CN111795938A - Method for measuring carboxyl content of nanocellulose by using multi-wavelength spectrum technology - Google Patents
Method for measuring carboxyl content of nanocellulose by using multi-wavelength spectrum technology Download PDFInfo
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Abstract
The invention discloses a method for measuring carboxyl content of nanocellulose by using a multi-wavelength spectrum technology. Methylene blue is adopted as a color developing agent to form an R-COOMB association with carboxyl of the nano-cellulose through ion exchange reaction, and a multivariate numerical analysis technology is combined to overcome the problem that a spectrum of the methylene blue and a spectrum of the generated association RCOOMB are completely overlapped in a wavelength range of 500-750 nm. The result shows that compared with the reference method, the method has better measurement precision and accuracy. Therefore, the method is simple, convenient and quick, and is suitable for measuring the content of carboxyl in the nano-cellulose.
Description
Technical Field
The invention belongs to the technical field of analysis and detection, and particularly relates to a method for determining carboxyl content of nanocellulose by using a multi-wavelength spectrum technology.
Background
Nanocelluloses (nanocelluloses) refer to cellulose functional materials with at least one dimension of 1-100 nm in size. Besides the characteristics of wide natural cellulose source, degradability, regeneration and environmental friendliness, the nanocellulose also has the unique advantages of high specific surface area, high strength, high Young modulus, high transparency and the like, so the nanocellulose has wide application prospects in the industries of cosmetics, coatings, biomedicines, energy and electronics and the like. In the preparation process of the nano-cellulose, a chemical method is adopted, such as: TEMPO (2,2,6, 6-tetramethyl piperidine-1-oxygen free radical) -NaBr-NaClO oxidation, persulfate oxidative degradation, periodate hydroformylation modification, sodium chlorite oxidation, carboxymethylation and other treatment processes are combined, hydroxyl on the surface of the nano-cellulose can be converted into carboxylic acid groups, and functional modification based on nano-cellulose materials can be favorably carried out, such as: endowing negative charge characteristic to the nano-cellulose, so that the nano-cellulose suspension is not easy to flocculate and precipitate; improving the dispersibility/compatibility of the nano-cellulose and a non-polar medium; provides an active site for protein/enzyme immobilization. In view of the fact that the carboxyl content of the nano-cellulose is directly related to the performance and application characteristics of products, the establishment of a method for quickly and accurately quantifying the carboxylic acid content is very important for the production, research and application of the nano-cellulose.
At present, conductometry titration based on the titration of strong and weak acid mixtures with a strong base is often used to determine the carboxyl content of cellulose samples. The method comprises the steps of dissolving a cellulose sample by using excessive hydrochloric acid to obtain a colloidal solution of the cellulose sample, back-titrating residual hydrogen ions in the solution by using a NaOH alkali standard solution, indicating the change of a signal in the titration process by using a conductive electrode, drawing a titration curve by using the conductive value to the addition of sodium hydroxide, and further converting the carboxyl content of the cellulose sample by using the difference of the volumes of two break-over points on the titration curve. Compared with an ash method, the method has simpler steps, but has poorer detection accuracy on the sample with low carboxyl content, because the acidified nano cellulose sample is more viscous, the mixing of the acidified nano cellulose sample and a titrant is influenced, the dropping speed of the NaOH alkali standard solution is limited, and although the problem can be solved by dilution, the leap point of the sample with low carboxyl content is difficult to determine.
In addition, some modern instrumental analysis methods (such as ion exchange-headspace gas chromatography, NMR method and the like) are used for reports on relevant researches on cellulose samples, but the chromatographic method needs ion exchange/hydrolysis pretreatment on the samples, so that the requirements of daily rapid detection in industry cannot be met, and the NMR method has high requirements on sample purity and expensive instruments, is not popularized in conventional laboratories and is difficult to popularize.
