NMR Study On The Aggregation Properties Of A Cationic Partially Fluorinated Surfactant DEFUMACl And Its CF/CH Mixed Systems

Fluorinated surfactants,with partially or per-fluorinated tails,differ from hydrogenated surfactants in several respects.Because of the larger volume and higher electro-negativity of fluorine than hydrogen,the introduction of fluorine atoms instead of hydrogen results in the specific properties of fluorocarbon surfactants:exceptional activity,thermal and biological inertness,high gas-dissolving capacity,low surface tension,excellent spreading characteristics,and high fluidity.These properties have triggered wide applications of fluorocarbon surfactants in our daily life, pharmaceuticals,and various industrial fields.Despite the wide technical interest of fluorocarbon surfactants due to their potential usefulness,with the large hydrophobicity and strong surface activity,the applications of pure fluorocarbon surfactants are quiet limited for the reason that a per-fluorinated surfactant uasually has not all the required properties for a given application,mainly because of its oleophobic properties.Hence,in practical applications,fluorocarbon surfactants are very often used in mixed systems with hydrocarbon surfactants and polymers to obtain some desired performance and unique features by playing with the surfactant ratio.The interaction between protein and surfactant has triggered extensive interest for many years.The exceptional chemical and biological inertia and hydrophobic and oleophobic character of fluorocarbon surfactant influence the interaction with protein, hence the mixtures are not only of particular importance in numerous biotechnological applications,in pharmaceutics and chemistry etc.,but also important in simulating biological systems,which expedites the penetration of biological technology into chemical industry and medicine field.The present knowledge of the aggregation behavior and physical chemistry in general of fluorinated surfactants reported was mainly about the per-fluorocarbon oxylates.Nonionic fluorocarbon surfaetants were dealt with in a serie of papers by Ravey.Very little has been published about cationic fluorocarbon surfactants.One reason is the difficulties in synthesis and purity.Cationic surfactants are scarce. Recently,with the rapid developments in synthetic technology and measurement methods,an increasing interest is directed towards the cationic fluorocarbon surfactants and their mixtures.The focus of this thesis has been on the properties of DEFUMACl,a novel-structured,partially fluorinated,cationic surfactant, diethanolheptadecafluoro-2-undecanolmethyl-ammonium chloride aqueous solutions; the interaction with conventional cationic hydrocarbon surfactants,C_nTACl(n=12, DTACl;n=14,TTACl;n=16,CTACl,and n=18,OTACl),as well as the interaction with water-soluble cationic Gemini EO-PO-EO triblocked copolymer and BSA by the combination of advanced techniques:¹H and ¹⁹F NMR,DLS,CD and surface tension. For comparison,parallel measurements have also been made on the interaction between DEFUMACl and nonionic EO-PO-EO blocked Pluorinic copolymer,F127 (PEO₉₉-PPO₆₇-PEO₉₉)and Pluorinic-R 17R4(PPO₁₄-PEO₂₄-PPO₁₄).Some important evidence for such complexation is obtained.These studies are of significance in making a good physical picture of the interaction between macromolecules and fluorocarbon surfactant.The present dissertation includes five sections.In the first section,the importance of study on fluorocarbon surfactants and their mixtures with hydrocarbon surfactants,copolymers and proteins,and the study progress on fluorocarbon surfactants and its mixtures are summarized,including methods employed in such studies.In the second section,the self-aggregation properties of DEFUMACl aqueous solutions are studied by surface tension and ¹⁹F NMR spectroscopy measurements. The effects of temperature and NaCl on the aggregation behavior of DEFUMACl are also investigated,obtaining such important physicochemical parameters,as cmc values at different temperature and counter-ionic binding degree(β)of