Competitive adsorption of nonionic surfactant and nonionic polymer on silica

Competitive adsorption of the nonionic polymer poly(ethylene oxide) (PEO) and the nonionic surfactant of the type poly(ethylene oxide) alkyl ether from aqueous solutions on a silica surface is examined. From one-component solutions, both species readily adsorb onto silica and, in the bulk of mixed (two-component) solutions, polymer-surfactant complexes are not observed. Because both species bind by the same mechanism to silica, subtle differences in layer structure, or other species-specific parameters, determine whether one or both of the species will adsorb. It was found that various surfactants can displace PEO up to a certain critical molecular weight. Surfactants with a high aggregation number, in bulk and on the surface, can displace PEO with a higher molar mass than surfactants with a low aggregation number. As the molar mass of the polymer increases, the time a surfactant needs to completely displace the polymer increases. We can explain both the existence of the critical molar mass and the decrease in adsorption kinetics with a shift in the critical surface association concentration (CSAC).     

Influence of a surfactant and electrolytes on adsorbed polymer layers

Solvent relaxation NMR and small-angle neutron scattering have been used to characterize adsorbed poly(ethylene oxide) (PEO) layers on silica at a range of surfactant and electrolyte concentrations. Below the critical aggregation concentration (cac), the results suggest that sodium dodecyl sulfate (SDS) interacts relatively weakly, perhaps analogously to a simple salt reducing the solvency of PEO. This is evidenced by a decrease in the adsorbed layer thickness combined with an increase in the bound fraction, although the total adsorbed amount is not greatly affected. The layer thickness goes through a minimum at the cac, after which further SDS addition results in the formation of PEO/SDS aggregates that repel each other and, hence, tend to desorb. The adsorbed amount therefore decreases, from 0.7 mg m(-2) initially to 0.2 mg m(-2) with 32 mM SDS. The aggregates that remain adsorbed also repel, and hence, there is an increase in the layer thickness and the persistence length, while the bound fraction is reduced. In comparison, the effects of electrolyte at the ionic strength studied are relatively minimal. There is, however, evidence that the repulsions between adsorbed PEO/SDS aggregates are partially screened, allowing them to approach each other more readily. This leads to a contraction of the adsorbed layer when the SDS concentration is sufficiently high.     

Influence of molecular characteristics of nonionic cellulose ethers on their interaction with ionic surfactant investigated by conductometry

The interaction between nonionic derivatives of cellulose, hydroxypropylmethyl cellulose (HPMC) and methyl cellulose (MC), and ionic surfactant, sodium dodecylsulfate (SDS) were investigated by conductometric titration method, at 30°C. Obtained titration curves show two break points: critical aggregation concentration (cac) defined as the concentration of SDS at which interaction starts, and polymer saturation concentration (psp) as the concentration at which interaction finishes. Changes of characteristic concentration breaks were determined in dependence on concentration and molecular characteristics of cellulose derivatives (degree of substitution (DS) and molecular mass, i.e. intrinsic viscosity). It was shown that the first break point, cac, is independent of polymer concentration; while the second break point, psp, increases as polymer concentration increases, as described by a linear correlation. The slopes of linear relationship justify the DS on the intensity of the cellulose derivatives–SDS interaction. Changes in the intrinsic viscosity of cellulose derivatives do not exhibit influence on the interaction with SDS.     

Adsorption of poly(ethylene oxide) at the air/water interface: a dynamic and static surface tension study

The adsorption of polyethylene oxide (PEO) homologues in a wide range of molecular weight (from M(PEO)=200 to 10(6)) at the air/aqueous solution interface was investigated by dynamic and static surface tension measurements. An approximate estimate for the lower limit of PEO concentration was given at which reliable equilibrium surface tension can be determined from static surface tension measurements. It was shown that the observed jump in the earlier published sigma-lg(c(PEO)) curves is attributable to the nonequilibrium surface tension values at low PEO concentrations. The adsorption behavior of short chain PEO molecules (M(PEO)1000) is similar to that of the ordinary surfactants. The estimated standard free energy of PEO adsorption, DeltaG(0), increases linearly with the PEO molecular weight until M(PEO)=1000. In this molecular weight range, DeltaG(0) was found to be approximately the fifth of the hydrophobic driving force related to the adsorption of a surfactant with the same number of methylene groups. In the case of the longer chain PEOs the driving force of adsorption is so high that the adsorption isotherm is near saturation in the experimentally available polymer concentration range. Above a critical molecular weight the PEO adsorption reveals universal features, e.g., the surface tension and the surface density of segments do not depend on the polymer molecular weight.     

