Surface Activity of Hydroxypropyl Methyl Cellulose at Different Interfaces

Abstract: The dynamic contact angle of hydroxypropylmethylcellulose (HPMC) powder on the water surface and the surface tension of HPMC solution on the air-liquid and oil-water interfaces were investigated under different concentrations and pH values. The results show that the wettability of HPMC powder is not strong, and it is not greatly affected by the pH value. When the pH = 4.00, the contact angle is the lowest at 86.380°; no matter at the gas-liquid interface or at the oil-water interface , the surface tension mainly depends on the conformational change of the HPMC molecular chain in the interface region, which is related to the solution pH value and solution concentration; for the HPMC solution with a mass concentration of 0.1%, the surface tension of the gas-liquid interface increases with the increase of the pH value. When the mass concentration is 0.3%-1.0% HPMC solution, the surface tension of the gas-liquid interface changes in a "∨" shape with the increase of pH, indicating that the conformational changes of HPMC molecular chains in the interface region are complex; oil-water The interaction between the HPMC at the interface and the oil molecules on the interface makes the conformational change of the HPMC molecular chain more complex; when the pH ≥ 6.00, the surface tension of the oil-water interface decreases with the increase of the pH value, while at pH < When 6.00, the influence of the concentration is greater, and the surface tension of the oil-water interface fluctuates up and down.

Key words:hydroxypropyl methylcellulose; surface tension; contact angle

Hydroxypropylmethylcellulose (HPMC) is a nonionic cellulose ether macromolecule. Due to its non-toxicity and good mechanical properties, it is widely used in the food and pharmaceutical industries. In the food industry, it is used as emulsifier, gelling agent, stabilizer, dispersant, thickener and film-forming agent, etc. In the pharmaceutical industry, it is used as a drug sustained release agent. In recent years, HPMC has been modified into pH-sensitive polymer compounds, such as hydroxypropylmethylcellulose acetate maleate and hypromellose trimellitate. It is about 3.0 and 3.5 ~ 4.5, which can be applied in the duodenum targeted drug delivery system.

HPMC has surface activity because of its hydrophobic methyl group and hydrophilic hydroxypropyl group on its main chain, so it can be adsorbed on the surface of the liquid, thereby reducing the surface tension of the liquid. The surface activity of polymers depends on the chemical structure, degree of substitution, size and flexibility of the molecular chain, as well as the structure and composition ratio of hydrophobic and hydrophilic groups.

wait. As a macromolecule, the surface adsorption behavior of HPMC is different from that of low-molecular surfactants, because the polymer chain segment is adsorbed on the liquid surface in a time-dependent manner and finally reaches an equilibrium state. increase. At the same time, when the macromolecules are adsorbed on the liquid surface, the chain segments present a column

shape, ring and tail. Columns are formed by the diffusion of lipophilic segments on the macromolecular chains to the surface, while rings and tails are formed by the hydrophilic segments of macromolecules entering the bulk solution.

The surface activity of HPMC is involved in many related industrial processes, such as dispersion, emulsification and wetting of food and pharmaceutical reagents. In solution systems, the interfacial properties of surface active substances have a major impact on dispersion and emulsification. For example, HPMC is widely used as a drug carrier in drug sustained release agents. Its wetting, gelation and dissolution processes in water , and the diffusion of hydration directly affects the wetting, dissolution and delivery of drugs. Therefore, the study of the surface activity of HPMC not only has theoretical value, but also has guiding significance for its practical application. In this paper, the properties of different concentrations of HPMC solutions at different pH values at the gas-liquid, oil-water and solid-liquid interfaces are discussed.

1. Experiment

1.1 Experimental materials and instruments

HPMC (pharmaceutical grade), total substitution degree is 0.92, molar substitution degree is 0.14, methoxyl content is 28.6%, hydroxypropyl content is 8.2%, produced by KIMA CHEMICAL CO.,LTD; Disodium hydrogen phosphate and citric acid are analytically pure, produced by Guangzhou Chemical Agent Factory; commercially available Luhua peanut oil, produced by Shandong Luhua Group Co., Ltd.

DCAT-21 interfacial surface tension meter, produced by Beijing Dongfang Defei Instrument Co., Ltd.; pHs-25 automatic potentiometric titrator, produced by Shanghai Kangyi Instrument Co., Ltd.

