Amphiphiles at Interfaces: Fundamentals and Applications in Dispersions
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The nanoscale organization of nonionic amphiphilic molecules in bulk aqueous solution and on interfaces leads to unique micro- and macroscopic material properties beneficial to a variety of applications and industries. A brief list includes fields/topics such as printing inks, mineral flotation, enhanced oil recovery, cleaning detergents, and drug delivery. The use of amphiphilic molecules is wide-spread but gaps in fundamental knowledge of their self-assembly and adsorption properties remain. In particular, the development of generalized guidelines for controlling the equilibrium amphiphile adsorbed amount and layer configuration is needed. These properties are driven by physical interactions between amphiphile, interface, and solvent. However, prediction of the self-assembly and adsorption behavior is not always straightforward. The nature of the interactions is specific to the class (structure and composition) of the amphiphile being considered. The adsorbing surface and solvent quality also dictate adsorption and self-assembly behavior.We present here an approach to control the aqueous adsorption and self-assembly behavior of nonionic amphiphiles by modulating the surface and solvent properties. A commercially available nonionic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer (PEO-PPO-PEO, Pluronic®) served as the adsorbing amphiphile, while spherical silica nanoparticles were used for a solid-liquid interface. We observed that the PEO-PPO-PEO block copolymer adsorbed onto the silica nanoparticles and formed micelle-like surface assemblies. The block copolymer concentration required for surface self-assembly (critical surface micelle concentration, csmc) was lower than that for self-assembly in the bulk. The csmc could be shifted by changing the amount of available nanoparticle surface area. The addition of short (200 and 600 Da) PEO homopolymers to aqueous silica nanoparticle dispersions increased the csmc and suppressed the adsorbed layer thickness of PEO-PPO-PEO on the surface by a few nm, possibly resulting in a heterogeneous mixed adsorbed layer of micelle-like block copolymer assemblies and adsorbed PEO homopolymers. Mono- and divalent salts were also found to increase the csmc. Divalent salts were more effective, requiring a lower amount in order to reach the maximum observed csmc. Beyond this point of maximum effectiveness, the salts affect micellization in the bulk.The aqueous micellization behavior of two homologous series of poly(ethylene oxide)-containing surfactants based on a C10-Guerbet hydrophobe was also investigated. The surfactants are described as alkyl ethoxylates and alkyl alkoxlyates and are commercially available under the trade name Lutensol® XP and XL, respectively. The latter incorporate propylene oxide (PO) units in the surfactant chain. Relationships between the surfactant composition/structure and micellization thermodynamics were drawn and compared with compositionally similar CiEOj surfactants with linear alkyl chains. Conclusions drawn from these data may enhance understanding of surfactant structure-property relationships required for industrial formulation.