Binding of surfactants to hydrophobically modified polymers

Recent progress in the understanding of the binding of surfactants to hydrophobically modified polymers (HMP), and the consequences of such binding, is reviewed. HMP are water-soluble polymers onto which low proportions of hydrophobic sidechains (hydrophobes) have been grafted. In an aqueous environment, the HMP hydrophobes associate among themselves and with added surfactant molecules into micelle-like aggregates. An HMP may therefore be considered as a ‘modified surfactant’, and the binding of surfactants to HMP is analogous to the mixed micellisation in mixed surfactant solutions. The binding isotherm gives the concentration of free (monomeric) surfactant and the stoichiometry of the HMP/surfactant complex at different total compositions. In mixtures involving ionic surfactants, it is found that the free surfactant often dominates, and gives important contributions to the ionic strength. Characteristic properties of HMP/surfactant mixtures may be related to stoichiometries of the mixed complexes. Thus, the maximum in solution viscosity, which is commonly found in HMP/surfactant mixtures, occurs at a similar hydrophobe stoichiometry (ratio of bound surfactant to HMP hydrophobe) for many different systems, although the total concentrations of surfactant at the maximum may vary by orders of magnitude, depending on the surfactant cmc. The solubility of a complex of oppositely charged HMP and surfactant is related to the charge stoichiometry of the complex. The phase separation/redissolution phenomena occurring in the bulk solution influence the HMP adsorption to surfaces and the forces between surfaces with adsorbed HMP.     

Interactions of hydrophobically modified polyelectrolytes with nonionic surfactants

Interactions of surfactants with hydrophobically modified polyelectrolytes in aqueous solutions are important in several applications such as detergency, cosmetics, food, and paints. Complexes formed in these systems raise some fundamental questions about the polymer-surfactant interactions that control their behavior. In this work, the interactions of a nonionic surfactant, penta-ethyleneglycol mono n-dodecyl ether (C(12)EO(5)), with a hydrophobically modified anionic polymer, poly(maleic acid/octyl vinyl ether) (PMAOVE), in aqueous solutions were studied using surface tension, viscosity, electron paramagnetic resonance (EPR) spectroscopy, light scattering, and fluorescence spectroscopic techniques. When the nonionic surfactant C(12)EO(5) was added to aqueous solutions of the anionic polymer PMAOVE, it was incorporated into the hydrophobic nanodomains of PMAOVE far below the the critical micelle concentration (cmc) of the surfactant. Two inflection points were observed corresponding to the critical complexation concentration (formation of mixed micelles composed of C(12)EO(5) and the octyl chains of PMAOVE) and the saturation concentration (saturation of the polymer with C(12)EO(5) molecules). Above the saturation concentration, the coexistence of pure C(12)EO(5) micelles and mixed micelles of PMAOVE and C(12)EO(5) was observed. Such a coexistence of complexes has major implications in their performance in colloidal processes.     

Rheological properties of solutions and gels of combined systems hydrophobically modified polyacrylamides-new viscoelastic cationic surfactants

The rheological properties (viscosity and dynamic elasticity modulus) of solutions and gels of combined systems composed of hydrophobically modified polymers containing main-chain charged groups or lacking these groups and of newly synthesized viscoelastic cationic surfactants with different amounts of OH groups and long nonpolar saturated and trans monounsaturated radicals have been studied. It has been shown that the maximum viscosity of solutions of polymer-surfactant complexes corresponds to the very low concentration of the surfactant. Regions where homogeneous and heterogeneous combined solutions and gels exist are considered. An increase in the viscosity of the combined solutions with temperature is discovered and discussed. The synergistic effect of polymers and surfactants is demonstrated, and a more efficient role of surfactants in the case of uncharged polymers is revealed. The roles of the number of OH groups and the length of the nonpolar radical in a surfactant molecule and a difference in the properties of systems containing surfactants with saturated and unsaturated radicals are considered.     

