Shibaev Andrey V, Aleshina Anna L, Arkharova Natalya A, Orekhov Anton S, Kuklin Alexander I, Philippova Olga E
Physics Department, Lomonosov Moscow State University, 119991 Moscow, Russia.
A.V. Shubnikov Institute of Crystallography, 119333 Moscow, Russia.
Nanomaterials (Basel). 2020 Nov 27;10(12):2353. doi: 10.3390/nano10122353.
Studies of the effects produced by the solubilization of hydrophobic substances by micellar aggregates in water medium are quite important for applications of viscoelastic surfactant solutions for enhanced oil recovery (EOR), especially in hydraulic fracturing technology. The present paper aims at the investigation of the structural transformations produced by the absorption of an aliphatic hydrocarbon (n-decane) by mixed wormlike micelles of cationic (n-octyltrimethylammonium bromide, C8TAB) and anionic (potassium oleate) surfactants enriched by C8TAB. As a result of contact with a small amount (0.5 wt%) of oil, a highly viscoelastic fluid is transformed to a water-like liquid. By small-angle neutron scattering (SANS) combined with cryo-TEM, it was shown that this is due to the transition of long wormlike micelles with elliptical cross-sections to ellipsoidal microemulsion droplets. The non-spherical shape was attributed to partial segregation of longer- and shorter-tail surfactant molecules inside the surfactant monolayer, providing an optimum curvature for both of them. As a result, the long-chain surfactant could preferably be located in the flatter part of the aggregates and the short-chain surfactant-at the ellipsoid edges with higher curvature. It is proven that the transition proceeds via a co-existence of microemulsion droplets and wormlike micelles, and upon the increase of hydrocarbon content, the size and volume fraction of ellipsoidal microemulsion droplets increase. The internal structure of the droplets was revealed by contrast variation SANS, and it was shown that, despite the excess of the cationic surfactant, the radius of surfactant shell is controlled by the anionic surfactant with longer tail. These findings open a way for optimizing the performance of viscoelastic surfactant fluids by regulating both the mechanical properties of the fluids and their clean-up from the fracture induced by contact with hydrocarbons.
研究胶束聚集体在水介质中增溶疏水性物质所产生的影响，对于粘弹性表面活性剂溶液在提高采收率（EOR）中的应用非常重要，特别是在水力压裂技术中。本文旨在研究阳离子（正辛基三甲基溴化铵，C8TAB）和富含C8TAB的阴离子（油酸钾）表面活性剂的混合蠕虫状胶束对脂肪烃（正癸烷）的吸收所产生的结构转变。与少量（0.5 wt%）的油接触后，高粘弹性流体转变为似水状液体。通过小角中子散射（SANS）结合低温透射电子显微镜（cryo-TEM）表明，这是由于具有椭圆形横截面的长蠕虫状胶束转变为椭圆形微乳液滴。非球形形状归因于表面活性剂单层内长链和短链表面活性剂分子的部分分离，为它们两者提供了最佳曲率。结果，长链表面活性剂可能优选位于聚集体较平坦的部分，而短链表面活性剂位于曲率较高的椭球体边缘。事实证明，这种转变是通过微乳液滴和蠕虫状胶束的共存进行的，并且随着烃含量的增加，椭圆形微乳液滴的尺寸和体积分数增加。通过对比变化SANS揭示了液滴的内部结构，结果表明，尽管阳离子表面活性剂过量，但表面活性剂壳的半径由具有较长尾部的阴离子表面活性剂控制。这些发现为通过调节流体的机械性能及其与烃接触引起的裂缝清理来优化粘弹性表面活性剂流体的性能开辟了一条途径。
