How does the magnitude of upper critical solution temperature (UCST) of phenol (hydroxy benzene)-aqueous NaCl system depend on the molar concentration (mol dm^-3) of the aqueous solution of NaCl added to it, determined using mutual solubility curve?

I feel that the most interesting part of studying experimental science is the way it connects real life observations and phenomenon with facts and principles we study in a text book. The genesis of this essay begins from an anecdote along the same idea, which I made much before I was introduced to the term – Extended Essay in the IB Diploma Program. Having passion for cooking, I observed something that really intrigued me; when I took oil in the pan and then added water to it, it did not mix and separated clearly into two distinct layers. But after heating it for some time, it existed as a single layer. In contrast, I have also observed that sometimes if either the amount of oil or water is too less in comparison to the other, they mix with each other at room temperature only. This observation immediately popped certain questions in my mind - Does the miscibility of a pair of liquids depend on the ratio of the mass/volume in which they are mixed or the temperature? What if I add something else in this mixture; will that affect the miscibility in any way?

To satiate the inquirer hidden inside, I explored more. After an effective search with the help of my supervisor, I came across few credible journals and research articles which taught me about something called ‘smart polymers’. The aforementioned are basically preparing polymers that are partially miscible with water and exhibit either LCST or UCST.

These polymers are mainly used in the field of pharmacy and metal extractions, detection of carcinogenic metal ions in foods and so on. What intrigued me the most was the fact that the properties of this partially miscible polymer-water system can be altered by adding various ionic electrolytes or organic compound to make them more effective and useful.

This urged me to explore more on how the mutual solubility of two liquids could be affected by addition of impurities. Further academic reading and consultation with my supervisor introduced me to the system of phenol and water as a common example of partially miscible liquids whose mutual solubility largely depends on temperature as well as the presence of external impurities. To make the discussion more specific and concise, I decided to conduct my Extended Essay in Chemistry based on the following research question - How does the magnitude of upper critical solution temperature (UCST) of phenol(hydroxy benzene)-aqueous NaCl system depend on the molar concentration(moldm^-3) of theaqueous solution of NaCl added to it, determined using mutual solubility curve?

Liquid-liquid system (mixture of two liquids) can be categorized into three types based on their miscibility (solubility of one of the liquid into the other).

Immiscible – If liquid A and liquid B when mixed together at room temperature form two distinct layers, the two liquids are considered to be completely immiscible with each other.

Example - pentane and ethanoic acid.

Miscible – If the two liquids A and B when mixed with each other at room temperature form one single layer, the  liquids are considered to be completely miscible with each other.

Example - water and ethanol.

Partially miscible – There are certain liquid pairs whose mutual solubility depends on the composition of the mixture and the temperature of the mixture. In this case, when the two liquids are mixed, they might form two distinct layers or one single layer depending on the temperature at which they are mixed and the ratio of volume in which they are mixed. Even if they form two distinct layers, the volume of the individual layers after mixing is either greater or smaller than the volume of the individual liquids before mixing. Such kind of liquid pairs are considered to be partially miscible.

A system of phenol and water is one such example of partially miscible liquids.

For any pair of liquids there exists a temperature, above which they are completely miscible with each other/form a clear single layer solution irrespective of the proportion in which they are mixed. This temperature is called Upper critical solution temperature (UCST). The phenol-water system exhibits Upper critical solution temperature.

Refer appendix A.1. for definition of LCST

In order to determine UCST of a partially miscible pair of liquids, I would have to record the temperature points at two instances –

• When the turbidity of the solution disappears i.e. completely mixes into a clear layer due to gradual heating.- t₁
•  When the turbidity re-appears due to cooling - t₂

The average miscibility temperature would be hence given by finding the average of the two temperatures t₁ and t₂. The average miscibility temperature is plotted against mass percentage of any one of the liquid to obtain the mutual solubility curve and the UCST is calculated from the maxima of the curve,

The following is a predicted graph of how the UCST value will be plotted –

Above t₁, the pair of liquids are completely miscible, whereas below t₂, the pair of liquids are completely immiscible irrespective of the proportion or ratio in which they are mixed with each other. Therefore, in between t₁ and t₂ the pair of liquids are partially miscible. The midpoint of this region would indicate the temperature at which the liquids are completely miscible.

The value of UCST showcase the extent of mutual solubility for any pair of liquids. As the UCST increases, it indicates that the mutual solubility for the pair of liquids is decreasing. Hence, as a pair of liquids become more soluble into each other, the value of UCST should decrease.

