Investigation On The Pseudo-polyanions Constructed By Polyvinylpyrrolidone-anionic Surfactants

Accompanying with synergetic effect,polymer and surfactant mixtures present excellent performance in aqueous solutions,and are widely applied in the fields of pharmaceutics,paints,foods,daily chemicals,oil recoveries and nanoscience.Researchers have successfully synthesized a variety of nanomaterials by applying the polymer-surfactant complex as micro-level or nano-level soft templates.Therefore,physicochemical properties and interaction mechanism of polymer-surfactant complex were widely studied in the past few decades and a great deal of achievements were obtained.However,solutions to several problems about the interaction between nonionic polymers and anionic surfactants are still absent.(1)Previous investigation of viscometry,conductometry,light scattering and neutron scattering on nonionic polymer-anionic surfactant systems found the complexes possess properties and structures of “pseudo-polyelectrolytes”.However,most identifications of the pseudo-polyelectrolyte in these studies were based on solution behaviors of the complexes or molecular chain size changing of the polymers.There is still a lack of direct evidence to confirm the exact existence of pseudo-polyelectrolyte and its chargeability in these systems.(2)In recent decades,various methods were developed focusing on the study of double-critical concentrations-critical aggregation concentration(c1)and polymer saturation point(c2)in the association between anionic surfactants and nonionic polymers.It was deduced that different species with varied aggregates would form in the complex solution among different concentration regions divided by c1 and c2.However,few effective means could exactly achieve the separation of different aggregates and the analysis of the equilibrium relationship of species in different concentration regions of polymer-surfactant solution for it is a thermodynamic equilibrium system.Moreover,the conclusion that free micelles would form beyond c2 is still a speculation derived through the varied solution behavior.(3)Previous researches mainly focused on the association interaction between polyvinylpyrrolidone(PVP)or polyethylene glycol(PEG)and strongly ionized anionic surfactants such as sodium dodecyl sulfate(SDS)and general laws of the association between nonionic polymers and anionic surfactants were obtained.However,few reports are found about the association between weakly ionized or multi-level ionized anionic surfactants and nonionic polymers,and the related knowledge is plain.That ascribes to the complex structure and the high pH dependence of the surfactants in aqueous solution.What’s more,the structure-activity relationship of the association between polymers and different types of surfactants also demands an in-depth exploration.It is of great theoretical significance for the study of polymer-surfactant association to solve the above problems concerning on the identification of pseudo-polyelectrolyte,equilibrium relationship of species and structure-activity relationship in the complex system.At the same time,to solve these problems plays a key role to optimize and expand the application of the complex in different fields.Based on the above mentioned problems,complexes of PVP-SDS,PEG-SDS,PVP-sodium dodecylbenzenesulfonate(SDBS),PEG-SDBS,PVP-dodecanoate(SD)and PVP-monolaurel phosphate(MLP)are detailed investigated by capillary electrophoresis(CE)combined with viscometry,conductometry and tensiometry in this paper.The main contents and conclusions are as follows: 1.Identification of the pseudo-polyanionsReduced viscosity of aqueous solutions of typical PVP-SDS and PEG-SDS complexes with different specific clusterization [Г] were examined by Ubbelohde viscometer.The result showed the complex solution exhibits “polyelectrolyte effect” that the reduced viscosity increases rapidly with decreasing concentration in dilute solution.Such a result comes down to the SDS bound micelles binding onto the polymer chains and charging the neutral polymers.Relative expansion ratio of the polymer chains obtained by fitting the Fuoss equation was found growing with the increasing [Γ] until the polymer chains were saturated associated by the SDS bound micelles.Experiment results also showed PEG-SDS complex with longer PEG chain presents more remarkable “polyelectrolyte effect” and the addition of salts would weaken this effect.In all,the study on the viscosity properties of PVP(PEG)-SDS complex provides preliminary evidence for its “pseudo-polyelectrolyte” structure.As an efficient,rapid and traceable way to achieve separation and analysis of species with different charge density in aqueous phase,CE was applied to study the PVP-SDS complex based on the strong UV absorption of PVP.The experiment provided direct evidence that PVP-SDS complex are pseudo-polyanions.PVP-SDBS,PVP-SD and