Interaction of surfactants with hydrophobic surfaces in nanopores.

Surfactant-induced wetting of hydrophobic nanopores is investigated. SDS micelles interact with the C18 layer on the nanopore walls with their hydrophobic tails, creating a charged wall lining with their head groups and inducing a breakthrough of the aqueous solution to wet the pores. The surface coverage of the surfactant molecules is evaluated electrophoretically. A surprising discovery is that pore wetting is achieved with 0.73 µmol/m(2) coverage of SDS surfactant, corresponding to only 18% of a monolayer on the walls of the nanopores. Clearly, the surfactant molecules cannot organize as a compact uninterrupted monolayer. Instead, formation of hemimicelles is thermodynamically favored. Modeling shows that, to be consistent with the experimental observations, the aggregation number of hemimicelles is lower than 25 and the size of hemimicelle is limited to a maximum radius of 11.7 Å. The hydrophobic tails of SDS thus penetrate into and intercalate with the C18 layer. The insight gained in the C18-surfactant interactions is essential in the surfactant-induced solubilization of hydrophobic nanoporous particles. The results have bearing on the understanding of the nature of hydrophobic interactions.

Tissue phantoms constructed with hydrophobic nanoporous silica particles.

We describe a protocol to create tissue phantoms with hydrophobic nanoporous particles. The nanopores of the particles are loaded with biological molecules at the desired compositions. Tissue phantoms are prepared by immersing dried particles into aqueous biological matrixes. The hydrophobicity of the pore surface prevents the solution from penetrating into the nanopores, thus preserving the designed molecular composition inside the particles. This protocol provides a unique approach to preparing biological systems in small domains, at micrometer and nanometer dimensions, with well-defined boundaries and tailored biological and optical properties. The nanoporous particle approach is easy when compared to the common preparation methods such as with polymers and vesicles as it involves direct loading of the biological molecules into the pores and does not require complex synthetic steps. The method is adaptable, with tunable pore and particle sizes, and robust, with a rigid boundary to protect the designed biological domain. In addition to tissue phantom preparation, this approach is applicable in systems where a well-defined biological domain is desired.