Noninteracting versus interacting poly(N-isopropylacrylamide)-surfactant mixtures at the air-water interface

Two polymer-surfactant mixtures have been studied at the air-water interface using neutron reflectivity and surface tension techniques. For the noninteracting system poly(N-isopropylacrylamide) (PNIPAM)/octaethyleneglycol mono n-decyl ether (C10E8), the adsorption behavior is competitive and driven purely by surface pressure (pi). When pi(polymer) > pi(surfactant), the surface layer consists of almost pure polymer, and for pi(polymer) < pi(surfactant), the polymer is displaced from the surface by the increasing pressure of the surfactant. Beyond the CMC, the polymer is completely displaced from the surface. For the interacting system PNIPAM/sodium dodecyl sulfate (SDS) where the two species interact strongly in the bulk beyond the critical aggregation concentration (CAC), the surface behavior is more original. Earlier neutron reflectivity studies investigated PNIPAM adsorption behavior where the SDS was contrast-matched to the solvent. In the present study, complementary measurements of SDS adsorption where PNIPAM is contrast-matched to the solvent give a complete view of the surface composition of the mixed system. At a constant polymer concentration, with increasing SDS, three main regimes are obtained. For C(SDS) < CAC, adsorption is governed by simple competition and PNIPAM is predominant at the interface. At intermediate SDS concentration (CAC < C(SDS) < x2, where x2 indicates the predominance of free SDS micelles), interfacial behavior is governed by bulk polymer-surfactant interaction. Adsorbed polymer is displaced from the interface to form PNIPAM-SDS complex in the bulk. SDS adsorption remains weak since most of the SDS molecules are used to form bulk polymer-surfactant aggregates. Further increase in SDS concentration results in continued displacement of PNIPAM and an abrupt increase in SDS adsorption. This is a result of saturation of bulk polymer chain with adsorbed micelles. Interestingly, beyond x2, PNIPAM is not completely displaced from the surface. A mixed PNIPAM-SDS adsorbed layer with enhanced packing of the SDS monolayer is formed.     

Ultrafast proton transfer of pyranine in a supramolecular assembly: PEO-PPO-PEO triblock copolymer and CTAC

Excited-state proton transfer (ESPT) of pyranine (8-hydroxypyrene-1,3,6-trisulfonate, HPTS) is studied in a polymer-surfactant aggregate using femtosecond emission spectroscopy. The polymer-surfactant aggregate is a supramolecular assembly consisting of a triblock copolymer (PEO)(20)-(PPO)(70)-(PEO)(20) (P123) and a cationic surfactant, cetyltrimethylammonium chloride (CTAC). ESPT of the protonated species (HA) in HPTS leads to the formation of A(-). The dynamics of ESPT may be followed from the decay of the HA emission (at approximately 440 nm) and rise of the A(-) emission (at approximately 550 nm). Both steady-state and time-resolved studies suggest that ESPT of HPTS in P123-CTAC aggregate is much slower than that in bulk water, in P123 micelle, or in CTAC micelle. The ratio of the steady-state emission intensities (HA/A(-)) in P123-CTAC aggregate is 2.2. This ratio is approximately 50, 12, and 2 times higher than that respectively in water, in P123 micelle, and in CTAC micelle. Retardation of ESPT causes an increase in the rise time of the A(-) emission of HPTS. In P123-CTAC aggregate, A(-) displays three rise times: 30, 250, and 2400 ps. These rise times are longer than those in CTAC micelle (23, 250, and 1800 ps), in bulk water (0.3, 3, and 90 ps), and in P123 micelle (15 and 750 ps). The rate constants for initial proton transfer, recombination, and dissociation of the ion pair are estimated using a simple kinetic scheme. The slow fluorescence anisotropy decay of HPTS in P123-CTAC aggregate is analyzed in terms of the wobbling-in-cone model.     

