Polymer–surfactant interactions: neutron scattering and reflectivity

Interaction of HPC with CTAB and Tween 40 at Water/Air and Water/Soya Oil Interfaces

The bulk and interfacial shear rheological behavior of aqueous solutions of biocompatible polymer HPC has been investigated in the presence of cationic CTAB and nonionic Tween 40 having the same chain length but different head groups. Steady-state bulk experiments depict two distinct regions in the rheogram (Newtonian followed by pseudoplastic). Dynamic experiments suggest that the stability of HPC hydrogels decreases with the increase in surfactant concentration. Interfacial steady shear tests of 2D monolayers of 1 wt % HPC and 1 wt % HPC with varying concentrations of Tween 40/CTAB show a non-Newtonian dilatant behavior at the solution-air interface. However, two distinct dilatant regions separated by a Newtonian region were observed for the same films at the solution-soya oil interface. The strength of films formed at the two interfaces decreases with the increase of surfactant concentration as observed from oscillatory interfacial tests. HPC interacts more strongly with CTAB than Tween 40 both in bulk as well as at the interfaces studied.

Alginic acid as a stabilizer of zirconia suspensions in the presence of cationic surfactants

The influence of hydrocarbon (CTAB), fluorocarbon (S-106-A) and silicone (C-Si) cationic surfactants: on stability, adsorption and electrokinetic properties of the alginic acid (AA)/zirconia (ZrO₂) suspensions was studied. The results obtained from the spectrophotometric measurements indicate on very high effectiveness of the surfactants in stabilization of the studied systems. This is due to the formation of multimolecular complexes between alginic acid and the surfactants. The existence of these complexes was confirmed by the surface tension and the zeta potential measurements. Presented studies also enabled the estimation of the conditions under which the complexes are effectively created and the determination of their character. These findings were also confirmed by the adsorption data. Moreover, the surface charge density measurements proved that the adsorption of AA or the AA/surfactant complexes changes the structure of the electrical double layer. The presented results may find applications in the fields of functionalized materials.

Substantial Enhancement of the Antioxidant Capacity of an α-Linolenic Acid Loaded Microemulsion: Chemical Manipulation of the Oil-Water Interface by Carbon Dots and Its Potential Application

Various active ingredients play a crucial role in providing and supplementing the nutritional requirements of organisms. In this work, we attempted to chemically manipulate the interfacial microstructure of oil-water microemulsions (ME) with carbon dots (CDs), concentrating on substantially enhancing the antioxidant capacity of α-linolenic acid (ALA). To this end, CDs were synthesized and introduced into an ME. The molecular interaction of surfactant with CDs was investigated by Fourier-transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). The microstructure of the ME was monitored by transmission electron microscopy (TEM) and cryo-electron microscopy (cryo-EM). The cryo-EM result showed the oil-water interface in the ME was better defined after the CDs were loaded, and ¹H NMR proved the CDs were distributed mainly at the interface. On the basis of these results, interfacial models were proposed. Final evaluation results demonstrated the stabilizing effect and oxidation-inhibition ability of the ALA-loaded ME was substantially enhanced after the introduction of the CDs, indicating a "turn off" effect of the interface. Interestingly, CDs do not affect the in vitro release of ALA, indicating a "turn on" effect of the interface. This work provided a successful interface manipulation with a nanocarrier that can be used for a large diversity of food nutraceuticals.

Real time monitoring of the minute dynamic variation at the crude oil–water interface

Minute mass variations caused by SDS injection and the corresponding interaction process at the interface.

Polymer-surfactant systems in bulk and at fluid interfaces

The interest of polymer-surfactant systems has undergone a spectacular development in the last thirty years due to their complex behavior and their importance in different industrial sectors. The importance can be mainly associated with the rich phase behavior of these mixtures that confers a wide range of physico-chemical properties to the complexes formed by polymers and surfactants, both in bulk and at the interfaces. This latter aspect is especially relevant because of the use of their mixture for the stabilization of dispersed systems such as foams and emulsions, with an increasing interest in several fields such as cosmetic, food science or fabrication of controlled drug delivery structures. This review presents a comprehensive analysis of different aspects related to the phase behavior of these mixtures and their intriguing behavior after adsorption at the liquid/air interface. A discussion of some physical properties of the bulk is also included. The discussion clearly points out that much more work is needed for obtaining the necessary insights for designing polymer-surfactant mixtures for specific applications.

