the present invention relates to the use of the supercritical fluid method to produce water - soluble high molecular - weight chitosan . the presently disclosed method modifies chitosan using supercritical fluid co 2 to enhance its water solubility in the ph range between 6 and 8 . referring to fig2 , the preparation method includes one or more of the following steps : first , a chitosan powder , an alkyl alcohol such as ethanol , an acid such as an organic acid or inorganic acid ( e . g . hydrochloric acid ), and water are introduced into a supercritical fluid vessel ( step 210 ). the chitosan powder contains input chitosan . next , the solution ( or the mixture ) in the supercritical fluid vessel is heated up to a high temperature ( step 220 ). the mixture is subsequently incubated at a supercritical pressure for time less than 3 hours ( step 230 ). the solution is depressurized ( step 240 ). the solution is then cooled to room temperature and the resulting dried powder is collected ( step 250 ). the resulting dried powder contains water - soluble polymeric or high molecular - weight chitosan . in some embodiments , gases other than co 2 can be employed in the supercritical fluid . the disclosed preparation method of the water - soluble high molecular - weight chitosan can use input chitosan having a molecular weight in the range 100 kda to 600 kda , or preferably in the range 50 to 100 kd , or in the range 20 kda to 50 kda . the input chitosan has a degree of aceylation higher than 50 %, preferably higher than 70 %, or higher than 85 %. the alkyl alcohol has a carbon number in a range c 3 to c 4 in the chain hydrocarbon chain , more preferably in the range c 2 to c 3 . the input chitosan can be placed in the supercritical fluid vessel with addition of a ternary solvent system composed of ethanol , hcl , and h 2 o in a 2 : 1 : 1 . 5 ratio . in some embodiments , salts such as nacl , cacl 2 and / or other organic co - solvents can be used . a critical variable in the experiments conducted was the ratio and the total mixture volume of the solvents ( ethanol , hcl , h2o ) to the volume of supercritical fluid extraction vessel ( supercritical co 2 ). in our experiments , this ratio did not exceed 60 %. the mixture in the supercritical fluid vessel can include from 10 % to 50 % of hydrochloric acid based on the volume of the mixture . the mixture in the supercritical fluid vessel can include from 5 % to 95 % of alkyl alcohol based on volume , or preferably 60 % to 80 %, or 50 % to 70 %, or 20 %- 50 %, or 15 - 30 % of alkyl alcohol based on volume . the mixture in the supercritical fluid vessel can include 10 %- 90 % of water based on volume , or preferably 20 %- 70 %, or 20 %- 40 % of water based on volume of the mixture . the temperature in the supercritical fluid vessel in step 230 can be in the range from 40 ° c . to 100 ° c ., or preferably from 60 ° c . to 100 ° c ., or from 90 ° c . to 100 ° c . the pressure in the supercritical fluid vessel in step 230 can be in the range 40 bar to 100 bar , preferably in the range 60 bar to 100 bar and more preferably in the range 90 to 100 bar . the incubation time in the supercritical fluid vessel in step 230 can be in the range 1 to 3 hours . the prepared water - soluble high molecular - weight chitosan can have an average molecular weight in the range from 500 kda to 10000 kda , or preferably in the range 1000 kda to 6000 kda , or in the range 1000 kda to 4000 kda . the molecular weight of the prepared water - soluble high molecular - weight chitosan can be higher than 2 times of the molecular weight of the input chitosan . the viscosity of 10 % prepared water - soluble polymeric solution can have viscosity from 500 cps to 5000 cps , or preferably in the range 1000 cps to 5000 cps , or in the range 2000 cps to 5000 cps . the prepared water - soluble high molecular - weight chitosan can have water solubility more than 1 % in weight / volume concentration , or preferably more than 2 %, or more than 4 %. preparation of high molecular - weight chitosan of average molecular weight 1 . 11 × 10 6 dalton five grams chitosan was mixed with 20 ml ethanol , 15 ml distilled water , and 10 ml hydrochloric acid inside the supercritical fluid vessel . the supercritical fluid vessel was heated up to 100 ° c . at a pressure of 100 bar for 1 hour . then solution is then depressurized and cooled to room temperature . the water - soluble chitosan obtained has high molecular - weight of an average value equal to 1 . 11 × 10 6 dalton as determined by application of mark - houwink equation ( fig3 a ). preparation of high molecular - weight chitosan of average molecular weight 2 . 