ARTICLE pubs.acs.org/Langmuir
Physicochemistry of Interaction between the Cationic Polymer Poly(diallyldimethylammonium chloride) and the Anionic Surfactants Sodium Dodecyl Sulfate, Sodium Dodecylbenzenesulfonate, and Sodium N-Dodecanoylsarcosinate in Water and Isopropyl AlcoholWater Media Suvasree Mukherjee,† Abhijit Dan,† Subhash C. Bhattacharya,† Amiya K. Panda,*,‡ and Satya P. Moulik*,† † ‡
Centre for Surface Science, Department of Chemistry, Jadavpur University, Kolkata-700032, West Bengal, India Department of Chemistry, University of North Bengal, Darjeeling-734013, West Bengal, India ABSTRACT: The physicochemistry of interaction of the cationic polymer poly(diallyldimethylammonium chloride) (PDADMAC) with the anionic surfactants sodium dodecyl sulfate, sodium dodecylbenzenesulfonate, and sodium N-dodecanoylsarcosinate was studied in detail using tensiometry, turbidimetry, calorimetry, viscometry, dynamic light scattering (DLS), and scanning electron microscopy (SEM). Fair interaction initially formed induced small micelles of the surfactants and later on produced free normal micelles in solution. The interaction process yielded coacervates that initially grew by aggregation in the aqueous medium and disintegrated into smaller species at higher surfactant concentration. The phenomena observed were affected by the presence of isopropyl alcohol (IP) in the medium. The hydrodynamic sizes of the dispersed polymer and its surfactant-interacted species were determined by DLS measurements. The surface morphologies of the solvent-removed PDADMAC and its surfactant-interacted complexes from water and IPwater media were examined by the SEM technique. The morphologies witnessed different patterns depending on the composition and the solvent environment. The head groups of the dodecyl chain containing surfactants made differences in the interaction process.
’ INTRODUCTION Complex formation between oppositely charged polyelectrolytes and surfactants has been an important subject of research for both fundamental and application reasons.17 Polymer surfactant mixtures are widely exploited in commonplace formulations to manipulate their performance behaviors. The ternary systems of surfactant, polymer, and water have potential for domestic, industrial, and technological applications, viz., foods, paints, drug delivery, coatings, laundry products, cosmetics, etc.8,9 In such applications, polymers are mainly used as viscosity modifiers and stabilizers. Oppositely charged polymer micellar aggregates can serve as models for polyioncolloid systems.10 The Coulombic polyioncolloid interaction guides the flocculation of inorganic materials important in water purification.11,12 Although the field is continuously being explored, information on combinations of different kinds is yet not adequate from the standpoint of fundamental understanding and applications. Polyelectrolytes interact strongly with oppositely charged amphiphiles and micelles in aqueous solution, which normally leads to liquidliquid phase separation (coacervation) or r 2011 American Chemical Society
liquidsolid phase separation (precipitation).13 A similar phase transition is also observed in polyelectrolytes with oppositely charged colloids, including proteins,14 dendrimers,15 etc. These phenomena have generated interest for both theoretical and technological reasons.16 During coacervation, a macromolecular aqueous solution separates into two immiscible liquid phases; the denser phase (relatively concentrated in macromolecules) is called the coacervate and is in equilibrium with the relatively dilute macromolecular liquid phase. For oppositely charged polyelectrolytesurfactant systems, coacervation, i.e., associative phase separation, yields a phase rich in both polymer and surfactant. Coacervation and the coacervate have importance in cosmetic formulations and pharmaceutical microencapsulation.12,17 Polyelectrolytemicelle coacervation is influenced by many factors: polymer properties, viz., its charge density, molecular weight, concentration, and molecular geometry, as well as micelle properties, viz., its surface charge density, size, and Received: January 4, 2011 Revised: February 28, 2011 Published: April 05, 2011 5222
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Langmuir concentration.18 Besides, the polymer to micelle mole ratio, the ionic strength, and the temperature also have important say in the process.18 Oppositely charged surfactants with fairly long hydrocarbon chains may also form ion pairs yielding coacervate-like phases17b in solution. In many applications, surfactants are used in combination with a variety of additives to achieve desired properties, with alcohols being frequently used as cosurfactants and cosolvents.19 The properties and structures of the aqueous solvent mixtures are thus modified, enhancing the solubility of ionic surfactants as the solutions become more hydrophobic with a concomitant decrease in polarity. The critical micelle concentration (cmc) increases by the cosolvent effect.20 An opposing effect can also occur as the alcohol decreases the surface charge density in the palisade layer (the region of the micelle around the hydrocarbon/ water interface between the head groups),21 promoting a decrease in the cmc (cosurfactant effect).22 In practice (especially in cosmetics and pharmaceutics), alcohol-like additives are used for better solubility, dispersity, and durability (or stability). The solvent isopropyl alcohol (IP) has fair use in pharmaceutical and medicinal preparations since it is nontoxic and biofriendly. It has liberal uses in the formulations of emulsions,23 microemulsions,24 and gels25 since it is miscible in water in all proportions. Poly(diallyldimethylammonium chloride) (PDADMAC), an important commercial water-soluble cationic functional polymer, is useful in water treatment as a flocculant or coagulant for the removal of organic and mineral contaminants such as arsenic, etc.26,27 It is used in the textile and paper industries for making antibacterial fiber and to improve the wet strength of papers.28 Staples et al. have studied the interaction of PDADMAC with sodium dodecyl sulfate (SDS) by neutron reflectivity and tensiometry.29 Magdassi et al. have recently reported the formation of unique micro- and nanostructures by the binding of SDS with PDADMAC.30 The structure of phase-separated, electrostatically neutral, cross-linked PDADMACanionic surfactant complexes (sodium alkyl sulfates) was studied by Chu et al.31 by small-angle neutron scattering (SANS). Kong et al. studied the interaction of PDADMAC and SDS by conductometry and rheometry techniques.32 Moroi et al. evaluated the critical aggregation concentration (cac) of the mixed solution of SDSPDADMAC by potentiometric and fluorimetric methods.33 Interaction of PDADMAC with mixed micelles of SDS/dodecanol ethoxylates (C12En) and SDS/TritonX-100 (TX-100) was studied in detail by Zhang and Dubin by turbidimetry, quasielastic light scattering, dialysis equilibrium, capillary electrophoresis, etc.34 The effect of temperature on the phase behavior of PDADMACSDS/TX-100 was studied over a wide range of surfactant compositions, ionic strengths, and polycation molecular weights using turbidimetry and dynamic light scattering by Dubin.34c The interaction between multiwalled carbon nanotubes (MWCNTs) and aqueous PDADMAC was studied by X-ray photoelectron spectroscopy (XPS) and photoacoustic Fourier transform infrared (PA-FTIR) techniques by Yang.35 Although the system PDADMACSDS was reported earlier, detailed physicochemical studies on the interaction of the polymerSDS and other surfactants are wanted. We have herein attempted a detailed study on the interaction of PDADMAC with three different anionic surfactants, SDS, sodium dodecylbenzenesulfonate (SDBS), and sodium N-dodecanoylsarcosinate (SDDS) (see Scheme 1 for the structures), of the same hydrocarbon chain length (C12) in aqueous medium