Interactions of a hydrophobically modified polymer with oppositely charged surfactants

The interaction of a hydrophobically modified anionic polymer (PMAOVE) with a cationic surfactant (DTAB) was studied using a multi-technique approach: turbidity, surface tension, and viscosity measurements, as well as EPR (5-doxyl stearic acid) and fluorescence (pyrene) probe techniques were used. In the investigated pH range (4-10), the cationic surfactant headgroups interact with the anionic carboxylic groups of the polymer backbone. In addition, nonpolar interactions of the surfactant chains with the n-octyl chains of PMAOVE stabilize the PMAOVE-DTAB complexes. Charge neutralization of the anionic polymer by the cationic surfactant leads to precipitation of the PMAOVE-DTAB complex at a certain DTAB concentration range. Further addition of DTAB causes a charge reversal of the complex and, subsequently, resolubilization of the precipitate. At an acidic pH (pH = 4), a second precipitation was observed, which is probably caused by conformational changes in the PMAOVE-DTAB complex. This second precipitate can be resolubilized by further addition of surfactant. At a neutral and basic pH, this second precipitation is absent. EPR analysis indicates that the surfactants form an ordered structure at the extended polymer chain at a neutral and basic pH, whereas at an acidic pH, a less ordered surfactant layer is formed on the coiled polymer with more hydrophobic microdomains.     

pH-dependence of the properties of hydrophobically modified polyvinylamine

A series of N-alkyl or N-benzyl substituted polyvinylamines (PVAm) were prepared and the properties of aqueous solutions were measured as functions of pH. The polymer solutions showed almost no surface activity under acidic conditions whereas surface tension was reduced to 40-50 mN/m around pH 9. Increasing either the degree of hydrophobic substitution or the hydrophobic chain length lowered the pH at which surface tension lowering was observed. Hydrophobic substitution also shifted plots of the degree of ionization versus pH toward lower pH which means lower pH values were required to achieve a given value of polymer charging. The hydrophobically modified PVAm associated in water giving species whose apparent diameter measured by dynamic light scattering decreased with increasing pH, whereas the electrophoretic mobilities of the associated species increased with decreasing pH. Although many hydrophobically modified and pH sensitive polymers have been described in the literature for applications in biomaterials, drug release and as pH sensitive surfactants, the hydrophobically modified PVAms are particularly attractive because they are easily prepared from commercially available polyvinylamines.     

Clouding and phase behavior of nonionic surfactants in hydrophobically modified hydroxyethyl cellulose solutions

Clouding phenomena and phase behaviors of two nonionic surfactants, Triton X-114 and Triton X-100, in the presence of either hydroxyethyl cellulose (HEC) or its hydrophobically modified counterpart (HMHEC) were experimentally studied. Compared with HEC, HMHEC was found to have a stronger effect on lowering the cloud point temperature of a nonionic surfactant at low concentrations. The difference in clouding behavior can be attributed to different kinds of molecular interactions. Depletion flocculation is the underlying mechanism in the case of HEC, while the chain-bridging effect is responsible for the large decrease of cloud point for HMHEC. Composition analyses for the formed macroscopic phases were carried out to provide support for associative phase separation for the case of HMHEC, in contrast to segregative phase separation for HEC. An interesting three-phase-separation phenomenon was reported in some HMHEC/Triton X-100 mixtures at high surfactant concentrations.     

Rheological properties of the aqueous solution for fluorocarbon‐containing hydrophobically modified sodium polyacrylic acid with various surfactants

The interaction of fluorocarbon‐ containing hydrophobically modified sodium polyacrylic acid (FMPAANa) (0.5 wt%) with various surfactants (anionic, nonionic and cationic) has been investigated by rheological measurements. Different rheological behaviors are displayed for ionic surfactants and nonionic surfactants. Fluorinated surfactants have stronger affinity with polyelectrolyte hydrophobes comparing with hydrogenated surfactants. The hydrophobic association of FMPAANa with a cationic surfactant (CTAB) and a fluorinated nonionic surfactant (FC171) is much stronger than with a nonionic surfactant (NP7. 5) and an anionic surfactant (FC143). Further investigation of the effects of temperature on solution properties shows that the dissociation energy E_m is correlated to the strength of the aggregated junctions.     

