Investigation of the physicochemical properties of aqueous solutions of sodium salt of carboxymethyl starch obtained in the solid phase by viscometric methods

doi:10.21203/rs.3.rs-3242681/v1

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Investigation of the physicochemical properties of aqueous solutions of sodium salt of carboxymethyl starch obtained in the solid phase by viscometric methods

https://doi.org/10.21203/rs.3.rs-3242681/v1

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By chemical modification of corn starch with sodium monochloroacetate in the solid phase, samples of sodium salt of carboxymethyl starch (Na-CMS) with different degrees of substitution (DS) were synthesized. In the synthesis of Na-CMS, solvents such as water, methanol, ethanol, butanol, and isopropyl alcohol were used as humectants in the reaction mixture. The viscosity of dilute aqueous solutions of Na-CMS was studied by capillary viscometry. It is shown that dilute solutions of Na-CMS exhibit properties characteristic of polyelectrolytes. The rheological properties of aqueous solutions of the synthesized Na-CMS samples were studied by the method of rotational viscometry. By changing the shear rate gradient over a wide range, the values of the shear stress and the effective viscosity of the solutions at different temperatures were determined. At the same time, the obtained data showed that aqueous solutions of Na-CMS belong to non-Newtonian liquids in the entire studied temperature range. It was also found that the dynamic viscosity of Na-CMS solutions decreases with an increase in their DS. The activation energy of the viscous flow of Na-CMS solutions was determined, the value of which shows that the polymer macromolecules interact with each other through hydrogen bonds.

sodium salt of carboxymethyl starch

degree of substitution

rotational viscometry

rheology

dynamic viscosity

Starch is a natural polysaccharide that is isolated from various raw materials of plant origin. The main sources of starch are annual cultivated plants, which ensures the annual renewal of its reserves in large volumes. Biodestructibility, non-toxicity, ease of chemical modification and other unique properties of this polymer determine its wide application in various areas of human life [1, 2, 3]. At the same time, starch does not dissolve in water under normal conditions, which limits its use. Therefore, modified starches are widely used, especially those that dissolve well in cold water [4, 5]. One of them is sodium solt of carboxymethyl starch (Na-CMS). Na-CMS is a polymer produced on an industrial scale and is used in oil and gas [6, 7], textile [8, 9, 10], pharmaceutical [11, 12] industries, is used as a stabilizer for aqueous solutions, suspensions and emulsions, flocculants, adhesive etc. [13, 14, 15]. In addition, aqueous solutions of Na-CMS are more stable and less prone to biodegradation compared to starch [16]. Na-CMS is mainly used in the form of aqueous solutions, both very dilute and viscous. Therefore, the study of physicochemical, especially hydrodynamic properties of Na-CMS aqueous solutions is of great practical importance. One of the simplest and most accessible methods for studying the properties of polymer solutions in a wide range of concentrations is viscometry. Therefore, the purpose of this work is to study the physicochemical properties of dilute and concentrated aqueous Na-CMS solutions with different degrees of substitution (DS) by capillary and rotational viscometry.

Materials

Starch from corn, (C₆H₁₀O₅)_n (practical grade, Sigma-Aldrich); sodium monochloroacetate (Na-MCA), ClCH₂COONa (98%, Sigma-Aldrich); methanol, CH₃OH (≥ 99.7%, Sinopharm Chemical Reagent Co., Ltd.); ethanol, C₂H₅OH (≥ 99.7%, Sinopharm Chemical Reagent Co., Ltd.); butanol, C₄H₉OH (99%, Component-Reaktiv); isoprapanol, C₃H₇OH (99%, Component-Reaktiv); sodium hydroxide, NaOH (pure for analysis, Lenreaktiv); distilled water was used.

Synthesis of Na-CMS

Synthesis of Na-CMS was carried out by carboxymethylation of corn starch with Na-MCA in the presence of NaOH in the solid phase method. Carboxymethylation of starch was carried out in a special mixer, 3 liters in volume, equipped with paired Z-shaped blades that rotate in different directions around a horizontal axis. This provides a high degree of friction of the reaction mixture. During the synthesis of Na-CMS, a certain amount of solvents, such as water, ethanol, methanol, butanol, and isopropanol, were also added to moisten the reaction mixture. Solvents were added by injection, in small doses to prevent the formation of a paste. This forms a powdery semi-dry product of white color.

