Structure-Property Relationships Of Extended Surfactants Based On Sodium Alkyl Sulfates

An extended surfactant（e-surfactant）is derived from a conventional surfactant by inserting an intermediate polarity region,the polypropylene oxide（PPO）-related spacer,in between the hydrophilic head and hydrophobic alkyl chain.Previous studies have been found that the introduction of PPO spacer in the central part of an e-surfactant molecule causes a smoother transition between lipophilic tail and hydrophilic anionic head,which makes e-surfactants produce the a middle phase microemulsion for long-chain polar oils,resulting in ultralow interfacial tension.The PPO chain provides lipophilicity for e-surfactant molecules,which makes e-surfactants behave low critical micelle concentration,improving the solubilization of small organic molecules.In addition,it has also been found indirectly that the PPO spacers are also somehow hydrated similar to a polyethylene oxide（PEO）spacer by investigating microemulsion behavior.These findings indicate that the all-round properties of e-surfactants are more excellent than those of their parent conventional surfactants,and thus behaving different structure-property relationships from the latter.It has been found that the interfacial properties of surfactants designed with special molecular frameworks are dominated by both molecular amphiphilicity and the surfactant shape at the interface.But the PPO spacer endow e-surfactants without a specific molecular framework,on the contrary,it makes them more flexible than their parent conventional surfactants.And it has also been found that e-surfactants have larger surface areas at the interfaces.It makes us wonder whether the excellent all-round properties of e-surfactants depend on the surface area or the unknown implicit surfactant shape at the interface.Secondly,the excellent properties of e-surfactants indicate that the molecular lipophilicity and polarity can be enhanced simultaneously by inserting PPO spacer in a conventional surfactant.It makes us wonder how the synergism of the PPO sapcers and other molecular blocks in e-surfactants to meet different demands of molecular amphiphilicity.Last but not least,if the experiments show that the properties of e-surfactants rely on both the molecular amphiphilicity and the unknown surfactant shape at the interface,and then there is the question of what is responsible for the excellent all-round properties of e-surfactants:the molecular amphiphilicity or the unknown surfactant shape.Based on the research status of e-surfactants and three questions above,the following studies were carried out in this paper,and the main results and conclusions are obtained as follows:（1）Although some commercial e-surfactants have been used in both theoretical research and application research,studing the structure-property relationships of commercial e-surfactants is not enough to answer the three questions mentioned above,and the structure of these e-surfactants also can not meet needs of the products used in daily chemical industry.In this paper,based on sodium alkyl sulfate surfactants as the parent conventional surfactants,twenty-nine kinds of molecular designed e-surfactants were synthesized by different branched alkyl or linear alkyl as hydrophobic groups together with the suitable PPO or PPO-b-PEO spacers to create a sample library,making it possible to systematically study the structure-property relationships of e-surfactants.The structures of twenty-nine target e-surfactants were confirmed by FT IR,¹H NMR and two-phase titration.The average adduct numbers of both propylene oxide and ethylene oxide of e-surfactants were calculated based on¹H NMR data.（2）The adsorption capacity of e-surfactants at the air/water surface and at the oil/water interface was measured by tensiometry,and the corresponding surface areas per adsorbed molecule were calculated according to Gibbs formula.On the basis of obtaining greater values for e-surfactants surface area per adsorbed molecule than conventional surfactants at the air/water surface and at the oil/water interface,a new concept was proposed that e-surfactant adopt a rugby ball shape at the interfaces.A new critical parameter,the shape factor(S=a_m/a_PP),was introduced to replace the traditional P parameter and further to evaluate rugby ball shaped surfactants.The results indicate that we can distinguish rugby ball shape of e-surfactants synthesized in this paper and other ones reported in literature from truncated cone-shaped ones by an S criterion.The rugby shape of e-surfactant is explained as follows:the PPO spacer is anchored by both the lipophilic tail towards air,oil phase,solid phase or micelle interior and the hydrophilic head towards water phase,it would have to arrange itself in somewhat“coiling”manner around the molecular long axis instead of random packing manner caused by the PPO coil collapsing by interface-induced interaction to produce dense packing at the monolayer adsorption or in the micelle.Further studies have found that the“extended”behavior of e-surfactant at the interface can be regulated in response to the external environment.