Surface activity and emulsification properties of hydrophobically modified dextrans

Neutral polymeric surfactants were synthesized by covalent attachment of hydrophobic groups (aromatic rings) onto a polysaccharide backbone (dextran). By changing the conditions of the modification reaction, the number of grafted hydrophobic groups per 100 glucopyranose units (substitution ratio) was varied between 7 and 22. In aqueous solution, these polymers behaved like classical associative polymers as demonstrated by viscometric measurements. The associative behavior was more pronounced when the substitution ratio increased. The surface-active properties of the modified dextrans were evidenced by surface tension (air/water) and interfacial tension (dodecane/water) measurements. In each case the surface or interfacial tension leveled down above a critical polymer concentration, which was attributed to the formation of a dense polymer layer at the liquid-air or liquid-liquid interface. Dodecane-in-water emulsions were prepared using the polymeric surfactants as stabilizers, with oil volume fractions ranging between 5 and 20%. The oil droplet size (measured by dynamic light scattering) was correlated to the amount of polymer in the aqueous phase and to the volume of emulsified oil. The thickness of the adsorbed polymer layer was estimated thanks to zeta potential measurements coupled with size measurements. This thickness increased with the amount of polymer available for adsorption at the interface. The dextran-based surfactants were also applied to emulsion polymerization of styrene and stable polystyrene particles were obtained with a permanent adsorbed dextran layer at their surface. The comparison with the use of an unmodified dextran indicated that the polymeric surfactants were densely packed at the surface of the particles. The colloidal stability of the suspensions of polystyrene particles as well as their protection against protein adsorption (bovine serum albumin, BSA, used as a test protein) were also examined.     

Microcalorimetric evidence of hydrophobic interactions between hydrophobically modified cationic polysaccharides and surfactants of the same charge

We synthesized and characterized a series of new polymers-hydrophobically modified cationic polysaccharides-based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-alkylammonium chloride groups randomly distributed along the polymer backbone. These polymers are good candidates for studying the hydrophobic effect on polymer/surfactant association. In previous papers we reported their interactions with oppositely charged surfactants. For further insight into the relative importance of the hydrophobic interaction in the association process now we studied the thermodynamics of the interaction of these hydrophobically modified polymers with surfactants of the same charge (DMRX/CnTAC) by isothermal titration calorimetry (ITC). In order to try to discriminate the solution behavior of these polymer/surfactant systems, we analyzed separately the interaction of unmodified dextran with ionic surfactants and the interactions between the corresponding cationic surfactants. The interaction enthalpies for DMRX/CnTAC systems were derived from a proposed thermodynamic model with equations that describe the polymer-surfactant interactions. The thermodynamic parameters for the DMRX/CnTAC aggregation process as well as surfactant micellization in the presence of the polymer were also calculated. From all the results we were able to ascertain the effect on the interactions of changing the alkyl chain length of the polyelectrolyte pendant groups or the surfactant. The importance of the polymer aggregation state on the mechanism of interaction was also addressed.     

Structural and rheological properties of hydrophobically modified polysaccharide associative networks

The phase behavior of hydrophobically modified chitosans (HMCs) in aqueous solution has been investigated using scattering and rheology experiments. We observed four regions on the phase diagram of the associative polymer: (i) a supernatant phase (unimers phase) at low polymer concentration; (ii) a dilute solution of intermolecularly bridged flowerlike micelles at intermediate concentration; (iii) an associative gel phase at high polymer content; and (iv) a phase separation. In the present paper, we discuss the structural and dynamical properties of the HMC associative networks (c > c*) at a fixed hydrophobic degree of substitution of 2% and fixed alkyl side chains (stickers) length C8 (domains iii and iv of the phase diagram). As the polymer concentration is increasing, a connecting network is formed from the percolation of bridges between micellar aggregates. In this regime, small-angle neutron scattering and light scattering measurements show that -50-nm flower aggregates are acting like junction points in the network. The effect of the concentration, the stress, and the shear on the structure of the network is discussed. In particular, we observe bridge-to-loop transitions and then the formation of microgels or a low-connected network under shear. Therefore, our results are compared to recent theoretical models and to the results reported for telechelic systems.     