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Scheme 1. Molecular Structures of the Cationic Polymer PDADMAC, SDS, SDBS, and SDDS
and that of PDADMAC and SDS also in IPwater medium by tensiometric, viscometric, turbidimetric, and microcalorimetric methods. The hydrodynamic sizes of the polymersurfactant aggregates in water and IPwater media were determined by the dynamic light scattering (DLS) method. The morphologies of the pure PDADMAC and its surfactant complexes were investigated by scanning electron microscopy (SEM) in their solventremoved states. The features at the airsolution interface, in the bulk, and in the solvent-removed state of the polymer and its complexes were ascertained and assessed. Such an elaborate study of the physicochemistry of PDADMAC and its complexes with the three surfactants constituted of the same alkyl chain but varied head groups has not been reported in the literature to the best of our knowledge. This would positively add to the understanding of the solution behaviors of the studied systems and their possible applications in pharmacy and technology related to various applications including abatement of pollution. Prevention of coacervate formation of SDS-interacted PDADMAC in the presence of TX-100 in salt solution was reported.36 We have herein studied prevention of the phase separation of the formed complex in aqueous medium with IP. The formation of coacervate makes the interaction between oppositely charged polymer and surfactant combinations useful with respect to drug encapsulation, synthesis of nanoparticles, etc.
’ EXPERIMENTAL SECTION Materials. The surfactant SDS was purchased from SRL, India, and the surfactants SDBS and SDDS were products of Fluka (Germany). All the surfactants were more than 99% pure and were used as received. The 5223
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Figure 1. (A) Tensiometric isotherm of SDBS in water (O), SDBS (f), and SDDS (Δ) in the presence of 0.005% (w/v) PDADMAC. (For the PDADMACSDDS isotherm, the right axis is y and the top axis is x.) Inset: Tensiometric isotherms of SDS (0) and SDBS (f) in the presence of 0.005% (w/v) PDADMAC. A nonscale to scale line drawing of the nature of the plot of the PDADMACSDS combination obtained by Staples et al.29a is shown for comparison. (B) ηR vs log [SDS] for the interaction of 0.005% (w/v) PDADMAC with SDS in water (right-pointing solid triangles). Temperature 303 K. cationic polymer PDADMAC of average molecular weight 275 000 and with 1701 units of cationic centers was a product of Aldrich (St. Louis, MO).37 IP used was an AR grade product of Sisco Research Laboratories (India). Doubly distilled conductivity water of specific conductance 24 μS cm1 at 303 K was used. All experiments were carried out at 303 ( 0.2 K, if not otherwise stated. Methods. Tensiometry. Tensiometric measurements were recorded with a calibrated du No€uy tensiometer (Kr€uss, Germany) by the ring detachment technique. A 10 mL volume of polymer solution of the desired concentration (or water) was placed in a thermostated double-walled container at 303 ( 0.2 K into which a stock surfactant solution of 3035 times higher concentration than the cmc was added in steps with a Hamilton microsyringe (allowing 20 min of equilibration time after each addition prior to the measurements being taken). The maximum volume change in the solution by the addition of surfactant solution was minor (within (23%), which negligibly affected the polymer concentration. Duplicate measurements were performed to check the reproducibility. The γ values were accurate within (0.1 mN m1. We observed that γ attains a constant value within 1520 min after the addition of surfactant solution and mixing for about another 5 min. Hence, all measurements were taken 20 min after the completion of mixing. Viscometry. Viscosity measurements were performed in a CannonFenske capillary viscometer with a clearance time of 66 s for 9 mL of water under thermostated conditiond at 303 ( 0.2 K. A polymer solution of the desired concentration was taken in a viscometer, concentrated surfactant solution [30(cmc)] was progressively added with a Hamilton microsyringe, the resulting solution was mixed thoroughly, allowing time for equilibration, and the flow time was then measured. Here also the volume change of the polymer solution was minor, negligibly affecting its concentration. Each measurement was duplicated, and the mean value was recorded. The results were associated with a standard deviation of (5%. The viscosity of the solution, relative to that of the surfactant solution (termed as the viscosity ratio ηR), was considered as a measure of the viscosity change to monitor the interaction effect. The observed ηR vs [surfactant] profiles were found to be smooth. The translucency/turbidity did not impart precipitates or floating materials to affect the capillary flow. We did not test the solution with prolonged standing to check the formation of precipitation, if any. The dynamic measuring condition (mixing and flowing) kept the solution homogeneous during the course of the measurements. Turbidimetry. A UVvis spectrophotometer, model 1601, from Shimadzu (Japan) operating in the dual-beam mode was employed for turbidity measurements using a matched pair of quartz cuvettes with a 1 cm path length under thermostated conditions at 303 ( 0.2 K. The solvent (water/waterisopropyl alcohol) was used as a blank. The
surfactant stock solution (as mentioned above) was progressively added as required with a Hamilton microsyringe into the sample cell (consisting of 2.5 mL of polymer solution of the desired strength); the solution was thoroughly mixed and allowed to equilibrate for 5 min. The percent transmittance (%T) in the wavelength (λ) range of 200700 nm was then recorded. The turbidity index (100 %T) at 300 nm was plotted against the surfactant concentration. The measurement errors were within (5%. Isothermal Titration Calorimetry (ITC). An OMEGA ITC microcalorimeter from Microcal (Northampton, MA) was used for calorimetric measurements. A concentrated solution of surfactant (30 times higher concentration than the cmc) was added for an injection duration of 30 s to 1.325 mL of polymer solution (in the calorimeter cell) at equal intervals of 240 s in multiple steps (3242 additions) under constant stirring (350 rpm) conditions. An identical polymer solution (1.645 mL) was taken in the reference cell. The heat released at each step of interaction of the surfactant with polymer was recorded, and the enthalpy per mole of surfactant added was calculated with the ITC Microcal Origin 2.9 software. The dilution of surfactant solution was also performed with the same injection matrix as that of the interaction experiment, placing water/waterisopropyl alcohol in the reference and the reaction cells. The enthalpy of the interaction, ΔH, was then plotted against the surfactant concentration. Each run was duplicated to check the reproducibility. Water was circulated within the calorimeter by a NESLAB RTE100 bath at a temperature within 5 °C lower than the experimental temperature. The temperature in the cell compartment of the calorimeter was automatically scanned to the desired temperature of the experiment, i.e., 303 ( 0.1 K. DLS. The size of the polymersurfactant aggregates in the aqueous solution and in waterisopropyl alcohol medium were measured using a Nano ZS Zetasizer (Malvern, United Kingdom) spectrometer. A HeNe laser beam of wavelength 632.8 nm was used, and the scattered intensity of light was recorded at 173°. The solutions were filtered three times through cellulose acetate paper of pore size 0.45 μm. SEM. SEM measurements were taken with an FEI scanning electron microscope (Quanta 200, The Netherlands). A drop of the sample solution after solvent removal was spin coated on a slab followed by goldpalladium coating under a pressure of 101 mbar.