DEFUMACl. The results show that cationic surfactant DEFUMACl has higher surface activity,with the cmc of DEFUMACl by ¹⁹F NMR is 3.40 mmol·L^-1,which is completely consistent with that obtained by surface tension method.With the increase of temperature,the cmc value decreases slightly at first and then increases slowly at higher temperature, and the cmc values of DEMUAC1 have a minimum value at T=～308 K.The addition of NaC1 in surfactant aqueous solution strongly decreases the cmc values of DEFUMACl systems,and the more the sum of NaCl is,the lower the cmc value of DEFUMACl salt solution.In the third section,the aggregation behavior and the interaction of four mixed systems for DEFUMACl mixing with cationic hydrocarbon surfactants, alkyltrimethylammonium chloride,C_nTAC1(n=12,14,16,and 18;where n=12, DTAC1;n=14,TTAC1;n=16,CTAC1,and n=18,OTAC1),were studied by ¹H and ¹⁹F NMR in detail.The results of ¹⁹F NMR measurements strongly indicate that in the three mixed systems of DEFUMACl/DTAC1,DEFUMACl/TTAC1,and DEFUMACl/CTAC1 at different molar fractions of fluorocarbon surfactantα_F (C_DEFUMACl/C_{DEFUMAC+CCnTAC1}with an increase of the total concentration(C_T=C_F+C_H),the mixed micelles at the first break point and the individual DEFUMACl micelles at the second break point form.However,three different types of micelles were determined in DEFUMACl and OTAC1 mixtures by ¹⁹F NMR measurements,the OTAC1-rich mixed micelles,the DEFUMACl-rich ones,and the individual DEFUMACl micelles, respectively.The chemical shifts of protonΔδ(¹H)for -CH₃ in the mixed systems of DEFUMACl/C_nTAC1(n=12,14,16,18)have different variation trends from ¹⁹F NMR measurements.For the two systems of DEFUAC1/DTAC1 and DEFUMACl/TTAC1, the mixed micelles at the first break point form.At the second break point,for lowerα_F,the DTAC1-rich and TTAC1-rich mixed micelles form with a strong down-field shift;however,for higherα_F,DEFUMACl-rich mixed micelles form with a strong up-field.For the other two systems of DEFUMACl/CTAC1 and DEFUMAC/OTAC1, mixed CH/CF-surfactant micelles form.And the chemical shifts of protonΔδ(¹H)of -CH₃ increase with an increase of the total concentration for the two mixtures.At higher total concentration,the larger effect of fluorinated chains of DEFUMACl on CH-chains was obvious,resulting in the strong up-field chemical shifts.In cationic fluorocarbon/hydrocarbon surfactant mixtures,the different kinds of micelles observed by ¹⁹F and ¹H NMR measurements could be caused by the increase in alkyl chain of hydrocarbon surfactants with different critical micelle concentration.In combination of the two theoretical models for mixing,it could be concluded that:for the four different chain-length hydrocarbon surfactants studied,in a certain range of total concentration,the two components of mixtures interacted with each other, forming mixed micelles in two completely different ways according to their molecular properties and cmc values:one is close to ideal mixing with the formation of one type of mixed micelles,such as DEFUMACl/DTAC1 and TTAC1 systems;the other is the demixing with the formation of two types of micelles,i.e.fluorocarbon-rich mixed micelles,and hydrocarbon-rich mixed miceUes,as DEFUMACl/CTAC1 and OTAC1 systems did.However,as the total concentration of the mixed systems were high enough,due to the initial respective interaction between fluorocarbon and hydrocarbon chains,the four systems tend to demix,forming individual micelles of corresponding component.That is to say,at high total concentration,there found individual DEFUMACl miceUes in all the four systems.These results may direct primarily toward acquiring an understanding of the mechanism of CF-CH mixtures in aqueous solution and secondarily to providing the more detail information on non-ideal mixing.In the forth section,mixed solutions of DEFUMACl with a novel structured cationic EO-PO-EO triblocked copolymer(Cat-Gemini copolymer)were investigated by ¹⁹F NMR spectroscopy,surface tension measurements and DLS techniques.The results show that DEFUMACl strongly interacts with Cat-Gemini