Effect of concentration of NaCl on percentage water absorption capacity of sodium polyacrylate hydrogels

How does the percentage absorbance of water by hydrogel beads (sodium polyacrylate crystals) from an aqueous solution of NaCl depends on the molar concentration of NaCl, determined using gravimetric analysis?

Hydrogel is an important and useful topic in synthetic chemistry. Starting from absorbing urine as diapers, absorbing blood as sanitary napkins to holding water for plants grown in desert areas in agriculture; they find an application in multiple sectors. Hydrogels are also used in medical field as a drug delivery system. Different life saving drugs are inserted and capsuled within hydrogels which after entering the blood vessels absorb fluids, expands and releases the drug molecule into the bloodstream. In short, hydrogels are polymers which can absorb and retain water for a long time. I came across this fact while trying hard to find an appropriate topic for my Chemistry Internal Assessment. The fact that the school laboratory is not accessible made the selection even tougher. However, referring to an experience I had and the inquiry emerged out of it made the task a little easier. Once, by mistake I dropped a diaper in a salt water which I was going to use for my gargle. To my surprise, I noticed that diaper did not absorb as well as it does for urine. This intrigued me and I wondered about the reason behind this. Table salt is sodium chloride which is also an integral constituent of urine. Does by any means the water absorption capacity of a diaper depends on that? Further research led me to know that the diapers contain hydrogels which are chemically cross chain polymers of sodium or potassium salts of acrylic acids and can act as water absorbent. Exploring a couple of research articles led me to know that the water absorption capacity depends on various factors like temperature, pH and presence of dissolved salts in water. Thus, I decided to explore the effect of concentration of sodium chloride in water on the percentage absorption of water by sodium polyacrylate crystals used as hydrogels.

Hydrogels are cross linked polymers of sodium or potassium salt of acrylate. Acrylic acid is an unsaturated (due to the presence of C = C) weak mono carboxylic acid. The conjugate base of the acid is an acrylate ion which is produced after losing a proton (H⁺) from the OH group of the acrylic acid. The acrylate can combine with alkali metal ions like-Na⁺ or K⁺ to form sodium or potassium acrylate.

CH₂ = CH-COOH (aq) ---------→ CH₂ = CH - COO - (aq) + H⁺ (aq)

\(Acrylic\ acid\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ acrylate\ anion\)

CH₂ = CH-COO- (aq) → ------[-----CH₂------CH(COOH)------]-----

\(Acrylate\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ chain\ polymerization\)

As already mentioned, the hydrogel is a cross linked polymers formed from chain of acrylate molecules linked together. Due to cross-linking, the chains are close to each other. The chains have the carboxylate group – (COO- ) projected outwards. The water molecules can enter into the gaps within the chain as the O atom with a negative charge on the carboxylate group makes a inter molecular Hydrogen bond with the H atom of water molecule. This is illustrated in the diagram below, where the chain of acrylate is shown in violet color and the water molecule is represented with H atoms in red color and the O atom in violet color. The H atoms are shown using zig zag lines of green color.

The quantitative measure of swelling of hydrogel can be expressed as the mass of water in grams absorbed by 100 g of dry hydrogel. It can be calculated using the formula given below:

Percentage water absorption capacity = \(\frac{mass \ of \ wet\ hydrogel\ (m_2) - mass\ of \ dry \ hydrogel\ (m_1)}{mass\ of \ dry\ hydrogel\ (m_1)} × 100\)

In a research article titled as – “ Preparation and Characteristics of Corn Straw-Co-AMPS-Co-AA Superabsorbent Hydrogel” by Xiangming Hu the correlation between salt concentration and swelling ratio (ratio of the mass of wet hydrogel and dry hydrogel) was investigated. As displayed in Figure-2, which is a screenshot of the graph in that paper, the swelling capacity has been found to decrease sharply as the concentration of NaCl increases from 0.00 moldm^-3 (control-only pure water) to 0.05 moldm^-3 and then decreases at a very slow rate with the increase in molar concentration of NaCl. Thus, more the concentration of NaCl, less the amount of water absorbed by the hydrogel beads.

The percentage absorbance of water has no correlation with molar concentration of the NaCl solution.

The percentage absorbance of water has a negative correlation with molar concentration of the NaCl solution.

As the concentration of NaCl increases, there are more Na⁺ ions in the medium which gets attached to the COO- group through an strong electrostatic force of attraction with the O atom carrying a negative charge. As a result, there are less number of COO- groups free to make inter molecular H bonds with the H atom of H₂O. This in turn reduces the number of water molecules that can be absorbed and thus reduces the percentage water absorbance of the hydrogel beads. Thus, a straight line with negative gradient is expected in a scatter plot of the percentage water absorbance against molar concentration of NaCl.

Molar concentration of aqueous solution of NaCl Aqueous solution of NaCl will be prepared by dissolving requisite mass of NaCl in distilled water. The concentration of Na⁺ in the urine of a normal male adult can reach up to 220 milliequivalent per L.

