In some papers, the acid is removed by repeated centrifuge until the supernatant is turbid, then the supernatant is collected and dialyzed to obtain CNWs. While in other papers, the supernatant is removed and the washed cellulose sediment (after several centrifugations) is diluted with water. Which process is correct? CNWs exist in supernatant or sediment after several centrifugations? which one should I collect? supernatant or sediment?
Thanks in advace
Dear Catherin
the two ways you mentioned are correct. you should collect the sediment after dialysis and centrifugation and do homogenization or soniaction to loose the aggregate. a final step of filtration with sintered funnel of size 1 will be required to a sure removal of un hydrolyzed particle (not nano size).
(sorry for me english, I´m not native)
After the hydrolysis process you need to washed (generally by centrifugal process, but have cases that only a simple basic neutralization is made) in order to remove the remaining acid products (i.e HCl or H2SO4) of the media. In this step, under pH 4, the cellulose nanocrystals dont have good interactions with water and will preciptate easly even a low centrifugal force. But, when you achieve a media that the surface of cellulose nanocrystals will have good interactions, the particles will became more stable and will need a higher centrifugal force to preciptate. (*Note: after each centrifugal process is recomend to homogenize the preciptate in the aqueous media to do another wash)
When supernatant became turbid, the pH of the media is about 4 or higher. In the case of use of H2SO4 is more easy, due the surface is esterified by H2SO4 converting some -OH groups to SO3- groups. The SO3, when pH>4 will have charge and will have eletrostatic repulsion between cellulose surfaces. The cellulose nanocrystals obtained by HCl will need a ultrasonification for desagregated, but will not stay stable for much longer (the OH groups have good interactions and complementarity surface) and will need a new ultrasonification step to use.
Therefore, probably will remain a preciptate phase too. The two phases will separate the hydrolysed cellulose by size, or surface charge density.
A final dyalysis process generally is necessary to remove remaining acid products highly adsorbed to the surface and another produts as olygossacharides.
If you not have need to lyophilise, is more appropriate to mantain in aqueous suspension with CH3Cl (1 drop to 250 mL) to conserve.
If you need to lyophilise, is recomend that you diminish eventually aggregation (will became more laborius to resuspend it). For this purpose you could freeze in concentrations about 1mg.mL^-1 and do quickly by N2 liquid.
If you need the references ask me.
see: http://link.springer.com/article/10.1007%2Fs10570-015-0626-y
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