Polymer-surfactant Aggregates Research Articles

Detailed physicochemical study and thermodynamic aspects of the interaction between nonionic cellulose derivative hydroxyethyl cellulose and anionic surfactant sodium N-dodecanoyl sarcosinate in aqueous media

BackgroundThe scheme of this work was to study the interaction between hydroxyethyl cellulose (HEC), a non-ionic, water soluble biopolymer and sodium N-dodecanoyl sarcosinate (SDDS), a novel, environmentally friendly amino acid-based anionic surfactant in aqueous medium. The study of this type of interaction is relatively less complicated compared to that between a polymer and a surfactant with opposite charges. MethodsThe critical micelle concentration (CMC) of HEC-induced SDDS has been determined by various physicochemical methods such as tensiometry, conductometry, fluorimetry and microcalorimetry. Several thermodynamic and surface parameters related to the binding of SDDS with HEC have been assessed. The aggregation number (Nagg) of SDDS micelles in the presence of non-ionic HEC polymer has been measured by the steady-state fluorescence method. The hydrodynamic diameter (Dh) and zeta potential (ζ) of the polymer-surfactant aggregate have been measured by the dynamic light scattering method. The interaction study has further been amplified by detecting the change in the surface morphology of the polymer due to surfactant binding using Field Emission Scanning Electron Microscopy (FESEM) and high resolution transmission electron microscopy (HR-TEM) imaging in solvent-free state, as well as visualization of the HEC-SDDS micelle in the solution phase using fluorescence microscopy technique. Significant findingsFrom a physicochemical point of view, hydrophobic synergism seems to be important for understanding the interaction pattern of HEC-SDDS combination. In each scenario, the CMC of SDDS decreases with an increase in weight percentage of HEC. The aggregation number (Nagg) of SDDS micelles is also found to decrease with an increase in weight percentage of HEC in the medium. The hydrodynamic diameter (Dh) and zeta potential (ζ) of the polymer-surfactant aggregate, these two parameters have been found to be strongly dependent on the amphiphile concentration. Finally, investigations of the surface morphologies by taking Field Emission Scanning Electron Microscopy (FESEM), high resolution transmission electron microscopy (HR-TEM) and fluorescence microscopy techniques assist the experimental data.

A phase transition approach to elucidate the propagation of shear waves in viscoelastic materials

In the field of acoustics, a medium has traditionally been considered a liquid if shear waves cannot propagate. For more complex liquids, such as those containing polymer chains or surfactant aggregates, this definition begins to be unclear. By adopting a rheological model-independent approach, this work investigated by means of dynamic elastography, the liquid–solid phase transitions in viscoelastic liquid media. When the storage shear modulus G′ dominated the loss shear modulus G″, a minimal shear wave attenuation frequency region was defined and the medium was considered solid. When G″ dominated G′, the shear waves were strongly attenuated and the medium was considered liquid. The investigated medium, an aqueous solution of xanthan gum, behaved as a bandpass filter with transition bands, showing liquid–solid–liquid behavior from low to high frequency. During these transitions bands, shear waves still propagated but highly attenuated. The limiting values where shear waves were no longer observed were identified as the low and high cutoff frequencies. Finally, the ability of various rheological models to predict the phase transition frequencies and describe the dispersion curves was tested. A three-element rheological model, the Jeffreys model, was required to accurately fit the experimental response of the medium at different concentrations over the entire frequency range. Shear wave propagation methods can overcome the technical limitations of traditional rheometry and explore higher frequencies, rarely investigated in viscoelastic liquids.

