Unusual Adsorption at the Air–Water Interface of a Zwitterionic Carboxybetaine with a Large Charge Separation

Abstract

The structures of layers of three different dodecylcarboxybetaine surfactants adsorbed at the air-water interface have been determined by neutron reflection. The zwitterionic compounds differed in the length of the spacer separating the quaternary ammonium and carboxylate groups, which was (CH2)1, (CH2)4, or (CH2)8. The limiting area per molecule was found to be 45, 52, or 84 Å(2), respectively, and compared reasonably with results from surface tension showing that the Gibbs prefactor is 1 in each case. Isotopic labeling was used to distinguish between the position of the alkyl and spacer groups in the layer. The spacer was found to be well-immersed in water for the (CH2)1 and (CH2)4 spacers but significantly above water for the (CH2)8 spacer. The distribution of the (CH2)8 spacer along the surface normal was found to be similar to that of the dodecyl group; i.e., it projects out of the water, contrary to an earlier hypothesis that it forms a loop. Comparison of the overlap of water with dodecyl and spacer groups also indicates that the (CH2)8 spacer is well out of the water. This in turn suggests that the anionic carboxylic acid group, which is dissociated in solution, is not ionized in the adsorbed layer. A further observation is that the dodecylcarboxybetaine with the (CH2)8 spacer reaches surface saturation at one-tenth of the critical micelle concentration. This is highly unusual and is attributed to the long spacer destabilizing the micelle relative to the surface layer.

Studies on Interfacial Tensions of Ionic Surfactant and Alkyl Sulfobetaine Mixed Solutions

“…Zwitterionic surfactants contain both one anionic group and one cationic group. As a result of the special molecular structure, zwitterionic surfactants show a lot of unique properties. ,− Commonly, they have low toxicity, high tolerance of temperature and salinity, wide pH adaptation, and high foamability. Hence, the studies of zwitterionic and ionic surfactant mixed systems for EOR have received the attention of researchers.…”

Molecular Organization of an Adsorbed Layer: A Zwitterionic, pH-Sensitive Surfactant at the Air/Water Interface

“…Zwitterionic surfactants have been investigated less but present some particularly interesting behavior. Packing of zwitterionic surfactants, which generally have bulky headgroups, leads to compact micelles and different interfacial layers. − A consequence of the recent studies has been the understanding that the surface concentration of a planar interface with adsorbate that is limited by packing of molecules does not necessarily remain constant beyond the critical micelle concentration. The critical micelle concentration rather reflects the assembly of surfactants in solution.…”

Synergisms in Binary Mixtures of Anionic and pH‐Insensitive Zwitterionic Surfactants and Their Precipitation Behavior with Calcium Ions

“…The strong synergistic interaction between anionic and zwitterionic surfactants could be due to the strong electrostatic attraction between these molecules [34]. Many previous studies have been devoted to pH-sensitive zwitterionic surfactants, for instance alkyl betaines, which are able to accept a proton in acidic solution to transform into a cationic form and thus react with anionic surfactants and other anions [1,6,[35][36][37][38]. However, studies on pH-insensitive zwitterionic surfactants have rarely been reported.…”