Binding of sodium dodecyl sulfate and hexaethylene glycol mono-n-dodecyl ether to the block copolymer L64: electromotive force, microcalorimetry, surface tension, and small angle neutron scattering investigations of mixed micelles and polymer/micellar surfactant complexes

The interactions of sodium dodecyl sulfate (SDS) with the triblock copolymer L64 (EO13-PO30-EO13) and hexaethylene glycol mono-n-dodecyl ether (C12EO6) were studied using electromotive force, isothermal titration microcalorimetry, differential scanning microcalorimetry, and surface tension measurements. In certain regions of binding, mixed micelles are formed, and here we could evaluate an interaction parameter using regular solution theory. The mixed micelles of L64 with both SDS and C12EO6 exhibit synergy. When L64 is present in its nonassociated state, it forms polymer/micellar SDS complexes at SDS concentrations above the critical aggregation concentration (cac). The cac is well below the critical micellar concentration (cmc) of pure SDS, and a model suggesting how bound micelles are formed at the cac in the presence of a polymer is described. The interaction of nonassociated L64 with C12EO6 is a very rare example of strong binding between a nonionic surfactant and a nonionic polymer, and C12EO6/L64 mixed micelles are formed. We also carried out small angle neutron scattering measurement to determine the structure of the monomeric polymer/micellar SDS complex, as well as the mixed L64/C12EO6 aggregates. In these experiments, contrast matching was achieved by using the h and d forms of SDS, as well as C12EO6. During the early stages of the formation of polymer-bound SDS micelles, SDS aggregates with aggregation numbers of approximately 20 were found and such complexes contain 4-6 bound L64 monomers. The L64/C12EO6 data confirmed the existence of mixed micelles, and structural information involving the composition of the mixed micelle and the aggregation numbers were evaluated.     

Shear and extensional rheology of solutions of mixtures of poly(ethylene oxide) and anionic surfactants in ionic environments

Interactions between a high molecular weight poly(ethylene oxide) (PEO) and the anionic surfactant sodium dodecyl benzene sulfonate (SDBS) in aqueous solutions were investigated by shear and extensional rheometry. Results for mixtures between PEO and sodium dodecyl sulfate (SDS) are also presented for comparison purposes. Addition of anionic surfactants to PEO solutions above the critical aggregation concentration (CAC), at which micellar aggregates attach to the polymer chain, results in an increase in shear viscosity due to PEO coil expansion, and a strengthening of interchain interactions. In extensional flows, these interactions result in a decrease of the critical shear rate for the onset of the characteristic extension thickening of the PEO solutions that is due to transient entanglements of polymer molecules. The relaxation times associated with these transient entanglements are not directly proportional to the shear viscosity of the solutions, but rather vary more rapidly with surfactant concentration. In the presence of an electrolyte, coil contraction results in lower shear viscosities and a decrease in the extension thickening effects at surfactant concentrations just beyond the CAC. The relaxation times associated with transient entanglement reach a minimum at the same surfactant concentration as the shear viscosity, which indicates that coil contraction is responsible for the observed effects in both types of flow. However, the increase in extensional-flow entanglement relaxation times is much more abrupt than the decrease in shear viscosity. All these results point to a greater sensitivity of extensional flows on the molecular conformation of PEO/surfactant complexes.     

Dynamic adsorption of weakly interacting polymer/surfactant mixtures at the air/water interface

The dynamic adsorption of polymer/surfactant mixtures containing poly(ethylene oxide) (PEO) with either tetradecyltrimethylammonium bromide (C(14)TAB) or sodium dodecyl sulfate (SDS) has been studied at the expanding air/water interface created by an overflowing cylinder, which has a surface age of 0.1-1 s. The composition of the adsorption layer is obtained by a new approach that co-models data obtained from ellipsometry and only one isotopic contrast from neutron reflectometry (NR) without the need for any deuterated polymer. The precision and accuracy of the polymer surface excess obtained matches the levels achieved from NR measurements of different isotopic contrasts involving deuterated polymer, and requires much less neutron beamtime. The PEO concentration was fixed at 100 ppm and the electrolyte concentration at 0.1 M while the surfactant concentration was varied over three orders of magnitude. For both systems, at low bulk surfactant concentrations, adsorption of the polymer is diffusion-controlled while surfactant adsorption is under mixed kinetic/diffusion control. Adsorption of PEO is inhibited once the surfactant coverage exceeds 2 μmol m(-2). For PEO/C(14)TAB, polymer adsorption drops abruptly to zero over a narrow range of surfactant concentration. For PEO/SDS, inhibition of polymer adsorption is much more gradual, and a small amount remains adsorbed even at bulk surfactant concentrations above the cmc. The difference in behavior of the two mixtures is ascribed to favorable interactions between the PEO and SDS in the bulk solution and at the surface.     

