Self-assembly Of Gemini Surfactants At Air/water Interface

Gemini surfactants have complicated and novel properties due to their unique molecular structures. The adsorption and spreading at air/water interface are the important aspects of the self-assembly behaviors. The spacer group connects the two amphiphilic moieties, which enhances both the alkyl chain density and the charge density in a gemini molecule. Thus, it should be more sensitive to the electrolytes in the aqueous solution. So far, no studies haven been reported focusing on the effect of electrolytes on the self-assembly of gemini surfactants, though it is obviously of importance and significance to this investigation for understanding the peculiar properties of gemini surfactants. On the other hand, binary mixtures of surfactants have always attracted close attention of the researchers due to both the academic and the application causes. The most of synthesized gemini surfactants were soluble, so few studies were focused on the insoluble monolayers at the air/water interface. So far, no report was on the mixed monolayers of two gemini surfactants. Thus, in this work, we first aimed the effect of NaBr on the adsorption of C₁₂-s-C₁₂·2Br at air/water interface. Then, the mixed monolayers composed of two gemini surfactants 2C₁₂-s-2C₁₂·2Br and C11pPHCH at air/water interface were examined. Some interesting observations and results in the present work are described as follows.1. Gemini surfactant C₁₂-s-C₁₂·2Br is more sensitive to NaBr than the corresponding conventional surfactant C₁₂TABr.2. The effect of NaBr on the effectiveness (γcmc) of C₁₂-s-C₁₂·2Br in surface tension reduction varies with the length of spacer (s). When the spacer is short (s=2), the distance between the two ionic heads is constrainedly shortened and the dissociation degree of the ionic head is reduced, which leads to the small change of surface-active of C₁₂-s-C₁₂·2Br with the concentration of NaBr. When the spacer is long (s=12), the spacer moves to the airside of the interface and adopts a wicket-like conformation due to the increase of solution polarity by addition of NaBr. Hence, C₁₂-s-C₁₂·2Br molecules pack more tightly at the surface, which results in the decrease ofγcmc.3. The effect of NaBr on the efficiency (C20) of C₁₂-s-C₁₂·2Br in surface tension reduction varies with the length of spacer (s). This is attributed to the variation of the distances between the two ionic heads of C₁₂-s-C₁₂·2Br due to the wicket-like conformation of the spacer. The synergism between the two tails of one molecule increases firstly with s and then decreases, which results in a maximumγcmc at s=6.