In the previous reports, the carboxyl group content on the solid fiber was measured by the methylene blue method. The method is based on that after carboxyl on fiber and Methylene Blue (MB) reach adsorption equilibrium, the carboxyl content on the fiber is indirectly and quantitatively detected by measuring the absorbance of the residual methylene blue in filtrate. However, unlike the above samples, nanocellulose is completely water-soluble, its carboxylic acid groups are also water-soluble with methylene blue complex (R-COOMB), and the spectrum of methylene blue itself and of carboxylic acid groups on nanocellulose with methylene blue complex are largely overlapping, so it is not possible to determine the component content in an actual mixture by quantitatively detecting the single component with absorption. In the invention, a multi-wavelength spectrometry method based on spectral change of carboxylic acid groups in methylene blue combined nano-cellulose is provided for rapidly determining the carboxyl content of the nano-cellulose, and an effective means is provided for related research and application of determining the carboxyl content of the nano-cellulose.
Disclosure of Invention
In order to overcome the defects and shortcomings in the prior art, the invention aims to provide a method for measuring the carboxyl content of nanocellulose by using a multi-wavelength spectrum technology.
The purpose of the invention is realized by the following technical scheme:
a method for measuring carboxyl content of nano cellulose by using a multi-wavelength spectrum technology comprises the following steps:
(1) scanning the full spectrum of the blank sample with deionized water by using an ultraviolet-visible spectrophotometer (UV-Vis), and correcting an absorbance 0 point;
(2) scanning a full spectrum of a Methylene Blue (MB) aqueous solution by using UV-Vis to obtain a pure spectrum diagram of the methylene blue (I);
(3) scanning the full spectrum of the nano-cellulose aqueous solution by using UV-Vis to obtain a spectrogram of the nano-cellulose aqueous solution;
(4) according to the ratio of MB to nano-cellulose of 20-40 mu mol: 100-200 mg, mixing the MB aqueous solution with the nano-cellulose aqueous solution to obtain a sample to be detected, and scanning by UV-Vis to obtain a spectrogram of a product RCOOMB formed by association of MB and carboxyl on the nano-cellulose;
(5) according to the ratio of MB to nano-cellulose of 20-40 mu mol: 1-5 mg, mixing the MB aqueous solution with the nano-cellulose aqueous solution to obtain a sample to be detected, and scanning by UV-Vis to obtain a binary component mixed spectrogram (IV) of unassociated MB and an association product RCOOMB in the sample to be detected;
wherein the concentration of the MB aqueous solution in the step (2) is the same as the concentration of the MB in the sample to be detected in the steps (4) and (5);
the concentration of the nano-cellulose aqueous solution in the step (3) is the same as that of the nano-cellulose in the sample to be detected in the step (5);
(6) introducing all absorbance data within a certain wavelength range of a pure spectrum (I), a nano-cellulose spectrum (II), a spectrogram of RCOOMB and a mixed spectrogram of binary components (MB and RCOOMB) in the step (5) into a multi-component system multi-wavelength spectrum matrix formula as follows:
wherein A isj λi、kjAnd eλiRespectively represent the component j at the wavelength lambdaiTaking 1,2 as an i term, taking N as an N term, taking 250 as an N term, and taking 2 as an M term; m is the slope of a spectrum chart of the nano cellulose with the wavelength within the range of 500-750 nm;
then using mathematical solving software Polymath to fit k1And k2A value;
(7) according to the fitted k1The carboxylic acid content in the nanocellulose sample was calculated from the following formula:
wherein, CMBIs the molar concentration of the aqueous MB solution in the step (5); c is the mass concentration of the nano-cellulose, and the unit is mg/L.
Preferably, the concentration of the Methylene Blue (MB) aqueous solution in the step (2) and the concentrations of the Methylene Blue (MB) solutions in the samples to be detected in the steps (4) and (5) are both 10-20 mu mol/L, and the optimal concentration is 15.65 mu mol/L. The concentration of the methylene blue aqueous solution is too large to exceed the detection range of the ultraviolet spectrum, and the sensitivity is too low when the concentration is too small.