NMR investigations of self-aggregation characteristics of SDS in a model assembled tri-block copolymer solution

The present work was undertaken with a view to understand the influence of a model non-ionic tri-block copolymer PEO-PPO-PEO (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)) with molecular weight 5800 i.e., P123 [(EO)(20)-(PO)(70)-(EO)(20)] on the self-aggregation characteristics of the anionic surfactant sodium dodecylsulfate (SDS) in aqueous solution (D(2)O) using NMR chemical shift, self-diffusion and nuclear spin-relaxation as suitable experimental probes. In addition, polymer diffusion has been monitored as a function of SDS concentration. The concentration-dependent chemical shift, diffusion data and relaxation data indicated the significant interaction of polymeric micelles with SDS monomers and micelles at lower and intermediate concentrations of SDS, whereas the weak interaction of the polymer with SDS micelles at higher concentrations of SDS. It has been observed that SDS starts aggregating on the polymer at a lower concentration i.e., critical aggregation concentration (cac=1.94 mM) compared to polymer-free situation, and the onset of secondary micelle concentration (C(2)=27.16 mM) points out the saturation of the 0.2 wt% polymer or free SDS monomers/micelles at higher concentrations of SDS. It has also been observed that the parameter cac is almost independent in the polymer concentrations of study. The TMS (tetramethylsilane) has been used as a solubilizate to measure the bound diffusion coefficient of SDS-polymer mixed system. The self-diffusion data were analyzed using two-site exchange model and the obtained information on aggregation dynamics was commensurate with that inferred from chemical shift and relaxation data. The information on slow motions of polymer-SDS system was also extracted using spin-spin and spin-lattice relaxation rate measurements. The relaxation data points out the disintegration of polymer network at higher concentrations of SDS. The present NMR investigations have been well corroborated by surface tension and conductivity measurements.     

Investigation of the interaction between sodium dodecyl sulfate and cationic polymers

Aggregation properties of sodium dodecyl sulfate (SDS) on a cationic hydroxyethyl cellulose, Polyquaternium-10 (PQ-10), of low charge density were studied by potentiometric and pyrene fluorescence methods and compared with those of poly(diallyldimethylammomium chloride) (PDADMAC) of high charge density. The critical aggregation concentration (cac) was measured with the potentiometric method and further confirmed with the fluorescence method. The former was found to be more accurate. The value of the cac for the SDS/PQ-10 system was measured at 100, 200, and 400 ppm polymer and at 288.2,298.2, and 308.2 K. They showed almost the same cac value, 0.04 mmol dm-3. The I1/I3 value of the pyrene fluorescence spectrum in the SDS/PQ-10 system at higher SDS concentration was smaller than that in SDS/PDADMAC solution and much larger than that of water. From the binding isotherm by the potentiometric method, the free DS- concentration (Cf) and the bound DS- concentration (Cb) could be evaluated with ease over the SDS concentration range above the cac. The aggregation number of DS- aggregates for both the above polymers was evaluated from the fluorescence quenching method using the values of Cf and Cb from the potentiometric method. Because Cf in the SDS/PQ-10 system above the cac did not maintain a constant value contrary to that in the SDS/PDADMAC system but increased quite a lot, Cb should not be regarded as [SDS] - cac above the cac. The aggregation number in the SDS/PQ-10 system increased almost linearly with increasing total concentration of SDS, while that in the SDS/PDADMAC system reached a plateau. With increasing temperature, the aggregation number of the SDS/PDADMAC system decreased more rapidly than that of the SDS/PQ-10 system.     

