| Amphiphilic molecules in aqueous solutions can self-organize into a wide variety of aggregates such as micelles,vesicles,lamellae,and sponge structures,in which vesicles have been widely studied duo to their special structure.Generally,vesicle formation are observed in aqueous solutions of multi-chained amphiphiles,mixed amphiphiles,and amphiphilic polymers.Simple single-chain amphiphiles(SCAs)are not considered to be able to form vesicle,where fatty acid vesicle is an exception.Recent studies revealed that other SCAs,such as monoalkyl phosphates,alkyl sulfates,and alkyl sulfonates,can also form vesicles in single-component systems,but the mechanism and characteristics of vesicles formation are still unclear.Furthermore,according to the primitive relevance of SCAs,SCAs vesicles are favorable to be the protocell model for exploring the origin of life.Primitive SCAs such as monoalkyl phosphate(MAPs),alkyl sulfate,and alkyl sulfonic acid/salt have been less involved,while fatty acid vesicles are widely used at present.The formation mechanism and characteristics of SCAs vesicles were studied,to improve the understanding of the SCAs aggregation behavior and to provide information for exploring the origin of life.MAPs are one type of important anionic SCAs,but their aqueous solubility are very poor as the alkyl chain is long(carbon number?12).The vesicle structure can be formed at a low C,coexisting with precipitates.Thus,an interesting question raised is how to improve the solubility of MAPs and to obtain their homogeneous vesicle solutions.In this dissertation,sodium dodecylphosphate(SDP)and 4-dodecylbenzene sulfonic acid(DBSA)were selected as single-chain weakly acid/salt surfactant models.Firstly,the solubilization of SDP by short chain alcohols and guanidine salts(GuSalts)was investigated.Then,the aggregation behavior of SDP and DBS A in aqueous solution was studied,especially the formation and properties of their vesicles.The mechanism of vesicle formation was discussed,to improve the understanding of the aggregation behavior of weak acid/salt SCAs single-component system and to provide information for protocell membrane mimicking and the basis for the practical application of these aggregates.The main research contents and conclusions of the dissertation are as follows:(1)Aggregation behavior of SDP in alcohol/water mixtruesThe Krafft point of SDP is ca.40?,resulting in a quite low aqueous solubility at room temperature(i.e.,ca.0.17 mg·L-1 or 0.55 ?M).A series of short chain alcohols with different structures(with different alkyl lengths,alcohol hydroxyl positions,and alcohol hydroxyl numbers)were selected to investigate their effects on the water solubility of SDP.Then,the aggregation behavior and mechanism were studied.The results indicate that the solubility of SDP in water can be improved(up to 223.9 g·L-1 or 746.3 mM)by alcohols with suitable dielectric constants(11-25)or alkyl chains(2-7 carbons).The isotropic phase,and homogeneous vesicle systems can be formed in SDP/alcohol/H2O ternary systems at low SDP concentration.With the increase of SDP concentration,vesicles can be transformed into dendritic aggregates and micelles,which is independent of alcohol structure.The critical vesicle concentration(CVC)of SDP is about 0.3 mM,and the vesicles are of a unilamellar structure,with a size of-80 nm and membrane thickness of?3.81 nm.The molecular structure of alcohol had no significant effect on the structure and morphology of vesicles.The possible mechanism for the formation of SDP vesicles is the synergistic effect of the solubilization of alcohol for SDP and the H-bonding between SDP molecules.SDP homogeneous vesicles provide a representative for the single-component SCAs vesicle system,and also provide information for the investigation of the MAPs protocell model.(2)Study on the properties of SDP/alcohol/water vesicle systemsThe SDP vesicles formed in the mixed solvents of n-butanol(NBT)/water and n-pentanol(NPT)/water were studied,respectively.The effects of temperature,divalent metal ions(Mg2+,Ca2+),pressure and shear on the stability of vesicles were investigated.The membrane permeability of the SDP vesicles was examined by means of conductivity method and fluorescence probe technique.Especially,the SDP vesicles were used to construct spatially segregated enzyme-loaded compartments to host a cascade enzyme reaction.SDP vesicles show remarkable stability upon long-term storage,exposure to high temperature(80?),and freeze-thawing(-20,-196?