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.     

Thermodynamic characterization of the interaction behavior of a hydrophobically modified polyelectrolyte and oppositely charged surfactants in aqueous solution: effect of surfactant alkyl chain length

We have used a precision isothermal titration microcalorimeter (ITC) to measure the enthalpy curves for the interaction of a hydrophobically modified polyelectrolyte (D40OCT30) with oppositely charged surfactants (SC(n)S) in aqueous solution. D40OCT30 is a newly synthesized polymer based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-octylammonium chloride groups randomly distributed along the polymer backbone with degree of substitution of 28.1%. The employed anionic surfactants are sodium octyl sulfate (SC(8)S) and sodium tetradecyl sulfate (SC(14)S). Microcalorimetric results along with turbidity and kinematic viscosity measurements demonstrate systematically the thermodynamic characterization of the interaction of D40OCT30/SC(n)S. A three-dimensional diagram with the derived phase boundaries is drawn to describe the effect of the alkyl chain length of surfactant and of the ratio between surfactant and pendant groups on the interaction. A more complete picture of the interaction mechanism for D40OCT30/SC(n)S systems is proposed here.     

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.     

Thermodynamics of micellization for partially fluorinated cationic gemini surfactants and related single-chain surfactants in aqueous solution

A series of partially fluorinated cationic gemini surfactants and their corresponding monomeric surfactants have been studied by isothermal titration microcalorimetry. The critical micelle concentration (CMC) and enthalpy of micellization (DeltaH(mic)) were obtained from calorimetric curves. The CMCs of the gemini surfactants are much lower than those of the corresponding monomeric surfactants and decrease with an increase in the number of fluorine atoms on the hydrophobic chain. The micellization of partially fluorinated cationic gemini surfactants is much more exothermic than that of the corresponding monomeric surfactants. Because of the incompatibility of hydrocarbon spacer and partially fluorinated chain, DeltaH(mic) values of the surfactants with a C6 spacer are more negative than those of the surfactants with a C12 spacer. The variations in the architecture of the fluorocarbon chain segments may be the reason of the irregularities in the change of DeltaH(mic) for the gemini surfactants. Moreover, the contribution of the enthalpy generally increases with an increase in the number of fluorine atoms.     

Interaction between oppositely charged polyelectrolytes in aqueous solution

Oppositely charged polyelectrolytes interact in solution, forming polyelectrolyte complexes, which often appear as gel-like precipitates. This kind of complex formation was studied by means of calorimetric and rheological measurements. The enthalpy effects, though being fairly small, give some information about the binding strength of counterions to the macroion. We studied the system poly(p-styrene sulfonate)/poly(trimethylammonium-2-ethyl methacrylate) (PSS-PTMA), varying systematically the low molar mass counterions of PSS. In every case, the maximum of enthalpy was found around a 1:1 (mol:mol monomer units) composition of the complexes, with the shape of enthalpy versus composition-curve indicating a stoichiometric interaction. The maximum enthalpy decreased with increasing atomic mass of the counterion when the alkaline metal salts of PSS were used and no change was made on the side of the cationic polyelectrolyte. The salts of the alkaline earth metals gave a distinctly higher enthalpy. On the contrary, viscosity measurements showed a very broad minimum as a function of composition, indicating that the formation of non-stoichiometric complexes is also occurring. The conclusion of these observations is that the complex formation is stoichiometric with respect to the monomeric units, but not necessarily stoichiometric with respect to the entire macromolecules.     

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.     

Phase separation in aqueous mixtures of hydrophobically modified cellulose derivatives with their nonmodified analogues

Mixtures of hydrophobically graft-modified cellulose derivatives and their nonmodified analogues have been studied in aqueous solution. A qualitatively similar behavior was found in the phase behavior of nonionic as well as of cationic polymer systems. Over a large range of total polymer concentrations and mixture ratios the solutions phase separated into two phases of similar polymer concentration, with one of the phases enriched in the hydrophobically modified polymer. From the manufacturing process the cellulose derivatives investigated are likely to contain polymer chains with a rather continuous distribution in degrees of substitution and, possibly, substitution patterns. This causes a complex phase behavior that cannot be adequately described by a ternary representation. The multicomponent nature became apparent from composition analyses of the phases in equilibrium. It may thus be more appropriate to view the phase separation as a fractionation. A phase of small relative volume with a highly enhanced hydrophobe content (compared to the original hydrophobically modified polymer sample) was created. This was particularly obvious in more dilute solutions. Sometimes the phase separation was difficult to observe because the phases in equilibrium had similar polymer concentrations and, therefore, similar refractive indices. The observations presented here call for the attention of producers and users of these types of polymers. Received: 6 July 2000 Accepted: 6 September 2000     