’ RESULTS AND DISCUSSION Interaction in Aqueous Medium. Tensiometry. For polymer amphiphile interaction, tensiometry has a distinction; it can reveal the interfacial behavior of the amphiphile in relation to the processes in the bulk. The polymer PDADMAC was mildly 5224
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Table 1. Critical Concentrations of SDS, SDBS, and SDDS at Different Courses of Interaction with Different PDADMAC Concentrations at 303 K in Aqueous Solution Obtained from Different Methodsa,b SDS
SDBS
SDDS
[PDADMAC], % (w/v)
C*
Cs
Cf
0.005
0.13
0.50
14.8
0.010 0.020
0.27 0.37
1.1 2.1
17.0 18.2
0.005
0.12
0.62
14.9
0.005
0.07
0.48
0.7
0.010
0.09
0.97
0.8
0.020
0.47
1.95
0.9
C*
Cs
Cf
C*
Cs
Cf
0.07
0.28
6.6
0.09
0.32
27.5
0.33 0.66
0.80 1.54
7.9 8.5
0.14 0.22
0.40 0.66
28.2 30.1
0.31
6.3
Tensiometry
Viscometry 0.08 Microcalorimetry
SDS
SDBS
[PDADMAC], % (w/v)
T1 (C*)
T2 (Cs)
T3
T4 (Cf)
0.005 0.010
0.12 0.30
0.48 0.98
6.5 6.7
14.8 16.0
0.020
0.40
1.90
6.8
17.2
T1 (C*)
SDDS
T2 (Cs)
T3
T4 (Cf)
T1 (C*)
T2 (Cs)
T3
T4 (Cf)
0.05 0.30
0.26 0.75
2.2 2.6
6.9 8.0
0.08 0.12
0.37 0.48
7.8 7.9
27.0 27.7
0.58
1.50
2.9
8.7
0.20
0.75
8.1
29.2
Turbidimetry
All critical concentrations are expressed as millimolar. Standard deviations: tensiometry, (5%; viscometry, (8%; microcalorimetry, (6%; turbidimetry, (4%. b The average molecular weight of PDADMAC is 275 000.37 The charge unit is 1701. For charge balance, 0.01% (w/v) polymer 0.62 mM surfactant anion. a
surface active. A 0.1% (w/v) polymer reduced the surface tension (γ) of water by 3 mN m1. As a representative of the studied surfactants, the results for PDADMACSDBS and PDADMACSDDS are illustrated in the main plot of Figure 1A. For pure surfactant, the surface tension showed the expected behavior, with an abrupt change of slope at the cmc. With progressive addition of SDBS to the 0.005% (w/v) polymer solution, γ decreased to 0.07 mM, then increased, and maximized at 0.28 mM wherefrom γ started declining again and leveled off at 6.6 mM SDBS. The first minimum in the plot is the cac, denoted by C* (wherefrom formation of the polymersmall SDBS micelle complex starts). The increase in γ in the post-C* stage was caused by the entry of the micelle-loaded polymer complex from the interface to the bulk. The process maximized at [SDBS] Cs (where the polymermicelle complexation becomes complete). Thereafter, the added SDBS monomers started to get adsorbed at the airwater interface; γ declined and leveled off at Cf with formation of free micelles in solution. This was the extended cmc (Cf) of the surfactant SDBS. The C*, Cs, and Cf values are presented in Table 1 for the studied surfactants at varied [PDADMAC]. C* and γC* increased and decreased, respectively, with an increase in [PDADMAC]. For SDDS, there appeared inflections corresponding to the points C*, Cs, and Cf ; the γlog[SDDS] plot for PDADMACSDDS interaction was less expressive than that with SDBS and SDS (inset of Figure 1A). It should be mentioned here that Asnacios et al.38 attributed the γ reduction due to polymer surfactant complex formation at the interface as a result of the electrostatically driven cooperativity. The Cs values increased with increasing [PDADMAC] following the mass balance requirement. The
ionic surfactant-interacted neutralized polymer complexes became hydrophobically self-associated in solution, forming a turbid dispersion. Broad peaks in the tensiometric profile at intermediate surfactant concentration between C* and Cf were observed for poly-NIPAM [poly(N-isopropylacrylamide)]/SDS mixtures,39 poly(styrenesulfonate) (PSS)/dodecyltrimethylammonium bromide (DTAB) mixtures,40 etc. A more pronounced and analogous tensiometric variation was reported by Merta and Stenius41 for the combination of SDS and cationic starch. For higher [PDADMAC], the tensiometric profile remained flat for a certain concentration range before rising to Cs. Staples et al. and Green et al. observed similar results for low molecular weight PDADMACSDS29 and lysosymeSDS systems.42 At Cf and beyond, the coacervates tended to disintegrate and solubilized in the micellar solution, which resulted in a decrease in the turbidity of the solution. The Cf values increased with increasing [PDADMAC]; more SDBS was required to saturate the increased amount of PDADMAC present in the system. Similar results can also be found for other polymersurfactant systems.6,43,44 The tensiometric response for the interaction of PDADMACSDS was similar to that of PDADMACSDBS with an initial reduction in γ, followed by a peak in the intermediate range of surfactant concentration. Consolidated results are shown in Table 1. In this connection, we refer to the earlier reports of Staples et al.29a and Penfold et al.,45 where the tensiometric profiles of the PDADMACSDS system were found to be much different between the first and the second minima with the current notations C* and Cf, respectively. Between C* and Cf the rise was very high and almost perpendicular; the heights of Cs almost reached the level of γ close to 5225
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Figure 2. (A) Turbidity (0) and enthalpy (9) profiles for the interaction of 0.005% (w/v) PDADMAC with SDS. Inset: Expanded turbidity profile to show the low [SDS] effect. The axis scales are the same as those of the main plot. (B) Turbidity profile of the interaction of 0.005% (w/v) PDADMAC with SDBS. Inset: Same as for the PDADMACSDDS combination. Inset in inset: Expanded turbidity profile to show the low [SDDS] effect. Temperature 303 K.