To purify the resulting product, it was first dissolved in water until a homogeneous solution was formed, then the solution was poured into ethanol or isopropanol, while Na-CMS precipitated as a white precipitate. The process is repeated at least three times. Purified from impurities, Na-CMS is dried in an oven at a temperature of 70–80°C to constant weight.

The IR spectrum of the Na-CMS was recorded using a FT-IR spectrometer (Nicolet iS50, Thermo Fisher Scientific) in the range of 4000–450 cm^− 1.

Determination of the DS of Na-CMS

The DS of the Na-CMS samples was determined according to the procedure [17] according to the equation:

$$\text{D}\text{S}=\frac{162\text{*}{\text{n}}_{COOH}}{{\text{m}}_{ds}-58\text{*}{\text{n}}_{COOH}}$$

;

$${\text{m}}_{ds}=\frac{(1-{\text{W}}_{\text{w}\text{a}\text{t}\text{e}\text{r}})}{100}\text{*}{\text{m}}_{s}$$

;

$${\text{n}}_{COOH}=({V}_{b}-V)*{C}_{HCI}*4$$

;

where: 162-is the molar mass of an anhydroglucose unit (AGU), g/mol; n_COOH-is the amount of COOH, mol; 58-is the net increase in the mass of an AGU for each carboxymethyl group substituted, g/mol; m_ds-is the mass of dry sample calculated from known sample mass m_s, g; w_water-the water content, %; V_b-is the volume of HCl used for the titration of the blank, cm³; V-is volume of HCl used for titration of the sample, cm³; c_HCl-is the HCl concentration, mol/dm³; 4-is the ratio of the total solution volume (100 cm³) and the volume taken for titration (25 cm³).

Capillary viscometry

The viscosities of dilute aqueous solutions of Na-CMS were determined using an Ubbelohde-type glass capillary viscometer. The studies were carried out in a water thermostat maintaining the temperature with an accuracy of 0.02°C. The flow time of pure solvent from the viscometer was 95 seconds.

Relative (η_r), specific (η_sp), and reduced (η_red) viscosities of polymer solutions were determined by the equations:

η _r = τ/τ₀; η_sp = η_r-1; η_red = η_sp/C

where: τ-outflow time of the polymer solution from the capillary, s; τ₀-solvent outflow time from the capillary, s; C-concentration of the polymer solution, %.

Rotational viscometry

Rheological studies were carried out on a rotational viscometer "Rheotest-2" type RV (Rheotest Medingen GmbH, Germany) using a cell system of coaxial cylinders S/S2 in a wide range of velocity gradient (γ) and temperature according to the method [18].

The effective viscosity (η_eff) of the solution was calculated by the expression:

η _eff = Ζα/γ = σ/γ

where: σ-shear stresses, Pa·s; Ζ = 0.58–cell constant; α–instrument reading, measured at different values of γ, s^− 1.

Study of the influence of the conditions for obtaining Na-CMS on its DS

As is known, there are several methods for the synthesis of Na-CMS, among which the main ones are the modification of starch in solution, suspension, and solid phase [19]. With the solid phase method for obtaining Na-CMS, the DS of the resulting polymer is usually lower than that of polymers obtained by other methods. But this method is more technological and economical. Since the consumption of solvents will be minimal, also less energy will be required to dry the resulting product. When starch carboxymethylation by the solid phase method, it was found that when the initial components are mixed in a dry state, products with low DS are formed (Table 1). Therefore, when mixing the initial components, the reaction mixture was moistened by adding a dosed amount of solvents (no more than 10–15% by weight of the dry mixture). The solvents were added to the dry mixture by the method of injection, in several approaches with a certain interval. Water, methanol, ethanol, butanol, isopropanol, as well as their aqueous solutions of various concentrations were used as solvents. It was found that the moistening of the reaction mixture leads to a sharp increase in the DS of Na-CMS. In this case, the most significant increase in the DS of the polymer was observed when using aqueous solutions of alcohols. The experiments were carried out at room temperature (∼25°C). The resulting solid products had up to 20% moisture, which quickly dried at 50°C in 1–2 hours.