Coadsorption and surface forces for selective surfaces in contact with aqueous mixtures of oppositely charged surfactants and low charge density polyelectrolytes

The coadsorption of a positively charged polyelectrolyte (with 10% of the segments carrying a permanent positive charge, AM-MAPTAC-10) and an anionic surfactant (sodium dodecyl sulfate, SDS) on silica and glass surfaces has been investigated using optical reflectometry and a noninterferometric surface force technique. This is a selective coadsorption system in the sense that the polyelectrolyte does adsorb to the surface in the absence of surfactant, whereas the surfactant does not adsorb in the absence ofpolyelectrolyte. It is found that the total adsorbed amount goes through a maximum when the SDS concentration is increased. Maximum adsorption is found when the polyelectrolyte/surfactant complexes formed in bulk solution are close to the charge neutralization point. Some adsorption does occur also when SDS is present in significant excess. The force measured between AM-MAPTAC-10-coated surfaces on approach in the absence of SDS is dominated at long range by an electrostatic double-layer force. Yet, layers formed by coadsorption from solutions containing both polyelectrolyte and surfactant generate long-range forces of an electrosteric nature. On separation, adhesive interactions are found only when the adsorbed amount is low, i.e., in the absence of SDS and in a large excess of SDS. The final state of the adsorbed layer is found to be nonhysteretic, i.e., independent of the history of the system. The conditions for formation of long-lived trapped adsorption states from mixed polymer-surfactant solutions are discussed.     

Adsorption of hydrophobically modified anionic starch at oppositely charged oil/water interfaces

In this paper we study the adsorption at cationic emulsion droplets of starch which had been hydrophobically modified with octenyl succinic anhydride (OSA), a modification which also renders the starch anionic. Emulsions were formed with didodecyldimethylammonium bromide (DDAB) after which the OSA-starch was added. The emulsions were separated by centrifugation and the surface load of OSA-starch was determined through serum depletion. The results show the adsorbed amounts can become very high, in some cases reaching approximately 40 mg/m2. The surface load correlates positively with the surface charge density of the starch which depends on the degree of substitution, rms radius and molar mass. Furthermore, the surface load obtained depends on the ratio between polymer surface charge density and the interface charge density which could be varied experimentally by combining various amounts of DDAB and dioleoyl phosphatidylcholine (DOPC) in the formation of the emulsion. The very high surface loads should correspond to very thick adsorbed layers. Thus, OSA-starch should be appropriate for encapsulation applications provided a suitable adhesion substance is employed.     

Interactions of polyelectrolyte brushes with oppositely charged surfactants

Using Brownian dynamics simulations, we study the effect of the charge ratio, the surfactant length, and the grafting density on the conformational behavior of the complex formed by the polyelectrolyte brush with oppositely charged surfactants. In our simulations, the polyelectrolyte chains and surfactants are represented by a coarse-grained bead-spring model, and the solvent is treated implicitly. It is found that varying the charge ratio induces different morphologies of surfactant aggregates adsorbed onto the brush. At high charge ratios, the density profiles of surfactant monomers indicate that surfactant aggregates exhibit a layer-by-layer arrangement. The surfactant length has a strong effect on the adsorption behavior of surfactants. The lengthening of surfactant leads to a collapsed brush configuration, but a reswelling of the brush with further increasing the surfactant length is observed. The collapse of the brush is attributed to the enhancement of surfactants binding to polyelectrolyte chains. The reswelling is due to an increase in the volume of adsorbed surfactant aggregates. At the largest grafting density investigated, enhanced excluded volume interactions limit the adsorption of surfactant within the polyelectrolyte brush. We also find that end monomers in polyelectrolyte chains exhibit a bimodal distribution in cases of large surfactant lengths and high charge ratios.     