amphiphile-free water (cf. Figure 2 of ref 29a and Figure 2 of ref 45). In the inset of Figure 1A, we have documented our findings on SDS and SDBS together with the nature of the findings of Staples et al.29a (an arbitrary γ vs log C line drawing profile presented for comparison). A clear difference of our results from those of Staples et al. is witnessed. Penfold et al.45 proposed a model and the fitting parameters required in describing the isotherm pattern. While the attempt was novel, the noncomparable nature of our findings with theirs generated confusion in the results reported earlier. Viscometry. It is known that aggregation and configuration aspects of colloids and polymers in solution can be ascertained by the methods of light scattering, viscosity, etc. The viscosity method can probe the bulk complexation process in terms of the configuration changes of the polymersurfactant combination. The viscosity ratio, ηR, vs [SDS] plot for the 0.005% (w/v) PDADMAC solution is depicted in Figure 1B. A well-formed bell-shaped profile resulted with three distinct transitions corresponding to C*, Cs, and Cf of values 0.13, 0.6, and 14.9 mM SDS, respectively. The initial decrease in viscosity up to C* indicated a compaction of the polymer chain owing to the neutralization of the positive charges by the negative surfactant heads. The large increase in ηR beyond C* depended on the factors (1) expansion of the polymer chain by electrostatic repulsion among the attached charged polymersmall SDS micelle complex, (2) formation of double-stranded complexes for effective stabilization, and (3) generation of their aggregates in solution. The viscosity increase maximized at Cs. In the post-Cs region, the increased anionic environment caused disintegration/deaggregation of the complex into smaller ensembles to produce decreased viscosity with a decrease in solution translucency. The viscosity reached a low value at Cf and declined mildly from that point wherefrom free micelles prevailed in solution. Such a phenomenon was observed for polyacrylamidemethyl propanesulfonate46 and inulinATAB (alkyltrimethylammonium bromide)43 interaction. A bell-shaped viscometric profile was also reported for ethyl hydroxyethyl cellulose (EHEC) and SDS mixtures.47 The comparable nature of the viscometric profile with an initial trough followed by a shallow crest and a moderate but distinct decline was also observed for the 0.005% (w/v) PDADMAC and SDBS combination. The interaction of PDADMAC with SDDS produced only slight changes in the viscosity of the solution. The above results suggested that the molecular structure of the amphiphiles has a say in the physicochemistry of the PDADMACamphiphile interaction. The critical
concentrations obtained from viscosity measurements are summarized in Table 1 with a model representation of the complex morphology in solution in Figure 1B. Turbidimetry. In the studied interacting system, the phenomenon of aggregation of the polymeranionic surfactant complex produced turbidity in solution. The turbidimetric plot of the PDADMACSDS combination at 0.005% (w/v) PDADMAC is shown in Figure 2A. An inflection point, T1, arose at a small [SDS] ≈ 0.12 mM, beyond which the turbidity (100 %T) increased (inset, Figure 2A). The inflection point closely resembled C* obtained by tensiometry and viscometry. The phenomenon rose steeply with an inflection/break at T2 with a convex profile and a maximum at T3 (Figure 2A main plot). The value of T2 was found close to Cs, which leveled off at T4 Cf. SDBS produced the same pattern (Figure 2B) with T2 (Cs) well separated from T3. The pattern for SDDS interaction was different on the first half (left side of the inset, Figure 2B) where it was more curved between T2 and T3. The turbidity[surfactant] profiles were surfactant type dependent. From the observed inflections, the first two and the last on the whole matched with C*, Cs, and Cf registered by other methods. The values are shown in Table 1 for comparison. The turbidity features of the studied systems were not simple, particularly the maximum point (T3) found in all the cases. Structural changes in the assembled complex molecules could be a possible reason for this. According to Dubin et al.,48 the electrophoretic mobility of such complexes approached zero in the vicinity of the turbidity maximum. In their opinion the progressive binding of the micelles to the polymer eventually led to charge neutralization; the higher order aggregation of these electrically neutral complexes produced multipolymer complexes that scattered light strongly.36,48b After saturation, the complexes acquired a net negative charge; then they dissociated by way of repulsion, and the turbidity decreased and became constant at T4 as seen in Figure 2A by the dispersive effect of the free larger micelles formed in the system. T4 was close to Cf obtained from tensiometry and viscometry. A similar observation of a decrease in turbidity by the action of excess surfactant was reported.49 The T1, T2, T3, and T4 values for different PDADMAC concentrations in the presence of SDS are listed in Table 1. In terms of surfactant, the extents of turbidity for PDADMACsurfactant systems followed the order SDS < SDBS < SDDS. It should be mentioned that Bai et al.50 studied the kinematic viscosity of a hydrophobically modified cationic polyelectrolyte (D40OCT30) interacting with oppositely charged surfactants in aqueous solution to confirm the 5226
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Figure 3. (A) Enthalpy profile (g) of interaction of 0.02% (w/v) PDADMAC with SDS. (B) Same as for the 0.02% (w/v) PDADMACSDDS combination. In both cases, the solid line represents the dilution enthalpogram of the pure surfactant in water. Temperature 303 K.
