Langmuir Monolayers Of Quaternary Ammonium Gemini Surfactants And Amphiphilic Block Copolymers Containing PEO At Air-Water Interface

Of the newly developed surfactants, gemini surfactants consist of two hydrophobic chains and two hydrophilic groups covalently attached through a spacer. Gemini surfactants have superior performances over their corresponding conventional surfactants, such as much lower critical micelle concentration, higher efficiency in reducing surface tension, much easier forming into micelle, lower Krafft point, much better wetting ability and so on. Recently, gemini surfactants have become a hot topic of colloid science and industry in the world due to their unusual properties and potential applications, in which quaternary ammonium gemini surfactants are important cationic surfactants. In comparison with the conventional single-chain surfactants, quaternary ammonium gemini surfactants have prominent characters, such as higher surface activity, more strong sterilization ability, better biodegradation ability, lower toxicity etc., so various academic groups worldwide have continued the synthesis, characterization of structure and physicochemical properties. Gemini surfactants are very expensive, so the large scale application is restricted. A practicable solution to this problem is to compound other surfactants with gemini surfactants, so that it could produce obvious synergistic effect. Therefore extensive attention has been paid for the surface properties of the mixed system between gemini surfactants and other surfactants. The behavior of monolayers for gemini surfactants have been investigated at the interface by use of Langmuir balance, and the aggregation morphology have been observed in virtue of the advanced instruments. It is important of theoretical and practical signification in surface science.LB membrane technology is an effective method of obtaining orderly molecular ultra thin film. It can assemble on the level of molecule and can prepare monolayer or multilayer that the thickness is exact controlled and the structure is definitive and the property is anisotropic. LB film is the tightest and almost disfiguration-free ultrathin molecular film prepared for the moment. Langmuir balance is not only the most common instrument in preparing LB film, but also an important means researching the molecular dimension and arrangement mode. Many scientists studied monolayer and multilayer of small molecule since 1930. But for small molecules the heat resisting property and mechanical strength of LB are not good, so the application is limited. Monomolecular film and LB membrane of polymer have also been investigated, and it has been made a scientific breakthrough. Among amphiphilic polymers, polyethylene oxide (PEO)s are of biodegradability and surface activity, and it is extensively applied in biotechnology and medicine material. Recently the surface behavior and aggregation morphology of monolayers for graft, block and star copolymers containing PEO at interface have been extensively and intensively studied.In this article monolayers of quaternary ammonium gemini surfactants and amphiphilic block copolymers containing PEO were formed at air-water interface using Langmuir balance. The molecular conformation of quaternary ammonium gemini surfactants was analyzed by theÏ€-A isotherms of monolayers, and the monolayer stability was further discussed by way of the static or dynamic surface elasticity. The mixed monolayers for quaternary ammonium gemini surfactants with other appropriate surfactants were explored in order to research their compounded ability. Based on theÏ€-A isotherms, the deviation of the mean molecular area from ideal curves, mixed surface excess free energy and interaction parameters of each mixed monolayer at different surface pressure were analyzed, and the feature of monolayers at air-water interface was discussed. Furthermore, the monolayers for amphiphilic block copolymers containing PEO with different proportional hard segments were investigated at air-water interface. It was found from theÏ€-A isotherms that the cohesion of monolayer is increased with the proportion of hard segments in amphiphilic block copolymer. The static and dynamic surface elasticity of monolayers for amphiphilic block copolymers were compared, and the cyclic curve at different surface pressure, the relationship between surface pressure and time were discussed. The highlight of results in the paper is as follows:Poly(ethylene oxide) alkyl ether, C₁₈E₂₀, was in combination with each of three n-fatty alcohols (1-octanol, n-dodecanol and n-octadecanol) respectively as a model of nonionic surfactant and polar organic substance composites, which was spread onto air-water interface and formed a mixed monolayer on a Langmuir trough. The stability of each mixed monolayer was analyzed by use of theÏ€-A isotherms at various mole fractions of the C₁₈E₂₀-fatty alcohol pair. It was found that the C₁₈E₂₀ monolayer exhibits characters of a liquid expanded monolayer, with a collapse pressure 30 mN/m. The stability of the C₁₈E₂₀ monolayer could be enhanced by combining with fatty alcohols, based on two points. The one is the expanding degree of the monolayer decreases as the surface pressure of the mixed monolayer is under the inflexion of theÏ€-A curve, and the other is that the plateau length of the curve decreases after the surface pressure is above the inflexion of theÏ€-A curve, comparing with the pure C₁₈E₂₀ monolayer. The condensability of the mixed monolayer increases with the mole ratio of fatty alcohol, and with the hydrophobic chain length of the alcohol up to that of C₁₈E₂₀. For almost all of theÏ€-A isotherms of the mixed monolayers, the inflexion and the plateau always appeared near