Preferably, the concentration of the nano-cellulose aqueous solution in the step (3) and the concentration of the nano-cellulose in the sample to be detected in the step (5) are both 0.5-2.5 mg/L.
Preferably, the solubility of the nano-cellulose in the sample to be detected in the step (4) is 50-100 mg/L.
Preferably, the concentration of the MB aqueous solution in the step (4) is 20-40 mu mol/L, the concentration of the nano-cellulose aqueous solution is 100-200 mg/L, and the volume ratio of the MB aqueous solution to the nano-cellulose aqueous solution is 1: 1.
Preferably, the concentration of the MB aqueous solution in the step (5) is 20-40 mu mol/L, the concentration of the nano-cellulose aqueous solution is 1-5 mg/L, and the volume ratio of the nano-cellulose aqueous solution to the nano-cellulose solution is 1: 1.
Preferably, the wavelength range in the step (6) is 500-750 nm, and the fitting condition satisfies the minimum mean square error principle (c)Minimum).
Compared with the prior art, the invention has the following advantages and beneficial effects:
(1) the method is adopted to determine the carboxyl content of the nano-cellulose, and the problem of overlapping of an MB spectrum and a generated associate RCOOMB spectrum is solved by utilizing the ion exchange reaction between the MB and the nano-cellulose carboxyl sodium and the multivariate numerical analysis technology.
(2) The method for determining the carboxyl content of the nano-cellulose widens the limitation of the original methylene blue method only aiming at solid fibers, is suitable for soluble nano-cellulose, and can be extended to the detection of the carboxyl content of other oxidized and carboxylated modified natural high molecular substances by the same principle.
(3) The method for measuring the carboxyl content of the nano-cellulose has the advantages of simple and quick experimental operation and objective and accurate detection result.
Drawings
FIG. 1 is a spectrum of the nanocellulose solution of example 1.
FIG. 2 is a spectrum of MB and RCOOMB in example 1.
FIG. 3 shows the predicted spectrum (calculated by fitting equation 1 in step (6)) and the actual measured spectrum (obtained in step (4)) in example 1.
Detailed Description
The present invention will be described in further detail with reference to examples and drawings, but the embodiments of the present invention are not limited thereto.
Those who do not specify specific conditions in the examples of the present invention follow conventional conditions or conditions recommended by the manufacturer. The raw materials, reagents and the like which are not indicated for manufacturers are all conventional products which can be obtained by commercial purchase.
The following examples used instrumentation and reagents: model 8453 uv-vis spectrophotometer (Agilent, usa), quartz cuvette (optical path 10mm, Agilent, usa), 20mL headspace vials, magnetic stirrer, precision balance, pipette guns (5mL, 1mL, 100 μ L) at different ranges.
The reagents used were: methylene blue; the cellulose nano-fiber prepared by a mechanical method is marked as CNF; the valerolactone (GVL) pulp and the bleached sulfate pulp are respectively prepared into corresponding nanocelluloses in a laboratory by a mild TEMPO oxidation method, and the nanocelluloses are respectively marked as GTCNF and KTCNF.
Example 1
In this example, the carboxyl content of the nanocellulose prepared by different oxidation processes is determined by the following specific steps:
(1) the full spectrum was scanned with an ultraviolet-visible spectrophotometer (UV-Vis) using deionized water as a blank to correct the absorbance 0 point.
(2) Mixing 5mL of MB aqueous solution with the concentration of 31.3 mu mol/L and 5mL of deionized water, and scanning by UV-Vis to obtain a pure spectrum (r) of methylene blue.
(3) Mixing the nano-cellulose aqueous solution with the concentration of 3mg/L with deionized water with the same volume, and scanning the full spectrum by using UV-Vis to obtain a spectrogram of the nano-cellulose aqueous solution;
(4) uniformly mixing 5mL of MB aqueous solution with the concentration of 31.3 mu mol/L and 5mL of nanocellulose aqueous solution with the concentration of 168mg/L, and scanning by UV-Vis to obtain a spectrogram of RCOOMB which is a product of association of MB and carboxyl on the nanocellulose.