Thermodynamics of micellization of alkylimidazolium surfactants in aqueous solution

Alkylimidazolium salts are a very important class of compounds. So far, calorimetry has hardly been used to characterize their solution behaviour. The enthalpies obtained from indirect methods have an intrinsic large uncertainty, and nowadays it is clear that calorimetry is the most sensitive technique for directly measuring the thermodynamic properties of aggregation.In this work, isothermal titration calorimetry (ITC) was used along with conductivity to determine the thermodynamics of aggregation of 1-alkyl-3-methylimidazolium chlorides ([C_nmim]Cl, n = 8, 10, 12, and 14) in aqueous solution. The critical micelle concentrations, cmc, were obtained from conductivity and calorimetry, and the enthalpies of micelle formation, ΔH_mic, were derived from the calorimetric titrations. From conductivity, we could also derive the values for the degree of ionisation of the micelles (α), the molar conductivity (Λ^M) of the [C_nmim]Cl micellar species and the molar conductivity at infinite dilution (Λ^∞) for the [C_nmim]⁺ cations.Values are therefore reported for the enthalpy (ΔH_mic), the Gibbs free energy (ΔG_mic) and entropy (ΔS_mic) changes for micelle formation. Further, the aggregate sizes and aggregation numbers were obtained by light scattering (LS) measurements.The observed variation of the thermodynamic properties with the alkyl chain length is discussed in detail and compared with the traditional cationic surfactants 1-alkyl-trimethylammonium chlorides, [C_nTA]Cl. The difference in the values of the thermodynamic parameters for both types of surfactants is here related to the structure of their head groups.     

Gel formation in systems composed of drug containing catanionic vesicles and oppositely charged hydrophobically modified polymer

The aim of this study was to explore if mixtures of drug containing catanionic vesicles and polymers give rise to gel formation, and if so, if drug release from these gels could be prolonged. Catanionic vesicles formed from the drug substances alprenolol or tetracaine, and the oppositely charged surfactant sodium dodecyl sulphate were mixed with polymers. Three polymers with different properties were employed: one bearing hydrophobic modifications, one positively charged and one positively charged polymer bearing hydrophobic modifications. The structure of the vesicles before and after addition of polymer was investigated by using cryo-TEM. Gel formation was confirmed by using rheological measurements. Drug release was studied using a modified USP paddle method. Gels were observed to form only in the case when catanionic vesicles, most likely with a net negative charge, were mixed with positively charged polymer bearing lipophilic modifications. The release of drug substance from these systems, where the vesicles are not trapped within the gel but constitute a founding part of it, could be significantly prolonged. The drug release rate was found to depend on vesicle concentration to a higher extent than on polymer concentration.     

Aggregation of hydrophobically modified polysaccharides in solution and at the air-water interface

The present article focuses on the comparative study of physicochemical properties of two ionic pullulan derivatives modified by 10 or 35 C8 chains per 100 anhydroglucose units, named CMP10C8 and CMP35C8, respectively. In aqueous solutions, these derivatives exhibited an associative behavior as evidenced by pyrene fluorescence spectroscopy. This phenomenon, which stems from intra- and/or intermolecular interactions between the hydrophobic groups grafted on the polymer backbone, results in the formation of more or less condensed aggregates depending on the C8 ratio and the ionic strength of the media. The hydrophobically modified pullulans also displayed surface properties. Their adsorption at the air-solution interface was assessed from surface tension measurements. The results showed that both hydrophobized polymers adsorb in a coil conformation occupying a large interfacial molecular area. The comparison of these molecular areas indicated that CMP35C8 adopts a more shrunken conformation at the interface than CMP10C8, due to stronger intramolecular interactions. The stability of the adsorbed monolayer under bulk dilution was investigated by ellipsometric measurements. Whereas bulk dilution had no effect on the stability of the adsorbed CMP35C8 film, it provoked significant changes in the adsorbed CMP10C8 monolayer. The stability of the CMP35C8 monolayers was attributed to the existence of intermolecular associations between the adsorbed coils.     