Enhanced copper cyanide complex precipitation using polymer-surfactant aggregates

Precipitation of metal cyanide complexes using quaternary ammonium salts has been used in cyanidation solutions after gold leaching. Due to the high residual load of metal cyanide complexes and ammonium salts, this method does not apply to effluents containing moderate or low cyanide concentrations. To overcome these limitations, in this study, we introduce poly(sodium 4-styrenesulfonate) (PSS) into a solution containing cetyltrimethylammonium bromide Sinpharm (CTAB), to form polymer-surfactant aggregates (PSAs) and enhance the removal efficiency of Cu and TCN. 93.8% and 97.0% of Cu and TCN, respectively, were removed from the synthetic solution (Cu 0.5 mM and CN 1.5 mM) under optimum dosage of 150 ppm PSS and 0.8 mM CTAB, resulting in 92.2% of substrate usage. The addition of 3.0 mM CTAB without PSS resulted in only 46.3% of substrate usage, even though the removals of Cu and TCN were 92.2 and 93.6%, respectively. The presence of thiocyanate ions significantly reduced the removal efficiency due to the competitive interaction with PSAs, whereas the pH of the solution and the temperature did not have any noticeable impact. Tests performed with an industrial effluent corroborated the results obtained with synthetic solutions: PSAs react with copper cyanide to form a precipitate that can be separated from the solution.

Interfacial Structure and Electrostatics Related to Solute Activity in a Model Anionic-Surfactant/Polymer Self-Assembly.

Polymer/surfactant composites are used in industry as an excipient for water-insoluble solutes. Such enhanced dissolution ability of composite media is related to the spontaneous formation of pre-micellar polymer surfactant aggregates (PS) at a magnitude of order lower than the surfactant critical micelle concentration in water. Combining electrochemical and spectroscopic studies, we investigate the microscopic interfacial structure (i.e., interface electrostatics and surface polarity) of PS formed in composite media. We establish that in a composite system, a mere change in the polymer concentration at a fixed surfactant concentration makes possible to regulate the counter-ion binding ability, surface potential, surface charge density, packing and surface polarity of the PS interface. Our study shows that the higher dissolution of water-insoluble nonionic solutes in composite media is driven by the depressing of surface charge density and polarity of the PS interface. A similar modulation of the PS interface acts as a barrier for the passive relocation of water-soluble charged solutes into the PS pseudo-phase. The time-resolved fluorescence anisotropy study allows us to underline the effect of surface charge modulation on the dynamical aspects of solutes at the PS interface.

Phase Behavior and Rheological Properties of Poly(vinyl alcohol)/AES/Water System.

In this work, the phase behavior of the poly(vinyl alcohol) (PVA)/alkyl ethoxysulfate (AES)/water ternary system is investigated at 25 °C. The PVA/AES/water ternary phase diagram is conducted which shows that there are two main phases corresponding to the solid phase and the hexagonal liquid crystalline phase (H) in the ternary system. Besides these two phases, a high-viscosity liquid phase (L2) and a micellar phase (L1) can also be found in the phase diagram, although they just occupy small areas. Polarizing optical microscopy and small-angle X-ray scattering are used to characterize the different lyotropic liquid crystal types. Moreover, the viscosity distribution and oscillation tests are also performed by means of the rheometer. High elastic modulus (G') and viscous modulus (G″) can be found in the H and the L2 phase, whereas both moduli are low in the L1 region. The PVA/AES/water ternary phase diagram provides a good guide for accelerating the selection of the detergent formula, whereas the rheological tests provide an application guidance for industrial operations. Beyond tis, the L1 region is considered to be a reasonable range for slurry making because of its good fluidity and low viscoelasticity. This research enriches the content of polymer-surfactant aggregates and promotes the development of solid detergent manufacturing industry.

