Kawasaki Hideya, Garamus Vasil M, Almgren Mats, Maeda Hiroshi
Department of Chemistry, Faculty of Sciences, Kyushu University, Fukuoka, Japan.
J Phys Chem B. 2006 May 25;110(20):10177-85. doi: 10.1021/jp061335r.
A double-tail amine oxide surfactant, di-n-decylmethylamine oxide (2C10MAO), was prepared, and the effects of protonation on aggregate structure were examined by small-angle neutron scattering (SANS), cryo-transmission electron microscopy (cryo-TEM), turbidity, electric conductivity, and solubilization of an oil-soluble dye at various degrees of neutralization, X, defined as the mole ratio of HCl/2C10MAO. The surfactant makes an L(2) phase in the nonprotonated state (X = 0) in water. The L(2) phase is in equilibrium with an aqueous L(1) phase. On protonation, unilamellar vesicles (ULVs) are formed over a wide range of compositions (0.05 < X< 0.4-0.5 at C = 10 mM) as observed by cryo-TEM. At X = 0.2, the ULV is stable over a wide concentration range (3 mM < or = C < 0.1 M), but an L(alpha) phase replaces the vesicle phase at C > 0.1 M. SANS results show that the mean radius of the ULV is about 25 nm and the bilayer thickness is about 2 nm, consistent with the extended configuration of the alkyl chains of the surfactant. An important contribution to the enhanced stability of the bilayer structures over the L(2) phase is suggested to be the translational entropy of the counterions. The enhanced stability of the bilayers diminishes as the counterion concentration increases either by an increase of X or by the addition of a salt. When the counterion concentration exceeds a critical value, the ULV solutions transform into the L(2) phase (or L(2)/L(1) two-phase system at low surfactant concentrations). The critical composition X is about 0.4-0.5 in water, but it is below 0.4 in D(2)O. The critical NaCl concentration is below 5 mM at X = 0.2. The stability of ULVs against multilamellar vesicles is ascribed partly to undulation forces and partly to the adjustable nature of the spontaneous curvature of amine oxide monolayers. The characteristics of the ULV of the surfactant remain the same within a temperature range 25-50 degrees C at X = 0.2. An iridescent lamellar phase and possibly an L(3) phase were observed in a very narrow X range (0 < X < 0.02) prior to the vesicle phase.
制备了一种双尾氧化胺表面活性剂二正癸基甲基氧化胺（2C10MAO），并通过小角中子散射（SANS）、冷冻透射电子显微镜（cryo-TEM）、浊度、电导率以及在不同中和度X（定义为HCl/2C10MAO的摩尔比）下油溶性染料的增溶作用，研究了质子化对聚集体结构的影响。该表面活性剂在水中非质子化状态（X = 0）时形成L(2)相。L(2)相与水相L(1)相处于平衡状态。通过cryo-TEM观察发现，质子化时，在很宽的组成范围内（C = 10 mM时，0.05 < X < 0.4 - 0.5）会形成单层囊泡（ULV）。在X = 0.2时，ULV在很宽的浓度范围内（3 mM ≤ C < 0.1 M）是稳定的，但当C > 0.1 M时，L(α)相取代了囊泡相。SANS结果表明，ULV的平均半径约为25 nm，双层厚度约为2 nm，这与表面活性剂烷基链的伸展构型一致。双层结构相对于L(2)相稳定性增强的一个重要贡献被认为是抗衡离子的平移熵。随着X增加或添加盐导致抗衡离子浓度增加，双层结构增强的稳定性会降低。当抗衡离子浓度超过临界值时，ULV溶液会转变为L(2)相（或在低表面活性剂浓度下转变为L(2)/L(1)两相体系）。在水中，临界组成X约为0.4 - 0.5，但在D(2)O中低于0.4。在X = 0.2时，临界NaCl浓度低于5 mM。ULV相对于多层囊泡的稳定性部分归因于波动力，部分归因于氧化胺单层自发曲率的可调节性质。在X = 0.2时，表面活性剂ULV的特性在25 - 50℃的温度范围内保持不变。在囊泡相之前的一个非常窄的X范围（0 < X < 0.02）内观察到了一个彩虹色层状相，可能还有一个L(3)相。