PVP-MLP pseudo-polyanions were also detected between c1 and c2 under specific conditions by CE.According to the strong UV absorption of SDBS,PEG-SDBS complex were also verified as pseudo-polyanions by CE.The obtained effective electrophoretic mobility(μe)further demonstrated that surface charge density of the pseudo-polyanions increases with increasing [Γ].2.Equilibrium relationship of speciesTake the advantage of SDBS presenting the characteristics to be detected by UV detection in CE,whereas SDS not,three related systems of SDBS,PVP-SDBS and PEG-SDBS were studied by the combination of three CE modes including capillary zone electrophoresis(CZE),micellar electrokinetic capillary chromatography(MEKC)and vacancy affinity capillary electrophoresis(VACE).UV probe of SDBS makes it possible for the first time to provide the direct experimental evidence for the free surfactant micelle emerging beyond c2,and further makes clear the equilibrium relationship of the polymer-related and the surfactant-related species in different SDBS concentration(cSDBS)regions divided by c1 and c2.SDBS molecule coexists with nonionic polymer without association interaction as cSDBS < c1,and coexists with the polymer-SDBS complex as c1 < cSDBS < c2,neutral polymer may still exist as cSDBS is a little higher than c1.Eventually SDBS molecule coexists with the newly emerging free SDBS micelle and certainly the already formed polymer-SDBS complex as cSDBS > c2.The concentration thresholds and equilibrium relationship of species judged by CE were proved in accordance with those results and deduction observed by tensiometry and conductometry.3.Mechanism and structure-activity relationship of the association interactionBased on the results of dissociation degree of counter-ion and electrophoretic mobility of the complexes for different systems under the addition of salt and different pH conditions,obtained by tensiometry,conductometry and CE,both adding salt and reducing p H value were verified favorable for the association interaction between nonionic polymers and anionic surfactants.It further showed that cations play the role of counter-ion bridging in the association of PVP with anionic surfactants including SDS,SDBS,SD and MLP.Other than the cation bridging,hydrogen ions act as hydrogen bonding bridges in weakly ionized surfactant systems.The hydrogen ion dependence of association interactions were reinforced with the increase of pKa values corresponding to different anionic surfactants.Therefore,the electrostatic interaction is the main driving force in the association interaction between nonionic polymers and anionic surfactants,and the hydrophobic interaction enhances the aggregation behavior of the surfactant molecules.According to the results of effective electrophoretic mobility(μe),polyelectrolyte property of different pseudo-polyanions complies with the following strength order: PVP-SD > PVP-SDBS > PVP-SDS > PVP-MLP,which is related to the degree of ionization(α)and the specific saturation capacity of clusterization([Г∞])of the pseudo-polyanions.Both the experimental and theoretical analysis results approved that μe μ α[Г∞].The association efficiency of different systems was analyzed by the [Г∞] values.It was found that PVP-SD system with pH < 9.0 had higher association efficiency than other systems due to the strong hydrogen bonding in the complex.The association efficiency between PVP and MLP was the lowest due to the great steric hindrance of the head-groups for MLP.Thus PVP-MLP presents weak association interactions only at pH < 7.4.The addition of polymer would reduce the micellar concentration from cmc to c1.According to the reduction degree of micellar concentration(cmc-c1)/ cmc,the association effectiveness of polymer-surfactant complexes were observed as the following sequence: PVP-MLP > PVP-SDS > PVP-SDBS > PVP-SD。All the above findings for different PVP-anionic surfactant systems are the combined results of hydrogen bonding,counter-ion bridging,hydrophobic interaction and steric hindrance of the head-groups of surfactants under the different salt and pH conditions.The order of impacts on the association interaction is steric effect > hydrogen bonding > counter-ion bridging.In summary,CE and other methods proved that not all anionic surfactants would associate with nonionic polymers,and in the event of association,nonionic polymer-anionic surfactant pseudo-polyanions would exactly form,in which counter-ion bridging and hydrogen bonding play important roles.This work not only has important theoretical significance for the study of polymer-surfactant system,but also presents a certain theoretical significance to guide the controllable synthesis of nanomaterials.The pH-dependent study of the association interaction between different types of anionic surfactants and nonionic polymers would help guide the application of these systems in different environments.