Solvation dynamics of DCM in a polypeptide-surfactant aggregate: gelatin-sodium dodecyl sulfate

Solvation dynamics of 4-(dicyanomethylidene)-2-[p-(dimethylamino)styryl]-6-methyl-4H-pyran (DCM) is studied in a polypeptide-surfactant aggregate consisting of gelatin and sodium dodecyl sulfate (SDS) in potassium dihydrogen phosphate (KP) buffer. The average solvation time (tauS) in gelatin-SDS aggregate at 45 degrees C is found to be 1780 ps, which is about 13 times slower than that in 15 mM SDS in KP buffer at the same temperature. The fluorescence anisotropy decay in gelatin-SDS aggregate is also different from that in SDS micelles in KP buffer. DCM displays negligible emission in the presence of gelatin in aqueous solution. Thus the solvation dynamics in the presence of gelatin and SDS is exclusively due to the probe (DCM) molecules at the gelatin-micelle interface. The slow solvation dynamics is ascribed to the restrictions imposed on the water molecules trapped between the polypeptide chain and micellar aggregates. The critical association concentration (cac) of SDS for gelatin is determined to be 0.5 +/- 0.1 mM.     

Effect of Membrane Potential on the Binding of Merocyanine 540 to Human Erythrocytes

Illumination of erythrocytes in the presence of merocyanine 540 (MC540) resulted in changed binding characteristics of MC540, i.e. a red shift in the emission maximum of bound dye with an increase in the relative fluorescence quantum yield. Aluminum phthalocyanine tetrasulfonate-mediated photodynamic treatment, before addition of MC540, resulted in a comparable change in the MC540-binding characteristics with, in addition, an increase in the concentration of MC540 in the membrane. Both photodynamic treatments induce depolarization of the red cell membrane, with a dose dependency comparable to that of changed MC540 binding. Also depolarization, induced by incubation of the cells with A23187 in the presence of Ca₂+ in high [K+] buffer, resulted in similar changes in the MC540 binding characteristics. These results indicate a relation between photodynamically induced membrane depolarization and changed MC540-binding characteristics. Hyperpolarization induced by incubation with A23187 in low [K+] buffer resulted in decreased binding of MC540. In accordance, the MC540-mediated photodamage to red cells decreased upon hyperpolarization of the cells. The results indicate that the binding of MC540 to erythrocytes is strongly dependent on the membrane potential and that hyperpolarization of the membrane could be a possible protection mechanism for erythrocytes against MC540-mediated photodynamic damage.     

Interaction between casein and sodium dodecyl sulfate

The interaction of the anionic surfactant sodium dodecyl sulfate (SDS) with 2.0 mg/ml casein was first investigated using isothermal titration calorimetry (ITC), dynamic light scattering (DLS), and fluorescence spectra. ITC results show that individual SDS molecules first bind to casein micelles by the hydrophobic interaction. The micelle-like SDS aggregate is formed on the casein chains when SDS concentration reaches the critical aggregation concentration (c1), which is far below the critical micellar concentration (cmc) of SDS in the absence of casein. With the further increase of SDS concentration to the saturate binding concentration c2, SDS molecules no longer bind to the casein chains, and free SDS micelles coexist with casein micelles bound with SDS aggregates in the system. DLS results show that the addition of SDS leads to an increase in the hydrodynamic radius of casein micelles with bound surfactant at SDS concentration higher than 4 mM, and also an increase in the casein monomer molecule (or submicelles) at SDS concentration higher than 10 mM. Fluorometric results suggest the addition of SDS leads to some changes in the binding process of hydrophobic probes to casein micelles.     

Mixed adsorption of poly(vinylpyrrolidone) and sodium dodecylbenzenesulfonate on kaolinite

The mixed adsorption of the nonionic polymer poly(vinylpyrrolidone) (PVP) and the anionic surfactant sodium dodecylbenzenesulfonate (SDBS) on kaolinite has been studied. Both components adsorb from their mixture onto the clay mineral. The overall adsorption process is sensitive to the pH, the electrolyte concentration, and the amounts of polymer and surfactant. Interpretation of the experimental data addresses also the patchwise heterogeneous nature of the clay surface. In the absence of PVP, SDBS adsorbs on kaolinite by electrostatic and hydrophobic interactions. However, when PVP is present, surfactant adsorption at 10(-2) M NaCl is mainly driven by charge compensation of the edges. The adsorption of PVP from the mixture shows similar behavior under different conditions. Three regions can be distinguished based on the changing charge of polymer-surfactant complexes in solutions with increasing SDBS concentration. At low surfactant content, PVP adsorbs by hydrogen bonding and hydrophobic interactions, whereas electrostatic interactions dominate at higher surfactant concentrations. Over the entire surfactant concentration range, polymer-surfactant aggregates are present at the edges. The composition of these surface complexes differs from that in solution and is controlled by the surface charge.     