Local mobility of micelles of new cationic surfactants and their interactions with hydrophobically modified polyacrylamides

The local dynamics and organization of micelles of new long-chain cationic surfactants with saturated hydrocarbon fragments (from C₁₆ to C₂₂) are investigated via the EPR spin-probe technique. The local mobility of spin probes in the hydrocarbon core of a micelle changes insignificantly, while the order parameter noticeably increases with lengthening of the hydrocarbon fragment of the surfactant molecule. The specific features of the interaction of the surfactants with network junctions of the gels formed by two types of hydrophobically modified polyacrylamides??either containing charged groups (sodium acrylate) in the backbone or lacking these groups??are studied. In both cases, the local mobility of network junctions of the gel increases after the introduction of the surfactant (C₁₈). Moreover, for surfactant with a long alkyl group (C₂), the microscopic viscosity of the gel based on the uncharged polymer decreases, although the local mobility of the network junctions increases. Possible causes of the observed specific features are discussed.     

Altering associations in aqueous solutions of a hydrophobically modified alginate in the presence of beta-cyclodextrin monomers

The formation of associative networks in semidilute aqueous solutions of hydrophobically modified alginate (HM-alginate) is dependent on intermolecular hydrophobic interactions. Addition of beta-cyclodextrin (beta-CD) monomers to the system provides decoupling of these associations via inclusion complex formation with the polymer hydrophobic tails. This results in a dramatic decrease in the viscoelastic response of the system and a more extended local structure of the polymer chains, as shown by small-angle neutron scattering (SANS) measurements. The zero-shear viscosity decreases about an order of magnitude when the beta-CD concentration is increased from 0 to 12 mm. The lifetime of the associative network decreases strongly with increasing levels of beta-CD addition. These findings clearly demonstrate that the hydrophobic association effect is efficiently reduced as the amount of beta-CD is increased. In the framework of drug delivery, this effect may be useful to improve the release of therapeutic molecules that can be entrapped in the polymer matrix.     

Wetting properties of aqueous solutions of hydrophobically modified inulin polymeric surfactant

The contact angles of aqueous solutions of a polymeric surfactant namely hydrophobically modified inulin (INUTEC®SP1) were measured on hydrophilic and hydrophobised quartz glass surfaces using the sessile drop technique. These measurements showed a large difference (>10°) between the advancing contact angle θ ₁ (that is measured immediately after placing the drop on the surface) and the constant contact angle θ ₂ (that is measured 30 minutes after placing the drop). In all the results only the contact angle θ ₂ was subsequently measured. θ versus INUTEC®SP1 concentration C _s curves were obtained at various NaCl concentrations both on hydrophilic and hydrophobic glass surfaces. On hydrophilic glass surface the θ versus C _s curves showed a maximum at a concentration range of 10^–6 to 2?×?10^–5 mol dm^-3 INUTEC®SP1. These curves were shifted to lower values as the NaCl concentration was increased. On such hydrophilic surface the INUTEC®SP1 molecule adsorbs with the polyfructose loops and tails oriented towards the surface leaving the alkyl chains in solution. Saturation adsorption with this orientation occurs at 2?×?10^–5 mol dm^-3 INUTEC®SP1. However, the contact angles remain quite small (<18°) indicating the presence of several hydrophilic glass patches uncovered by surfactant molecules. At C _s?>?2?×?10^–5 mol dm^-3 θ decreases with further increase of the INUTEC®SP1 concentration reaching 5° at the Critical Association Concentration (CAC) of the polymer. This indicates the formation of a bilayer of INUTEC®SP1 molecules with the alkyl chains hydrophobically attached to those of the first layer. On a hydrophobic glass surface, adsorption of INUTEC®SP1 occurs by multi-point attachment with the alkyl chains on the surface leaving the hydrophilic polyfructose loops and tails dangling in solution. This results in a gradual decrease of the contact angle with increase in INUTEC®SP1 concentration, reaching a plateau value (>85°) between 2?×?10^–5 and 2?×?10^–4 mol dm^-3. The large contact angles obtained on adsorption of the polymeric surfactant on a hydrophobic surface indicate the presence of several uncovered hydrophobic patches. These results give a reasonable picture of the adsorption and orientation of the INUTEC®SP1 molecules on both hydrophilic and hydrophobic solid surfaces.     