1 milli-equivalent of Na⁺ = 0.023 g of Na^{+ 9}

220 milli-equivalent of  Na⁺ = (220 × 0.023 g) = 5.06 g of Na⁺

\(5.06 g\text{ of }Na^+ = \frac{5.06 \ (mass)}{23\ (mass\ of\ 1 \ mole\ of\ Na^+)} = 0.22 \, \,moles \, \text{ of}  \, \,Na^+\)

NaCl dissociates according to the equation,

NaCl (aq) -----→ Na⁺ (aq) + Cl-(aq)

Thus, moles of Na⁺ = moles of NaCl

This means that according to the maximum value of biological interval, the maximum moles of NaCl in 1 L of urine sample is 0.22 which makes it a 0.22 moldm^-3 solution of NaCl. Here, reference has been drawn to urine as it contains water as a solvent with NaCl as the solute along with other substances. Thus, to mimic the composition of urine in terms of NaCl, the concentration of the aqueous solution of NaCl used has been varied in the range of 0.10 moldm^{- 3}, 0.15 moldm^-3, 0.20 moldm^-3, 0.25 moldm^-3 and 0.30 moldm^-3.

The percentage water absorption capacity at room temperature.

The mass of the dry hydrogel and the mass of the wet hydrogel will be measured using a digital mass balance. The percentage water absorption capacity will then be calculated using the equation given below:

Percentage water absorption capacity = \(\frac{mass\ of\ wet\ hydrogel\ (m_2)\ - mass \ of\ dry\ hydrogel\ (m_1)}{mass\ of\ dry\ hydrogel\ (m_1)} × 100\)

Quantitatively, the percentage swelling represents the mass of water in g absorbed by 100 g of the hydrogel.

• Temperature:

As temperature increases, the rate in which the chains of the acrylate unfold becomes faster and thus it absorbs more water in the same time. Thus, all trials were conducted at the room temperature. The room temperature was measured using an infrared thermometer and all trials were conducted on the same day to avoid error due to fluctuations of temperature.

• Time of exposure:

Longer the time, the hydrogels are immersed in water, more the number of water molecules that can get inside the bulk of the hydrogel. Thus, the hydrogel was soaked in water for 10 minutes in all cases. A stop-watch in the mobile-phone was used to monitor this.

• Physical conditions:

As reported in a research article, it has been found that water absorbing capacity of the hydrogel may also depend on physical factors like presence of electrical field, magnetic field as well. Thus, any electrical or magnetic device was not kept in vicinity to the beaker that had the hydrogel soaked in water.

• Volume of water used:

​​​​​​​More the volume of water used, more the concentration gradient between the water and the hydrogel, more the flow of water molecules from the water to that in the bulk of hydrogel. Thus, the same volume of water was used in all trials. 20.00 ± 0.05 cm³ of water was used in all trials and a graduated pipette was used to control the volume.

• Source of hydrogel:

​​​​​​​For all the trials, the same brand of diaper was used-Pampers. The beads of sodium polyacrylate was taken out from these diapers for the investigation. The same brand was used as variation in the brand used may alter the composition of the hydrogel.

• Wear a safety masks and a protective clothing.
• The knife was used with utmost care and under the supervision of an adult.
• Hair must be always kept tied while performing the investigation.
• The work-station must be always kept clean and organized.

Minimum amount of chemicals was used. Any toxic chemicals were not used.

All waste and materials were disposed of safely into a particular waste bin.

• The diaper was taken out of the packet.
• The paper stick pads were removed.
• A scissor was taken and the central part of the diaper was cut carefully.
• The hydrogel beads were taken out.

• A 100 cm³ glass beaker was taken.
• Distilled water was added till the mark of 100 cm³ from the distilled water bottle.
• A watch glass was taken and placed on the top pan of the digital mass balance.
• The reading of the balance was tared to 0.00 ± 0.01 g.
• NaCl was transferred using a spatula from the packet to the watch glass until the balance reads 0.58 ± 0.01 g (0.01 moles).
• The weighed solid was transferred exactly from the watch glass to the distilled water in the beaker.
• A glass rod was used to stir the solution and dissolve the solid completely.
• 2.00 ± 0.01 g of hydrogel beads was weighed on a watch glass using a spatula on the digital mass balance.
• Another 100 cm³ glass beaker was taken.
• 20.00 ± 0.05 cm³ of the NaCl solution was added to it using a 20.00 cm³ graduated pipette.
• The weighed hydrogel beads were transferred to it.
• The stop-watch was started in the mobile phone.
• After 10.00 minutes, the content of the beaker was filtered using a funnel, a filter paper and a 150.00 cm3 conical flask.
• The filtrate was discarded and the swollen hydrogel beads collected as a residue on the filter paper was allowed to dry in the air. The ceiling fan was switched on to make this process faster.
• The swollen hydrogel was transferred from the filter paper to a watch glass placed on a top pan digital mass balance and the mass was recorded.
• Steps 10-15 were repeated for four more times to collect data in five trials.
• Steps 1-16 were repeated using other masses of NaCl – 0.88 ± 0.01 g (0.010 moles), 1.17 ± 0.01 g (0.020 moles), 1.46 ± 0.01 g (0.025 moles), 1.75 ± 0.01 g (0.030 moles).