Interactions between chitosan and SDS at a low-charged silica substrate compared to interactions in the bulk--the effect of ionic strength

The effect of ionic strength on association between the cationic polysaccharide chitosan and the anionic surfactant sodium dodecyl sulfate, SDS, has been studied in bulk solution and at the solid/liquid interface. Bulk association was probed by turbidity, electrophoretic mobility, and surface tension measurements. The critical aggregation concentration, cac, and the saturation binding of surfactants were estimated from surface tension data. The number of associated SDS molecules per chitosan segment exceeded one at both salt concentrations. As a result, a net charge reversal of the polymer-surfactant complexes was observed, between 1.0 and 1.5 mM SDS, independent of ionic strength. Phase separation occurs in the SDS concentration region where low charge density complexes form, whereas at high surfactant concentrations (up to several multiples of cmc SDS) soluble aggregates are formed. Ellipsometry and QCM-D were employed to follow adsorption of chitosan onto low-charged silica substrates, and the interactions between SDS and preadsorbed chitosan layers. A thin (0.5 nm) and rigid chitosan layer was formed when adsorbed from a 0.1 mM NaNO3 solution, whereas thicker (2 nm) chitosan layers with higher dissipation/unit mass were formed from solutions at and above 30 mM NaNO3. The fraction of solvent in the chitosan layers was high independent of the layer thickness and rigidity and ionic strength. In 30 mM NaNO3 solution, addition of SDS induced a collapse at low concentrations, while at higher SDS concentrations the viscoelastic character of the layer was recovered. Maximum adsorbed mass (chitosan + SDS) was reached at 0.8 times the cmc of SDS, after which surfactant-induced polymer desorption occurred. In 0.1 mM NaNO3, the initial collapse was negligible and further addition of surfactant lead to the formation of a nonrigid, viscoelastic polymer layer until desorption began above a surfactant concentration of 0.4 times the cmc of SDS.     

Influence of aquo-organic solvent media on the self-aggregation of sodium dodecyl sulfate (SDS) and its interaction with polyvinylpyrrolidone (PVP)

Influence of mixed aquo-organic solvents viz. water-dimethyl sulfoxide (DMSO), water-formamide (FA), water-dioxane (DX), and water-ethylene glycol (EG) on the micellization of sodium dodecylsulfate (SDS) alone and in presence of neutral polymer polyvinyl pyrrolidone (PVP) was studied. Interaction with PVP initially witnessed formation of critical aggregation concentration (CAC) in the favor of formation of induced small micelles of SDS at a concentration lower than the normal critical micelle concentration (CMC), and later found the formation of normal micelles with extended critical micelle concentrations (CMC_e) in solution. The SDS-PVP interaction depended on the nature and composition of the mixed solvents. Besides CAC and CMC_e, the maximum Gibbs surface excess at the interface (Γ _max), the minimum area (A _min) of the dissociated amphiphile anion, and enthalpy of micellization (ΔH _m ⁰ ) were also determined. Configurational state of PVP in aquo-organic media was investigated by the methods of viscometry, dynamic light scattering (DLS), and scanning electron microscope (SEM) methods. The [η] and Huggins constant (k _H) were considered to ascertain the overall configuration of PVP in solution. The complexes were formed and aggregated at different stages of their molecular composition. The aggregate sizes were determined by DLS, and the surface morphologies in the solvent removed states were examined by SEM. With reference to bulk and interfacial phenomena, polymer-surfactant interaction is thus considered to be important, and the detailed study herein under taken for SDS-PVP combination and PVP alone in mixed aquo-organic solvent media is a new sort of attempt. Figure

DX and DMSO influenced [η] of PVP, SDS interacted PVP enthalpogram and the SEM image of the PVP in 10 wt% DX     

