Aqueous-organic phase transfer of nanoparticles: The effects of molecular structures of cationic surfactants

The phase transfer efficiencies of silica and Au nanoparticles (NPs) using four different types of cationic surfactants have been investigated. The pH value has a significant effect on the phase transfer efficiencies of NPs because the amount of surface charge of NPs will increase with the increase of pH values. At the optimal pH value, the phase transfer efficiencies follow the order of: 1–2 type >2–2 type >1–1 type >3–2 type. Meanwhile, the order of the phase transfer efficiencies agrees with the results of the contact angle. According to these results, we define a parameter P, which is the hydrophobicity per unit surface area induced by the adsorption of surfactants. The increase in the P value suggests that more hydrophobicity has been introduced to NPs per unit surface area, thus enhances the phase transfer efficiencies of NPs. Owing to the shorter alkyl chain, DTAB and MQAS12 cannot transfer the silica NPs to the organic phase, while DTAB, BQAS12, and MQAS12 cannot transfer Au NPs to the organic phase. Although other surfactants can transfer NPs successfully to the organic phase, only 1–2 type surfactants can effectively disperse NPs in the organic phase, which is also related to the properties of the organic phase. [Display omitted] •The pH value has a significant effect on the phase transfer efficiencies of NPs.•The ratio of carbon chain to hydrophilic headgroup determines transfer efficiency.•The greater the P value, the higher the phase transfer efficiency of NPs is.•Only 1–2 type surfactants can effectively disperse NPs in the organic phase.