Interaction among branched polyethylenimine (PEI), sodium dodecyl sulfate (SDS) and metal cations during copper recovery from water using polymer-surfactant aggregates

The application of polymer–surfactant aggregates (PSAs) to recover heavy metal ions from water is a novel treatment process for aqueous metallic effluents. To better employ this strategy, branched polyethylenimine (PEI), sodium dodecyl sulfate (SDS) and copper ions were selected to investigate their interaction in water and to improve the recyclable PSAs process for metal removal and recovery. Electrostatic association between PEI and SDS caused the formation of PSAs, and the addition of SDS to a partially protonated PEI solution caused a pH increase; however, re-dispersal of PSAs could be achieved via an increase in pH. Precipitation of PSAs depended on pH, SDS/PEI concentration ratio and the total concentration of PEI; the optimal SDS/PEI ratio decreased as pH increased, and a higher concentration of PEI showed a greater potential to precipitate. PEI formed a strong complex with Cu2+, with the most stable complex at a PEI/Cu chelation ratio of 4. Acidification decreased the chelation capacity of PEI to Cu2+, because of the competition from protons for amino groups. Complexation with Cu2+ in turn reduced the proton buffer capacity of PEI in a non-acid solution. The removal of Cu2+ increased by increasing the total PEI concentration, or by increasing pH from 1 to above 4. Ionic strength and hardness had no marked effect on Cu2+ removal using the PSAs process. Following the initial interaction among PEI, SDS and Cu2+, the Cu2+ could then be released from the PSAs by acidification and the reuse of the PSAs material could be achieved by alkalization. Copper removal and recovery were still up to 98 % and 88 % after three reuse cycles of the PSAs process, respectively.

Effect of HPAM hydrolysis degree on catanionic mixtures of DTAB/HPAM: A coarse-grained molecular dynamic simulation

Self-assembly of polymer and surfactant mixtures has attracted great interests because of its potential applications, ranging from tertiary oil recovery to drug delivery. However, the mechanism by which the hydrolysis degree of polymers promotes morphology transition is still experimentally challenging. In this study, self-assembly of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) and anionic polyelectrolyte partially hydrolyzed polyacrylamide (HPAM) mixture was investigated by coarse-grained molecular dynamics (CG MD) simulation. In the polymer-surfactant mixtures, a sphere-to-rod transition of aggregates was obtained with the increasing concentration of DTAB. The increase of HPAM hydrolysis degree can promote the morphological transition from spherical into rod-like aggregates. Our simulation revealed that the flexibility of HPAM chain and the electrostatic attraction between DTAB headgroups (N(CH3)3+) and HPAM carboxylate ions (COO−) exerted a crucial role in promoting the morphological transition of aggregates. These simulation results brought to light the molecular-level information of sphere-to-rod transitions in polymer-surfactant aggregates. This work is expected to trigger further studies on morphology transition of polymer-surfactant mixtures.

An efficient microalgal biomass harvesting method with a high concentration ratio using the polymer-surfactant aggregates process

The high cost and energy consumption related to the downstream harvesting and dewatering process is one of the most important bottlenecks limiting the commercial production of microalgal bioenergy. In this study, a novel microalgal biomass harvesting technique has been developed using polymer surfactant aggregates (PSAs). This approach has been applied to three different microalgal strains and two cynobacterial strains with a recovery efficiency of over 80%. In particular, the recovery efficiency of Chlorella sp. ZTY4 with a biomass concentration of 1.43g·L−1 can be as high as 99.9% using 360mg·L−1 poly (acrylic acid) (PAA) and 4mM (1432mg·L−1) cetylpyridinium chloride (CPC). In addition, with this PAA and CPC dosage, the recovery efficiency of Chlorella sp. ZTY4 remains above 90% for biomass concentrations up to 2.5g·L−1. Furthermore, the water content in the harvested biomass is below 70% with a corresponding concentration ratio of 231. The total flocculation time needed for this technique is 20min. The optimum dosage ratio for PAA to CPC ranges from 90 to 100mg/mmol. Based on these results, an efficient harvesting method with a high concentration ratio is proposed to simplify the whole downstream harvesting and dewatering processes of microalgal biomass.