/25?)treatments,but the resistance to divalent cations(Mg2+,Ca2+)is low.The vesicles transform to tube-like structures(nanotubes)with diameters of about 500 nm and lengths up to several micrometers under high pressure(3-60 bar)and/or shearing(10-500 s-1)treatments.These results indicate that the obtained SDP vesicles are thermodynamics metastable.The vesicular membrane exhibits size-selective permeability,and the vesicles support a cascade reaction to achieve the chemical signaling communication.These SDP vesicles display some of the basic functional properties of probiotic membrane,which can be used in the study of protocell model.This work improves the understanding of the features of SDP vesicles and provides information for their applications in biomembrane simulation and microreactor.(3)Aggregation behavior of SDP in GuSalts/water mixturesThe effects of five GuSalts on the aqueous solubility of SDP were investigated.The homogeneous vesicle system was obtained and the formation mechanism was discussed.In particular,the effect of temperature on the structure of aggregates was investigated,and the transformation between vesicles and ?-gels was observed.It was studied that the permeability and microviscosity of the vesicles and the rheology of the?-gels phase,to improve the understanding of the basic properties of the aggregates.The solubility of SDP can be significantly improved(up to 15.7 g·L-1 or 52.3 mM)at 25? by five GuSalts including guanidine hydrochloride(GuCl),guanidine sulfate(GuSO4),guanidine sulfamate(GuSO3),guanidine phosphate(GuPO4),and guanidine carbonate(GuCO3).The maximum solubility of SDP is related to the type of acids as guanidine counterions,decreasing in order GuCl,GuSO4,GuSO3,GuPO4,and GuCO3.Being independent of the type of GuSalts,vesicles are formed spontaneously in the isotropic phase,with a CVC of?1.0 mM.SDP and GuSalts can form "bridging dimer",which plays a key role in the formation of vesicles.The vesicles have a unilamellar structure,with a size of-80 nm and the membrane thickness of?3.81 nm,and exhibit size-selective permeability.The microviscosity of SDP vesicle membranes is 35.79-49.34 mPa·s at 25 ?.A temperature-induced reversible transition between vesicles and?-gels was observed for the SDP/GuSalt/H2O systems when the SDP content is higher than 20 mM,with a transition temperature of ca.22 ?.The ?-gels are composed of vesicles and bilayer sheets,whose water content could reach 98 wt%,showing the similar viscoelasticity to conventional gels.This work deepens the understanding of the aggregation behavior of MAPs and also provides valuable information for their practical applications.(4)Aggregation behavior of DBS A in waterThe aggregation behavior of DBS A in aqueous solution was studied,and the effect of concentration was mainly investigated.The structure,stability and membrane permeability of the vesicles were characterized,especially the effect of wet-dry cycle on the aggregate structure was studied,and the related mechanism was discussed.The aim was to deepen the understanding of one-component SCAs aggregation behavior.The results show that the concentration-driven stepwise aggregation behavior of DBS A in water.With an increase in the DBS A concentration,micelles are first formed,and then transformed partially into vesicles that coexist with micelles in the solution,with a critical micelle concentration(CMC)and CVC of?0.53 and 2.14 mM,respectively.The vesicles are of a unilamellar structure,with a size of?80 nm and the membrane thickness of?2.87 nm,and their membranes exhibit size-selective permeability.In addition,the vesicles show remarkable stability upon long term storage,exposure to high temperature(80?),and freeze-thaw cycles(-20?/25?).The H-bonding interaction between DBS A species and the interdigitated structure of alkyl chains in bilayers play a key role in the formation and stability of DBSA vesicles.Interestingly,we found that the wet-dry cycle for the micelle and unilamelar vesicle solutions can induce a micelle-to-vesicle transition and an obvious increase in the size of the original vesicles,accompanied by the formation of some multilamellar vesicles.Probably,bilayer stacking structures are formed during the drying process of the micelle/vesicle solutions,which transform into larger and multilamellar vesicles during the rehydration.This study shows that the wet-dry cycles promote the formation of vesicular structure of one-component SCAs single-component system,which provides a new way for the preparation of pure SCAs vesicle system. |
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