Solubilization of octane in cationic surfactant-anionic polymer complexes: effect of polymer concentration and temperature

Polymers may alter the ability of oppositely charged surfactant micelles to solubilize hydrophobic molecules depending on surfactant-polymer interactions. This study was conducted to investigate the effects of polymer concentration and temperature on the solubilization thermodynamics of an octane oil-in-water emulsion in mixtures of an anionic polymer (carboxymethyl cellulose) and cationic cetyltrimethylammonium bromide (CTAB) surfactant micelles using isothermal titration calorimetry (ITC). Results showed that the CTAB binding capacity of carboxymethyl cellulose increased with increasing temperature from 301 to 323 K, and correspondingly the thermodynamic behavior of octane solubilization in CTAB micelles, either in the absence or presence of polymer, was found to depend on temperature. The addition of carboxymethyl cellulose caused the solubilization in CTAB micelles to be less endothermic, and increased the solubilization capacity. Based on the phase separation model, the solubilization was suggested to be mainly driven by enthalpy gains. Results suggest that increasing concentrations of the anionic polymer gave rise to a larger Gibbs energy decrease and a larger unfavorable entropy increase for octane solubilization in cationic surfactant micelles.

Thermal gelation of aqueous hydroxypropylmethylcellulose solutions with SDS and hydrophobic drug particles

The thermal gelation of hydroxypropylmethylcellulose (HPMC) solutions has been studied as a function of sodium dodecyl sulfate (SDS) concentration with and without griseofulvin, a model particulate BCS Class II drug by rheological measurements of gelation temperature (Tgel), steady-state viscosity (η) at 25 °C, and ζ-potential. Polymer adsorption on the drug was demonstrated by a decrease in η and potential in the absence of SDS. Griseofulvin had a synergistic effect on gelation which was attributed to an effective spanning of associated hydrophobic polymeric regions through interactions with the adsorbed polymer. Adding SDS offsets this effect on Tgel shielding hydrophobic interactions. Higher SDS concentrations had no effect on the particles surface as evidenced by constant ζ-potential and Tgel. Yet, polymeric chains are saturated and larger surfactant aggregates account for the increase in viscosity. Understanding the gelation mechanism and complex interactions of HPMC with surfactants and drugs is necessary for the design of pharmaceutical products and optimization of their performance properties.

Coassembly of poly(ethylene glycol)-block-poly(glutamate sodium) and gemini surfactants with different spacer lengths

The coassembly of poly(ethylene glycol)-b-poly(glutamate sodium) copolymer (PEG113-PGlu100) with cationic gemini surfactants alkanediyl-α,ω-bis-(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)SN(CH3)2C12H25]Br2 (designated as C12CSC12Br2, S = 3, 6, and 12) have been studied by isothermal titration microcalorimetry, cryogenic transmission electron microscopy, circular dichroism, small-angle X-ray scattering, zeta potential, and size measurement. It has been shown that the electrostatic interaction of C12CSC12Br2 with the anionic carboxylate groups of PEG113-PGlu100 leads to complexation, and the C12CSC12Br2/PEG113-PGlu100 complexes are soluble even at the electroneutral point. The complexes display the feature of superamphiphiles and assemble into ordered nanosheets with a sandwich-like packing. The gemini molecules which were already bound with PGlu chains associate through hydrophobic interaction and constitute the middle part of the nanosheets, whereas the top and bottom of the nanosheets are hydrophilic PEG chains. The size and morphology of the nanosheets are affected by the spacer length of the gemini surfactants. The average sizes of the aggregates at the electroneutral point are 81, 68, and 90 nm for C12C3C12Br2/PEG113-PGlu100, C12C6C12Br2/PEG113-PGlu100, and C12C12C12Br2/PEG113-PGlu100, respectively. Both C12C3C12Br2/PEG113-PGlu100 and C12C12C12Br2/PEG113-PGlu100 mainly generate hexagonal nanosheets, while the C12C6C12Br2/PEG113-PGlu100 system only induces round nanosheets.

Adsorption of anionic surfactants in a nonionic polymer brush: experiments, comparison with mean-field theory, and implications for brush-particle interaction

The adsorption of the anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl benzene sulfonate (SDBS) in poly(ethylene oxide) (PEO) brushes was studied using a fixed-angle optical flow-cell reflectometer. We show that, just as in solution, there is a critical association concentration (CAC) for the surfactants at which adsorption in the PEO brush starts. Above the critical micelle concentration (CMC) the adsorption is found to be completely reversible. At low brush density the adsorption per PEO monomer is equal to the adsorption of these surfactants in bulk solution. However, with increasing brush density, the number of adsorbed surfactant molecules per PEO monomer decreases rapidly. This decrease is explained in terms of excluded volume interactions plus electrostatic repulsion between the negatively charged surfactant micelles. Experimentally, a plateau value in the total adsorption is observed as a function of grafting density. The experimental results were compared to the results of an analytical self-consistent field (aSCF) model, and we found quantitative agreement. Additionally, the model predicts that the plateau value found is in fact a maximum. Both experiments and model calculations show that the adsorption scales directly with the polymerization degree of the polymers in the brush. They also show that an increase in the ionic strength leads to an increase in the adsorbed amount, which is explained as being due to a decrease in the electrostatic penalty for the adsorption of the SDS micelles. The adsorption of SDS micelles changes the interactions of the PEO brush with a silica particle. This is illustrated by atomic force microscopy (AFM) measurements of the pull-off force of a silica particle from a PEO brush: at high enough PEO densities, the addition of SDS leads to a very strong reduction in the force necessary to detach the colloidal silica particle from the PEO brush. We attribute this effect to the large amount of negative charge incorporated in the PEO brush due to SDS adsorption.

