extent of interfacial area generated is much faster, it accordingly limits the concentration of surfactant molecules diffusing to the interface. In such 20 ACS Paragon Plus Environment

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conditions, the micelles from the bulk should disintegrate with compatible faster rate to resupply monomers to the interface. For capillary method used in our study, the rate of interface generation is less compared to the hand shaking method, where the high shear rate ultimately controls the foaming process. Due to slow interfacial area generation, it accordingly allows significant concentration of molecules for diffusion to decrease the interfacial tension at the gas-liquid interface. It can also be argued that this method allows sufficient time for the dynamic surface tension of polymeric surfactant solutions to reach equilibrium surface tension. The interfacial tension at the air-water interface can be correlated with the change in the interfacial area as per the equation51,53,

ܹ = γ(DST). ∆A

…… [1]

where, γ(DST) = dynamic interfacial tension, W is the work done, and ∆A= change in interfacial area. For series-1, as the equilibrium tension for P103 (32.4 mN/m) is lower than P105 (34.5 mN/m) and F108 (37.5 mN/m), the order of the foam generation follows the order as: P103 > P105~F108. For series-2, P123 (33.2 mN/m) shows higher foamability compared to F127 (36.8 mN/m) due to lower equilibrium surface tension and presumably less stable micelles allowing significant diffusion of molecules for the adsorption at the air-water interface. The observed foam height for P103 and P123 was almost identical, which can again be correlated with their almost similar EST values (See table-1). The adsorption of Pluronic® molecules at the air-water interface of the bubble is illustrated in Scheme-2.


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Scheme-2: Proposed adsorption pattern of Pluronic® molecules at the air-water interface of bubble These observations can also be attributed to the lower dynamic surface tension values, presumably due to faster disintegration of micelles and accordingly increased concentration of free molecules at the interface. The foam stability is a function of molecular packing at the air-water interface, and layering of micelles in foam lamellae.21 The actual mechanism leading to film rupture is not known, but coalescence of bubbles, liquid drainage in thin film between air bubbles, bubble size variation and bubble inconsistency may lead to foam collapse.54 The foam stability is also higher for P123 and P103. The considerable high foam generation and foam stability for P123 and P103 is an indication of optimum rate of micellar kinetics leading to faster diffusion of molecules at the film interface, which also prevents the drainage of water and provides high stability. We conclude that lower HLB block copolymers again show significantly higher surface activity compared to higher HLB copolymers for a respective series. 4. Conclusion: The aqueous solutions of different Pluronic® block copolymers (P103, P105, P123, F108 and F127) are investigated for their kinetics of adsorption at air-water and PTFE-water interface using dynamic surface tension (DST) in relation to wetting of PTFE powder by water penetration in packed powder, sedimentation rate in aqueous dispersions, dynamic surface tension, equilibrium surface tension, contact angle, foam stability and foamability. A 22 ACS Paragon Plus Environment

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molecular mechanism is proposed based on surface activity and adsorption kinetics of Pluronics at PTFE-water interface. With increase in PEO contribution, the DST increases gradually due to enhanced partitioning in water. Among the block copolymers under investigation, P123 depicts highest stability (lowest rate of sedimentation) for PTFE dispersion and highest water penetration. The water penetration decreases with increase in polymer concentration due increased bulk viscosity and internal friction. The contact angle results are in good agreement with the wettability data, where low HLB copolymers depict maximum reduction in contact angle. The foamability and foam stability confirm critical PPO molecular weight as governing factor for the diffusion of molecules at the air-water interface, where Pluronic® with higher PPO molecular weight demonstrate preferential adsorption at the air-water interface. For a given series of block copolymers with same PPO molecular weight, the wettability and dispersion stability are maximum for low HLB Pluronic®, which increases with further increase in PPO molecular weight due to greater surface activity and presumably faster kinetics of adsorption. These results are of sustained interest for the scientists working in the technological applications involving newly created interfaces e.g. particle stabilization and wetting in porous media. For the first time, we are showing comparison of PPO/PEO molecular ratios and molecular weights on DST, EST, contact angle, wetting of packed PTFE powder, foamability and sedimentation rate of dispersion.

Acknowledgements: Authors (VS and BB) thank Department of Science and Technology (DST), New Delhi for research grant in form of DST Fast Track project No: SB/FT/CS080/2013. References: 1) Schick, M. (Ed.), Nonionic Surfactants: Physical Chemistry, Dekker, New York and Basel, 1987.


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Graphical abstract:


Correlation of Dynamic Surface Tension with ... - ACS Publications

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