Viscosity of aqueous solutions of polysaccharides and hydrophobically modified polysaccharides: Application of Fedors equation

Amphiphilic polysaccharides have been obtained by hydrophobic modification of a neutral bacterial polysaccharide, dextran. Various amounts and types of aliphatic hydrocarbon groups have been attached to dextran.The solution behaviour of unmodified dextran samples and amphiphilic dextran derivatives is characterized by viscometric measurements. The overall viscosity behaviour of unmodified polysaccharides is described up to C × [η] = 3, using the equation of Fedors [Fedors RF. Polymer 1979;20:225] which involves only a concentration parameter. The latter is shown to depend on the hydrodynamic volume of the macromolecules in solution.The equation of Fedors is shown to conveniently estimate the viscosity behaviour of amphiphilic dextran derivatives up to C × [η] = 1. The interdependence between Fedors parameter and other viscometric characteristics (intrinsic viscosity, Huggins coefficient) is evidenced. These results are extended to the data of other authors.     

Phase behavior in mixtures of cationic and anionic surfactants in aqueous solutions

Phase behavior of cationic/anionic surfactant mixtures of the same chain length (n=10, 12 or 14) strongly depends on the molar ratio and actual concentration of the surfactants. Precipitation of catanionic surfactant and mixed micelles formation are observed over the concentration range investigated. Coacervate and liquid crystals are found to coexist in the transition region from crystalline catanionic surfactant to mixed micelles.The addition of oppositely charged surfactant diminishes the surface charge density at the mixed micelle/solution interface and enhances the apparent degree of counterion dissociation from mixed micelles. Cationic surfactants have a greater tendency to be incorporated in mixed micelles than anionic ones.     

Characterization of complexation and phase behavior of mixed systems of unmodified and hydrophobically modified oppositely charged polyelectrolytes

This paper reports turbidity, rheology, zeta potential, and rheo-small angle light scattering measurements on aqueous mixtures of oppositely charged and hydrophobically modified hydroxyethylcellulose derivatives (HM-HEC(?) and HM-HEC(+)) and mixtures of oppositely charged hydroxyethylcellulose (HEC(?) and HEC(+)). The experiments were restricted to the one-phase region, i.e., at mixing ratios before and after the two-phase area. The associative phase separation behavior usually observed when mixing oppositely charged polyelectrolytes was undetectable in the mixtures of the polyelectrolytes without attached hydrophobic groups. Upon modification of HEC by incorporation of pendant hydrophobic groups and by introducing charges of negative or positive sign (HM-HEC(?) and HM-HEC(+)), the mixtures showed phase separation over a certain mixing interval, revealing the existence of large polyelectrolyte complexes. The zero shear viscosity was strongly dependent on both the hydrophobicity of the polymers and the mixing ratio, increasing significantly with hydrophobic modification of polyelectrolytes. The strong enhancement of the turbidity and the viscosity drop as the two-phase area is approached suggest the formation of fragmented non-connected complexes. This work demonstrates that if the oppositely charged polyions have a hydrophilic character, it is not necessary that the attractive Coulombic forces induce insoluble polyelectrolyte complexes.     

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.     

Three-phase separation in nonionic surfactant/hydrophobically modified polymer aqueous mixtures

Three-phase separation for Triton X-114 or Triton X-100 solutions with addition of hydrophobically modified hydroxyethyl cellulose was investigated experimentally. When the surfactant concentration was high enough, the solution slightly above the cloud point could separate into three macroscopic phases: a cloudy phase in between a clear phase and a bluish, translucent phase. The rate of phase separation was very low with the formation of the clear and cloudy phases followed by the emergence of the bluish phase. The volume fraction of the cloudy phase increases linearly with the global polymer concentration, whereas the volume fraction of the bluish phase increases linearly with the global surfactant concentration. Composition analyses found that most of the polymer stayed in the cloudy phase, as opposed to most of the surfactant in the bluish phase. The interesting phase behavior can be explained by an initial associative phase separation followed by a segregative phase separation in the cloudy phase.     

Supramolecular systems based on cationic surfactants: an influence of hydrotropic salts and oppositely charged polyelectrolytes

Russian Chemical Bulletin - For cationic surfactant 4-aza-1-dodecyl-1-azoniabicyclo[2.2.2]octane bromide (DABCO-16), a significant decrease in the critical micelle concentration (CMC) is shown in...