(5) Uniformly mixing 5mL of MB aqueous solution with the concentration of 31.3 mu mol/L and 5mL of nano-cellulose aqueous solution with the concentration of 3mg/L, and scanning by UV-Vis to obtain a binary component mixed spectrogram (r) of residual MB and association product RCOOMB in the solution to be detected.
(6) Introducing absorbance data of the MB spectrum (i), the nano-cellulose spectrum (ii), the RCOOMB (iii) and a mixed spectrum (iv) of binary components (MB and RCOOMB) in the solution to be tested into a multi-wavelength spectrum fitting formula (1):
wherein A isj λi、kjAnd eλiRespectively represent the component j at the wavelength lambdaiTaking 1,2 as the i, 250 as the N and N, and 2 as the M; m is the slope of a spectrum chart of the nano cellulose with the wavelength within the range of 500-750 nm;
then using Polymath mathematical solving software to fit k1And k2The value is obtained.
(7) According to the fitted k1The carboxylic acid content of the nanocellulose sample can be calculated from equation (2). The calculation formula of the carboxyl content in the nano-cellulose sample is as follows:
wherein, CMBIs the molar concentration of the aqueous MB solution in step (5) (concentration when not reacted with nanocellulose); c is the mass concentration of the nano-cellulose, and the unit is mg/L.
To verify the accuracy of the method, the carboxyl content of the nanocellulose prepared from two different raw material pulps by different oxidation processes was measured by the spectroscopy method described herein (performed under the conditions of example 1) and the conventional conductivity titration method, respectively, and the results are shown in table 1. As can be seen from the table, the relative error of the detection results of the two methods on the samples is less than 4.23%, which indicates that the method is suitable for the determination of the carboxyl content in the nano-cellulose sample.
TABLE 1 comparison of methods
The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.
Claims (8)
1. A method for measuring carboxyl content of nano cellulose by using a multi-wavelength spectrum technology is characterized by comprising the following steps:
(1) scanning the full spectrum of the blank sample with deionized water by an ultraviolet-visible spectrophotometer, and correcting an absorbance 0 point;
(2) scanning the full spectrum of the methylene blue aqueous solution by using UV-Vis to obtain a pure spectrum diagram of methylene blue;
(3) scanning the full spectrum of the nano-cellulose aqueous solution by using UV-Vis to obtain a spectrogram of the nano-cellulose aqueous solution;
(4) according to the ratio of MB to nano-cellulose of 20-40 mu mol: 100-200 mg, mixing the MB aqueous solution with the nano-cellulose aqueous solution to obtain a sample to be detected, and scanning by UV-Vis to obtain a spectrogram of a product RCOOMB formed by association of MB and carboxyl on the nano-cellulose;
(5) according to the ratio of MB to nano-cellulose of 20-40 mu mol: 1-5 mg, mixing the MB aqueous solution with the nano-cellulose aqueous solution to obtain a sample to be detected, and scanning by UV-Vis to obtain a binary component mixed spectrogram (IV) of unassociated MB and an association product RCOOMB in the sample to be detected;
wherein the concentration of the MB aqueous solution in the step (2) is the same as the concentration of the MB in the sample to be detected in the steps (4) and (5); the concentration of the nano-cellulose aqueous solution in the step (3) is the same as that of the nano-cellulose in the sample to be detected in the step (5);
(6) introducing all absorbance data of a pure spectrum (i), a nano-cellulose spectrum (ii), a spectrogram of RCOOMB and a binary component mixed spectrogram (ii) in a certain wavelength range into the following multi-component system multi-wavelength spectrum matrix formula:
wherein A isj λi、kjAnd eλiRespectively represent the component j at the wavelength lambdaiTaking 1,2 as the i, 250 as the N and N, and 2 as the M; m is the slope of a spectrum chart of the nano cellulose with the wavelength within the range of 500-750 nm;
then using mathematical solving software Polymath to fit k1And k2A value;
(7) according to the fitted k1The carboxylic acid content in the nanocellulose sample was calculated from the following formula:
wherein, CMBIs the molar concentration of the aqueous MB solution in the step (5); c is the mass concentration of the nano-cellulose, and the unit is mg/L.
