Wang Zi, Li Peixun, Ma Kun, Chen Yao, Webster John R P, Campana Mario, Yan Zifeng, Penfold Jeff, Thomas Robert K
School of Science, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China; ISIS Facility, Rutherford Appleton Laboratory, STFC, Chilton, Didcot, Oxon OX11 0QX, UK.
ISIS Facility, Rutherford Appleton Laboratory, STFC, Chilton, Didcot, Oxon OX11 0QX, UK.
J Colloid Interface Sci. 2021 Sep;597:223-232. doi: 10.1016/j.jcis.2021.03.183. Epub 2021 Apr 8.
The formation of surface multilayer structures, induced by the addition of multivalent counterions in dilute surfactant solutions, has been widely observed in a range of anionic surfactants. The phenomenon is associated with the ability to manipulate surface properties, especially in the promotion of enhanced surface wetting, and in the presence of an extensive near surface reservoir for rapid surface delivery of surfactant and other active components.
In the single alkyl chain anionic surfactants, such as sodium dodecysulfate, SDS, sodium alkylethoxylsulfate, SAES, and alkylestersulfonate, AES, surface multilayer formation is promoted by trivalent counterions such as Al, and is generally not observed with divalent counterions, such as Ca or with monovalent counterions. In the di-alkyl chain anionic surfactant, dodecylbenzenesulfonate, LAS, surface multilayer formation now occurs in the presence of divalent counterions. It is attributed to the closer proximity of a bulk lamellar phase, resulting in a greater tendency for surface multilayer formation, and hence should occur in other di-alkyl chain anionic surfactants.
Aerosol-OT, AOT, is one of the most commonly used di-alkyl chain anionic surfactants, and is extensively used as an emulsifying, wetting and dispersing agent. This paper reports on predominantly neutron reflectivity, NR, measurements which explore the nature of surface multilayer formation of the sodium salt of AOT at the air-solution interface with the separate addition of Ca and Al counterions.
In the AOT concentration range 0.5 to 2.0 mM surface multilayer formation occurs at the air-solution interface with the addition of Ca or Al counterions. Although the evolution in the surface structure with surfactant and counterion concentration is broadly similar to those reported for SDS, SAES and AES, some notable differences occur. In particular the surfactant and counterion concentration thresholds for surface multilayer formation are higher for Ca than for Al. The differences encountered reflect the greater affinity of the di-alkyl chain structure for lamellar formation, and how the surface packing is controlled in part by the headgroup structure and the associated counterion binding affinity.
在稀表面活性剂溶液中添加多价抗衡离子诱导形成表面多层结构，这在一系列阴离子表面活性剂中已被广泛观察到。该现象与操纵表面性质的能力有关，特别是在促进增强的表面润湿性方面，以及在存在大量近表面储库以实现表面活性剂和其他活性成分的快速表面递送方面。
在单烷基链阴离子表面活性剂中，如十二烷基硫酸钠（SDS）、烷基乙氧基硫酸钠（SAES）和烷基磺酸酯（AES），三价抗衡离子如Al会促进表面多层结构的形成，而二价抗衡离子如Ca或一价抗衡离子通常不会导致这种现象。在二烷基链阴离子表面活性剂十二烷基苯磺酸钠（LAS）中，现在在二价抗衡离子存在的情况下会出现表面多层结构的形成。这归因于本体层状相更接近，导致表面多层结构形成的趋势更大，因此应该在其他二烷基链阴离子表面活性剂中也会出现。
气溶胶-OT（AOT）是最常用的二烷基链阴离子表面活性剂之一，广泛用作乳化剂、湿润剂和分散剂。本文主要报道了中子反射率（NR）测量，该测量探究了在空气-溶液界面单独添加Ca和Al抗衡离子时AOT钠盐表面多层结构形成的本质。
在AOT浓度范围为0.5至2.0 mM时，添加Ca或Al抗衡离子会在空气-溶液界面形成表面多层结构。尽管表面结构随表面活性剂和抗衡离子浓度的演变与报道的SDS、SAES和AES大致相似，但仍存在一些显著差异。特别是，Ca导致表面多层结构形成的表面活性剂和抗衡离子浓度阈值高于Al。所遇到的差异反映了二烷基链结构对层状形成的更大亲和力，以及表面堆积如何部分地由头基结构和相关的抗衡离子结合亲和力控制。