Definite mass (5.00 ± 0.01g) of phenol was taken in a test tube and the volume of aqueous NaCl  added to it was varied to vary the mass percentage of phenol. Subsequently, different concentration of NaCl solutions were added to the test tube as an impurity. The test tube was then heated in a water bath to record the temperature (t₁) at which the turbidity disappears i.e. a single layer in formed. It is then allowed to cool down to record the temperature(t₂) at which turbidity re-appears (separation into two distinct layers). This method is chosen as it is easy and convenient to perform in a school laboratory.

The UCST of a pair of liquids can also be determined by studying the variation of the morphology of the molecules in the mixture using SEM(Scanning electron microscopy) and dynamic light scattering method. The pair of liquids has to be heated and at various temperature, the morphology of the mixture has to be investigated. At UCST, both the liquids will undergo a significant change in their morphology and behave as an emulsion. This method was not viable for me as it involve the use of high end apparatus which are not available in a school laboratory.

Both phenol and water has inter molecular H bonding in the pure state due to the presence of polar O-H bonds in them. On mixing, these molecules will break the H bonds within their own system and form inter molecular H bonds with each other to become mutually soluble. The formation of inter molecular H bonds between them depends on two factors. - 

• If the amount of one component in the system is significantly lesser than the other, they will form the H bond easily and become soluble with each other even at a low temperature.
•  In case, both the components are present in nearly equal proportion, the temperature of the system has to increase to provide more kinetic energy to the molecules and break the H bond within their own system to form H bond with each other. Thus, as the ratio of the components in the mixture approaches towards 1:1, they become mutually soluble only above a particular temperature which is considered as the UCST value of that system. 

The current investigation deals with a phenol-aqueous NaCl system which is actually a ternary system phenol, water and NaCl but it may be looked upon as a binary system of phenol and water with NaCl as an impurity in it, as the solutions of NaCl used are dilute.

The UCST of a pair of liquids decreases as we add an impurity which is soluble in both the liquids while it increases as we add an impurity, soluble in one of the liquids and insoluble in other. NaCl is an ionic compound which is soluble in water but not in phenol. Hence, presence of NaCl in a phenol-water system would increase the value of UCST. As we add NaCl, more water molecules are engaged in hydrating the Na⁺ and Cl^- ions formed from disassociation of NaCl. Thus, amount of H₂O molecules available to hydrate phenol molecule decreases and hence the mutual solubility of phenol and water decreases, which in turn increases the UCST.

The molar concentration of NaCl (aq) added to phenol is the independent variable. The solutions used are of concentration – 0.0 moldm^-3 (only distilled water; used as control), 0.5 moldm^-3, 0.7 moldm^-3, 0.9 moldm^-3, 1.1 moldm^-3 and 1.3 moldm^-3. All these solutions were prepared by adding requisite mass of NaCl weighed in an electronic mass balance in a 100cm³ volumetric flask. A graduated measuring cylinder was used to measure the volume of water added.

The temperature at which turbidity disappears (formation of one single layer) and reappears (separation into two distinct layers) of the phenol-NaCl(aq) system was noted down by taking the mixture in a test tube, heating it in a water bath and allow it to cool down after that. A stainless steel temperature probe coupled with a Lab Quest was used to record the temperature. The average of these two temperature values was used as average miscibility temperature and plotted against the mass percentage of phenol used to determine the UCST from the mutual solubility curve. In this way, the UCST value of the phenol-NaCl(aq) system was determined as the dependent variable for all values of concentration of NaCl(aq).

• Mass of phenol added – 5.00 ± 0.01 g
• Apparatus used to record temperature-stainless steel temperature probe
• Surface area – same test tube (20 cm³, hard glass) was used in all trials.
• Apparatus used to record mass – electronic mass balance.
• Percentage purity of reagents – both phenol and NaCl used were from the same reagent bottle.

Refer to Appendix A.2.- for the significance and method of controlling these variables.

There is no correlation between the UCST of phenol-aqueous NaCl system and the molar concentration of aqueous NaCl used.

There is a positive correlation between the UCST  of phenol-aqueous NaCl system and the molar concentration of aqueous NaCl used.

Regression analysis will be done to test the hypotheses to be accepted.

Refer to Appendix A.3.

Refer to Appendix A.4.

• Wear protective glasses, gloves and a lab coat at all times in order as phenol is a very corrosive compound and can cause extreme burns and blindness.
• The procedure should be carried out in a well ventilated environment so as to minimize any sort of inhalation caused due to heating of phenol.
• The phenol should be stored in a cool environment and disposal of phenol waste must be carried out judiciously, should be placed in a labeled glass bottle with a tightly secured lid.

Phenol has widespread harmful environmental effects. All phenol waste were diluted and disposed off safely into a waste chemical bin. All glass apparatus used were washed with chromic acid before reuse.