Reversible phase transition from vesicles to lamellar network structures triggered by chain melting

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Original entry by A.J. Kumar, APPHY 225 Fall 2009


Yuwen Shen, Jingcheng Hao, Heinz Hoffmann and Zhonghua Wu. Soft Matter, 2008, 4, 805–810


vesicle, phase transitions, lamellar network, chain melting, catanionic surfactant


In this article, the authors report the observation of a reversible phase transition from densely packed multilamellar vesicles of cationic and anionic (catanionic) surfactant with an amount of salt (NaBr) to network structures. The transition was triggered by chain melting. Catanionic surfactant vesicles in aqueous solution are of great interest because of their potential applications in drug-delivery, micro-reactors to produce colloids, and cosmetics. A more extensive knowledge of the phase behavior of such materials will greatly advance the ability of scientists to engineer useful materials that exploit phase behavior.

This article also gives insight into exactly how to experimentally characterize phase transitions in soft matter. The authors created mixtures of different amounts of NaBr and mixed them with a solution of salt-free catanionic veisicular aqueous solution. As temperature was increased, visual observations both with and without polarizers were used to observe phase behavior. Conductivity, turbidity, and viscosity were measured. Addionally, Fourier transform infrared spectroscopy measurements were used to monitor the chain-melting process. Small-angle X-ray scattering (SAXS) measurements, polarizing microscopy measurements, and TEM imaging was also done. The polarizing microscopy measurements revealed birefringent lamellar structures. The TEM images provided direct evidence of the transition from multi-lamellar vesicles to three-dimensional lamellar networks.

Figure 1: FF-TEM micrographs of the top precipitates for two phases of precipitates and L1-phase. The two samples: 100 mmol <math>L_1</math> TTAL–80 mmol <math>L_1</math> NaBr (a) and 100 mmol <math>L_1</math> TTAL–100 mmol <math>L_1</math> NaBr (b).11 A typical polarizing micrograph of the top precipitates for the sample of 100 mmol <math>L_1</math> TTAL–100 mmol <math>L_1</math> NaBr was inserted in the FF-TEM image.
Figure 2: TEM micrograph of birefringent solution of 100 mmol <math>L_1</math> TTAL and 60 mmol <math>L_1</math> NaBr at <math>70^o</math> C.

The surfactant used was tetradecyltrimethylammonium laurate (TTAL). In salt free solution, TTAL forms mostly unilamellar vesicles. The addition of salts change the balance of the driving forces due to the presence of ions. As NaBr is added, the solution changes from unilamellar vesicles to densely packed multi-lammellar vesicles. As temperature increased, a phase transition was observed as the vesicles broke down and formed a three dimensional lammelar network, which led to dramatic changes in the solutions physical characteristics.

Figure 3: Polarizing micrographs of 100 mmol <math>L_1</math> TTAL and 80 mmol <math>L_1</math> NaBr at (a) T ¼ <math>25^o</math> C (T < Tm, precipitates), (b) <math>55^o</math> C (T ¼ Tm, birefringent solution), and (c) <math>70^o</math> C (T > Tm, birefringent solution).
Figure 4: Phase behavior induced by increasing temperature for two systems of 100 mmol <math>L_1</math> TTAL–80 mmol <math>L_1</math> NaBr and 80 mmol <math>L_1</math> NaBr aqueous solution. Conductivity data of the two systems were inserted. The conductivity was measured after the sample was kept for two weeks at the equilibrium temperature. Samples of two phases of precipitates–L1-phase at lower chain melting and birefringent phase above Tm are inserted in the phase diagram.

Similar results have been reported for lipids but the authors claim that this report is the first time that phase conversion from catanionic surfactant vesicles to bilayer networks triggered by chain-melting has been observed.

Soft Matter Connection

Phase transitions in soft matter offer a large amount of unexplored potential. The effect of surfactants to create micelles and a zoo of other structures creates room for all types of interesting and potentially useful phase behavior. If we can gain a more comprehensive understanding of the phases and metastable phases accessible in surfactant solutions, polymers, and colloids, we will be able to move into a whole new realm of materials design, with the potential to have huge impacts in drug delivery, consumer goods, and more.

This article adds an important piece to the vast foundation that must be laid to begin mapping out and understanding the phase behavior of some of these soft matter materials.