redissolution of coacervates with a decrease in turbidity beyond a critical point, leading to the formation of gel. The presently studied system compositions formed turbidity which faded without showing gel morphology; higher [polymer] could lead to gel formation, which we did not investigate. Microcalorimetry. This technique was successfully used for thermodynamic understanding of the interaction between oppositely charged surfactants and polyelectrolytes, such as SDS and poly(ethyleneimine) (PEI),51 SDS and hydrophobically modified polyelectrolyte (D40OCT30),52 DTAB and poly(acrylic acid),53 DTAB and methacrylic acid/ethyl acrylate copolymers,54 CTAB and carboxymethyl cellulose,6 SDBS and SDDS with JR400 and LM200,55,49a etc. The sign and magnitude of the enthalpy changes for the overall process are governed by the superposition of various contributions such as electrostatic binding, hydrophobic interaction between the polymer and the adjacent surfactant alkyl chains, changes in the state of hydration/dehydration of both the involved surfactant and the polymer molecule, release and rearrangement of counterions, conformational changes of the polymer molecule, etc.49a The experimental enthalpograms, therefore, represent the resultant values with shares from different involved processes. The enthalpogram at 303 K for the interaction of 0.02% (w/v) PDADMAC with SDS is presented in Figure 3A. The initial kink in the plot at ∼0.32 mM appeared equivalent to C* and was followed by a bend at 1.9 mM (Cs), maximization at 7.8 mM SDS, and then a decline to level off in the region g13 mM SDS, much lower than Cf registered by other methods. It was observed that between Cs and the maximum, the SDS dilution enthalpogram to some extent differed from the interaction course; it then totally merged with the latter. It was interesting also that the courses of the interaction enthalpograms at polymer concentrations of 0.005%, 0.01%, and 0.02% (w/v) were different up to the Cs state; afterward all overlapped along with that of SDS (plots not shown). The heats absorbed or released with different involved processes made their results strikingly equivalent. The PDADMAC [0.02% (w/v)]SDDS enthalpogram is displayed in Figure 3B along with the SDDS dilution profile. A minimum at 0.9 mM (Cs) followed by a sigmoidal course was observed which merged with the SDDS dilution course at 25 mM < Cf = 30.1 mM (obtained by tensiometry, Table 1). Interestingly, the SDS-interacted enthalpogram at 0.005% (w/v) PDADMAC was comparable in nature with the turbidity profile at the same polymer concentration up to T3 (exemplified in Figure 2A). Similar observations were reported for SDS/TX-100-interacted PDADMAC systems in 0.4 M NaCl by Dubin.48b For lack of
distinct stages for C*, Cs, and Cf, the estimation of enthalpies for the different stages was not attempted. In overall comparison, the PDADMAC interaction with SDS (Figure 3A) was evenly poised between exothermicity and endothermicity whereas that with SDDS was mainly endothermic (Figure 3B). Nizri et al.30 also reported exothermic binding of SDS with PDADMAC at a low SDS/PDADMAC ratio that diminished with increasing ratio. In salt (NaCl) medium, the binding process became more exothermic. Microcalorimetry was not sensitive enough to assess the very low resultant enthalpy of interaction of PDADMAC with SDBS. We herein reiterate that since calorimetry records the resultant heat of all the physical chemical processes operative in the system, without a clear knowledge of the contributing processes energy rationalization of the present results is not possible. Complex and varied nature of ITC curves was observed in aqueous medium for the oppositely charged interacting polymer and surfactant systems, viz., PEIs with SDS,56 D40OCT30 with sodium octyl sulfate (SC8S),50 D40OCT30 with SDS,52 and anionic polyelectrolytes poly(sodium styrenesulfonate)s (NaPSSs) and poly(sodium acrylate)s (NaPAAs) of different molar masses with cationic ammonium gemini surfactant (C12C6C12Br2) and DTAB.49b Bai et al.50 presented microcalorimetric characterization of the interaction between a cationic polyelectrolyte (chemically modified dextran) and sodium octyl and decyl sulfate with a proposed mechanism of interaction. On the thermograms, cac, Cp (critical precipitation (coacervation) concentration), and Cnc (charge nuetralization concentration), i.e., the maximum phase separation state, were identified and marked. Redissolution of coacervate at CR and formation of gel followed by free micelle formation in the medium (CM) were also located. The boundary conditions adopted in the present study produced no gel morphology; there was translucency or formation of mild turbidity. The thermodynamic behavior of the system studied by Bai et al. was different from that of the system we herein studied. The interaction process and the associated morphology changes by the progressive addition of surfactant in the PDADMAC solution are shown as model representations in Figures 1B and 6. Morphology of the PolymerSurfactant Complex. SEM images tell about the surface morphology of a material whose formation from a sample solution is guided by thermodynamic, kinetic, and steric effects of molecules and their assemblies. It is a complex affair to scientifically describe the patterns formed from the materials. The amphiphile-assembled samples are of special categories by themselves. To have a direct observation of the 5227
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Langmuir morphology of the polymer surfactant aggregates (in the solventremoved states), the SEM images of 0.005% (w/v) PDADMAC and SDS/SDBS/SDDS complexes at Cf were taken (Figure 4). Entities of different shapes and sizes were found. Pure PDADMAC formed large aggregated structures (Figure 4A). By interaction with SDS, the morphology changed to a perforated sheetlike structure (Figure 4B).30,57 The average pore size was 200 nm. PDADMAC and SDBS reacted to form a globular bush headlike morphology (Figure 4C) in which small individual spheres clustered with loose connectivity. Such assemblies fre-
Figure 4. SEM images showing the morphologies of surfactant-interacted PDADMAC in water: (A) pure PDADMAC, (B) PDADMACSDS, (C) PDADMACSDBS, (D) PDADMACSDDS. In all cases, [PDADMAC] = 0.005% (w/v) and [surfactant] = Cf were used. Scale bars: 5 μm for (A) and (B); 4 μm for (C) and (D). The same samples were used in the DLS study.