SDS与PVP对碘化银溶胶稳定性的影响

研究了SDS、PVP及二者的混合物对AgI溶胶稳定性的影响。SDS主要通过表面活性负离子在AgI质点表面上的吸附使ζ电势升高,提高溶胶的稳定性。低浓度的PVP使AgI溶胶敏化,高浓度时又通过吸附层的空间稳定效应使其稳定。在PVP与SDS的混合溶液中,AgI溶胶的稳定性显著增加。根据PVP与SDS在固液界面上的相互作用讨论了这一增效作用。     

Micro-DSC and rheological studies of interactions between methylcellulose and surfactants

The effects of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), on the gelation of methylcellulose (MC) in aqueous solutions have been investigated by micro differential scanning calorimetry (micro DSC) and rheology. Methylcellulose had a weight average molecular weight of 310,000 and a degree of substitution of 1.8. The concentration of MC was kept at 0.5 wt % (0.016 mM) and 1 wt % (0.032 mM), and the concentration of CTAB in the MC solutions was varied from 0 to 0.6 wt % (16.5 mM). Upon heating, a single endothermic peak, which is due to the hydrophobic association and gelation of MC, shifts to lower temperatures with increasing CTAB for CTAB < or = CMC (0.93 mM or 0.034 wt %), and then it shifts to higher temperatures lineally with CTAB for CTAB > CMC. At the same time, the endothermic enthalpy decreases with increasing CTAB concentration. Even though CTAB shows a significant "salt-in" effect on the gelation of MC, it does not affect the pattern of the sol-gel transition as well as the gel strength of MC. At the highest concentration of CTAB, 0.60 wt %, MC is still able to form a gel. At a given ratio of CTAB/MC, the effect of CTAB on MC becomes stronger when the MC concentration is lower. The results for the MC-CTAB system are compared with an ionic surfactant, SDS and the significant differences in affecting the gelation of MC between two surfactants are recognized.     

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.     

The role of the carboxylate head group in the interaction of sodium dodecanoate with poly(ethylene oxide) investigated by electrical conductivity, viscosity, and aggregation number measurements

The binding of sodium dodecanoate (SDoD) to poly(ethylene oxide) (PEO) in aqueous solution was investigated and compared with the well-known polymer-surfactant complexes formed between PEO and sodium dodecyl sulfate (SDS). Electrical conductivity measurements indicated that the concentration ratio of bound SDoD to PEO (on monomer basis) was greater than that for the system PEO-SDS. However, the aggregation numbers of the micelles supported on the polymer chain are practically constant and similar for both surfactants at concentrations lower than the polymer saturation point. The difference in binding capability is explained in terms of a larger PEO coil expansion upon complexation of SDoD than in the case of SDS. An increase in the polymer surface favors the binding of SDoD to PEO in aqueous solution. This conclusion is supported by the results of the viscometric studies of PEO-surfactant solution.     

The influence of surfactants on latex flocculation with poly(diallyldimethyl ammonium chloride)

Aqueous latex was flocculated by mixtures of poly(diallyldimethyl ammonium chloride), PDADMAC, and anionic surfactants. Sodium dodecyl sulfate, (SDS), and Aerosol OT influenced flocculation whereas nonionic Tergitol NP-10 did not. The flocculation domains were correlated with properties of the polymer-surfactant complexFlocculation was never observed above the CMC of the corresponding surfactant solution without polymer or latex. At SDS concentrations greater than 10^–3.6 M the flocculation boundary corresponded to the first appearance of insoluble polymer-surfactant complex which was characterized by dynamic light scattering and microelectrophoresis. Under these conditions latex (diameter 570 nm) and dispersed polymer-surfactant complex particles (diameters between 30 and 2 000 nm) displayed simultaneous homo and heteroflocculation. The boundaries of the flocculation domains at low surfactant concentration were determined by the ratio of polymer to latex and by the net electrostatic charge of the soluble polymer-surfactant complex. On the other hand, the mechanisms controlling flocculation boundaries in the dispersed polymer-surfactant domain require further clarification.     

