量子化学方法研究阴离子表面活性剂在气液界面上的吸附

用量子化学方法中的密度泛函理论,在B3LYP/6-3IG水平上,对十二烷基硫酸钠（SDS）阴离子表面活性剂与水分子形成的水合物CH₃（CH₂）₁₁OSO₃ˉ（H₂O）_n（n=0～7）进行结构优化和频率计算。从分子水平上研究了CH₃（CH₂）₁₁OSO₃ˉ在气液界面上与水分子的相互作用。计算结果表明:（1）7个水分子与极性头均采用1:1型和2:1型,即极性头中一个氧原子或2个氧原子与水分子以氢键形式构成水合层;（2）CH₃（CH₂）₁₁OSO₃中的氧原子与水分子中的氧原子最短氢键的键长（O-O键长）在0.27～0.31 nm之间,H…O键长在0.19～0.21 nm之间,O-H…O键角在140°～167°之间,均属于中强氢键:（3）水合物R（S-O）平均键长比表面活性剂单体分子分别增长了,说明形成水合物后S-O间的键减弱;（4）结合能D₀从64.04 kJ/mol增加到.428.29 kJ/mol,说明随着水分子数的增加,所获得的7种水合物的稳定性依次增强,表明最终形成的水合层是稳定的:（5）随着水分子数增加疏水基链长收缩,亲水基总电荷增加,C12-O13-S14的键角增大;（6）由于烷烃链带有了弱电荷,使胶束内核带有了部分极性,此种极性介于烷烃油相和水相的极性之间,利于表面活性剂在溶液中的聚集。

Investigation of adsorption of surfactant at the air-water interface with quantum chemistry method

Density functional theory（DFT）of quantum chemistry was used to optimize the configuration of the anionic surfactant complexes CH₃（CH₂）₁₁OSO₃¹（H₂O）_n（n=0⁷） and calculate their molecular frequencies at the B3LYP/6-31G level.The interaction of CH₃（CH₂）₁₁OSO₃¹ with 1 to 7 water molecules was investigated at the air-water interface with DFT.The results showed that:（l） 1:1 and 2:1 type of ion-pair were formed in which one oxygen atoms or two oxygen atoms from the polar group in the surfactant bound with one water molecule;（2） The shortest hydrogen-bonded（O...O） from oxygen atom in the CH₃（CH₂）₁₁OSO₃¹ to oxygen atom in the water are in the range of 0.27～0.31 nm,H...O bond length is equal to 0.19-0.21 nm,0-H...0 bond angle is equal to 140°～167°,the strength of H-bonds belongs to medium;（3） S-O average bond length of CH₃（CH₂）₁₁OSO₃¹（H₂O）_n complexes are predicted to be increasing compared with CH₃（CH₂）₁₁OSO₃;（4） The hydration shell was formed in the form of H-bond between the hydrophilic group of CH₃（CH₂）₁₁OSO₃¹ and 7 waters,binding free energy increases from 64.04 kJ/mol to 428.29 kJ/mol with increasing water molecules,it revealed that the hydration shell was stable;（5） The increase of the number of water molecules will cause increases of the total charge of hydrophilic group and C₁₂-O₁₃-S₁₄ bond angle;（6） This calculation also showed that the chain had a weak negative charge in the micelle resulting in the core of the micelle having polarity.This core polarity of the micelle is somewhere between the oil polarity and the water phase polarity,which favors surfactant aggregation in solution.