Interactions between sodium dodecyl sulphate and non-ionic cellulose derivatives studied by size exclusion chromatography with online multi-angle light scattering and refractometric detection

The novel approach described allows to characterise the surfactant-polymer interaction under several sodium dodecyl sulphate (SDS) concentrations (0-20 mM) using size exclusion chromatography (SEC) with online multi-angle light scattering (MALS) and refractometric (RI) detection. Three different cellulose derivatives, hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC) and hydroxyethyl cellulose (HEC), have been studied in solution containing 10 mM NaCl and various concentrations of sodium dodecyl sulphate. It is shown that this approach is well suited for successful application of both Hummel-Dreyer and multi-component light scattering principles and yields reliable molecular masses of both the polymer complex and the polymer itself within the complex, the amount of surfactant bound into the complex as well as appropriate values of the refractive index increment (dn/dc)micro, of both the complex and the polymer in question. The more hydrophobic derivatives HPC and HPMC adsorbed significantly more SDS than HEC. The inter-chain interactions close to critical aggregation concentration (cac) were clearly seen for HPC and HPMC as an almost two-fold average increase in polymer molecular mass contained in the complex.     

Solubilization of n-alkylbenzenes in aggregates of sodium dodecyl sulfate and a cationic polymer of high charge density (II)

The solubilization property of the aggregate composed of sodium dodecyl sulfate (SDS) and a cationic polymer (polydiallyldimethylammonium chloride, PDADMAC) was investigated. From the binding isotherm, the increasing free SDS concentration (Cf) above the critical aggregation concentration (cac) was clearly confirmed and used to calculate the Gibbs free energy change of solubilization. The maximum additive concentration of the alkylbenzene solubilizates remained almost constant around their aqueous solubilities below the cac and then increased with increasing SDS concentration above the cac and with decreasing alkyl chain length of the solubilizates. Also, their solubility increased with increasing temperature over the concentration range of the surfactant examined. Because the monomeric DS- concentration in the aqueous phase (Cf) increased with the SDS concentration above the cac in the SDS/PDADMAC system, Cf was evaluated from the binding isotherm to calculate the change in the Gibbs energies of transfer of the solubilizates using the phase separation model. The Gibbs energy change for the solubilizates decreased with increasing temperature and increasing alkyl chain length. The decrease in the Gibbs energy per CH2 group (DeltaGCH2 degrees) was favored by an increase of temperature, and it was larger in magnitude than that for micelles of single-surfactant systems. From the values of DeltaH degrees and TDeltaS degrees, the solubilization of alkylbenzenes into SDS/PDADMAC was found to be entropy-driven.     

Ideal Mixing of Polymer-Surfactant Complexes of Polyethylene Oxide and Sodium Dodecyl Sulfate Plus Sodium Dodecanoate

Mixtures of the anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecanoate (SDoD) were investigated regarding their ability to bind to a hydrophilic nonionic polymer, polyethylene oxide (PEO). By electrical conductivity measurements, the parameters with respect to the onsets of surfactant aggregation were determined in the presence of 0.06 M PEO (critical aggregation concentration) and in its absence (critical micelle concentration). It was found that both plots of these parameters for the multicomponent mixtures against molar fraction of SDoD showed an ideal mixing behavior. The same technique was used to estimate the degree of ionization as a fundamental parameter relating to the interfacial composition of mixed aggregates. In addition, in order to monitor changes in polymer-surfactant structures, we used steady-state quenching fluorescence measurements to characterize the sizes of PEO-SDS/SDoD complexes at different compositions of the complex mixture. Copyright 1999 Academic Press.     

Solubilization of 1-butanol in a sodium dodecyl sulfate-poly(ethylene oxide) system by NMR and conductivity at 298.1 and 283.1 K

 The effects of adding 0.1 molal 1-butanol to the aqueous SDS system at 298.1 K and the aqueous PEO–SDS system at 298.1 and 283.1 K have been studied. NMR NOESY experiments on the PEO– SDS–1-butanol system in D₂O were obtained. NMR self-diffusion experiments and measurements of NMR chemical shifts and specific conductivity were carried out on the samples, i.e. on samples with PEO and without PEO. The addition of 1-butanol to an aqueous SDS–PEO system decreases the critical aggregation concentration (c.a.c). Determination of the second critical concentration (c ₂) depends on the method of measurements, i.e. the molecular species monitored. Conductivity measurements will give c ₂ as the SDS concentration where free micelles (micelles not bound to the polymer) are formed. PEO self-diffusion measurements, on the other hand, determine c ₂ as the SDS concentration where the polymer is saturated with SDS. Both the c.a.c and the c ₂ decrease upon 1-butanol addition. However, the c ₂ value exhibits a larger decrease than the c.a.c value. Thus, the amount of polymer bound surfactant molecules decreases upon addition of 1-butanol. Micellar solubilization of 1-butanol starts at c.a.c., but the solubilization capacity is low until the surfactant concentration reaches c ₂, where the increase in solubilization is significant. Thus, solubilization data can be used to detect c ₂, the concentration where free micelles form. Received: 21 July 1997 Accepted: 9 February 1998     