Metallic anion removal from dilute aqueous solutions using polymer-surfactant aggregate process: Effect of surfactant chain length

A novel process was developed to remove metallic anions from dilute aqueous solutions using polymer-surfactant aggregates (PSAs). This process uses a cationic surfactant to form micelle-like aggregates on oppositely charged polymer chains, thus forming PSAs. The PSAs can bind with dilute anions and self-flocculate. Subsequently, the anion-loaded PSA flocculates can be separated from the solution by gravity settling. The concentration of chemical residue, however, is relatively high in the treated water. To improve the quality of treated water and reduce the chemical usage, two different surfactants containing a 16-carbon chain have been investigated individually. The results show that an extended chain surfactant can decrease the concentration of chromate after treatment from 7ppm (0.06mM) to 0.5ppm (0.004mM) and the concentration of residual surfactant from 1mM to 0.02mM, using the same polymer. The PSAs formed with an extended chain surfactant work effectively at a salinity below 300mM KCl and a temperature below 50°C. These results suggest that the PSAs formed with a longer chain surfactant enhance the effectiveness of treating dilute metallic anions, and at the same time lead to much less surfactant residue in the treated water. Such a PSA process has potential application in the efficient removal of dilute metallic anions during process aqueous effluent treatment.

Removal of metallic ions at the parts per billion level from aqueous solutions using the polymer-surfactant aggregate process

The polymer-surfactant aggregate process has been developed to remove metallic ions from aqueous solutions for which the ions are typically at the mg/l (parts per million) concentration level. In some cases, such as extremely valuable or highly toxic metals, the concentration of metals may exist at the μg/l (parts per billion) level. It is, nevertheless, essential to be able to apply such a process to treat them cost-effectively. In this paper, cadmium ions are removed from a 560 ppb aqueous solution down to a concentration of 17 ppb using 2 ppm polyethylenimine (PEI) and 0.02 mM sodium dodecyl sulphate (SDS). At such a low optimum dosage of polymer and surfactant (the removal agent), the loading percentage on the removal agent increases with the concentration of Cd(II) to as high as 39%. However, at this low optimum dosage of the removal agent, the removal efficiency of Cd(II) and the usage efficiency of the removal agent both have a lower tolerance toward salinity effects. This contrasts with the higher salinity tolerance corresponding to the higher optimum dosages commensurate with higher metallic ion concentrations.

Metallic anion recovery from aqueous streams and removal agent recycle in the polymer–surfactant aggregate process

In a previous contribution, a new application of the polymer–surfactant aggregate (PSA) process was developed; the removal of metallic anions from dilute aqueous solutions. A subsequent pH adjustment method has been developed to recover the metallic anions from the flocculated PSAs into a concentrated solution and then to recover the polymer (PAA) and surfactant (MTAB) for recycling. The PSA is a colloidal structure that is formed by micelle-like aggregates associating with the oppositely charged polymer chains. The PSA can then bind with metallic anions, and will eventually flocculate out of the solution under colloidal destabilisation. In the work presented, a small volume of 0.1M NaOH is firstly added to the flocs to leach out the bound metallic anions in 15min, and then a coarse filter is used to separate out the basic solution containing 5–20 times more concentrated metallic anions than the original effluent. After the metallic anion recovery, the flocs can be completely dissolved in a small volume of 0.05M H2SO4. This acidic solution, containing PAA and MTAB, is then reused in the next treatment cycle; meanwhile, the pH of the feed is adjusted to 5.3 by adding NaOH. The results show that the recovery efficiency of CrO42− at an optimum pH of 12 is 94%, and the recovery efficiency of PAA–MTAB at its optimum pH of 1.4 is 94%. The kinetics of the recovery process is quick; both the basification and acidification steps can be completed within 15–20min. In addition, the removal efficiency of 0.2mM CrO42− solution remains at the same level when using previously recycled PAA and MTAB (with a small makeup of the MTAB to cover leakage at the metallic anion removal stage). In short, the sequential pH adjustment method is able to recover and concentrate the metallic anions from the flocculated PSAs, and then recover the removal agent for recycling into the process with little deterioration of removal ability.