Cooperative tuneable interactions between a designed peptide biosurfactant and positional isomers of SDOBS at the air-water interface

Rationally designed peptide biosurfactant AM1 was mixed with sodium dodecyl benzene sulfonate (SDOBS) to self-assemble a mixed surfactant-biosurfactant layer at the air-water interface. Under optimal conditions in the presence of Zn2+, the interfacial elasticity of the mixed layer was approximately 5-fold higher than for biosurfactant alone. Two head positional isomers, SDOBS-2 and SDOBS-6, were compared for their ability to enhance interfacial film strength. SDOBS-6 forms a stronger layer with AM1 than does SDOBS-2. The highest interfacial elasticity of the AM1/SDOBS-6 layer was 640 mN m(-1) whereas the maximum value for the AM1/SDOBS-2 layer was 440 mN m(-1). Neutron reflection was used to investigate the structure of AM1/SDOBS films at varied bulk SDOBS concentrations. Both deuterated and nondeuterated SDOBS-2 and SDOBS-6 were used to provide contrast variation. It was shown that there is cooperative interaction between AM1 and SDOBS at low SDOBS concentration in the presence of 100 microM Zn2+, promoting AM1 adsorption atthe interface to form a two-layered structure of AM1 resulting in a mechanically strong interfacial film. In the presence of EDTA, only a single AM1 layer was formed at the same SDOBS concentration, and the film did not show lateral force transmission capability. Further increasing the SDOBS concentration to a molar excess of > 10x decreased the peptide population at the interface and resulted in a mechanically weak layer. Compared to SDOBS-6, SDOBS-2 depletes AM1 at a lower bulk concentration. These results demonstrate that the film strength of a self-assembled surfactant-biosurfactant mixed layer can be fine tuned by changing the isomer type and concentration of surfactant and by adding or removing metal ions.

Polymer-vesicle association

Mixed polymer-surfactant systems have been intensively investigated in the last two decades, with the main focus on surfactant micelles as the surfactant aggregate in interaction. The main types of phase behavior, driving forces and structural/rheological effects at stake are now fairly well understood. Polymer-vesicle systems, on the other hand, have received comparatively less attention from a physico-chemical perspective. In this review, our main goal has been to bridge this gap, taking a broad approach to cover a field that is in clear expansion, in view of its multiple implications for colloid and biological sciences and in applied areas. We start by a general background on amphiphile self-assembly and phase separation phenomena in mixed polymer-surfactant solutions. We then address vesicle formation, properties and stability not only in classic lipids, but also in various other surfactant systems, among which catanionic vesicles are highlighted. Traditionally, lipid and surfactant vesicles have been studied separately, with little cross-information and comparison, giving duplication of physico-chemical interpretations. This situation has changed in more recent times. We then proceed to cover more in-depth the work done on different aspects of the associative behavior between vesicles (of different composition and type of stability) and different types of polymers, including polysaccharides, proteins and DNA. Thus, phase behavior features, effects of vesicle structure and stability, and the forces/mechanisms of vesicle-macromolecule interaction are addressed. Such association may generate gels with interesting rheological properties and high potential for applications. Finally, special focus is also given to DNA, a high charge polymer, and its interactions with surfactants, and vesicles, in particular, in the context of gene transfection studies.

Effect of polymer-surfactant association on colloidal force

We investigate the forces between emulsion droplets in the presence of neutral polymer-surfactant complexes. The polymer used in our experiment was statistical copolymer of polyvinyl alcohol. The anionic surfactant used is sodiumdodecyl sulphate, the cationic surfactants are cetyltrimethylammonium bromide and tetradecyltrimethylammonium bromide, and the nonionic surfactant is nonylphenol ethoxylate (NP10). It has been found that the force profiles in the presence of surfactant-polymer complexes follow an exponential scaling with a characteristic decay length, close to the radius of gyration of the polymer alone. A continuous increase in the onset of repulsion is observed in the case of all three ionic surfactants, whereas no such variation was noticed in the case of nonionic surfactant, NP10. The experimental observations suggest that in the presence of charged surfactant molecules or micelles, the neutral polymer chain at the interface is converted into partial polyelectrolytes, where the charges on the chain repel each other and the electrostatic repulsion collectively leads to chain stretching. These results suggest that the associative polymers can be potential candidates for making the emulsions stable for a sufficiently long period.