1.2 Experimental method

1.2.1 Preparation of HPMC solution

Disodium hydrogen phosphate-citric acid buffer solutions with pH values of 3.00, 4.00, 5.00, 6.00, 7.00, and 8.00 were prepared. Then use corresponding buffer solutions to prepare HPMC solutions with mass fractions of 0.1%, 0.3%, 0.5%, 0.7%, and 1.0%. Equilibrate for 24 h at 4℃.

1.2.2 Measurement of dynamic contact angle

Put filter paper at the bottom of the two tubes of the DCAT-21 interfacial surface tensiometer (the tube attached to the instrument), and then install the HPMC powder of the same quality, slightly vibrate until the height does not change significantly, and ensure that the two tubes The heights are all 4 cm to achieve the same powder density. First, use one of the tubes to measure the contact angle of HPMC on the surface of n-hexane, and use the WASHBURN method to calculate the capillary constant C. Then use another tube in the aqueous solution Measure the contact angle of HPMC on the water surface, and input the C value, and calculate the contact angle (CA value) by software SCAT32.

1.2.3 Surface tension measurement of air-liquid interface

Use the DCAT-21 interface surface tensiometer, select the platinum sheet method, set the immersion depth to 3 mm, the sampling frequency to 5 Hz, and set "if the difference between the surface tension recorded twice in continuous time is less than 0.03 mN/m Stop the measurement when ", take the average value of the last 50 measurement points. The temperature is kept at (30 +0.5) C, each sample is measured twice, and the results are averaged. Generally, the difference between the two is +0.5mN/m.

1.2.4 Surface tension measurement of oil-water interface

First measure the surface tension of peanut oil, and then measure the surface tension of HPMC solution in another glass vessel. After that, inject peanut oil on the surface of HPMC solution until the platinum sheet is submerged, and measure the surface tension of the oil-water interface. Measure each sample Twice, take the average value, the difference between the two is generally +0.5 mN/m.

2. Results and Discussion

2.1 The effect of pH value on the contact angle of HPMC powder in liquid

From the dynamic contact angle of HPMC powder on the water surface, it can be seen that the contact angle of HPMC powder is less than 90°, but close to 90°. Therefore, HPMC powder can be wetted on the water surface, but the wettability is not strong. At different pH The contact angle between the HPMC powder and the water surface changes slightly under the pH value, and the lowest contact angle is obtained at pH = 4.00, which is 86. 380°.

Since the filter paper placed at the bottom absorbs water, the water absorption time of HPMC powder is defined as after 1 s. The adsorption process of HPMC powder on the water surface is very fast at different pH values, but the equilibrium adsorption amount is not large, showing hydrophobic characteristics. This is the same as the report in the literature. When the pH value is small (3.00, 4.00), the adsorption rate is also small; when pH=4.00, the wetting effect of HPMC powder on the water surface is relatively more obvious; when pH=3.00, Although the mass increase process of HPMC powder on the water surface is relatively long, but the increase is not large; when pH ≥ 5.00, the adsorption rate of HPMC powder on the water surface is faster, but the adsorption amount changes little.

The wettability of a solid is determined by the chemical composition and microscopic geometry of the surface. There are both hydrophobic methyl groups and hydrophilic hydroxypropyl groups on the HPMC molecular chain, and the substituents and compositions on each structural unit are not complete. The same. At different pH values, the conformation of the HPMC molecular chain on the powder surface will change, and some hydrophobic methyl groups will move and hide in the lower layer of the powder surface. At the same time, some hydrophilic hydroxypropyl groups will be exposed to the powder The surface of the powder changes the ratio between the hydrophobic group and the hydrophilic group on the powder surface, and the transformation between the conformations will affect the wettability of the HPMC powder in liquids with different pH values. However, the total substitution degree of HPMC used in the experiment in this paper is 0.92, the molar substitution degree is 0.14, and the substitution degree of hydroxypropyl group is relatively low, which means that within the scope of the experiment in this paper, the change between the two conformations of the molecular chain The effect caused is small. Therefore, in this paper, the pH value has little effect on the CA value of HPMC.

2.2 Effect of pH value on the surface tension of the gas-liquid interface of HPMC solutions with different concentrations

Within the experimental range, for the HPMC solution with a mass fraction of 0.1%, the surface tension of the gas-liquid interface increases slightly as the pH value increases; for the HPMC solution with a mass fraction of 0.3% and the mass fraction of For 0.5%, 0.7% and 1.0% HPMC solutions, the surface tension of the gas-liquid interface has a similar trend with the pH value, showing a "V" shape, and the lowest value is at pH = 4 with the concentration increasing. .00, pH = 5.00, pH = 5.00 and pH = 6.00, that is, as the concentration increases, the pH value corresponding to the lowest point of surface tension also increases, which shows that the surface of HPMC solution at this time The molecular chain conformation is in a complex state.