By varying the ratio of the initial components, the reaction time, and the type of solvent used, more than 50 samples of Na-CMS with DS from 0.09 to 0.95 were obtained. Some results obtained during the experiments are shown in Table 1.

Table 1

Influence of the ratio of the initial reagents and the nature of the solvent used to moisten the reaction mixture on the DS of the resulting Na-CMS (Reaction time 30 min, temperature 25 °С)
№	Molar ratio of reagents, AGU^∗:NaOH: Na-MCA	Solvent used to moisten the reaction mixture	DG of Na-CMS
1	1:1:1	No moisture	0.09
2	1:1,5:1,5	No moisture	0.13
3	1:1:1	Water	0.38
4	1:1,5:1,5	Water	0.52
5	1:1:1	Ethanol (anhydrous)	0.41
6	1:1:1	Ethanol (80%)	0.54
7	1:1,5:1,5	Ethanol (80%)	0.68
8	1:1,5:1,5	Butanol (80%)	0.73
9	1:1,5:1,5	Methanol (anhydrous)	0.62
10	1:1,5:1,5	Methanol (80%)	0.84
11	1:1,5:1,5	Isopropanol (80%)	0.95

^∗ AGU-in the calculations, the molecular weight of the anhydroglucoside unit, the elementary unit of starch macromolecules, was used.

As can be seen from the data given in Table 1, the use of alcohols and their aqueous solutions when moistening the reaction have a catalytic effect on the reaction of starch carboxymethylation. In this case, the catalytic effect of the alcohols used during starch carboxymethylation increases in the following order: ethanol<butanol<methanol<isopropanol.

The identification of the obtained Na-CMS was carried out on the basis of the analysis of its FTIR spectra (Fig. 1). In contrast to starch, in the FTIR spectrum of Na-CMS in the region of 1696 cm^–1, an intense absorption band of the C = O bond of the carboxymethyl group appears. And in the region of 1280 cm^–1, an intense band of the stretching vibration of the C–O bond is observed, the band in the region of 1198 cm^–1 characterizes the vibrations of the C–O–C simple ether bond [20].

Properties of dilute aqueous solutions of Na-CMS

As is known, in highly dilute solutions, polymer macromolecules are practically separated from each other. This allows, based on the study of the viscosity of dilute polymer solutions, to obtain information about the shape of a single macromolecule in a solution and the changes occurring in it under the influence of various factors. Therefore, in this work, viscometric studies of dilute aqueous solutions of Na-CMS were carried out. The viscosity of the solutions was determined using a capillary viscometer with an Ubbelohde tube, while the concentration of the polymer in the solution did not exceed 0.1%. One of the quantitative indicators associated with the structure and conformation of a macromolecule is the reduced viscosity. Therefore, in the work, the change in the shape of macromolecules under the influence of various factors was assessed based on the change in the value of this parameter. The effect on the conformation of Na-CMS macromolecules of such factors as changes in solution concentration, temperature, pH value, and DS of polymer was studied. The data obtained during the study of the influence of the concentration of the Na-CMS solution on its reduced viscosity are shown in Fig. 2.

As can be seen from Fig. 2, with a decrease in the concentration of the polymer solution, a sharp increase in its reduced viscosity is observed. Such an anomalous behavior of the reduced viscosity of the solution is characteristic of polyelectrolytes. This phenomenon is called polyelectrolyte swelling of macromolecules [21]. This is due to the fact that when the polymer solution is diluted, the macromolecules unfold and acquire a more voluminous shape.

Studies have shown that the dependence of the reduced viscosity of the Na-CMS solution on the DS has the form of an extreme curve, the maximum of which passes through DS≈0.8 (Fig. 3). This shows that above the DS of 0.8, changes occur in the structure of the polymer macromolecule, which lead to an increase in its flexibility. And the increase in viscosity to the DS of 0.8 is associated with the unfolding of the macromolecule due to an increase in the repulsive forces between like-charged –COO⁻ groups.

Experiments have shown that with increasing temperature, the reduced viscosity of the Na-CMS solution decreases (Fig. 4). This indicates that an increase in temperature leads to an increase in the flexibility of Na-CMS macromolecules and they fold, as a result of which the size of the macromolecule coil decreases.

