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Featured researches published by Na Li.


Green Chemistry | 2006

A cyclic voltammetric technique for the detection of micro-regions of bmimPF6/Tween 20/H2O microemulsions and their performance characterization by UV-Vis spectroscopy

Yanan Gao; Na Li; Liqiang Zheng; Xueyan Zhao; Shaohua Zhang; Buxing Han; Wanguo Hou; Ganzuo Li

As green solvents, ionic liquids (ILs) can be substitutes for traditional organic solvents, and form useful microemulsions with water. Microemulsions consisting of the IL, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), the non-ionic surfactant Tween 20 and water were formed at 30.0 °C, and the phase behavior of the ternary system was investigated. Three regions of the microemulsions: water-in-bmimPF6 (W/IL), bicontinuous, and bmimPF6-in-water (IL/W) were identified by cyclic voltammetry using potassium ferrocyanide K4Fe(CN)6 as an electroactive probe. The polarity of the microemulsion environment was investigated by UV-Visible spectroscopy using methyl orange as a probe. Use of the ionic compound K3Fe(CN)6 in UV-Visible measurements revealed that the bmimPF6/Tween 20/H2O microemulsions could solubilize salt species into the microemulsion droplets. Moreover, the solubilization of riboflavin in the microemulsions was also shown by UV-Visible spectra. These results show that these IL-based microemulsions have potential in the production of metal nanomaterials, in biological extractions or as solvents for enzymatic reactions.


Langmuir | 2011

Aggregation Behavior of Long-Chain N-Aryl Imidazolium Bromide in Aqueous Solution

Lijuan Shi; Na Li; Han Yan; Yanan Gao; Liqiang Zheng

The aggregation behavior of three long-chain N-aryl imidazolium ionic liquids (ILs), 1-(2,4,6-trimethylphenyl)-3-alkylimidazolium bromide [C(n)pim]Br (n = 10, 12, and 14), in aqueous solutions was systematically explored by surface tension, electrical conductivity, and (1)H NMR. A lower critical micelle concentration (cmc) for the N-aryl imidazolium ILs is observed compared with that for 1,3-dialkylimidazolium ILs [C(n)mim]Br, indicating that the incorporation of the 2,4,6-trimethylphenyl group into a headgroup favors micellization. The enhanced π-π interactions among the adjacent 2,4,6-trimethylphenyl groups weaken the steric hindrance of headgroups and thus lead to a dense arrangement of [C(n)pim]Br molecules at the air-water interface. An analysis of the (1)H NMR spectra revealed that the introduced 2,4,6-trimethylphenyl group may slightly bend into the hydrophobic regions upon micellization. The micelle formation process for [C(n)pim]Br (n = 10, 12, and 14) was found to be enthalpy-driven in the investigated temperature range, which is attributed to the strong electrostatic self-repulsion of the headgroups and the counterions as well as the π-π interactions among headgroups. Strong, stable fluorescence properties are presented by the new N-aryl imidazolium ILs, indicating their potential application in the field of photochemistry.


Langmuir | 2009

Temperature-Induced Microstructural Changes in Ionic Liquid-Based Microemulsions

Yanan Gao; Na Li; Liane Hilfert; Shaohua Zhang; Liqiang Zheng; Li Yu

In the present contribution, results concerning the effect of temperature on the nonionic surfactant Triton X-100 based 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4)-in-cyclohexane and bmimBF4-in-toluene ionic liquid (IL) reverse microemulsions are reported. Dynamic light scattering (DLS) along with freeze-fracture transmission electron microscopy (FF-TEM) measurements revealed that the sizes of single microemulsion droplets increased with increasing temperature. However, a decreased temperature led to the appearance of droplet clusters, which have also been observed previously when the single microemulsion droplets were swollen by added bmimBF4 to a certain extent (Gao, Y. A.; Vogit, A.; Hilfert, L.; Sundmacher, K. ChemPhysChem, 2008, 9, 1603-1609). Compared to traditional aqueous microemulsions, IL microemulsions revealed relatively high temperature-independence. The droplet-shaped microstructure was always kept in a large range of temperature. The temperature-independence is ascribed to the temperature-insensitive electrostatic attraction between the solubilized bmimBF4 and Triton X-100, which was considered to be the driving force for solubilizing bmimBF4 into the cores of Triton X-100 aggregates. Two-dimensional rotating frame nuclear Overhauser effect (NOE) experiments (ROESY) further confirmed the microstructural change with temperature.