copolymer,and three peculiar break points at given copolymer concentrations have been defined: corresponding to the critical association concentration of DEFUMACl,P₁;the concentration where cationic Gemini copolymer molecules become saturated by DEFUMACl micelles,P^*;and the concentration where DEFUMACl micelles and DEFUMACl/Cat-Gemini copolymer mixtures coexist,P₂.In contrast,anionic surfactant SDS shows relatively weaker interaction with the same copolymer.For comparison,two types of one-chain nonionic blocked copolymer EPE-type F127 and PEP-type 17R4 solutions mixed with DEFUMACl were also studied.For straight single chain copolymer F127 and 17R₄ mixed systems,one can observe the similar three break points.But there still existed some interesting differences between the Gemini branched-chain copolymer and the single chain ones.P₁ values for the three copolymers decrease along the sequence of 17R4,F127 and Cat-Gemini copolymer, suggesting Gemini copolymer demonstrate the most stronggest interaction with the cationic CF surfactant,while the 17R4 take relatively weaker interaction with DEFUMACl.The results also provided us useful information on the interaction sites between CF-surfactant with different copolymers.The distributions of hydrodynamic radius, from DLS clearly show that bimodal distributions are obtained in 1 wt% Cat-Gemini copolymer systems with and without DEFUMACl.Without DEFUMACl, the two modes originate from two types of diffusing complexes,one with the averaged ～3.4 nm,and the other～60 nm.On the other hand,in DEFUMACl/Cat-Gemini mixtures,the larger mode increased in to～134 nm. The observed results suggest that in pure 1 wt%Cat-Gemini copolymer aqueous solution,the fast mode corresponds to the monomer of copolymer,and the second mode to the copolymer micelle.A small amount of DEFUMACl could associate with copolymer forming larger complexes.These results obtained in this work should broaden the useful information for a better understanding of the mechanism of interaction and the behavior of surfactant-polymer mixtures.In the fifth section,the interaction of DEFUMACl with bovine serum albumin (BSA)was investigated by Circular Dichrosium(CD)and ¹⁹F NMR spectroscopy measurements at room temperature.As a combination of electrostatic and hydrophobic contributions,a quite complex behavior between DEFUMACl and BSA has been observed.Comparing to DTAC1,DEFUMACl has much stronger binding ability with BSA to induce the denaturation of BSA at very lower fluorocarbon surfactant concentration,reflected in a clear decrease in the ellipticity content,whereα-helix structure decreased sharply andβ-sheet conformation increased as the concentration of BSA is high enough(such as above 0.5 g/L).As the concentration of DEFUMACl increased up to its cmc,BSA chain becomes saturated by DEFUMACl molecules,then after the content ofα-helix andβ-sheet of BSA keep constant.While the tertiary structure lies in the relatively loose state as the secondary structure of BSA was disrupted.No protective effect was observed in present systems,which were well reported in conventional hydrocarbon anionic surfactant and some in cationic Gemini surfactant with protein systems.The above behavior is consistent with corresponding changes in the behavior of fluorocarbon surfactant in bulk solution by ¹⁹F NMR measurements,and obtained three critical concentrations as DEFUMACl interacted with BSA,corresponding to the concentration at which DEFUMACl starts to bind on the BSA,P₁;the concentration where BSA molecules become saturated by DEFUMACl micelles,P^*,and the concentration where individual DEFUMACl micelles formed,P₂.At lower DEFUMACl concentration,DEFUMACl molecules interact with BSA mainly by electrostatic forces;as the concentration of DEFUMACl reached P^*,hydrophobic interaction between fluorocarbon chains of DEFUMACl took the main role,and bead-like complexes were formed between BSA and DEFUMACl aggregates.Further addition of DEFUMACl to P₂,individual DEFUMACl micelles formed,and DEFUMACl monomers,DEFUMACl/BSA complexes,and DEFUMACl micelles reached equilibrium.