Synthesis and aqueous solution properties of hydrophobically modified anionic acrylamide copolymers

In this investigation, hydrophobically modified polyacrylamide with low amounts of anionic long‐chain alkyl was synthesized by the free radical polymerization in deionized water. This water‐soluble copolymerization method is more convenient compared with the traditional micellar copolymerization methods. The copolymers were characterized using Fourier transform infrared, ¹H NMR, and the molecular weight and polydispersity were determined using gel permeation chromatography. The solution behavior of the copolymers was studied as a function of composition, pH, and added electrolytes. As NaCl was added to solutions of AM/C₁₁AM copolymers or pH was lowered, the shielding or elimination of electrostatic repulsions between carboxylate groups of the C₁₁AM unit lead to coil shrinkage. The steady shear viscosity and dynamic shear viscoelastic properties in semidilute, salt‐free aqueous solutions were conducted to examine the concentration effects on copolymers. In addition, the shear superimposed oscillation technique was used to probe the structural changes of the network under various stresses or shear conditions. We prepared hydrophobically modified polyacrylamide with N‐alkyl groups in the aqueous medium. The advantage of this method is that the production is pure without surfactants. These results suggest that the unique aqueous solution behavior of the copolymers is different from conventional hydrophobically associating acrylamide. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2465–2474, 2008     

Systems based on the hydrophobically modified poly(ethylene imines) and surfactants: Aggregation and catalysis

The aggregation in aqueous systems of hydrophobically modified poly(ethylene imines) and their mixtures with cationic surfactants, as well as the catalytic activity of these compositions in the hydrolysis of O-4-nitrophenyl-O-ethylchloromethylphosphonate, are studied by conductometry, tensiometry, light-scattering, and viscometry methods. Critical association concentration, hydrodynamic radii of polymer and polymer-colloid aggregates formed in such systems, as well as kinetic parameters of hydrolysis are determined.     

Thermodynamics of self-assembling of hydrophobically modified cationic polysaccharides and their mixtures with oppositely charged surfactants in aqueous solution

Microcalorimetric techniques, combined with turbidity measurements, were used to study the thermodynamics of self-assembling of hydrophobically modified cationic polysaccharides and their mixtures with oppositely charged surfactants in aqueous solution. The studied polyelectrolytes were a series of polymers based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-alkylammonium chloride groups randomly distributed along the polymer backbone. The parameters for their micellization process are evaluated from the results of the observed dilution enthalpy curves and compared with those of the related cationic surfactants (DTAC and CTAC). The microcalorimetric results for the mixed systems (polyelectrolytes with oppositely charged surfactants) are used along with turbidity measurements to characterize systematically the thermodynamics of their interaction. The phase behavior is described and the interaction enthalpies are derived from the differences between the observed enthalpy curves with and without polyelectrolyte. Therefore, we discuss in detail the effect of changing the alkyl chain length of polyelectrolyte pendant groups, the molecular weight of the dextran backbone, and the temperature of the measurements on the interactions between polyelectrolyte and surfactant.     