The change of pH value will affect the conformation of the molecular chain. For HPMC, under acidic conditions, more H + will interact with the O atoms in -OH on the HPMC molecular chain, making the molecular chain charged, resulting in molecular chain tends to stretch, at this time, the adsorption of HPMC on the surface of the solution is mainly the hydrophobic molecules in the HPMC molecules.

For the methyl group of water, due to the stretching of the chain segment, the hydrophobic group forms a longer "column" arrangement on the surface of the solution, so the surface tension decreases; as the pH value of the solution increases, the OH - ions in the solution increase, The H + adsorbed around the HPMC molecular chains will be neutralized, which will make the HPMC molecular chains curl, and the surface tension of the solution will increase with the increase of the pH value due to the reduction of the "column" arrangement.

The conformation of macromolecular chains in solution has an important influence on the macromolecular interface, but macromolecular chains have many conformational forms in solution, and it is very difficult to analyze these conformations in detail. The impact on the interface properties of macromolecules comes from the average conformation or the conformation with the highest probability of occurrence, not only the pH value, the properties of the chains, such as the degree of substitution, the release of substitutions, the interaction between chains, etc., will affect the molecular chains of HPMC. Conformation.

2.3 Effect of pH value on oil-water interface surface tension of different concentrations of HPMC solution

The surface tension of the HPMC solution oil-water interface is not only related to the pH value, but also related to its concentration. In the HPMC solution, when the mass fraction was 0.1%, the surface tension of the oil-water interface fluctuated slightly with the increase of the pH value before pH = 6.00, and then continued to decrease; the mass fraction of HPMC As the fraction increases (0.3%-0.7%), the surface tension of the oil-water interface basically decreases with the increase of pH value, and decreases in the region of pH = 5.00-6.00. The trend slows down, and then continues to decrease; when the mass fraction increases to 1.0%, the surface tension of the oil-water interface increases slightly and then decreases with the increase of pH value.

Oil has better solubility for hydrophobic groups, and hydrophobic groups (such as methyl groups) in HPMC that enter the surface layer interact more with glycerides, free fatty acids, and phospholipids in the oil on the surface layer. , the hydrophilic hydroxypropyl groups are regularly arranged in the water phase, so the HPMC can be arranged more orderly on the oil-liquid interface, thereby reducing its surface tension. As the pH value increased, although the molecular chains of HPMC tended to curl, the dissolution of oil on its hydrophobic chains made more HPMC molecular chains diffuse to the oil-water interface, making the HPMC molecules more densely arranged in the oil-water interface. interface, so that the surface tension of the oil-water interface decreases rapidly after the pH value is greater than 6.00. This effect is particularly prominent when the mass fraction of HPMC is between 0.3% and 0.7%.

When the mass fraction is very small (such as 0.1%), the force between HPMC molecular chains is small, and the conformational changes of molecular chains are more abundant, but after all, the concentration is low and the influence is limited. The surface tension of the interface presents little fluctuations up and down. When the mass fraction of HPMC increases to 1.0%, when the pH value is less than 5.00, because the concentration increases and the molecular chains of HPMC are stretched, its migration speed is hindered, and it is different from the organic matter occupied in the surface oil. Glycerides, free fatty acids, and phospholipids have limited competition, and the number of methyl or hydroxypropyl groups migrating to the oil-water interface is small, resulting in an increase in the surface tension of the oil-water interface. After the pH value is greater than 6.00, Since the HPMC chain tends to be more coiled, its migration speed to the interface is accelerated, and the number of HPMC molecules arranged at the oil-water interface increases, resulting in a decrease in surface tension.

3. Conclusion

Under the experimental conditions in this paper, the wettability of HPMC powder is not strong, and it is less affected by the pH value. Both the air-liquid interface and the oil-water interface are complex physical and chemical systems with different properties. On the two interfaces, HPMC molecules can diffuse, adsorb or rearrange, and their surface properties depend on the conformation of HPMC on the interface. The research in this paper shows that the interfacial properties of HPMC are largely affected by the nature and quantity of hydrophobic groups. No matter at the air-liquid interface or at the oil-water interface, the surface tension of HPMC solutions with different concentrations shows different changes with pH value. With the same trend, HPMC exhibits complex conformations on the interface. Since oil has more surface active components than water, the surface properties of HPMC are more dependent on its surface concentration, making HPMC molecules more effectively compete with the surface active components in oil.