The Interactions Between Amphiphilic Molecules-bile Salts And Cationic Surfactants

The interaction and synergism among mixed system of the oppositely charged surfactants havegreatly changed the properties of suface and bulk phase, which leads to their widespread applications. Theconstruction and the structural regulation of biocompatible aggregates is a hot topic in many areas, such assolution, colloids, biology, medicine and so on, and also is the key in the development of novel type of drugcarriers self-associated by amphiphilic molecules. Therefore the studies on the biocompatibility, the thermalstability, and the regulation of aggregation morphology of amphiphilic molecules should becomprehensively considered from various factors–the amphiphilic molecular conformation, the propertiesof hydrophobic groups, the size of polar groups, and charge density, etc. So far, the related thermodynamicparameters obtained are isolated and quite rare, so that it is urgent to carry out systemic work inthermodynamics. In this work, from the perspective of thermodynamics, we studied the thermodynamics ofmolecular self-assembling for mixed systems containing bile salts by using microcalorimetry, along withconductivity and turbidity measurements. The research content and results are as follows:1. The calibration of Isothermal Titration CalorimeterIsothermal Titration Calorimetry (ITC) is one of the experimental methods that can be useddirectly to study the intermolecular weak interactions. It is also a main method of thermodynamic researchused in this work. Therefore, first of all a nano-isothermal titration calorimeter with a thermostat (TAM III)was electrically and Chemically calibrated to evaluate the thermodynamic behavior of molecularself-assembliing of single and mixed surfactants in aqueous solution. Electrical calibration of thisinstrument showed that its precision is better than±0.09%. Its accuracy was tested by measuring thereaction heat of tris-hydroxymethylaminomethane (Tris), employed often as a calorimetric standardsubstance, with hydrochloric acid. The resulting value ((-47.48±0.12) kJ·mol-1) agreed well with that in theliterature.2. The thermodynamics of molecular self-assembling of amphiphiles in aqueous solutionThe thermodynamic characterizations of molecular self-assembly for the synthesized dimersurfactants and biosurfactants in aqueous solution were studied by ITC and conductivity measurements. So far due to a few directly calorimetric studies on bile salt biosurfactants, the thermodynamic properties oftheir micellization were less characterized systematically and quantitatively. In particular, directcalorimetric evidences will be necessary for the understanding of thermodynamic control factors forpre-micelle and micelle formation, and further for providing the deep insight into the thermodynamicmechanism of their self-assembling that still is in debate. The main results obtained in this aspect are asfollows:(1) For the series of dimer surfactants C12CSC12Br2(S=2,6,10) and its monomer quaternaryammonium salt DTAB and bile salts (NaC, NaDC), we obtained their thermodynamic properties ofmicellization, and studied the thermodynamic mechanism of micelle formation.(2)The experimental results of NaDC obtained from ITC show that the observed enthalpy curvesof NaDC in low concentration range and relatively high concentration range, there are two obvious ΔHobsbreaks, indicating that there are two kinds of aggregation morphogenetic of NaDC in aqueous solution. Theenergetic factors obtained from ITC support the NaDC stepwise aggregation model. Thus we analyzed anddiscussed the thermodynamic mechanism of intermolecular interaction in the two–step model, indicatingthat the process of premicellar formation is driven by entropy, and the process of micellar formation isdriven jointly by entropy and enthalpy.3. The thermodynamics and phase behavior of the mixed systems of oppositely chargedsurfactantsIntermolecular interactions of the biosurfactants, bile salts include those between hydrophobicsurfaces, hydrogen bonds on the hydrophilic surface, and hydrogen bonds and electrostatic repulsionbetween carboxyl polar heads. These interactions also have an important influence on the mixed surfactantsystems. Both the establishment of a suitable thermodynamic model and the regulation on theaggregation morphology of the