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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CH_3(CH_2)_(11)OSO_3ˉ(H_2O)_n(n=0～7)进行结构优化和频率计算。从分子水平上研究了CH_3(CH_2)_(11)OSO_3ˉ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)CH_3(CH_2)_(11)OSO_3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D_0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。
Using the density functional theory (QDD) in quantum chemical method, the hydrate CH_3 (CH_2) _ (11) OSO_3ˉ, which is formed by the anionic surfactant of sodium dodecyl sulfate (SDS) (H_2O) _n (n = 0 ~ 7) for structural optimization and frequency calculation. At the molecular level, the interaction of CH_3 (CH_2) _ (11) OSO_3 with water at the gas-liquid interface has been studied. The results show that: (1) The seven water molecules and the polar head adopt 1: 1 type and 2: 1 type, that is, one oxygen atom or two oxygen atoms in the polar head and the water molecule form hydrogenated layer in the form of hydrogen bond ; (2) The bond length (OO bond length) between the oxygen atom in CH_3 (CH_2) _ (11) OSO_3 and the oxygen atom in water molecule is between 0.27 and 0.31 nm, and the length of H · O bond is between 0.19 ~ 0.21 nm, the OH ... O bond angle is between 140 ° ~ 167 °, all belong to the strong hydrogen bond: (3) The average bond length of the hydrate R (SO) is higher than that of the surfactant monomer molecules, (4) The binding energy D_0 increased from 64.04 kJ / mol to 428.29 kJ / mol, indicating that with the increase of water molecules, the stability of the seven hydrates obtained in turn (5) As the number of water molecules increases, the hydrophobic chain length shrinks and the total hydrophilic group charge increases, and the bond angle of C12-O13-S14 increases. (6) Chain with a weak charge, the micelle core with a partial polarity, the polarity between the paraffin oil phase and the aqueous phase polarity, which will help the surfactant in solution aggregation.