Electric Birefringence of Aqueous Solutions of a Rigid Polyelectrolyte. Polarization Mechanisms and Anomalous Birefringence Signals

We present an electric birefringence study of aqueous solutions of an intrinsically rigid polyelectrolyte, xanthan, and of mixed solutions with an oppositely charged surfactant, DTAB. The stationary electric birefringence as well as built-up and decay processes give information on the polarization mechanisms of the polyelectrolyte and on the role played by counterions. By adding a cationic surfactant (DTAB), that replaces the small Na+ counterions of the polymer, we could obtain further insights into the role of condensed counterions in these mechanisms. In some cases, anomalous birefringence signals appear, revealing a new polarization mechanism likely due to field-induced interactions within polymer–surfactant aggregates.

Removal of metallic anions from dilute aqueous solutions by polymer–surfactant aggregates

A new application of polymer–surfactant aggregates has been developed to remove dilute metallic anions, such as CrO42− and Fe(CN)63−, from aqueous solutions. This process uses anionic polymers, such as poly(sodium 4-styrenesulfonate) (PSS) or poly(acrylic acid) (PAA), as a back-bone structure onto which cationic surfactants, such as myristyl trimethyl ammonium bromide (MTAB), form micelle-like aggregates. The resulting structures, called polymer–surfactant aggregates (PSAs), have the ability to remove anions from solutions, and to form larger flocculated aggregates through a process of intermolecular association. The flocculated aggregates can then be separated out from the solution through a settling or coarse filtration step. In the work presented here, the results show that 99% of 0.1mM Fe(CN)63− and 80% of 0.2mM CrO42− can be removed at optimum dosages. The change in conductivity with the change in surfactant concentration is plotted to reveal changes in the state of the system, and to detect the point of formation of the PSAs. The results indicate that the PSAs, which form at surfactant concentrations well below the critical micellar concentration, are responsible for removing the metallic anions. This process is effective for metallic anion removal over a pH range from 4.5 to 6 and a temperature range from 5 to 50°C. Furthermore, both salt and organic contaminants do not significantly interfere with the anion removal efficiency. Such a PSA process thus has potential applications for the efficient removal of dilute anions, and metallic anions in particular, during process effluent water treatment.

Recovery of heavy metal ions and recycle of removal agent in the polymer–surfactant aggregate process

Abstract In a previous work, a novel effluent treatment process was developed and applied to remove heavy metal ions from dilute aqueous solutions using polymer–surfactant aggregates (PSAs) (Shen et al., 2015). A subsequent pH adjustment method for the polymer–surfactant aggregate process has been developed to recover the heavy metal ions from the flocculated PSAs into a concentrated solution and to regenerate the PSAs for recycle. The PSA is a colloidal structure that is formed by micelle-like aggregates associating with the oppositely charged polymer chains. The PSA can then bind with heavy metal ions and precipitate out of the solution. In the work presented, the flocs are firstly acidified using a small volume of 0.05 M H2SO4. The bound metal ion are leached out in 15 min, and pass through a coarse filter to report as a metallic ions solution 20–50 times more concentrated than the original effluent. After the acid leaching, the flocs can be completely dissolved in a small volume of 0.1 M NaOH. This basic polymer and surfactant solution is then reused in the next treatment cycle; meanwhile, the pH of the solution is neutralised by adding H2SO4. The results show that the removal efficiency of 11.2 ppm Cd(II) solution remains above 97% after 5 cycles without the need for make-up of the removal agent. The recovery efficiencies of Cd(II), Zn(II), and Cr(III) remain above 91%, 86% and 73%, respectively, after 6 cycles. In addition, a MATLAB simulation shows that the accumulations of Na2SO4 are stabilised at 280 ppm after the third cycle. Thus, the pH adjustment method is able to recover and concentrate the metal ions from the flocculated PSAs, and also to regenerate the removal agent such that it can be recycled with little deterioration of removal ability.