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quently occur in highly cross-linked gels.58 Figure 4D shows the aggregates of the SDDS-interacted PDADMAC complex wherein uniformly nonassociated spherical bodies of average size 150 nm prevailed. The patterns formed in the systems in IP water medium will be discussed in the next section. Interaction in Isopropyl AlcoholWater Medium. In aqueous medium, the polymer concentration used was 0.005 0.020% (w/v), which was much lower compared to that of other studied polymersurfactant systems.43,55 This was done to have a low level of the coacervate formation in the system. Li et al.36 studied interaction of PDADMAC with the mixed TX-100/SDS system in 0.4 M NaCl solution to restrict formation of coacervate, visible turbidity, and precipitation in solution. We have used IPwater medium with varied extents of IP to provide nonpolarity in the medium for easier dispersion of the interacted products. The PDADMACSDS interaction was studied in the presence of IP (1060% (v/v)) at higher polymer concentrations of 0.050.20% (w/v). Tensiometry was nominally employed in this study since fair surface activity of IP could offset the effect of SDS on the surface tension.59 It was observed that the presence of IP distinctly affected the physicochemistry of PDADMACSDS interaction. Results by Tensiometry, Microcalorimetry, and Turbidimetry. A general comprehension of the interaction of PDADMACsurfactant systems by different techniques is presented in Figure 5. In Figure 5A large differences between the tensiometric isotherms without and with 10% IP in water were observed. Although the γlog [SDS] plot obtained in IPwater was much less sharp than in aqueous medium, evidence for C*, Cs, and Cf was observed. Higher percentages of IP were not used for low interfacial sensitivity to derive relevant information. In Figure 5B, also representing 10% IPwater medium, there were three transitions in both turbidimetry and microcalorimetry
Figure 5. Results of interaction of 0.1% (w/v) PDADMAC with SDS in IPwater medium studied by different methods at 303 K. (A) Tensiometric isotherms in water (9) and in 10% IPwater medium (0). (B) Enthalpy profile (9) and turbidity profile (O) in 10% IPwater medium (x-axis, top). (C) Enthalpy profile (2) and turbidity profile (*) in 40% IPwater medium. Inset: Showing the way to estimate ΔHs, ΔHI, and ΔHf from the enthalpogram. The axis scales are the same as those for the main plot. (D) Turbidity profile in varying IPwater medium: f, 10% IP (x-axis, top); Δ, 40% IP; O, 50% IP; `, 60% IP. 5228
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Table 2. Critical Concentrations of SDS at Different Courses of Interaction with Different PDADMAC Concentrations at 303 K in Varying IPWater Medium Obtained from Different Methodsa,b A (0.1% (w/v) PDADMACSDS in IPWater with Varying Concentration [% (v/v)] of IP) microcalorimetryc [IP] (%)
turbidimetry
viscometry
Cs (ΔHs)
CI (ΔHI)
Cf (ΔHf)
T1 (C*)
T2 (Cs)
T3 (CI)
C*
Cs
Cf
10
1.0 (3.2)
2.3 (22.3)
8.6 (4.3)
0.03
0.9
2.1
20
1.1 (8.1)
3.4 (20.6)
8.6 (1.7)
0.05
1.0
3.2
40
1.5 (12.6)
4.4 (19.2)
8.7 (2.3)
0.07
1.3
4.3
0.06
1.4
7.2
50
2.9 (2.6)
6.5 (3.9)
0.12
2.7
6.1
0.11
2.4
7.5
0.37
2.9
8.1
0.41
3.0
7.8
60
B (PDADMACSDS with Varying PDADMAC Concentration [% (w/v)] in 50% IPWater) microcalorimetryc [PDADMAC] [% (w/v)]
Cs (ΔHs)
CI (ΔHI)
0.05
2.0 (0.7)
0.10
2.9 (2.6)
0.20
3.8 (6.2)
turbidimetry Cf (ΔHf)
viscometry
T1 (C*)
T2 (Cs)
T3 (CI)
C*
Cs
Cf
4.5 (3.0)
0.12
1.9
4.2
0.11
1.8
7.1
6.5 (3.9)
0.12
2.7
6.1
0.11
2.4
7.5
7.7 (5.7)
0.16
3.6
7.2
0.11
3.5
7.5
All critical concentrations are expressed as millmolar. Standard deviations: microcalorimetry, (6%; turbidimetry, (4%; viscometry, (8%. b For 0.1% (w/v) PDADMAC in water, the C*, Cs, and Cf values were 0.46, 2.1, and 23.0 mM, respectively, by tensiometry; the same values in 10% IPwater medium were 0.8, 3.2, and 17.0 mM, respectively. c Enthalpy values are expressed in kilojoules per mole of SDS and are presented in parentheses under microcalorimetry. a
courses. Those were T1 (C*), T2 (Cs), and T3 for turbidimetry and Cs, CI, and Cf for calorimetry. Unlike aqueous medium, the monomer binding feature (with the manifestation of C* witnessed in other methods and at 0.02% (w/v) polymer concentration in microcalorimetry (Figure 3A) was not visible in the enthalpy course (Figure 5C) in 40% IPwater medium. In the beginning, the addition of SDS in the PDADMAC solution yielded an exothermic ΔH value with the formation of a trough at Cs and a faint increase in translucency. In the turbidity profile, below Cs, the location of T1 (C*) was also always not distinct. The initial exothermic regime essentially manifested binding of the polymer-induced small SDS assemblies cooperatively with the polymer reaching the trough at Cs that mildly increased with [IP] (Table 2A). Similar results were reported by Griffith.60 Along with the binding, associated hydrophobic and nonspecific interactions in the system yielded a resultant endothermic enthalpy change up to CI (4.4 mM), which then declined, breaking at Cf (start of free SDS micelle formation in solution). In 50% IP, for all PDADMAC concentrations, the conformational changes in the polymer and the reorganization of the bound micelles in the chain were least to make the resultant enthalpogram less structured (illustration not shown). In the inset of Figure 5C, the rationale for estimation of the associated enthalpies for the three distinct stages Cs, CI, and Cf are presented with the entry of results with notations ΔHs, ΔHI, and ΔHf, respectively, in Table 2B. The presence of IP affected the overall process; it lengthened and flattened the enthalpy courses. Additionally, in turbidimetry, the value increased from T2 (Cs) with the addition of SDS and saturated at T3. Here in IPwater medium, the complex PDADMAþDS formed a stable translucent solution without precipitation in the studied range of concentration (0.050.2% (w/v) polymer) and solvent composition (1060% IPwater (v/v)). With increasing IP concentration (%) in the medium, the turbidity profiles followed a similar trend with a forward shift (Figure 5D). The T1 (C*), T2
