Polyampholyte-surfactant film tuning in reverse microemulsions

The pH-dependent influence of two different strongly alternating copolymers [poly(N,N'-diallyl-N,N'-dimethylammonium-alt-N-phenylmaleamic carboxylate) (PalPh) and poly(N,N'-diallyl-N,N'-dimethylammonium-alt-3,5-bis(carboxyphenyl)maleamic carboxylate) (PalPhBisCarb)] based on N,N'-diallyl-N,N'-dimethylammonium chloride and maleamic acid derivatives on the phase behavior of a water-in-oil (w/o) microemulsion system made from toluene-pentanol (1:1) and sodium dodecyl sulfate was investigated. It was shown that the optically clear phase range can be extended after incorporation of these copolymers, leading to an increased water solubilization capacity. Additionally, the required amount of surfactant to establish a clear w/o microemulsion depends on the pH value, which means the hydrophobicity of the copolymers. Conductivity measurements show that droplet-droplet interactions in the w/o microemulsion are decreased at acidic but increased at alkaline pH in the presence of the copolymers. From differential scanning calorimetry measurements one can further conclude that these results are in agreement with a change of the position of the copolymer in the interfacial region of the surfactant film. The more hydrophobic PalPh can be directly incorporated into the surfactant film, whereas the phenyl groups of PalPhBisCarb flip into the water core by increasing the pH value.

Polyelectrolyte–surfactant complex: phases of self-assembled structures

We study the structure of complexes formed between ionic surfactants (SF) and a single oppositely charged polyelectrolyte (PE) chain. For our computer simulation we use the "primitive" electrolyte model: while the polyelectrolyte is modeled by a tethered chain of charged hard sphere beads, the surfactant molecules consist of a single charged head bead tethered to a tail of tethered hard spheres. A hydrophobic attraction between the tail beads is introduced by assuming a Lennard-Jones potential outside the hard-sphere diameter. As a function of the strengths of both the electrostatic and the hydrophobic interactions, we find the following scenario: switching on and increasing the electrostatic forces first leads to a stretching of the PE and then by condensation of SF to the formation of a complex. For vanishing hydrophobic forces this complex has the architecture of a molecular bottle-brush cylindrically centered around the stretched PE molecule. Upon increasing the hydrophobic attraction between the SF tails, a transition occurs inverting this structure to a spherical micelle with a neutral core of SF tails and a charged corona of SF heads with the PE molecule wrapped around. At intermediate hydrophobicity there is a competition between the two structures indicated by a non-monotonic dependence of the shape as function of the Coulomb strength, favoring the cylindrical shape for weak and the spherical micellar complex for strong interaction.

Polymer surfactant kinetics using surface plasmon resonance spectroscopy dodecyltrimethylammonium chloride/polyacrylic acid system

Kinetics of polymer surfactant interactions and the effect of surfactant binding on the conformational dynamics of the polymer were explored in this work using surface plasmon resonance spectroscopy. Polyacrylic acid was modified with thiol to varying degrees so as to force the polymer to form different loop sizes upon adsorption on the gold SPR sensor surface. Dodecyltrimethylammonium chloride in solution was flowed over the polymer-coated sensor surface and the binding was followed in real time. It was found that control of the loop size of the polymer on the solid surface enabled in turn the control of surfactant binding, with the largest loop allowing the maximum amount of surfactant to bind and vice versa. The kinetic plot of the binding showed three distinct segments. The first segment followed convective-diffusive kinetics. The second and third segments followed first-order kinetics with the second rate being significantly faster than the first one. Careful analysis of the second segment showed that it is possible to divide it into two different segments, each following a first-order kinetics, with the second rate being slightly slower than the first one suggesting a gradual slow down of the reaction due to convolution from the polymer conformational changes. Mechanistically, the sudden increase in the rate for the third segment of surfactant binding implies that the polymer matrix is opening up so as to incorporate more surfactant molecules. This was attributed to the formation of charged double surfactant species the repulsive interaction of which prevented the polymer network from imploding. Studies using unmodified polymers suggested the possibility of sudden conformational rearrangement in the polymer network, with progress in surfactant binding. Furthermore, the reflectance of the SPR spectrum was found to increase upon surfactant binding, implying that there is a decreased efficiency of coupling of the incident radiation into the surface plasmon mode of the metal, which suggests that the surfactant actually penetrated the polymer matrix.

Binding of molecules to DNA and other semiflexible polymers

A theory is presented for the binding of small molecules such as surfactants to semiflexible polymers. The persistence length is assumed to be large compared to the monomer size but much smaller than the total chain length. Such polymers (e.g., DNA) represent an intermediate case between flexible polymers and stiff, rodlike ones, whose association with small molecules was previously studied. The chains are not flexible enough to actively participate in the self-assembly, yet their fluctuations induce long-range attractive interactions between bound molecules. In cases where the binding significantly affects the local chain stiffness, those interactions lead to a very sharp, cooperative association. This scenario is of relevance to the association of DNA with surfactants and compact proteins such as RecA. External tension exerted on the chain is found to significantly modify the binding by suppressing the fluctuation-induced interaction.

Interaction of Mixed Surfactants with Bacterial Lipopolysaccharide

The interaction of lipopolysaccharide isolated from Klebsiella O3 strain with cationic and cationic-nonionic mixed surfactants has been studied by turbidimetry and by spectrophotometric, spectrofluorometric, viscosity, and conductance measurements. The cationic surfactants benzyldimethyl-n-hexadecylammonium chloride (BDHAC), cetyltrimethylammonium bromide (CTAB), cetylpyridinium chloride (CpCl), dodecylpyridinium chloride (DpCl), and nonionic surfactants polyoxyethylene glycol-tert-octylphenyl ether (Triton X-100), polyoxyethylenesorbitan monolaurate (Tween-20), and polyoxyethylene (23) lauryl ether (Brij-35) were used in all the experiments as single surfactants and also as mixed surfactants of different compositions. Mixed surfactants were found to be more effective in binding with the lipopolysaccharide molecules. Micellar effects of cationic and cationic-nonionic mixed surfactants on binding with the anionic polymer are presented in this paper. The binding was found to be electrostatic in origin and also hydrophobic in nature to a certain extent. Copyright 1998 Academic Press.