Presence Of Micelles In Solution Research Articles

Redox-Induced Ion Pairing of Anionic Surfactants with Ferrocene-Terminated Self-Assembled Monolayers: Faradaic Electrochemistry and Surfactant Aggregation at the Monolayer/Liquid Interface

Oxidoreduction of self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate on gold in aqueous solutions of surface-active sodium n-alkyl sulfates (NaCnSO4) of 6, 8, 10, and 12 carbons is investigated by cyclic voltammetry and surface plasmon resonance. The effects of surfactant micellization and alkyl chain length on the redox response of the surface-tethered ferrocenes are examined. The SAM redox electrochemistry is sensitive to the surfactant aggregation state in solution. The nonideal behavior of the sodium alkyl sulfates at concentrations above the critical micelle concentration leads to a non-Nernstian variation of the SAM redox potential with concentration. The presence of micelles in solution results in decreased anodic-to-cathodic peak separations and anodic peak full widths at half-maximum. A longer alkyl chain length results in an increased ability of the alkyl sulfate anion to ion pair with the SAM-bound ferrocenium, resulting in oxidation of the ferrocene at lower potential. A comparison of the SAM redox potential at a fixed surfactant concentration of ideal behavior suggests a 4.5 × 10(4) difference in the ion-pairing abilities of the shorter-chain C6SO4(-) and longer-chain C12SO4(-). One-half of the available SAM-bound ferrocenes are oxidized in the NaCnSO4 electrolyte. Surfactant anions adsorb and assemble onto the SAM surface by specific ion-pairing interactions between the sulfate headgroups and oxidized ferrocenium species, forming an interdigitated monolayer in which the surfactant anions alternate between a heads-down and heads-up orientation with respect to the SAM. The work presented points to applications of ferrocenylalkanethiolate SAMs as anion-selective membranes, probes of micelle formation, and surfaces for the electrochemically switchable assembly of organosulfates.

Monte Carlo simulations of surfactant aggregation and adsorption on soft hydrophobic particles

Monte Carlo simulations with an explicit description of counterions are performed to investigate the adsorption of ionic surfactants at the interface between water and soft hydrophobic and penetrable particles. The surfactant molecules are represented at a coarse-grained level, their hydrophobic tails interact with each other through a Lennard-Jones potential, whereas their hydrophilic head and their counterions interact through a Coulombic potential. Two colloidal hydrophobic particles interact with the surfactant hydrophobic chains through a modified Lennard-Jones potential. By increasing the surfactant confinement between non-adsorbing colloidal particles, micellization is achieved and the micelle aggregation number is found to increase. Adsorption isotherms are determined for various interaction strengths between the surfactants and the particles. It is found that increasing this parameter increases the level of the adsorption plateau. The adsorbed surfactant molecules form conical aggregates which evolve into elongated structures by increasing the surfactant concentration and the strength of the interaction. The presence of micelles in solution is shown to be controlled by the level of adsorption and saturation of the hydrophobic particle surfaces. This study provides for the first time a comparison of surfactant micellization in solution and aggregate formation at one interface by considering hydrophobic and electrostatic interactions.

Adsorption of PEO−PPO−PEO Triblock Copolymers at the Solid/Liquid Interface: A Surface Plasmon Resonance Study

Poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−PEO) triblock copolymers are used in a wide range of industrial applications from detergents to pharmaceutical materials. We have employed surface plasmon resonance (SPR) to monitor the adsorption of these materials, from aqueous solutions to a model hydrophobic surface. The effect of varying the PPO or PEO block size of the copolymer on adsorption has been investigated, and a linear increase in SPR angle shift with increasing PPO or PEO chain length is observed. The SPR angle shifts show a greater increase in SPR angle with increasing PPO block size compared to an equivalent increase in PEO block size. This has been explained by considering the effect each block has on the dielectric properties of the adsorbed polymer at the interface. The presence of micelles in solution and their influence on Pluronic adsorption has also been considered, and adsorption from micellar solutions has been determined to be dependent on the PPO content ...