(Cs), T3 (CI), and Cf values are summarized in Table 2. It should be mentioned here that the polymer concentration used in the IPwater medium was considerably higher than that used in the aqueous medium. The interaction profiles in the two media thus produced several uncommon features (cf. Tables 1 and 2). Up to 40% IP at 0.1% (w/v) PDADMAC distinct stages for Cs, CI, and Cf were observed in the thermograms; the Cf stage became difficult to locate at higher IP concentration (%) (Table 2). Viscometry. The viscosity ratio ηR of PDADMAC was measured with progressive SDS addition in IPwater medium. For 0.1% (w/v) polymer at [IP] < 30%, the polymerSDS solutions were not easy flowing. We thus studied the viscosity behavior at [IP] = 4060% with depictions of the results in Figure 6 and Table 2. Three distinct inflections were observed in the profiles. The polymer configuration initially tended to become compact by the electrostatic interaction with DS at a number of its positive Me2Nþ centers. The ηR of the solution thereby progressively declined. The first break stood for the cac, i.e., C*. On further SDS addition, small induced micelles complexed with the polymer. The low polar IPwater medium decreased the counterion dissociation of micelles to make the complex ensembles more compact. The ηR declined further up to the binding saturation point, i.e., Cs. On further addition of SDS, the alkyl tails of the amphiphile monomers preferred to adhere to the hydrophobic regions of the polymer (rather than at the interface, which is favorably occupied by the IP molecules) with the head groups protruded out in the solution. This fairly made the ensembles studded with negative charges, causing expansion by way of electrostatic repulsion up to [SDS] = Cf. The measured ηR increased and remained practically unchanged beyond Cf, where the complexed polymer along with the free normal SDS micelles prevailed in the solution. Similar viscosity manifestations were also observed in other systems.49a,61 This trend in viscosity continued in the system until 50% IP was used in the medium. In 60% IP (inset, Figure 6), an exponential decrease in ηR was 5229
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Table 3. Average Hydrodynamic Diameter (dh) and Polydispersity Index (PDI) of PDADMAC and Its Interacted Products with Surfactants in Water and IPWater Media at 303 Ka,b dh/nm
PDI
P in water
235
0.41
P in 10% IP
186
0.28
P in 20% IP
263
0.20
P in 40% IP
485
0.38
PSDS
114
0.30
PSDBS
195
0.31
PSDDS PSDS (10% IP)
197 171
0.20 0.41
PSDS (20% IP)
192
0.10
PSDS (40%IP)
624
0.87
sample
Figure 6. ηR vs [SDS] for the interaction of 0.1% (w/v) PDADMAC with SDS in 50% IPwater medium (right-pointing solid triangles) at 303 K. Inset: Same plot for interaction in 60% IPwater medium. The axis scales are the same as those of the main plot.
Figure 7. SEM images demonstrating the morphology change of PDADMAC and the polymer (P)SDS complex in varying IPwater medium: (A) P in water (A1, PSDS in water), (B) P in 10% IPwater (B1, PSDS in 10% IPwater), (C) P in 20% IPwater (C1, PSDS in 20% IPwater), (D) P in 40% IPwater (D1, PSDS in 40% IPwater). [PDADMAC] = 0.005% (w/v). [SDS] = Cf. Scale bars: 20 μm for (A) and (B); 10 μm for (C) and (D); 5 μm for (A1), (B1), (C1), and (D1). The same samples were used in the DLS study.
observed with no distinct breaks. There the third stage of increase in ηR was absent; the medium nonpolarity reduced the electrostatic effect to affect the configuration of the complex. In IP water medium, C* was found to remain independent of the polymer concentration. The critical concentrations are tabulated in Table 2 with a model representation of the ensembles formed in Figure 6. Morphology of the Polymer and PolymerSDS Complex in IPWater. Figure 7 shows the SEM images of 0.005% (w/v) pure PDADMAC (panels AD) and PDADMACSDS (=Cf) complex (panels A1D1) prepared from water and IPwater media. In water, aggregated polymer molecules of varied geometries and large structures with an average size of 4 μm were formed (Figure 7A). In 10% IP, smaller polymer clusters of nearly globular geometries of average size 2 μm were found (Figure 7B). In Figure 7C, the polymer in 20% IP formed domains of flowerlike clusters with an average diameter of 5 μm. In 40% IPwater medium (Figure 7D), circular distorted
Errors in dh are (12%. b In all cases, [PDADMAC] = 0.005% (w/v) and [surfactant] = Cf. a
floral patterns of greater size appeared (810 μm). An increase in size from 186 to 485 nm in the presence of 1040% IP in water was revealed from DLS measurements (Table 3). On the whole, the sizes obtained by SEM were much larger than the DLS results because of growth (aggregation) during the solvent removal stage in the sample preparation process. In aqueous medium (Figure 7A1), the PDADMACSDS complex formed a planar porous sheetlike structure. In 10% IP medium, porous parallel arrays of tubelike structures were formed (Figure 7B1). In 20% IP, the structural arrangement looked like a porous mesh (Figure 7C1). In 40% IP, a wider tubular structure with a reduced number of pores resulted (Figure 7D1). The presence of IP changed the nearly random porous structure of the solvent-removed SDS-complexed PDADMAC prepared from water to parallel arranged structures of varied porosity prepared from IPwater medium. Striking morphology changes of PDADMAC as well as the PDADMACSDS complex from water to IPwater medium were witnessed. They represented manifestation of the concerted effect of weak forces in shaping up the polymer and polymersurfactant assemblies in the domains of thin dimensions. Dynamic Light Scattering. The results for 0.005% (w/v) PDADMAC and its interacted complexes with SDS, SDBS, and SDDS in water and IPwater media are presented in Table 3. The surfactant concentrations used in the measurements were equal to Cf. The results were consistent with moderate variations. The hydrodynamic diameter (dh) on the whole ranged between 100 and 200 nm, except for the pure polymer and its SDS-complexed product in 40% IPwater medium, where the size variation was 3-fold higher. The polydispersity indices of the reported systems were moderate to low, except for the PDADMACSDS complex in 40% IPwater medium. The polymer itself and its complexes with the surfactants were not expected to have well-defined shapes in solution. The dh values were the averages of assumed overall spherical geometry. The SEM results (Figures 4 and 7) witnessed much larger sizes compared to the DLS results; the former were aggregated species on the slide after removal of the solvent, whereas DLS represented native dispersions of the species in the solvent media. Cryo-TEM measurements could be a realistic method to decide on the shapes and sizes of the dispersions of the polymer and its complexes in the studied media. The formed polymersurfactant complex 5230
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Table 4. Comprehensive Presentation of Interaction Parameters of the Surfactants SDS, SDBS, and SDDS with PDADMAC [0.02% (w/v)] in Aqueous Medium at 303 Ka parameter
SDS
SDBS
SDDS
order
cmc
8.0
2.0
13.0
N
78
62
81
SDBS < SDS < SDDS
C*
0.37
0.66
0.22
SDDS < SDS < SDBS
SDBS < SDS < SDDS
Cs
2.10
2.54
0.66
SDDS < SDS < SDBS
Cf Cn
18.2 40
8.5 50
30.1 25
SDBS < SDS < SDDS SDDS < SDS < SDBS
T3
6.8
2.9
8.1
SDBS < SDS < SDDS
a
The cmc, C*, Cs, Cf, and T3 values are expressed as millimolar. Cn is expressed in percent.