2. The method for measuring carboxyl content of nanocellulose by using the multi-wavelength spectroscopy as claimed in claim 1, wherein the concentration of the methylene blue aqueous solution in the step (2) and the concentration of the methylene blue solution in the samples to be measured in the steps (4) and (5) are both 10-20 μmol/L.
3. The method for measuring carboxyl content of nanocellulose by using the multi-wavelength spectroscopy as claimed in claim 1, wherein the concentration of the nanocellulose aqueous solution in the step (3) and the concentration of nanocellulose in the sample to be measured in the step (5) are both 0.5-2.5 mg/L.
4. The method for measuring carboxyl content of nanocellulose by using the multi-wavelength spectroscopy as claimed in claim 1, wherein the solubility of nanocellulose in the sample to be measured in the step (4) is 50-100 mg/L.
5. The method for measuring carboxyl content of nanocellulose by using the multiple wavelength spectroscopy according to claim 1,2, 3 or 4, wherein said wavelength in step (6) is in the range of 500 to 750 nm.
6. The method for measuring carboxyl content of nano-cellulose by using the multi-wavelength spectroscopy as claimed in claim 5, wherein the concentration of the MB aqueous solution in the step (4) is 20-40 μmol/L, the concentration of the nano-cellulose aqueous solution is 100-200 mg/L, and the volume ratio of the MB aqueous solution to the nano-cellulose aqueous solution is 1: 1.
7. The method for measuring carboxyl content of nano-cellulose by using the multi-wavelength spectroscopy as claimed in claim 5, wherein the concentration of the MB aqueous solution in the step (5) is 20-40 μmol/L, the concentration of the nano-cellulose aqueous solution is 1-5 mg/L, and the volume ratio of the nano-cellulose aqueous solution to the nano-cellulose solution is 1: 1.
8. The method for measuring carboxyl content of nano-cellulose by using the multi-wavelength spectroscopy as claimed in claim 5, wherein the concentration of the methylene blue aqueous solution in the step (2) and the concentration of the methylene blue solution in the samples to be measured in the steps (4) and (5) are both 15.65 μmol/L.
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CN106198427A (en) * | 2016-07-26 | 2016-12-07 | 华南理工大学 | A kind of five length ultraviolet spectrographic techniques evaluating metal catalytic reducing agent reducing property |
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CN108760900A (en) * | 2018-03-20 | 2018-11-06 | 华南理工大学 | A kind of headspace gas chromatography measuring sodium carboxymethylcellulose degree of substitution |
CN108982489A (en) * | 2018-07-05 | 2018-12-11 | 广西大学 | A kind of biomass cellulose base Cu2+Detect the preparation method and application of material |
CN110954531A (en) * | 2019-12-11 | 2020-04-03 | 东莞理工学院 | Methylene blue colorimetric method for measuring concentration of gamma-PGA in solution |
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CN106198427A (en) * | 2016-07-26 | 2016-12-07 | 华南理工大学 | A kind of five length ultraviolet spectrographic techniques evaluating metal catalytic reducing agent reducing property |
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CN107576738A (en) * | 2017-09-08 | 2018-01-12 | 华南理工大学 | A kind of method of carboxyl-content in headspace gas chromatography detection nano-cellulose |
CN108760900A (en) * | 2018-03-20 | 2018-11-06 | 华南理工大学 | A kind of headspace gas chromatography measuring sodium carboxymethylcellulose degree of substitution |
CN108982489A (en) * | 2018-07-05 | 2018-12-11 | 广西大学 | A kind of biomass cellulose base Cu2+Detect the preparation method and application of material |
CN110954531A (en) * | 2019-12-11 | 2020-04-03 | 东莞理工学院 | Methylene blue colorimetric method for measuring concentration of gamma-PGA in solution |
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