（3）To highlight the hydrophobic contribution of PPO spacer to an e-surfactant molecule and find some novel water-soluble surfactants adapted for enhanced oil recovery,a series of molecular designed e-surfactants b-C₈P_pE_eS was synthesized by 2-ethylhexyl together with a suitable PPO-b-PEO spacer.Based on the experimental results that the PPO spacers make e-surfactants show lower surface tension,critical micelle concentration,ultralow interfacial tension,which not only makes it evident that extremely insufficient lipophilicity of the medium alkyl chain e-surfactants could be complemented by the PPO coil,but also determines that a matching PEO block can assist a PPO coil,resulting in higher lipophilicity and more pronounced rugby ball shape.In this paper,the experimental results also indicate that a positive correlation exists between surface area and p of the e-surfactants,and many interfacial properties have a positive or negative correlation with surface area or p,which reflects that the structure-property relationships of e-surfactants depend on the rugby ball shape.In addition,other results show that molecular design of b-C₈P_p E_eS has the potential to provide water-soluble surfactants for surfactant flooding as suitable soft interfacial materials.（4）To discuss the synergistic effect between PPO spacers and branched alkyl chains on molecular amphiphilicity and their structure-property relationships,a series of molecular designed e-surfactant b-C_cP_pS was synthesized by branched chain alkyl together with different PPO chains.The experimental results not only show that e-surfactants without a PEO chain also adopt pronounced rugby-ball shape at the air/water surface,but also indicate that a PPO spacer can adjust the amphiphilic contributions in view of different alkyl chain lengths,therefore,it is confirmed that different combinations of PPO spacers and branched alkyl chains make dynamic molecular amphiphilic contributions to e-surfactants.Some varieties of the b-C_cP_pS series behave better wettability,emulsifying power and lime soap dispersing power than conventional surfactants,which indicates that these surfactants have the potential be used in daily chemical industries,such as detergents.（5）To investigate the synergistic effect of PPO spacers and linear alkyl chains on molecular crystallinity and their structure-activity relationships,a series of molecular designed e-surfactant n-C_cP_p S was synthesized by linear alkyl together with different PPO chains to find samples adapted for household cleaning and personal care products without polyoxyethylene sulfates.The experimental results not only show that different combinations of PPO spacers and alkyl chains make dynamic molecular amphiphilic contributions to e-surfactants,but also indicate that e-surfactants with a long alkyl chain also have excellent low-temperature solubility.Therefore,it is confirmed that different combinations of PPO spacers and long alkyl chains can destroy the molecular crystallinity of e-surfactants.Some varieties of the n-C_cP_pS series have better detergency,emulsifying power and lime soap dispersing power than conventional surfactants,which indicates that n-C_cP_pS have a potential to substitute sodium laureth sulfates（SLE_eS）applied in household cleaning and personal care products.Based on the structure-property relationships of the e-surfactants（b-C₈P_pE_eS,b-C_cP_pS and n-C_cP_pS）,it is found that the excellent properties of e-surfactants are partly related to the rugby ball shape and partly to the dynamic molecular amphiphilicity,both of which are driven by the dynamic biphasic affinity of the PPO coil.Therefore,the properties of e-surfactants are dominated by the dynamic biphasic affinity of the PPO coil in response to the external environment in nature.（6）To overcome the problem of the poor hydrolysis stability of alkyl polyether sulfate surfactants,a microemulsion nanoreactor was constructed to solve the problem of both macro and micro mass transfer of micellar nanoreactor based on the sulfonation reaction characteristic of commercial SLE₃S by sulfite to synthesize its cousin sodium laureth-3sulfonate（SLE₃SO）.The experimental results show that both the macro and micro mass transfer of sulfonating agent in microemulsion nanoreactor are beneficial to increase the concentration and the sulfonation rate of SLE₃S,which solves the problems in micellar nanoreactor.In addition,six kinds of alkyl polyether sulfonate surfactants were successfully prepared in microemulsion nanoreactor,respectively.These findings indicate that microemulsion nanoreactor can be used to not only synthesize sulfonate anionic-nonionic surfactants,but also to prepare sulfonate e-surfactants with hydrolysis stability.