| As a new class of surfactant, gemini surfactant greatly shows more enhanced properties compared with the conventional surfactant, such as much lower critical micelle concentration (CMC), high surface activity, better wetting and foaming behaviors. Therefore, the properties of cationic gemini surfactants (m-s-m, m is the length of the hydrophobic chain, and s is the length of spacer, respectively; s represents the number of-CH2-in this dissertation) and their interaction with different materials, including anionic surfactant and DNA (deoxyribonucleic acid), at the air/water interface have been investigated in this dissertation by means of LB technology, atom force microscope, FT-IR spectra and circular dichroism spectra etc. In addition, the properties of biocompatible amphiphilic copolymers and their complex monolayer composed of gemini surfactants have been investigated.It is shown that the liquid-expanded films are formed at the air/water interface for cationic gemini surfactant, which is suggest that the cationic gemini surfactants are difficult to form a compact film. At the same time, the spacer has a significant effect on the interfacial properties of gemini surfactants. The spacer lies on the air/water interface and induces the linear increase of the limiting area and lift-off area with its length when it is rigid, while when the spacer is flexible(s> 6), the increasing hydrophobility of spacer lead to the tendency to escape away from the aqueous side of the interface, so that the arc shape conformation of the spacer and even the reverse U-shape conformation are formed. The AFM image shows that the surface micelles and the multilayer aggregates appear at the air/water interface, whereas it is also difficult to form homogenous monolayers at higher surface pressure. According to the results ofπ-A isotherms and contact angle of LB film, it is found that although the surface micelles and the multilayer aggregates appear at higher surface pressure, the major molecules lie nearly flat on the water surface.Gemini/SA (stearic acid) mixed monolayers exhibits negative deviation from ideal nmixing because of the electrostatic attractive interaction between gemini surfactant and stearic acid. It shows that the length of spacer group plays an important role in determining the surface properties of gemini/SA mixed monolayers. The most negative deviation shift to SA rich component with the spacer increase (XSA (the mole fraction of SA)=0.67 for s=3, XSA= 0.7 for s=4, XSA=0.8 for s=6-10, and XSA=0.75 for s=12). Furthermore, phase separation is observed in corresponding mixed monolayers:when s≤8, no phase separation is observed at all SA composition; when the spacer group increases up to 10, phase separation appears at XSA= 0.4-0.8. In particular, for s=12, phase separation appears in two composition regions of XSA= .4-0.75 and XSA= 0.75-0.85, respectively. The result of FT-IR spectra of LB films show that although SA is a weak acid, it ionizes completely when XSA≤0.67, while ionizes partially when XSA＞0.67.The interaction between DNA and surfactant at the air-water interface is the same as that of in solution. It is found that DNA molecule is condensed to-1 nm in gemini/DNA complex LB film, inducing the conformation of DNA changes from B-form conformation to aψ-phase. In addition, the length of spacers has a significant effect on the binding site between gemini surfactant and DNA, so that different aggregates are formed. When the spacer is rigid, and the intramolecular distance of N+-N+ of gemini mismatches that of P- - P- or the width of minor groove and major groove, only one ammonium group in a gemini surfactants can bind on one phosphate group in a DNA molecule, while the other one stretches out of this DNA. The later can easily interact with other DNA molecules at a random orientation during the deposition process. When s=6, the flexibility of the spacer in 18-6-18 lead to the match between the N+-N+ and P- - P- distance, which results in that 18-6-18 could anchor on one backbone in a DNA molecule. As s increases up to s＞6, the flexibility of the spacers increase, which makes N+-N+ distances match the width of minor groove in DNA molecule, so that gemini surfactants can interact on the complementary double strands in a DNA molecule. The results of AFM indicate that when spacer is rigid, the main morphology of gemini/DNA complex is fibril, while the main morphology is plate structure when spacer is flexible.Finally, the properties of biocompatible amphiphilic polyelectrolyte (PDMAEMAm-b-PAAn) and their complex with surfactants have been investigated at the air/water interface. PAA segment is ionized at basic condition, and PDMAEMA segment is hydrophobic chains, so that its surface activity increases with the increase of PDMAEMA segment. On the contrary, PDMAEMA segment is protonated, and PAA segment is relatively hydrophobic chains at acid condition. However, - COOH is a hydrophilic functional group in PAA segment. Thus, the surface activity decreases with the decrease of pH for PDMAEMAm-b-PAAn. In addition, surface aggregates are formed both at the basic and acid condition. Spherical aggregates are formed at basic condition, which is consisted of a PDMAEMA core surrounded by a water-soluble ionized PAA corona, and gemini surfactant can connect these spherical aggregates together to form necklace-like structure, while the traditional surfactant does not. On the contrary, random aggregates are formed by means of the hydrogen bond of intermolecular or intramolecular of PAA segment at acid condition.
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