Structure-Performance Relationship Between Aryl Spacer And Gemini Surfactants

Conventional surfactant molecules generally consist of two parts: a polar hydrophilic headand a nonpolar hydrophobic tail that are directly connected by chemical bond. Nowadays, aspacer as the third part has been tried to introduce into surfactant molecules, then so called"gemini" surfactant is obtained. Due to the spacer, gemini surfactant has the advantages ofmuch lower critical micelle concentration (cmc), higher effectiveness and efficiency ofsurface tension reduction, higher electrolyte tolerance, better water solubility. Surfactantflooding is one of the most appealing methods to enhance oil recovery (EOR) from depletedreservoirs after secondary recovery with water flooding. However, for the conventionalsurfactants used in EOR, there are some bottleneck problems, such as poor solubility in water,interfacial tension (IFT) not reduced sufficiently to affect the trapped oil without alkali, andsensitive to electrolyte, that prevents this technique from being successfully applying in thefield. Gemini surfactant has been considered as a perfect candidate for EOR due to itsexcellent and balanced properties.A surfactant containing aromatic nucleus energetically favours absorption at air/water oroil/water interface attributed to the aromatic nucleus has better compatibility with water,alkane and crude oil. And more importantly, the aromatic hydrocarbons are available from awealth of sources and thus cheap. So, most of surfactants used for EOR are derived fromaromatic hydrocarbons. Additionally, the aromatic nucleus has multi-active sites which can beused for synthesis to build a complex carbon skeleton. Therefore, one of the concerns in thisdissertation is, whether the aromatic nucleus can be used as a new molecular building block toprovide various hydrophobic precursors for preparing gemini surfactants?Producing ultra-low interface tension is one of the most important mechanisms for EORwith respect to surfactant flooding. The surfactant shape has been recognized as a majoradditional factor controlling the reduction of IFT, besides the hydrophilic lipophilic balance(HLB) of surfactants. It is well known that the rigid spacer has much more pronounced effecton the shape of gemini surfactants than the flexible one. The relationship between structureand performance has been focused on the gemini surfactant with flexible spacer. Therefore,another concern in this dissertation is, could we alter the rigidity of spacer by changing theconnection manner of two aromatic rings?For a successful surfactant flooding process, the achievement of ultra-low IFT between oiland surfactant solution is not the only requirement for surfactant. The oil reservoir water isoften salty, surfactants cannot generate the considerable reduction in interface tension assimply as described. Clearly, the compatibility of surfactants with reservoir brine is essentialto avoid plugging of the porous medium. However, most surfactants for EOR are poorsolubility in water and sensitive to electrolyte, especially divalent ions, mainly due to thelonger hydrophobic tail. It is difficult to accommodate these conflicting properties ofsurfactant by a simple variation of the nature of the hydrophilic head group and hydrophobicgroup (linear, branched, aliphatic, aromatic, etc.). Most previous works have been focused onthe relationship between the surfactant structure and ultra-low IFT, but the influence of the surfactant structure on the other concomitant properties was ignored. The nature of the spacerhas a pronounced effect on the properties of gemini surfactants. Thus, the third concern in thisdissertation is, whether the conflicting properties of surfactant can be accommodated byselecting a well-designed aromatic spacer?Aiming at the aforementioned three concerns, this dissertation mainly focuses on thefollowing:(1) Four kinds of hydrophobic precursors with different rigidity spacer were preparedusing the aromatic nucleus as a new molecular building block. Then4series and15novelsurfactants were synthesized based on the hydrophobic precursors: BHAM, geminisurfactants with a semi-rigid spacer containing two benzene rings connected by methylene,such as sodium salt of sulfonated bis (2-hydroxy-5-nonylphenyl) methane?SBHNM?sodiumsalt of sulfonated bis (2-hydroxy-5-alkylphenyl) methane? SBHDM)?sodium salt ofsulfonated bis (2-methoxy-5-nonylphenyl) methane?SBMNM); Sodium dialkylnaphthalenedisulfonate, DnDS for short, gemini surfactants with naphthalene directly as rigid spacer;Sodium nonylphenoxy polypropyleneoxide sulfates, PPnS for short, extended surfactantscombining the phenyl group and the PPO chain together as the intermediate polarity spacer,and so called extended-gemini surfactants bis(sodium nonylphenyl polypropyleneoxidesulfates) methane, B(PPnS) for short, containing both gemini spacer and extended spacer. Thestructures and purity of these products were confirmed by FT-IR,1H NMR, ESI-MS andelemental analysis. This synthetic strategy allows for the molecule design and development ofa wide variety of anionic gemini surfactants using low cost materials with environmentallyfriendly process.(2) The physic-chemical properties of these15novel surfactants were studied by surfacetension and conductometry. The results show that these surfactants with the spacer, especiallythe gemini surfactants, exhibit much lower cmc, higher effectiveness and efficiency of surfacetension reduction and better water solubility than conventional surfactants. It is interesting tonote that the extended-gemini surfactants B(PPnS) exhibit a second cmc.(3) The influence of aromatic spacer on adsorption of surfactants at interface wasinvestigated by calculating the area per molecule, and adsorption models were built accordingto Gibbs adsorption equation. The results show that, the aromatic spacers can significantlyalter the shape of gemini surfactants and play a major role in adsorption of gemini surfactants.DnDS gemini surfactants possess huge pseudo volume hydrophobic region due to the rigidnaphthalene, which induce the variation of configuration of surfactant molecule packing atinterface at different solution condition. A reverse absorption has been shown to occur forextended-gemini surfactants B(PPnS) where molecules packing close at interface at lowsurfactants concentration, while loose at high concentration.(4) The IFT between alkanes and surfactants solution were investigated by spinning–droptechnique. It is found that a surfactant with big pseudo volume hydrophobic region favorsproducing ultra-low IFT. Gemini surfactants DnDS, extended surfactants PPnS andextended-gemini surfactants B(PPnS) have excellent ability to produce the ultra-low IFT without alkali with good water solubility and high electrolyte tolerance, especially D16DS.(5) The electrolyte tolerances of surfactants were obtained by investigating precipitationboundary of surfactants solution upon the addition of NaCl or CaCl2. The hard tolerance ofSBMNM gemini surfactant is slight superior to that of corresponding monomeric surfactants.DnDS gemini surfactants possess ultra-high electrolyte tolerance which are much greater thanSBMNM attributed to their huge area per molecule and conjugated-system of naphthalene.In conclusion, a number of various anionic gemini surfactants were prepared using thearomatic nucleus as a new molecular building block with simple, efficient, low-cost and cleanprocess. The rigidity of spacer was adjusted by changing the connection maner of twoaromatic rings, and it is found that aromatic spacer is an important controlling factor toaccommodate the ultra-low IFT, good water solubility and excellent electrolyte tolerances in asingle surfactant molecule.