Interactions of cationic gemini surfactants with hydrophobically modified poly(acrylamides) studied by fluorescence and microcalorimetry

Steady-state fluorescence, time-resolved fluorescence quenching, and isothermal titration microcalorimetry have been used to study the interactions of cationic gemini surfactants alkanediyl-alpha,omega-bis(dodecyldimethylammonium bromide) (C(12)C(S)C(12)Br(2), S = 3, 6, and 12) with hydrophobically modified poly(acrylamide) (HMPAM) and unmodified poly(acrylamide) (PAM). Without addition of gemini surfactant, 0.2 wt % HMPAMs except PAM have already self-aggregated into hydrophobic aggregates. Different from single-chain surfactants, C(12)C(S)C(12)Br(2) have stronger interactions with HMPAMs to form surfactant/polymer aggregates, even with PAM. Addition of C(12)C(S)C(12)Br(2) may cause the disruption of HMPAM hydrophobic aggregates and the formation of mixed micelles. It is found that HMPAMs generate lower micropolarity of mixed micelles, larger values of enthalpy of interaction (DeltaH(ps)), and nearly constant values of Gibbs free energy of interaction (DeltaG(ps)). On the other hand, C(12)C(S)C(12)Br(2) with longer spacer brings out slightly lower micropolarity of mixed micelles, owing to the lower electrostatic repulsion between surfactant headgroups. Especially for C(12)C(12)C(12)Br(2), the values of DeltaH(ps) are much more endothermic and the values of DeltaG(ps) are much less negative. The weaker interactions of C(12)C(12)C(12)Br(2) with HMPAMs arise from the marked reduction of attraction between surfactant headgroups and polymer hydrophilic groups induced by its longer spacer.     

Emulsion polymerization of styrene and methyl methacrylate using a hydrophobically modified inulin and comparison with other surfactants

The use of a new class of graft polymer surfactants, based on inulin, in emulsion polymerization of poly(methyl methacrylate) (PMMA) and polystyrene (PS) particles is described. PS and PMMA were synthesized by emulsion polymerization, and stable particles with a high monomer content (50 wt %) were obtained with a very small amount of polymeric surfactant ([surfactant]/[monomer] = 0.0033). The latex dispersions were characterized by dynamic light scattering and by transmission electron microscopy to obtain the average particle size and the polydispersity index, and the stability was determined by turbidimetry measurements and expressed in terms of critical coagulation concentration. The last section gives a comparison of PMMA particles prepared by emulsion polymerization using classical surfactants from different types as emulsifiers with that obtained using the copolymer surfactant. It shows the superiority of INUTEC SP1 as it is the only one that allows stable particles at 20 wt % monomer content, with a smaller ratio [surfactant]/[monomer] = 0.002.     

Influence of hydrophobic groups on thickening and emulsification properties of hydrophobically modified polyacrylamides

Hydrophobically modified polyacrylamides can be used to enhance oil recovery in tertiary oil recovery process because they have good thickening and emulsification properties. Hydrophobically modified polyacrylamides with different hydrophobic groups were synthesized using micellar polymerization. Above CAC, elastic polymer gel is formed by the aggregation of hydrophobic groups. Hydrophobicity of hydrophobic groups plays a substantially important role in properties of HMPAMs solutions. Higher hydrophobicity of hydrophobic groups leads to more intensive intermolecular association and thus helps to enhance the apparent viscosity of HMPAMs solutions and form stronger elastic polymer gel network structures in HMPAMs solutions which can enhance the stability of the O/W crude oil emulsions stabilized by HMPAMs.     

Structural and rheological properties of hydrophobically modified alkali‐soluble emulsion solutions

Scattering and rheological experiments were carried out on hydrophobically modified alkali‐soluble emulsion solutions at pH ～ 7.3 as a function of the polymer concentration. The light scattering experiments revealed the existence of a liquidlike order for concentrations below approximately 0.1 × 10^?2g cm^?3 corresponding approximately to the close packing of microgels particles. Above this concentration, the zero‐shear viscosity rose sharply, whereas the ordering disappeared progressively. The results are discussed within the framework of gelation models of associating polymers. Diffusing wave spectroscopy experiments provided estimates of the high‐frequency modulus. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1985–1994, 2002