In this work, we also studied the influence of the pH medium on the reduced viscosity of the solution. To do this, polymer solutions were prepared with different pH environments by adding certain amounts of HCI and NaOH to it. Studies have shown that the dependence of the reduced viscosity on the pH medium of the solution passes through a maximum, the highest value of which corresponds to pH≈8 (Fig. 5).

As can be seen from Fig. 5, the higher the DS of polymer, the dependence curve has a more extreme character. The low values of the reduced viscosity of the solution in strongly acidic media can be explained by the shielding of mutual repulsion between like-charged COOH⁻ groups of H⁺ ions present in the solution in excess. As a result, the macromolecule will have a more folded shape. Correspondingly, Na⁺ are screening ions in strongly alkaline media. Thus, studies show that Na-CMS macromolecules, depending on the pH environment of the solution medium, are able to significantly change their conformations. In this case, the higher the DS of the polymer, the stronger will be the influence of various factors on the conformation of its macromolecules.

Rheological properties of aqueous solutions of Na-CMS

An increase in the concentration of the polymer solution leads to an increase in intermolecular interactions, while three-dimensional networks of bonds can form, which leads to gelation of the solution. And in other cases, fluctuation or stable associates of various shapes and sizes that are several orders of magnitude larger than ordinary molecules can be formed. Therefore, concentrated polymer solutions become very viscous, and external forces must be applied to study their fluidity. In this case, as is known, the properties of the solution in the study of its fluidity under the action of external forces are called its rheological properties.

In this work, the rheological properties of 10% Na-CMS aqueous solutions were studied using a rotational viscometer. The experiments were carried out in the range γ = 0.5–150 s^− 1 by generating a shear flow at 25°C, 40°C and 55°C. The data obtained are shown in Figs. 6 and 7.

As can be seen from the presented data, with an increase in the velocity gradient, the value of the shear stress increases (Fig. 6), and the effective viscosity of the Na-CMS solutions decreases (Fig. 7). The given curves of the dependence of σ and lnη_eff on γ have the form characteristic of non-Newtonian, pseudoplastic liquids [22, 23, 24]. This behavior of polymer solutions is due to the deformational change in the conformation of Na-CMS molecules in the flow. It can also be seen that an increase in temperature leads to a decrease in shear stress and effective viscosity of polymer solutions. This indicates a weakening of intermolecular friction in the shear flow with increasing temperature.

In order to determine the influence of the degree of substitution of Na-CMS on the rheological properties of its solutions, a number of experiments were carried out and the values of σ and η_eff solutions were determined at various γ. Some data obtained during the experiments are shown in Table 2.

Table 2

Values of σ and η_eff at different values of γ for Na-CMS solutions
γ, с⁻¹	DS of Na-CMS
	0.17		0.25		0.52		0.74		0.95
	σ, Pa	η_eff, Pa·s	σ, Pa	η_eff, Pa⋅s	σ, Pa	η_eff, Pa·s	σ, Pa	η_eff, Pa⋅s	σ, Pa	η_eff, Pa·s
25°C
3.00	36.15	12.05	28.41	9.47	21.72	7.24	18.57	6.19	14.67	4.89
27.0	132.03	4.89	88.29	3.27	61.02	2.26	54.00	2.00	40.23	1.49
40°C
3.00	29.61	9.87	21.45	7.15	15.48	5.16	12.75	4.25	10.23	3.41
27.0	90.45	3.35	57.78	2.14	48.06	1.78	39.42	1.46	30.24	1.12
55°C
3.00	23.34	7.78	15.66	5.22	10.74	3.58	5.90	2.90	5.34	1.78
27.0	78.03	2.89	53.19	1.97	48.06	1.78	36.72	1.36	18.36	0.68

As can be seen from the data given in Table 2, with an increase in the DS of Na-CMS, the effective viscosity of their solutions decreases. This dependence persists both at low and higher values of the velocity gradient. An increase in temperature also leads to a decrease in the effective viscosity of the solution, regardless of the DS of the polymer.

Under certain conditions, if we extrapolate γ→0 in rheograms (Fig. 8), then the condition lnη_eff = lnη, is satisfied, hence η_eff = η (25).

In this case, the value of η characterizes the viscosity of the solution in the absence of flow, that is, the coefficient of internal friction, which manifests itself as an offset of the thermal movements of the components of the liquid. Therefore, the parameter η can be considered as the “dynamic” viscosity of the solution. Thus, the values of the parameter η were determined for 10% aqueous solutions of Na-CMS samples with different DS values (Table 3).