the surface pressure 30 mN/m, which showed that a phase-separation occurs for the mixed monolayer at a higher surface pressure corresponding the collapse pressure point of C₁₈E₂₀. It was also observed that the mixed monolayer is in a favorable condition for the compatibility of the compounded surfactants, as the mole ratio of C₁₈E₂₀ and n-octadecanol is equal to 3:1.Quaternary ammonium gemini surfactants 12-2-4, 12-2-8, 12-2-12, 12-2-16 and corresponding conventional surfactant, dodecyl-trimethylammonium bromide (DTAB) were spread onto air-water interface, respectively, forming monolayer in KSV Langmuir trough. It was found that the molecular limited areas of 12-2-8, 12-2-12, 12-2-16 in each monolayer are greater than that of DTAB, but the molecular limited area of 12-2-4 is similar to DTAB, and that the collapsed surface pressures of gemini surfactants are higher than that of DTAB. It means that the molecular conformation of monolayer is different. For 12-2-4 the shorter hydrophobic chain may be under the water, but the other one may be perpendicular away from the water; the orientation of 12-2-4 is similar with that of DTAB. In contrast, for 12-2-8, 12-2-12 and 12-2-16, the two hydrophobic chains seem to break away water, and appear "U" conformation. The static surface elasticity of gemini surfactants are in turn of 12-2-4 < 12-2-8 < 12-2-16 < 12-2-12, moreover it is indicated that the static surface elasticity of gemini surfactants is relate to the length of hydrophobic chains, and that the greater the length of the hydrophobic chain is, the higher the static surface elasticity. The order of dynamic surface elasticity of gemini surfactants measured by the barrier oscillation method is: 12-2-4 < 12-2-8 < 12-2-16 < 12-2-12, with the similar values for 12-2-4 and DTAB.Mixed monolayers formed using a symmetric gemini surfactant 12-2-12, a dissymmetric gemini surfactant 12-2-16 and DTAB with C₁₈E₂₀ at air-water interface were investigated by a Langmuir trough. It was found from theÏ€-A isotherms for mixed monolayers that the expanding liquid state arises from the steric effect in the hydrophilic long chain of C₁₈E₂₀. The mean molecular area, mixed surface excess free energy and interaction parameters of each mixed monolayer at different surface pressure were analyzed. It is shown that each of mixed monolayers is miscible, and the positive interaction parameters of mixed monolayers for gemini surfactants with C₁₈E₂₀ are greater than those of mixed monolayers for DTAB and C₁₈E₂₀. It is also shown that the repulsion interaction between gemini surfactant and C₁₈E₂₀ in the mixed monolayer is strong at air-water interface, and the interaction intensity of mixed monolayers with C₁₈E₂₀ is in turn of 12-2-16 > 12-2-12 > DTAB.Mixed monolayer formed by each of 12-2-12, 12-2-16 and DTAB with octadecyl alcohol (C₁₈H₃₇OH) spread onto air-water interface was investigated with a Langmuir trough. It was found from theÏ€-A isotherms for mixed monolayers that the condensed state arise from the incorporation of C₁₈H₃₇OH. The mean molecular area, mixed surface excess free energy and interaction parameters of each mixed monolayer at different surface pressure were analyzed. It was shown that each of mixed monolayers was partly miscible, and the interaction parameters of mixed monolayer for gemini surfactants with C₁₈H₃₇OH were almost positive values greater than those of monolayers for DTAB with C₁₈H₃₇OH. It was also shown that the repulsion interaction between gemini surfactant and C₁₈H₃₇OH in the mixed monolayer was strong at the air-water interface, and the interaction intensity of mixed monolayers with C₁₈H₃₇OH is in turn of 12-2-16 > 12-2-12 > DTAB.Mixed monolayer formed by each of 12-2-12, 12-2-16 and DTAB with lauryl sodium sulfate (SDS) spread onto air-water interface was studied. When the mole fraction of SDS is 0.1～0.3, the cohesion of mixed monolayers for DTAB and SDS is stronger than that of DTAB, but the expanded state for mixed monolayers of 12-2-12 / SDS and 12-2-16 / SDS are increased in comparing with the corresponding gemini monolayers. As the mole fraction of SDS is within 0.7～0.9, the cohesion of each mixed monolayer for the three cationic surfactants with SDS is increased, respectively. It is related not only to the static attraction between anionic and cationic surfactants, but also to the reciprocity of homogeneity of molecules for three cationic surfactants. It was shown that the compounded character of mixed monolayers with SDS for the three cationic surfactants is 12-2-16 > 12-2-12 > DTAB.The monolayers for amphiphilic block copolymers containing PEO with different proportional hard segments were investigated at air-water interface. TheÏ€-A isotherm of hydrophobic hard segments in an amphiphilic block copolymer was discussed in case of ignoring the complex change for hydrophilic PEO chains at/or under the interface, and the theoretic area of hard segments was calculated. It was found from theÏ€-A isotherms that the cohesion of monolayer is strongly increased with the proportion of hard segments. The chain relaxation changes of monolayer can be seen from the relationship between surface pressure and time. The static surface elasticities of monolayers for amphiphilic block copolymers were similar each other, but for that of copolymer with 50% hard segments one little peak appears before the monolayer collapsed. Moreover, the cyclic curve at different surface pressure, the relationship between surface pressure and time were discussed. The dynamic surface elasticity of amphiphilic block copolymer with 50% hard segments is higher than those of other copolymers, which means that the reciprocity of hard segments is greater, so does the resisting distortion ability as the monolayer is interfered.