Removal of heavy metal ions from dilute aqueous solutions by polymer–surfactant aggregates: A novel effluent treatment process

A novel treatment process for effluent streams that employs polymer–surfactant complexation and flocculation has been developed and applied to remove dilute heavy metal ions, such as Cr(III), Zn(II) and Cd(II), from aqueous solutions. This process uses cationic polymers, such as poly diallyldimethylammonium chloride (PolyDADMAC) or poly ethylenimine (PEI), as a back-bone structure onto which anionic surfactants, such as sodium dodecyl sulphate (SDS), can form micelle-like aggregates. The resulting structures, called polymer–surfactant aggregates (PSAs), have the ability to remove heavy metal ions from solution, and to form larger flocculated aggregates through a process of intermolecular association. The flocculated aggregates can then be separated from the effluent stream through a settling or coarse filtration step. In the work presented here, it is shown that 99% of 11.2ppm Cd(II) was removed under optimum dosage using 40ppm PEI and 0.5mM SDS. Surface tension measurement results indicate that the removal of heavy metal ions occurs by the PSAs when the surfactant concentration is well below the critical micellar concentration. The results show that this process is effective for heavy metal ion removal at a pH range from 6 to 9, and is also effective in the presence of organic contaminants (acetone, ethanol and phenol) and in the presence of NaCl. Such a PSA complexation and flocculation process thus has potential application for the efficient removal of dilute heavy metal ions during process effluent water treatment.

Influence of polymer–surfactant aggregates on fluid flow

This paper describes the influence of interactions of poly(ethylene oxide) (PEO) with cationic cetyltrimethylammonium bromide (CTAB) micelles on drag reduction. Since the interactions between PEO and CTAB micelles alone are weak, salicylate ions were used as CTAB counterions. They facilitate formation of polymer–micelle aggregates by screening the electrostatic repulsions between the charged surfactant headgroups. The influence of polymer–surfactant interactions on drag reduction is of biomedical engineering importance. Drag reducing additives introduced to blood produce beneficial effects on blood circulation, representing a novel way to treat cardiovascular disorders. PEO is a blood-compatible polymer. However, it quickly mechanically degrades when subjected to high shear stresses. Thus, there is a need to search for other additives able to reduce drag, which would be more mechanically stable, e.g. polymer–surfactant aggregates. Numerical simulations of the flow were performed using the CFX software. Based on the internal structure of the polymer–surfactant solution, a hypothesis explaining the reason of increase of drag reduction and decrease in dynamic viscosity with increasing shear rate was proposed. It was suggested that the probable reason for the abrupt increase in friction factor, observed when the critical Reynolds number was exceeded, was the disappearance of the difference in the dynamic viscosity.

Interaction between two homologues of cationic surface active ionic liquids and the PEO-PPO-PEO triblock copolymers in aqueous solutions

The interaction of nonionic triblock copolymers of poly(ethyleneoxide) (PEO) and poly(propyleneoxide) (PPO) (PEOnPPOmPEOn) with a series of cationic surface-active ionic liquids in aqueous solutions have been investigated. The cationic surface-active ionic liquids include 1-alkyl-3-methylimidazolium bromide (CnmimBr, n = 8, 10, 12, 14, 16) and N-alkyl-N-methylpyrrolidinium bromide (CnMPB, n = 12, 14, 16). For different polymer-surfactant systems, the critical aggregation surfactant concentration (cac), the surfactant concentration to form free micelles (C m), and the saturation concentration of surfactant on the polymer chains (C 2) were determined using isothermal titration microcalorimetry (ITC) and conductivity measurements. The structure of the formed aggregates depended strongly on the hydrophobicity of the surfactant and the ratio of polymer/surfactant concentration. For C8mimBr, there were not any micelle-like surfactant–polymer clusters detected in the solution, and only micelles appeared. For other surfactants, the polymer–surfactant aggregates were formed in the solution, which was verified by the appearance of a broad endothermic peak in the ITC thermograms. The intensity of polymer–surfactant interaction increased with the hydrophobicity of the surfactants and the polymers but was not affected by the surfactant headgroups.