31 × 10 6 dalton five grams chitosan was mixed with 20 ml ethanol , 15 ml distilled water , and 10 ml hydrochloric acid inside the supercritical fluid vessel . the supercritical fluid vessel was heated up to 100 ° c . at a pressure of 100 bar for 2 hours . then solution is then depressurized and cooled to room temperature . the water - soluble chitosan obtained has high molecular - weight of an average value equal to 2 . 31 × 10 6 dalton as determined by application of mark - houwink equation ( fig3 b ). preparation of high molecular - weight chitosan of average molecular weight 1 . 9 × 10 6 dalton five grams chitosan was mixed with 20 ml ethanol , 15 ml distilled water , 10 ml hydrochloric acid , and additional 0 . 5 g nacl inside the supercritical fluid vessel . the supercritical fluid vessel was heated up to 100 ° c . at a pressure of 100 bar for 2 hours . then solution is then depressurized and cooled to room temperature . the resulting dried powder is collected . the water - soluble chitosan obtained has high molecular - weight of an average value equal to 1 . 9 × 10 6 dalton as determined by application of mark - houwink equation ( fig3 c ). preparation of high molecular - weight chitosan of average molecular weight 4 . 35 × 10 6 dalton ten grams chitosan was mixed with 40 ml ethanol , 30 ml distilled water , and 20 ml hydrochloric acid inside the supercritical fluid vessel . the supercritical fluid vessel was heated up to 100 ° c . at a pressure of 100 bar for 1 hour . then solution is then depressurized and cooled to room temperature . the water - soluble chitosan obtained has high molecular - weight of an average value equal to 4 . 35 × 10 6 dalton as determined by application of mark - houwink equation ( fig3 d ). a polymeric solution was prepared by dissolving 2 g of water - soluble high molecular - weight chitosan , produced by one of the above methods , in 100 ml of water . after 20 minutes of centrifugation , which removes air pulps from the polymeric solution , a measured volume of each polymeric solution ( 20 ml ) was poured into petri - dish , which was then kept at room temperature for 48 h . the dried films were peeled from the petri dishes . compositions of coating mixture prepared based on the water - soluble high molecular - weight chitosan ( modified chitosan ) a suspension of the coating mixture was prepared by adding the coating mixture ( 15 % to 25 % weight per weight ) to the volume of water ( 65 to 85 % weight per weight ) gradually and mixing for 45 minutes . the spraying suspension was filled into an atomizer . the uncoated tablets were introduced to the coating machine pan and warmed using hot air . the coating machine parameters were adjusted ; the tablets were sprayed to obtain a constant average weight ( the coating mixture represents a range from 2 % to 5 % to the total weight of the tablet ). white beeswax can be used to polish the tablets . one and half gram of the water - soluble high molecular - weight chitosan was dissolved in 5 ml of water . the resulting polymer solution is coated on a substrate such as textile , a fabric , and a medical bandage . antibacterial effect was tested against 2 bacterial strains ( e . coli 25922 as gram negative bacteria and staph . aureus 29213 as gram positive bacteria ). disk diffusion technique was employed with impregnation of known amount from test samples . negative controls were used with sterile disc impregnated with water only . the impregnated discs were kept at the center of agar plates , seeded with test bacterial cultures . the discs were then placed individually using a sterile forceps in appropriate grids which were marked on the undersurface of the plated petri plates and kept for incubation at room temperature ( 27 ° c .± 2 ) for 24 h . after incubation , plates were observed and showed clear zones of inhibition for the bacterial strains . the water - soluble high molecular - weight chitosan produced by the above described methods have been analyzed using several techniques . fig5 shows a solid state 13c - nmr spectrum of the modified water - soluble high molecular - weight chitosan . fig6 a and 6b show powder x - ray diffraction intensity distributions respectively for input chitosan ( 6 a ) and water - soluble high molecular - weight chitosan ( 6 b ). fig7 a and 7b show exemplified ftir spectra respectively for input chitosan ( 7 a ) and water - soluble high molecular - weight chitosan ( 7 b ). fig8 a and 8b show exemplified thermal analysis thermograms respectively for input chitosan ( 8 a ) and water - soluble high molecular - weight chitosan ( 8 b ). fig9 a and 9b show exemplified sem micrographs respectively for input chitosan ( 9 a ) and water - soluble high molecular - weight chitosan ( 9 b ).
a process for preparing a soluble high molecular - weight chitosan which comprises treating chitosan , water , alcohol , acid mixture with supercritical fluid to prepare a modified chitosan with high aqueous solubility .