Journal of Physical Chemistry B | 2009

Microstructures of Micellar Aggregations Formed within 1-Butyl-3-methylimidazolium Type Ionic Liquids

Yanan Gao; Na Li; Xinwei Li; Shaohua Zhang; Liqiang Zheng; Xiangtao Bai; Li Yu

Nonionic surfactant Triton X-100 was shown to aggregate and form micellar aggregation in ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF(4)) and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF(6)). The surface tension measurements revealed that the dissolution of Triton X-100 in ILs depressed the surface tension in a manner analogous to aqueous solutions, and a relatively higher critical micellar concentration (CMC) was obtained compared to that of water. Freeze-fracture transmission electron microscopy (FFTEM) shows that the micelles have an irregular droplet shape, which is larger than that formed in water. The micellar droplets preferred to assemble into larger clusters. (1)H NMR and two-dimensional rotating frame nuclear Overhauser effect (NOE) experiments (2D ROESY) show that the addition of Triton X-100 destroyed the ion pairs of pure ILs due to the electrostatic interaction between the positively charged imidazolium cation of ILs and the electronegative oxygen atoms of oxyethylene (OE) units of Triton X-100. The electrostatic interaction behaves similar to hydrogen bond that occurred between the OE units of nonionic surfactants and water molecules in aqueous micelles and cooperates with solvatophobicity, leading to the formation of IL micelles. The 2D ROESY analysis reveals that the microstructures of Triton X-100-based micelles in ILs are not regular spherical, which accords with the FFTEM image. Similar to the aqueous micellar systems, the hydrophobic interaction or solvatophobicity was found to drive the formation of micelles.


Langmuir | 2008

Aggregation behavior of polyoxyethylene (20) sorbitan monolaurate (tween 20) in imidazolium based ionic liquids.

Jiapei Wu; Na Li; Liqiang Zheng; Xinwei Li; Yanan Gao; Tohru Inoue

Surface tension measurements were carried out for the solutions of polyoxyethylene (20) sorbitan monolaurate (Tween 20) in 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF 4) and hexafluorophosphate (bmimPF 6) at various temperatures. Two transition points were found in the surface tension-concentration curves at each temperature. The freeze-fracture transmission electron microscopy revealed that two kinds of particles with different sizes are formed at the concentrations of each transition point. Thus, the surfactant concentrations of the two transition points are regarded as critical aggregation concentrations, CAC 1 and CAC 2. From the CAC values and their temperature dependence, we estimated the thermodynamic parameters of the aggregate formation, Delta G agg (0), Delta H agg (0), and Delta S agg (0). The thermodynamic parameters related to CAC 1 are almost independent of temperature. On the other hand, as for the aggregate formation at CAC 2, a positiveDelta S agg (0) contributes to a negative Delta G agg (0) at low temperature, while a negative Delta H agg (0) contributes to a negative Delta G agg (0) at high temperature. The behavior of the thermodynamic parameters as a function of temperature, combined with the variation of (1)H NMR chemical shifts of the bmim (+) protons as a function of the surfactant concentration, demonstrated that the aggregates formed at CAC 1 are nanodroplets of Tween 20 segregated from the solution phase, while those formed at CAC 2 are similar to the usual surfactant micelles formed in aqueous solution.


Langmuir | 2009

Aggregation behavior of a fluorinated surfactant in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid.

Na Li; Shaohua Zhang; Liqiang Zheng; Tohru Inoue

The cationic fluorinated surfactant, FC-4, unlike other surfactants, forms micelles in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimTf2N). Surface tension, freeze-fracture transmission electron microscopy, 19F NMR, 1H NMR, and Fourier transform infrared measurements revealed that (i) the FC-4 cation forms an ion pair with the Tf2N anion, (ii) the ion pairs undergo association to form premicellar aggregates, and (iii) the premicellar aggregates transform into micelles at the critical micelle concentration (CMC). The thermodynamic parameters for micelle formation derived from the temperature dependence of the CMC demonstrated that the solvophobic interaction between the solvophobic tails of the surfactant molecules is rather weak in bmimTf2N compared with other ionic liquids, in accordance with the observation that surfactants do not readily form micelles in bmimTf2N. The fact that FC-4 forms micelles in such an inconvenient solvent is attributed to the ion-pair formation between the surfactant cation and the ionic liquid anion.


Journal of Physical Chemistry B | 2008

Aggregation behavior of a fluorinated surfactant in 1-butyl-3-methylimidazolium ionic liquids.