Surfactant/Nonionic polymer interaction. A NMR diffusometry and NMR electrophoretic investigation

The interaction between the nonionic polymer poly(ethylene oxide) (PEO) of molecular weight 20,000 and surfactants of various types [sodium dodecyl sulfate (SDS), dodecyl trimethylammonium bromide, octyl beta-D-glucoside, and potassium laurate] has been investigated in an aqueous solution at 25 degrees C by 1H NMR pulsed-gradient spin-echo self-diffusion techniques. The SDS/PEO study was further complemented by component-resolved 1H NMR-based studies of the electrophoretic mobility of PEO and the alkyl part of SDS under the same measurement conditions. Through such combined studies, a much more complete picture of the binding and aggregation processes becomes accessible.     

Interaction of ethylene oxide-propylene oxide copolymers with ionic surfactants studied by calorimetry: random versus block copolymers

The present study used calorimetric techniques to follow the interaction of random and block ethylene oxide (EO)-propylene oxide (PO) copolymers with ionic surfactants. Features such as the intensity of the interaction (evaluated through their critical aggregation concentrations) and the profile of the isothermal titration calorimetry (ITC) curves were comparatively analyzed for random and block copolymers with similar composition (number of EO and PO units). Random copolymers displayed an interaction similar to that observed with other hydrophilic homopolymers with the additional characteristic that the intensity of the interaction increased with the increase in the copolymer hydrophobicity (as determined by its PO content), revealing that these copolymers display an intermediate behavior between PEO and PPO. For nonaggregated block copolymers (unimers) with large enough EO blocks (molar mass above 2000 g mol-1), ITC curves revealed that the anionic surfactant sodium dodecylsulfate (SDS) interacts with the PO and EO blocks almost independently, being more favorable with the PO block, which controls the critical aggregation concentration (cac) value. Effects of temperature and of the nature of the ionic surfactants on their interaction with these copolymers were found to agree with the previously reported trends.     

Surfactant--polymer aggregates formed by sodium dodecyl sulfate, poly(N-vinyl-2-pyrrolidone), and poly(ethylene glycol)

The interaction of sodium dodecyl sulfate (SDS) in aqueous solution with poly(N-vinyl-2-pyrrolidone) (M(w) = 55,000 g/mol) in the presence of poly(ethylene glycol) (M(w) = 8000 g/mol) is investigated by electrical conductivity, zeta potential measurements, viscosity measurements, fluorescence spectroscopy, and small-angle X-ray scattering (SAXS). The results indicate that SDS-polymer interaction occurs at low surfactant concentration, and its critical aggregation concentration is fairly dependent on polymer composition. The polymer-supported micelles have average aggregation numbers dependent on surfactant concentration, are highly dissociated when compared with aqueous SDS micelles, and have zeta potentials that increase linearly with the fraction of PVP at constant SDS concentration. The analysis of the SAXS measurements indicated that the PVP/PEG/SDS system forms surface-charged aggregates of a cylindrical shape with an anisometry (length to cross-section dimension ratio) of about 3.0.     

Interactions between Poly(ethylene Oxide) and Sodium Dodecyl Sulfate in Elongational Flows

This work investigates the elongational flow of aqueous solutions of mixtures of a high-molecular-weight poly(ethylene oxide) (PEO) and sodium dodecyl sulfate (SDS). The formation of micellar aggregates of SDS along the PEO chain results in an increase in the strength of the extension thickening of the PEO solutions. This is especially pronounced under conditions in which the PEO molecules form transient entanglements in the flow field. The minimum PEO concentration required to form intermolecular entanglements is substantially reduced in the presence of micellar aggregates. This effect becomes quantitatively less important in solutions with NaCl, which suggests PEO coil contraction due to electrostatic screening of micellar aggregates. However, once extension thickening starts in the presence of NaCl, the growth of pressure drop is more abrupt than without salt, which suggests stronger interactions between PEO coils with attached aggregates. The critical aggregation concentrations of PEO/SDS and PEO/SDS/NaCl solutions agree with those reported in the literature, which were obtained by means of different experimental techniques. However, the saturation of the surfactant effect is attained at lower surfactant concentrations than the polymer saturation point previously reported. This might reflect a low sensitivity of the extension thickening effect to the amount of surfactant bound to the polymerchain as the saturation point is approached. Copyright 2001 Academic Press.     