Plasma membrane properties involved in the photodynamic efficacy of merocyanine 540 and tetrasulfonated aluminum phthalocyanine

Merocyanine 540 (MC540)-mediated photodynamic damage to erythrocytes was strongly reduced when illumination was performed at pH 8.5 as compared to pH 7.4. This could be explained by high pH-mediated hyperpolarization of the erythrocyte membrane, resulting in decreased MC540 binding at pH 8.5. In accordance, the MC540-mediated photooxidation of open ghosts was not inhibited at pH 8.5. Photoinactivation of vesicular stomatitis virus (VSV) was not inhibited at pH 8.5. This suggests that illumination at increased pH could be an approach to protect red blood cells selectively against MC540-mediated virucidal phototreatment. With tetrasulfonated aluminum phthalocyanine (AIPcS4) as photosensitizer, damage to erythrocytes, open ghosts and VSV was decreased when illuminated at pH 8.5. A decreased singlet oxygen yield at high pH could be excluded. The AIPcS4-mediated photooxidation of fixed erythrocytes was strongly dependent on the cation concentration in the buffer, indicating that the surface potential may affect the efficacy of this photosensitizer. This study showed that altering the environment of the target could increase both the efficacy and the specificity of a photodynamic treatment.     

Supramolecular complex induced by the binding of sodium dodecyl sulfate to PAMAM dendrimers

Isothermal titration calorimetry (ITC) and dynamic light scattering (DLS) were employed to study the spontaneous supramolecular complexation of amine terminated PAMAM dendrimer (G3[EDA] PAMAM-NH2) induced by the binding of an anionic surfactant, sodium dodecyl sulfate (SDS). At pHor=10, the electrostatic binding ceased because the deprotonated PAMAM dendrimer was uncharged, and hence the surfactant-induced supramolecular assembly could not be formed.     

The solution behavior of poly(vinylpyrrolidone): its clouding in salt solution, solvation by water and isopropanol, and interaction with sodium dodecyl sulfate

This article deals with the solution properties of poly(vinylpyrrolidone) (PVP) in salt and surfactant environment. The cloud point (CP) of PVP has been found to be induced by the salts NaCl, KCl, KBr, Na2SO4, MgSO4, and Na3PO4. On the basis of CP values for a salt at different [PVP], the energetics of the clouding process have been estimated. The effect of the surfactant, sodium dodecyl sulfate (SDS), on the salt-induced CP has also been studied, and reduction in CP at low [SDS] and increase in CP at high [SDS] have been observed. The water vapor adsorption of PVP has been determined by isopiestic method. The results display a BET Type III isotherm whose analysis has helped to obtain the monolayer capacity of PVP and formation of multilayer on it. The solvation of PVP in a solution of water and a water-isopropanol mixture has been determined by conductometry from which contribution of the individual components were estimated. The interaction of PVP with SDS in solution led to formation of a complex entity, which has been studied also by conductometry adopting a binding-equilibrium scheme. SDS has been found to undergo two types of binding as monomers in the pre- critical aggregation concentration (CAC) range and as small clusters in the post CAC region. The stoichiometries of binding and binding constant were evaluated.     

Chlorpromazine and sodium dodecyl sulfate mixed micelles investigated by small angle X-ray scattering

Small-angle X-ray scattering (SAXS) studies are reported on the interaction of chlorpromazine (CPZ) with micelles of anionic surfactant sodium dodecyl sulfate (SDS). Isotropic solutions of SDS (40 and 100 mM) at pH 4.0, 7.0, and 9.0 in the absence and presence of CPZ (2-25 mM) were investigated at the National Laboratory of Synchrotron Light (LNLS, Campinas, Brazil). The data were analyzed through the modeling of the micellar form factor and interference function. The results evidence a micellar shape transformation from prolate ellipsoid to cylinder accompanied by micellar growth and surface charge screening as the molar ratio CPZ : SDS increases in the complex. Small ellipsoids with axial ratio nu=1.5+/-0.1 at 40 mM SDS grow and reassemble into cylinder-like aggregates upon 5 mM drug incorporation (1 CPZ : 8 SDS monomers) with a decrease of the micelle surface charge. At 10 mM CPZ : 40 mM SDS cylindrical micelles are totally screened with an axial ratio nu approximately 2.5. The data also indicate the presence of small prolate ellipsoids (nu=1.7+/-0.1) in solutions of 100 mM SDS (no drug) and micellar growth (nu approximately 2.0 and 4.0) when 10 and 25 mM CPZ are added to the system. In the latter case, the aggregate is also better represented by a cylinder-like form. Therefore, our results demonstrate that the axial ratio and shape evolution of the surfactant : phenothiazine complex are both SDS concentration and drug : SDS molar ratio dependent. The drug location close to the SDS polar headgroup region without disrupting in a significant way both the paraffinic hydrophobic core and the polar shell thickness is inferred. SAXS data made it possible to obtain the shapes and dimensions of CPZ/SDS aggregates.     