Na Li; Shaohua Zhang; Liqiang Zheng; Jiapei Wu; Xinwei Li; Li Yu

The aggregation behavior of a fluorinated surfactant (FC-4) was studied by surface tension measurements in 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF 4]) and hexafluorophosphate ([bmim][PF 6]) at various temperatures. A series of surface properties, including adsorption efficiency (p C 20), effectiveness of surface tension reduction (Pi CAC), maximum surface excess concentration (Gamma max) and minimum surface area/molecule (A(min)) at the air-water interface were estimated. By comparing the fluorinated surfactant with traditional surfactants, we deduced that the surface activity of the fluorinated surfactant in ILs was superior to the activity of other surfactants. From the CAC values and their temperature dependence, we estimated the thermodynamic parameters of aggregate formation. The thermodynamic parameters indicate that the aggregate of FC-4 in [bmim][BF 4] is a traditional micelle, while the aggregate of FC-4 in [bmim][PF 6] is nanodroplets composed of FC-4 molecules segregated from the solution phase. These results were further confirmed by (1)H NMR measurements.


Colloids and Surfaces B: Biointerfaces | 2011

Novel nanogels as drug delivery systems for poorly soluble anticancer drugs.

Na Li; Jinli Wang; Xingguo Yang; Lingbing Li

Two types of novel nanogels were prepared using shell cross-linking of Pluronic F127 micelles with polyethylenimine (PEI) (F127/PEI nanogel), and penetrating network of poly(butylcyanoacrylate) (PBCA) in Pluronic F127 micelles (F127/PBCA nanogel). Poorly soluble anticancer drug, paclitaxel (PTX) and 10-hydroxycamptothecin (HCPT), were used as model drugs and incorporated into nanogels. The results obtained from FT-IR spectroscopy confirmed that the drugs were molecularly dispersed in the nanogels. DLS measurements demonstrated that the nanogel size distribution was narrow with average diameter less than 200 nm. TEM images indicated that the nanogels were spherical in shape and had smooth surfaces. The drug-loaded nanogels showed sustained release profiles compared with the free drugs as revealed by in vitro release experiments. Cytotoxicity tests showed that the cytotoxicity of drug-loaded nanogels against cancer cell in vitro was much higher than that of the free drug. The data demonstrate that these novel nanogels improved stability towards dilution, increased solubility and showed better cellular uptake by cells compared with free drug.


Journal of Physical Chemistry B | 2008

Aggregation Behavior of Pluronic Triblock Copolymer in 1-Butyl-3-methylimidazolium Type Ionic Liquids

Shaohua Zhang; Na Li; Liqiang Zheng; Xinwei Li; Yanan Gao; Li Yu

Three amphiphilic poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) ethers triblock copolymers, denoted Pluronic L61 (PEO3PPO30PEO3), Pluronic L64 (PEO13PPO30PEO13), and Pluronic F68 (PEO79PPO30PEO79) were shown to aggregate and form micelles in ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6). The surface tension measurements revealed that the dissolution of the copolymers in ILs depressed the surface tension in a manner analogous to aqueous solutions. The cmcs of three triblock copolymers increase following the order of L61, L64, F68, suggesting that micellar formation was driven by solvatophobic effect. cmc and gamma cmc decrease with increasing temperature because hydrogen bonds between ILs and hydrophilic group of copolymers decrease and accordingly enhance the solvatophobic interaction. Micellar droplets of irregular shape with average size of 50 nm were observed. The thermodynamic parameters DeltaGm0, DeltaHm0, DeltaSm0 of the micellization of block copolymers in bmimBF4 and bmimPF6 were also calculated. It was revealed that the micellization is a process of entropy driving, which was further confirmed by isothermal titration calorimetry (ITC) measurements.


Journal of Physical Chemistry B | 2009

Organic Solvents Induce the Formation of Oil-in-Ionic Liquid Microemulsion Aggregations

Yanan Gao; Na Li; Shaohua Zhang; Liqiang Zheng; Xinwei Li; Bin Dong; Li Yu

The role of four organic solvents in the formation process of 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) based ionic liquid (IL) microemulsions is investigated. The results showed that the addition of Triton X-100 remarkably decreased the conductivity of bmimBF4. The added organic solvents provided a strong apolar environment for the hydrophobic tails of Triton X-100 and caused the surfactant molecules to aggregate into the interfacial film of oil-in-bmimBF4 (O/IL) microemulsions. As a result, the conductivities of the solutions were initially increased because the insulative Triton X-100 molecules were assembled, which corresponded to increasing the concentration of continuous bmimBF4 solutions. The hydrophobic interaction between the dispersed organic solvents and the hydrophobic tails of Triton X-100 may be the driving force for the formation of O/IL microemulsions. The droplets of O/IL microemulsions were successively swollen by organic solvents, and a bicontinuous IL-containing microemulsion was observed by freeze-fracture transmission electron microscopy for the first time. The current study can help in further understanding the ILs-containing microemulsions and thereby improve microemulsion theory.

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Li Yu

Shandong University

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Yanan Gao

Dalian Institute of Chemical Physics

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