mixed surfactant molecules are on the basis of understanding the extent ofthese interactions and the energetic balance. In this work, we studied the mixed systems of the oppositelycharged surfactants as follows:(1) the traditional cationic quaternary ammonium salt and anionic bile salts（DTAB+NaC，DTAB+NaDC）;(2) the dimer cationic surfactants and bile salt (C12CSC12Br2+NaC，C12CSC12Br2+NaDC). (1) We studied the thermodynamic behaviors of these mixed systems in the bile salt-richregions and rich-DTAB or rich-C12CSC12Br2regions by ITC. Thus we obtained a series of importantthermodynamic parameters-the critical micelle concentration, the enthalpy of mixed micelleand theenthalpy of interaction between two kinds of surfactants. From these thermodynamic parameters, weanalyzed the driving force of the processes of aggregation formation. The processes of micellization inrich-NaDC or rich-NaC regions are driven by entropy, and the processes of micellization in rich-DTAB orrich-C12CSC12Br2regions are driven jointly by entropy and enthalpy.(2) The molecular structure of the surfactant has important effect on the aggregation behavior ofthe mixed system. The results show that the synergy of the system, DTAB+NaDC is stronger than one ofDTAB+NaC. Compared with NaC, NaDC reduces a hydroxyl on the hydrophilic surface, so it has a smallercmc value, which directly affects the value of the cmcmic–mixin mixed systems having a bile salt.(3) The C12CSC12Br2/NaC system before the equalcharges show the aggregation behavior ofinmisciuble liquid crystal (LC) phase rather than forming the equally charged precipitation. After this, themixed system appeared to dissolve phenomenon, and finally the formation of positive charge-rich mixedmicelles. Further for C12C6C12Br2/NaDC mixed system with different molar ratios the results showed thatthe1:1ion pairs didn’t form, but equally charged complexes formed in lower C12C6C12Br2concentrationrange, We also studied the effect of the length of the spacer on the interactions of C12CSC12Br2(S=2,6,10)with NaDC. The wider composition range of liquid crystal phase was found. For the mixed systemcontaining C12C6C12Br2and C12C10C12Br2, themaximum turbidities all appear before the chargeneutralization concentration (cnc), and for the system containing C12C2C12Br2, the maximum turbidityappears after the cnc. Furthermore an important difference, compared with C12CSC12Br2/NaC system is thatin C12CSC12Br2/NaDC system there exists no critical point of redissolution before the cnc, suggesting thatthe presence of NaDC leads to the aggregate structure becoming more stable.In short we determined the phase boundaries for different mixed systems of oppositely chargedsurfactants using ITC, combined with conductivity and turbidity measurements, and further, analyzed theirphase behaviord of these mixed systems. Thus we proposed the thermodynamic model about interactionbetween two kinds of surfactants with asymmetric hydrophobic moieties of alkyl tail and rigid Face. 4. Thermodynamics of molecuar self-assembling of the mixed systems of oppositely chargedsurfactant with hydrophobic modified dextranWe constituted the oppositely charged systems of an anionic polyelectrolyte, dextran modifiedwith bile acid (Dex-CACOONa15) and a cationic surfactant DTAB or C12C6C12Br2respectively, andstudied the thermodynamic characterizations and the phase behaviors of aggregation in these mixedsystems by ITC, along with turbidity measurements. We discussed in detail the effects of the concentrationsof surfactant and polymer and the change of their mixing ratios on the aggregation morphology change andthe phase change, and further determined the phase boundaries and the thermodynamic parameters of theirinteractions. In particular in this part of work we followed the effects from various factors-the hydrophobicmoieties (NaC) of polymer, the structures of the surfactant molecules, and the concentrations ofamphiphiles, finally determining the driving forces of formation of the mixed aggregates.The resulting papers have been published in 《Acta Phys. Chim. Sin.》、《J. Chem.Thermodynamics》、《Chem.scichina.com》(3papers), and in Chinese and International thermodynamicconferences (6papers), a manuscript will be submitted soon.The work of the Master thesis is a part of the National Natural Science Foundation of China(Project No.21273061and21327003).