Morphological Control of Calcium Carbonate Particles by a Novel Polymer-Surfactant Aggregate

In this paper, calcium carbonate particles with unusual morphologies could be successfully synthesized by a precipitation reaction of sodium carbonate with calcium chloride in the presence of a novel polymer-surfactant aggregate consisting of poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) and sodium dodecyl sulfonate (SDS). The as-prepared products were characterized with scanning electron microscopy and X-ray diffraction. The results showed that the SDS concentrations, the molar ratio of CO32-to Ca2+(R), and the reactive temperature in the mixed system turned out to be important parameters for control of morphologies of the as-prepared CaCO3particles. Various unusual crystal morphologies, such as hollow microsphere, cubic, and rombehedra, etc., can be obtained depending on the experimental conditions. Moreover, the formation mechanisms of calcium carbonate with different morphologies were discussed.

Spectral Studies on the Binding Behavior of Cationic Dyes and Surfactants with Bacterial Polysaccharide of Klebsiella K43

The binding behavior of acidic capsular polysaccharide (SPS), isolated from Klebsiella serotype K43, with oppositely charged dyes and surfactants have been studied by way of absorbance and emission spectroscopic measurements. Each repeating unit of the SPS consists of three D-mannose, one D-galactose, and one D-glucuronic acid residue. The anionic polysaccharide exhibited chromotropic character and induced strong metachromasy in the cationic dye, pinacyanol chloride (PCYN) through the formation of a 1:1 stoichiometric complex. Evaluation of thermodynamic parameters, viz., changes in free energy (ΔG), enthalpy (ΔH), and entropy (ΔS), for the formation of dye-polymer complex and studies on the effect of different cosolvents were also evaluated to shed light on the binding nature as well as the extent of stability of the dye-polymer complex. Fluorescence of the cationic dye acridine orange (AO) was quenched with the progressive addition of SPS, which was found to be of Stern-Volmer type. Cationic surfactants in their pure form as well as in the mixed state with nonionic surfactant (Tween-20), replaced the dye bound to the polymer matrices; thus the original band intensities of the dyes could be reverted. Such studies revealed the involvement of both electrostatic as well as hydrophobic interactions between the dye-polymer as well as surfactant-polymer aggregates.

Physicochemical Studies on Surfactant Aggregation 1. Effect of Polyethylene Glycols on the Micellization of SDS

Interfacial adsorption and micellization of sodium dodecylsulfate (SDS) in aqueous liquid and solid polyethylene glycols (PEG) of varying chain lengths was studied by surface tension, conductance, and spectrophotometric methods. Both groups of PEGs enhanced the critical micelle concentration (CMC) of SDS. Fraction of counter ion binding (β), determined from conductance data, showed PEG chain length dependence. Surface excess (Γ max), minimum area per surfactant molecule (Amin) and free energy change of adsorption () were evaluated from surface tension data, while the free energy change of micellization () were determined from the CMC values. Effects of liquid and solid PEGs were similar, although slight differences were observed individually. In case of solid PEGs, in addition to CMC another premicellization/preaggregation process was observed. β, Γ max, and values decreased while Amin value increased with the with the increase in PEG concentration in water. Liquid PEGs behaved like cosolvent while the solid PEGs behaved like polymer, whereby initially polymer-surfactant aggregate were formed followed by the enhanced micellization. Results suggested that addition of PEG lowered the dielectric constant/polarity of the medium, which effectively stabilized the surfactant molecules in the bulk, and hence changed the micellization parameters.