Monolayers of novel Gemini amphiphiles with phthalimide headgroups at the air/water interface: pH and alkyl chain length effects

Abstract

A series of novel pH sensitive Gemini amphiphiles of N,N'-dialkyl-N,N'-di(ethyl-phthalimide) ethylenediamines (Di-CnPh, n=6, 8, 10, 12) was synthesized and characterized. It was found that relatively stable spread monolayers of Di-CnPh can be formed at the air/water interface once n≥6, and their stability was improved by increasing the hydrophobic chain length or adding electrolyte. The surface pressure-area (π-A) isotherms of Di-CnPh have been studied in detail by employing the axisymmetric drop shape analysis as penetration Langmuir balance. Furthermore, Brewster angle microscopy (BAM) was employed to confirm the monolayer phase transition from gaseous phase to liquid condensed phase via a liquid expanded phase. Moreover, dilatational rheological measurements of spread Di-CnPh monolayers at the air/water interface were performed, and the influence of the hydrophobic chain length and pH on the dilatational elasticity (ε) and viscosity (η) was also investigated. The results have suggested that the spread Di-CnPh monolayers are spontaneously elastic.