Phase Separation Phenomena Underlie the Preparation of Novel Particles

Phase Separation Phenomena Underlie the Preparation of Novel Particles 

As described above, interactions between oppositely charged surfactants and polyelectrolytes in aqueous solutions can lead to associative phase separation, where the concentrated phase assumes the form of a viscous liquid, gel, liquid crystal or precipitate. This behavior has been exploited to form gel particles, which have been prepared by drop-wise addition of cellulose-based polycation solution (chitosan or-, N,N,N-trimethylammonium derivatized hydroxyethyl-cellulose) to anionic (sodium dodecyl sulfate, sodium perfluorooctanoate) and cationic (cetyltrimethylammonium bromide/sodium perfluorooctanoate) surfactant solutions [76-80]. 

The formation of these DNA gel particles constitutes an example of strong associative phase separation. An indication of the strength of this interaction is the formation of a stronger film (or skin) constituted by the polyelectrolyte-surfactant complex. Preliminary results of S AXS measurements have supported the existence of an ordered structure formed on the hydrated skin of the obtained particles. SEM images of the cross-section of the particles have given evidence for the existence of a shell structure, its formation being more pronounced in the case of ss-DNA. The capsule shells obtained may be considered as physical networks in which surfactant micelles form polycationic-multianionic electrostatic complexes as crosslink points. 

Determinations of the degree of DNA entrapment show loading efficiency (LE) values of CTAB-DNA particles higher than 99%, confirming the effectiveness of DNA entrapment in the surfactant solution. DNA loads of up to 2% were achieved. The binding of CTAB to DNA depends of the secondary structure of the polyelectrolyte. Quantification of the surfactant-DNA complexes in the particles shows significative differences (52% versus 79% in particles formed with native ds-DNA and denatured ss-DNA, respectively). 

Differences in CTAB-DNA interactions between the secondary structures of DNA are displayed during the swelling behavior experiments in the presence of high salt content. While CTAB-dsDNA particles placed in 150 mM NaBr monotonously dissolve with time, particles formed with denatured DNA show an initial swelling and dissolve only after 600 h. The observed response is related to the capacity to form stronger DNA-surfactant complexes in the latter system, to which both higher flexibility and amphiphilic character contribute. 

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