Adsorption of sodium dodecyl sulfate at the hydrophobic solid/aqueous solution interface in the presence of poly(ethylene glycol): dependence upon polymer molecular weight

The polar orientation and degree of conformational order of sodium dodecyl sulfate (SDS) adsorbed at the hydrophobic octadecanethiol/aqueous solution interface in the presence of poly(ethylene glycol) (PEG) has been investigated as a function of the surfactant concentration and the molecular weight of the polymer. Sum frequency generation (SFG) vibrational spectroscopy was employed to obtain spectra of interfacial surfactant; weak SFG signals from interfacial polymer were also detected for polymer molecular weights of 900 and above. The phase of the SFG spectra indicated that both the surfactant and polymer had a net orientation of their CH2 and/or CH3 groups toward the hydrophobic surface. Spectra of SDS in the presence of mixed polymer/surfactant solutions showed increasing conformational order as the surfactant concentration was raised. At the lowest surfactant concentrations, the spectra of SDS were weaker in the presence of the polymer than in its absence. All PEG molecular weights investigated, with the exception of PEG 400, gave rise to significant inhibition of ordered surfactant adsorption below the critical micelle concentration. The greatest inhibitory effect was noted for PEG 900. Probing interfacial PEG specifically through the use of perdeuterated SDS revealed that the polymer spectral intensity decreased monotonically as the surfactant concentration was increased for all polymer molecular weights where a PEG spectrum was apparent. These findings are interpreted in terms of the displacement of preadsorbed polymer as the surfactant concentration increases. This result is compatible with observations of adsorption from SDS/PEG solutions at solid/solution and solution/air interfaces made using other techniques.     

Swelling of poly(ethylene oxide) gel in aqueous solutions of sodium dodecyl sulfate with added sodium chloride

The swelling behavior of poly(ethylene oxide) (PEO) gels in aqueous solutions of sodium dodecyl sulfate (SDS) with and without NaCl was investigated. In the absence of NaCl, PEO gels with different degrees of cross-linking began to swell from a concentration lower than the critical micelle concentration (cmc) of SDS, then showed sigmoidal enhancements of swelling in a higher SDS concentration region until the degrees of swelling reached maximum values. The SDS concentration at which the swelling began to appear was in reasonable agreement with the critical aggregation concentration (cac) value reported for the aqueous PEO system. For the cases where NaCl was present, the swelling behavior of PEO gel was different from that when NaCl was absent in the following way. The concentrations where the swelling begins to appear, and hence those where the degree of swelling rises steeply, decreased with an increase in NaCl concentration. The ultimate degrees of swelling at higher concentration regions also decreased with an increase in the NaCl concentration. The lowering of the SDS concentrations at which the PEO gel began to swell is in line with the decreases in the cmc of SDS solutions containing NaCl and also with the decreases in the cac of PEO solution. Electronic Publication     

Laser light scattering and isothermal titration calorimetric studies of poly(ethylene oxide) aqueous solution in presence of sodium dodecyl sulfate

The aqueous solution of poly(ethylene oxide) (PEO) in the presence of different concentrations of sodium dodecyl sulfate (SDS) was examined by laser light scattering and isothermal titration calorimetric techniques. A small fraction of PEO aggregates were found to coexist with unimeric PEO chains in dilute solution. The presence of monovalent salt does not alter the hydrodynamic properties of PEO in aqueous solution. Addition of a monovalent anionic surfactant, such as SDS, induces cooperative binding of surfactant monomers to PEO backbones at SDS concentrations ranging from 4.0 mM (critical aggregation concentration) to 16.5 mM (saturation concentration). The hydrodynamic radius of PEO unimers decreases initially and then increases with SDS concentration, resulting from the structural reorganization of the PEO/SDS complex. Beyond the saturation concentration, the hydrodynamic radii of PEO/SDS complex are independent of SDS concentration.