Novel fluorescent probe as aggregation predictor and micro-polarity reporter for micelles and mixed micelles

Aggregational behaviour of micelles sodium dodecyl sulphate (SDS and Triton X-100, TX-100 both in pure and mixed form) and micelle like aggregates such as polymer-surfactant system [polymer poly(vinyl pyrrolidone), PVP]-SDS have been studied by using fluorescence characteristics of a newly synthesized probe. The critical micelle concentration (CMC) values determined at various surfactant compositions are lower than the ideal values indicating a synergistic effect. The value of the interaction parameter for the surfactant mixture has been determined which agrees well with the value calculated according to molecular thermodynamic theory. The total aggregation number of surfactant in mixed micelle shows a drastic variation in the SDS mole fraction range 0 < or = alpha1 < or = 0.3 and beyond the range it remains practically constant. Molar-based partition coefficients for the dye between the micellar and aqueous phase have been determined and a non-linear variation is obtained for the mixed micellar system. Variations of micro-polarity in the mixed micellar region have been investigated as a function of surfactant composition and results have been explained in terms of a suitable realistic model.     

Indirect fluorescence of aliphatic carboxylic acids in nonaqueous capillary electrophoresis using merocyanine 540

A method for the analysis of aliphatic carboxylic acids (ACAs) in nonaqueous capillary electrophoresis (NACE) in conjunction with indirect laser-induced fluorescence (ILIF) using merocyanine 540 (MC 540) is described. Performing the analysis in organic solvent is advantageous when using MC 540, because of its greater quantum yield in aprotic solvent. To achieve a high dynamic reserve (DR) and optimize resolution, we have tested a number of aqueous mixtures containing alcohols and acetonitrile (ACN). The optimum buffer for the analysis of C2-C18 ACAs, in terms of sensitivity, resolution, and speed, is an aqueous mixture of 40% ACN, 30% ethanol, and 1 mM Tris at apparent pH 7.4 (adjusted with ascorbic acid). Under this condition, the DR is greater than 1000, thereby the limits of detection for acids are in the range of sub-microM to microM. Linear plots show that the dynamic ranges for the analysis of ACAs are at least two decades in concentration, with regression coefficients all greater than 0.98. The relative standard deviations of the migration times and peak heights for all ACAs are less than 2.0%. Furthermore, this simple and cost-effective method has been applied to the analysis of marine lipid concentrate, with the concentrations of 1.67+/-0.03 and 4.50+/-0.05 mM (n = 5) for C14 and C16 acids, respectively, in a tablet of marine lipid concentrate sample.     

Titration microcalorimetry of poly(vinylpyrrolidone) and sodium dodecylbenzenesulphonate in aqueous solutions

Microcalorimetric titrations are carried out on solutions containing the anionic surfactant sodium dodecylbenzenesulphonate (SDBS), and mixtures of SDBS and the uncharged polymer poly(vinylpyrrolidone) (PVP). Measurements are taken at different temperatures. Micellisation of SDBS is driven by hydrophobic bonding. The interaction enthalpy of mixed PVP/SDBS systems shows clearly a consecutive endothermic and exothermic region with increasing surfactant concentration. The endothermic part can be looked upon as an incremental binding isotherm and reflects the number of surfactant molecules involved in the association process. The exothermic region features inverse hydrophobic bonding behaviour. This is related to the flexible nature of the adsorbent, i.e. the polymer. Electrostatic repulsion between neighbouring surfactant molecules causes at increased surfactant concentrations structural rearrangements of the polymer-surfactant complexes. This is accompanied by losing inter- and intrachain linking and entropy gain since the expanded complexes can move more freely. Additional surfactants continue to adsorb on the vacant hydrophobic adsorption sites. The influence of the initial amount of polymer and the electrolyte concentration support our proposals.