Adsorption Of The Bola Histidine-based Surfactant (h <sub> 2 </ Sub> D), Its Complex System Of Gas / Liquid Interface

The widespread use of surfactants in the huge variety of fields (such as detergency, moistening, emulsification, dispertion and foam formation), is typically based on their adsorption behavior at the air-water interface, and hence, the study of this behavior is an important subject that scientists always concern. For bolaform surfactants, the study of adsorption behavior currently is only about the adsorption equilibrium state, and the study on the dynamic adsorption process and adsorption in the external perturbation has never been reported.In this paper, a novel bolaform surfactant, 1, 12-dihistidine diaminododecane (H₂D), with biodegradability and biological compatibility, was selected as the research object. The adsorption behaviors at the air-water interface of H₂D and the mixed systems of H₂D/small surfactant(CTAB) and H₂D/polymer-based surfactant (chitosan-based surfactant) were studied by surface tension, dynamic surface tension, surface dilational viscoelasticity and other methods. The followed are the main results.1. In H₂D/salt system, H₂D adopted a reversed U-shaped conformation once the adsorption saturation state was reached. Two relaxation processes were observe for H₂D, which are fast process and slow process. The former one was caused by the molecular diffusion from bulk to interface and the latter one was caused by the relaxation of H₂D molecules at interface. Moreover, the dynamic adsorption behavior of H₂D was related to the aging time, solution concentration, electric of H₂D, the type and concentration of electrolyte, and temperature. In the range of the experimental conditions, the stability of H₂D membrane was the best when the concentration of salt was 0.3 mol/L and pH value was 8.2.2. For H₂D/CTAB mixed system, the critical micelle concentration (cmc) was slightly lower than that of pure CTAB system and a minimum surface tension was appeared. Moreover, when the concentration of the mixed solution is less than cmc, its ability to reduce the surface tension was improved, and the surface dilational viscoelasticity and relaxation process were different from pure system, suggesting both the H₂D and CTAB affect the interface properties. Howerver, when the concentration of mixed solution was higher than cmc, the main constituent adsorbed at the air-water interface was CTAB.3. For N-lauryl-O-carboxymethyl chitosan (C₁₂-OCMCS) system, the dynamic surface tension measurements showed that there were four characteristic dsorption stages: induction stage (lag stage), post-induction stage, final stage and equilibrium stage. The surface dilational viscoelasticity menifested that C₁₂-OCMCS started to form ring structure at the air-water interface with the increase of the ageing time and there were a maximium values for both the elastic storage and loss moduli. The limit elastic modulus and viscous modulus at characteristic relaxation time were also increased first and then decreased with the increase of ageing time. Compared with the case at lower concentration, at higher concentration, C₁₂-OCMCS molecules were diffused faster and needed shorter time to reach the induction stage and post-induction stage. But at higher concentration, more gelatinous structures existed at the air-water interface, thus the time needed for reaching the final stage was longer. NaCl can shield the electrostatic repulsion among C₁₂-OCMCS molecules, thus, making the time needed for reaching the threes stages shorter than without NaCl system.In C₁₂-OCMCS/H₂D solution system and C₁₂-OCMCS/H₂D/salt solution system, the results of dynamic surface tension and surface dilational viscoelasticity showed that they were adsorpted at the air-water interface through both electrostatic and hydrophobic interactions and formed a composite film. The stability of the composite film was higher than that in C₁₂-OCMCS system and lower than that in H₂D system formed in the system of lower concentration of C₁₂-OCMCS mixed with H₂D. In this case, C₁₂-OCMCS molecules were adsorbed as loose"brush"at the air-water interface. The formation of complex in this system depended on electrostatic attraction which made the relaxation of molecular rearrangement disapper. When higher concentration of C₁₂-OCMCS was mixed with H₂D, the molecular density of C₁₂-OCMCS and H₂D were increased at the interface beacause of electrostatic attraction, thus the interaction causing segment exchange in the surface sub-layer was decreased. The characteristic frequencies of fast relaxation and slow relaxation for the mixed system were close to those for pure C₁₂-OCMCS system. But the proportion of slow relaxation was larger than that of fast relaxation. The value of dilational viscoelasticity for that mixed system in the presence of NaCl was lower than that for each pure system. The main interaction of C₁₂-OCMCS and H₂D was hydrophobic. The effect aroused by the interaction among C₁₂-OCMCS molecular chains of the relaxation process because of interaction between C₁₂-OCMCS and H₂D with NaCl was weaker than that without NaCl.