coacervates of SDS, SDBS, and SDDS were of supramolecular dimensions. It should be mentioned here that comparable dimensions of mixed micelles of SDS/TX-100-interacted PDADMAC products in 0.4 M NaCl were also reported by Li et al.36 Surfactant Nature Dependent Interaction Features. The three surfactants used in this study possessed identical hydrophobic tails (Me(CH2)n) and different head groups (sulfate, benzenesulfonate, and sarcosinate anions) with a common Naþ counterion. Their self-assembly and interaction with the cationic polymer PDADMAC was expected to be different depending on the nature of the head group. The herein reported results have corroborated this. A comprehensive and tentative discussion on this issue is presented below with reference to comparative data illustrated in Table 4. It was observed that the aqueous cmc, aggregation number (n), extended cmc or Cf, and T3 followed the same sequence, SDBS < SDS < SDDS; C*, Cs, and Cn (charge neutralization) followed the order SDDS < SDS < SDBS. In the latter three parameters (C*, Cs, and Cn) the positions of SDBS and SDDS were reversed. Considering the complexity of the processes, and the involvement of electrostatic, hydrophobic, solvation, change in solvent structure, and other nonspecific effects, the observed trends were not much different. The head group of SDBS, i.e., benzenesulfonate, was bulkier than the sulfate in SDS and thus had a lower charge density for easier self-assembly than SDS (the head group of SDBS was also more hydrophobic than that of SDS because of the presence of the aromatic ring). The sarcosinate head group with well-separated three oxygen and one nitrogen centers was more solvated and required a higher concentration to assemble. This principle guided their free assembly formation. During interaction with the polymer, other factors also became operative, making the reversal of the places of SDBS and SDDS; the latter assembled easier on the polymer segments than the former. In this connection, it may also be mentioned that, at Cf, a perforated sheet, a bush head assembly, and isolated dots of the PDADMAC complex were observed for SDS, SDBS, and SDDS, respectively, from SEM measurements. The particle hydrodynamic dimension of nearly 129 nm was observed for SDS by DLS measurements, which was nearly 200 nm for both SDBS and SDDS in aqueous solution. These differences, we consider, were controlled by the overall electrical charge of the complex, leading to their folding and aggregation. The SDS complex was less facile to such conditions than the SDDS and SDBS products.
’ CONCLUSION The interaction study of the cationic polymer PDADMAC with anionic surfactants is limited in the literature. It has been mostly done with SDS. There are several studies reported as well with mixed surfactants. In addition to SDS, we have herein used SDBS and SDDS, where the nonpolar dodecyl tail is attached to different head groups. Like normal findings, the cac or C* (the point of origin of small polymer-induced micelles), binding of the small aggregates with the polymer that maximizes at Cs, and subsequent formation of free micelles in solution at Cf were evaluated. On average, to augment the cac, the required charge neutralization for PDADMAC (average molecular weight 275 000) was 25%, 40%, and 50% for SDDS, SDS, and SDBS, respectively. The reported interaction parameters in the aqueous medium (Table 1) were on the whole consistent among the different methods used except microcalorimetry. The findings from calorimetry resulted with the influences of processes other than direct binding of the surfactants to the PDADMAC in solution. Such results by calorimetry in the IPwater medium were more consistent with those found from other methods. The enthalpies of interaction with SDS at lower SDS/PDADMAC mole ratios were found to be exothermic both by us and by Nizri et al.30a A gross difference of our tensiometric study with that of Staples et al.29a was observed wherein the maximum (Cs) in γ between C* and Cf occurred within a very short range of [SDS]. The rise was high, very close to the γ of water, and almost perpendicular. The rise in γ we observed was moderate and broad-dome-shaped as reported by Goddard1 and others.49a Penfold et al.45 tried to explain the results in terms of a model. Since we did not obtain such a striking difference in the tensiometric profiles for the PDADMACSDS combination, we are at present not in a position to make a pragmatic judgment on the issue and the validity of the model. It thus remains open for future investigation. The viscosity, turbidity, and DLS measurements evaluated the nature and dimensions of the formed PDAMADCsurfactant complexes and their association and hydrodynamic sizes in the aqueous and IPwater media. The attachment of surfactant monomers, the induced micelles bound to the polymer, and the association of the complexes formed and their configurational changes under the influence of higher surfactant concentration, etc. shown in Figures 1B and 6 speak for the interaction complexity. In the IPwater medium the formation of coacervate was minimized at [IP] > 30%. The observed dimensions by DLS were comparable with the sizes reported in the literature36 (except in 40% IPwater medium, where they were larger). The SEM measurements produced differences in the solvent-removed patterns of the polymer and its SDS-interacted products formed in water and IPwater media. ’ AUTHOR INFORMATION Corresponding Author
*Phone: þ91 94 3334 7210 (A.K.P.); þ91 33 2414 6411 (S.P. M.). Fax: þ91 35 326 9901 (A.K.P.); þ91 33 2414 6266 (S.P. M.). E-mail:
[email protected] (A.K.P.); spmcss@yahoo. com (S.P.M.).
’ ACKNOWLEDGMENT S.M. thanks the Council of Scientific and Industrial Research, Government of India, New Delhi, for a Senior Research 5231
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Langmuir Fellowship. S.P.M. thanks the Indian National Science Academy for an Honorary Scientist position. We thank Department of Science & Technology, Government of India, New Delhi, for funding. We extend our appreciation to Prof. K. P. Das, Department of Chemistry, Bose Institute, Kolkata, West Bengal, India, for DLS measurements and Mr. S. C. Mycap, Bose Institute, for SEM measurements.
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