Ionic Liquid, The Structure Of The Chitosan Derivatives Of Worm-like Micelles

Due to the high viscoelastic properties, wormlike micelles are widely used in many areas, such as coatings, cosmetics and oil recovery. Recently, several researches have demonstrated that organic salts and some polymers can considerably increase the viscoelasticity of wormlike micelles, thus expanding its application fields. So, it is believed that looking for additives, which could increase the viscoelastic properties of wormlike micelles, is vital from both theory and application points-of-view.It is well-known that organic salts can usually decrease the electrostatic interactions among surfactant molecules so that the micelles grow rapidly, and the viscoelastic properties of micelles increase. Ionic liquids are a class of organic salts that are liquid at temperatures below100℃and they possess many distinctive properties such as negligible vapor pressure, wide potential window, high thermal stability, high viscosity, and good conductivity. Thus, in this paper, bmimBF4is chosen initially as an example of ionic liquid to study its influence on the structure of wormlike micelles. But, contrary to conventional organic salts, bmimBF4can’t increase the viscoelastic properties of wormlike micelles but induces a wormlike-rod/spherical micellar transition. So, the effects of O-carboxymethylchitosan and its hydrophobically modified derivations on the structure of wormlike micelles are studied later. It has been found that these polymers could increase the viscoelastic properties of wormlike micelles obviously and their interactions can be adjusted by varying the alkyl chain length, electrostatic interactions and temperature, which provide us sufficient information about the structure changes of wormlike micelles. The main results are as follows:(1) The structural changes of wormlike micelles composed of Tween80and Brij30in the presence of a common ionic liquid1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) were studied. Upon addition of bmimBF4, the viscosities of the wormlike micelle systems decrease, or in other words, more Brij30is needed to attain a similar viscosity value. The changes of the storage/loss modulus and the relaxation time also show that the viscoelastic properties of the wormlike micelles decrease with increasing bmimBF4concentration. We believe that these results are attributed to a wormlike-rod/spherical micellar transition in the wormlike micelle solution, which is further confirmed by FF-TEM images. The location of bmimBF4in mixed micelles was studied to explain the structure transition in the above systems. Additionally, similar results are also observed in CTAB/NaCl and SDS/Bij30/H2O wormlike micelles.(2) The enhancement of wormlike micellar structure after the addition of O-carboxymethylchitosan (OCMCS) or hydrophobically modified O-carboxymethylchitosan (hm-OCMCS) has been studied by rheology and FF-TEM. The results show that the viscoelastic properties of the wormlike micelles composed of Tween80and Brij30increase significantly after the addition of OCMCS. However, as the OCMCS molecules are hydrophobically modified, the viscosity enhancement is reduced and even diminishes with an increase of the length of aliphatic chains. FF-TEM was also used to provide us a direct investigation of the microstructure changes of wormlike micelles after the addition of OCMCS and hm-OCMCS. Combined with the rheology behavior of OCMCS and hm-OCMCS, we believe that these results are originated from a greater degree of intra-aggregation of polymers after hydrophobic modifications, which results in a weaker interaction between polymers and micelles. Additionally, the influence of temperature on the rheological behavior of polymer/wormlike micelles systems also demonstrates the intra-aggregation of polymers is the main impact factor in these systems.(3) The effect of hydrophobically modified O-carboxymethylchitosan on the structure of oppositely charged CTAB micelles has been studied by rheology, cryo-SEM and1H NMR. The results show that the viscoelastic properties of CTAB micelles increase significantly after the addition of C8-OCMCS. When CTAB content is lower, C8-OCMCS could induce the formation of complex network structure and when CTAB content is higher, or in other words, the network structure of wormlike micelles has already existed, C8-OCMCS could strengthen it. Based on cyco-SEM and1H NMR experiments, we hypothesize that these results should be attributed to the following reasons:(1) C8-OCMCS could decrease the electrostatic interactions among CTAB molecules inducing the micellar growth;(2) the C8chains could thread through and link the micelles, inducing the formation of more rigid complex network structure.(4) The effects of hydrophobically modified O-carboxymethylchitosan (Cn-OCMCS, n=4,6,8) on the viscoelastic properties of oppositely charged CTAB/NaCl wormlike micelles have been studied. The rheological data show that addition of C4-OCMCS decreases the viscoelastic properties of wormlike micelles. But, as the length of alkyl chain increases, the influence of Cn-OCMCS becomes opposite gradually, as shown by a slight increase of viscoelastic properties of wormlike micelles induced by C6-OCMCS and a larger increase induced by C8-OCMCS. From the detailed examination of how Cn-OCMCS (n=4,6,8) influences the FF-TEM images of wormlike micelles and1H chemical shifts of surfactant molecules, we posit a possible mechanism to explain these results, which lies on the length of alkyl chains on polymer molecules. In the mixed system containing C4-OCMCS, the C4chain is too short to anchor into the micelles and the electrostatic interactions between CTAB and C4-OCMCS may remove some surfactant molecules from the structure of wormlike micelles, thus the viscoelastic properties of admixtures decrease. But, for C6-OCMCS and C8-OCMCS, intercalation of the hydrophobic stickers into the wormlike micelles physically cross-links the system, so that the viscoelastic properties of admixtures increase. Because the cross-link effect is proportional to the alkyl chain length, the effect of C6-OCMCS is weaker than that of C8-OCMCS.(5) The effect of hydrophobically modified O-carboxymethylchitosan (C4-OCMCS, C8-OCMCS) on the rheological behavior of like-charged SDS/Bij30/H2O wormlike micelles has been studied by rheology, FF-TEM and1H NMR. The results show that in the presence of C4-OCMCS and C8-OCMCS less Brij30is needed to attain a similar viscosity value, but the maximum viscosity is also decreased. However, no obvious interaction is observed between polymer and wormlike micelles in1H NMR experiments. Compared with the effect of NaCl on the viscoelastic properties of wormlike micelles, we believed that C4-OCMCS or C8-OCMCS plays similar role to salt in this system.