Table 3

Values of η of Na-CMS solutions at different temperatures
T, °С	DS of Na-CMS
T, °С	0.17	0.25	0.52	0.74	0.95
25	20.6	18.4	14.1	10.3	8.4
40	14.2	13.7	9.1	7.5	6.3
55	10.7	9.6	6.5	5.1	4.2

As can be seen from the data presented in Table 3, in all cases, the dynamic viscosity of Na-CMS solutions decreases with an increase in their DS. This may be due to the fact that with an increase in the DS of the polymer, intermolecular interactions decrease. A decrease in the dynamic viscosity of solutions with increasing temperature can be explained by an increase in the thermodynamic flexibility of macromolecules.

The data obtained make it possible to calculate the value of the activation energy (E_a) of the viscous fluidity of Na-CMS solutions using the Arrhenius-Frenkel-Eyring equation [26, 27, 28]:

$$\eta =A exp\left(\frac{{-E}_{а}}{RT}\right)$$

;

where: Е_а–activation energy, kJ/mol; R-is the universal gas constant, 8.314 J/mol·K; T-is temperature, K; A-is the pre-exponential Arrhenius factor.

From the linear dependence of lnη on 1/T, the values E_a of the viscous fluidity of Na-CMS solutions having different DS were calculated (Fig. 9), which are shown in Table 4.

Table 4

Values E_a of the viscous fluidity of Na-CMS solutions with different DS
DS of Na-CMS	0.17	0.25	0.52	0.74	0.95
Е_а, kJ/mol	17.8	17.7	21.0	19.0	18.0

As can be seen from the data in Table 4, the values of E_a for the viscous fluidity of Na-CMS solutions with different DS are very close to each other. Based on the value of E_a, it can be assumed that the polymer macromolecules in solution are linked to each other via hydrogen bonds [29, 30].

In this work, samples of Na-CMS were synthesized by carboxymethylation of starch with Na-MCA by the solid phase method. It has been shown that moistening the reaction mixture with water and alcohols leads to an increase in the DS of the formed Na-CMS. In this case, alcohols and their aqueous solutions exhibit a catalytic effect on the carboxymethylation reaction. The physicochemical properties of dilute and concentrated Na-CMS solutions were studied by viscometric methods. The effect on dilute solutions of Na-CMS of such factors as changes in solution concentration, temperature, pH value of the medium, and DS of polymer has been studied. It is shown that the behavior of dilute Na-CMS solutions is in many respects similar to the behavior of polyelectrolytes in aqueous solutions. In the study of the rheological properties of 10% aqueous solutions of Na-CMS by the method of rotational viscometry, it was revealed that they exhibit a non-Newtonian nature of the flow in a shear flow. At the same time, an increase in the DS of Na-CMS and temperature is accompanied by a decrease in their dynamic viscosity. This shows that an increase in the DS of Na-CMS and temperature leads to an increase in the flexibility of macromolecules. Based on the determination of the values of the activation energy of the viscous flow of polymer solutions, it is assumed that the polymer macromolecules are linked to each other by hydrogen bonds.

Author contributions Muzaffar Makhkamov: methodology, resources, supervision, investigation, further text modifications and text finalization. Sanjar Saparov: investigation, methodology, writing original draft, calculations. All authors contributed to the study conception and design. All authors read and approved the final manuscript.

Availability of data and material The datasets during and analyzed during the current study available from the corresponding author on reasonable request.

Funding The authors did not receive support from any organization for the submitted work.

Ethics approval and consent to participate Not applicable.

Conflict of interest The authors declare no competing financial interest.

All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.

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Investigation of the physicochemical properties of aqueous solutions of sodium salt of carboxymethyl starch obtained in the solid phase by viscometric methods

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Abstract

Figures

Introduction

Materials and methods

Materials

Synthesis of Na-CMS

Determination of the DS of Na-CMS

Capillary viscometry

Rotational viscometry

Results and discussion

Study of the influence of the conditions for obtaining Na-CMS on its DS

Properties of dilute aqueous solutions of Na-CMS

Rheological properties of aqueous solutions of Na-CMS

Conclusion

Declarations

References

Additional Declarations

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