INTRODUCTION:

Surface tension of a liquid is defined as the force in dynes acting along the surface of the liquid at right angle to any line in the surface one centimeter in length¹. Interfacial tension is the force per unit length existing at the interface between two immiscible liquid phases². The interfacial tension between two immiscible liquids has been determined by various methods such as,

1. Dunoy ring method.

2. Drop weight method.

3. Maxi pull force method

4. Pendant drop method

5. Spinning drop method

6. Wilhelmy plate method.

The measurement of interfacial tension between immiscible liquids of equal density using a glass capillary tube has been reported by S.B. Reddy Karri and K. Mathur³. Interfacial tension between water and benzaldehyde has been reported by L.A. Girifalco and R.J. Good⁴. In the present investigation a capillary of uniform radius was used to determine the interfacial tension between water and the organic liquid lighter than water, namely, benzene, toluene, p-xylene and n-hexane.

MATERIAL AND METHODS:

Instrument:

Spectrophotometer: JASCO V-630

Sonicator: D4 Surgicals of India

Materials:

Organic liquids benzene, toluene, p-xylene and n-hexane were of BDH grade and were distilled before use, Water used was deionized distilled water. The liquids were degassed using sonicator. Paracetamol supplied by s-d Fine Chem-limited was used, Capillary used was Pyrex glass capillary.

METHOD:

The major difficulty in using this method was to get a glass capillary of uniform radius (bore size). The capillaries were prepared in the laboratory using a pyrex glass tube which was heated on a burner and drawn into fine capillary. The success of getting capillary of uniform radius was about 5%. The capillary of radius 0.025cm was used which was washed with distilled water, acetone and absolute alcohol. Light suction was applied to draw these liquids into the capillary. The capillary was dried in the oven at 60^oC. The radius of the capillary was determined by spectrophotometric method.

Spectrophotometric method to determine the radius (r) of the capillary:

Paracetamol solution of concentration c₁ (1gm/l) was prepared in aqueous 0.1N NaOH solution. The capillary end was dipped into the paracetamol solution and removed. The length of the solution column drawn into the capillary was measured and the solution in the capillary was transferred to 10ml volumetric flask under air pressure. This was repeated three times. The three solution column lengths l_1,l_2,l₃ were added to give total length “l” in cm. The solution in the volumetric flask was diluted to 10ml with 0.1N NaOH to give paracetamol solution of concentration c₂gm/l. The absorbance reading of the standard solution (1 gm/l) and that of diluted solution were taken at 257nm using 0.1N NaOH solution as the reference using aquartz cell of 1cm path length (b=1cm)

Calculation for the radius of the capillary:

As per Beer-Lamberts Law,

A=abc

where A=absorbance, a=absorptivity at 257nm, b=path length of the cell=1cm and c=concentration of paracetamol solution in gm/l,

A₁=absorbance of paracetamol solution of concentration c₁, A₂=absorbance of diluted paracetamol solution of concentration c₂ingm/l. The value of A₂ was used to calculate the concentration c₂ using the equation A₂=abc₂

Since concentration is inversely proportional to the volume (dilution), we get

(1)

Capillary being cylindrical, V₁ = πr²l

(2)

Substituting the values of c₂, c₁and l in the equation (2) the radius ‘r’ of the capillary was calculated. The data is given in Table 1. The radius of the capillary was found to be 0.025cm (average of the three readings).

Table 1: Radius of the capillary (Spectrophotometric Data)

Sr. No.	c₁gm/l	l cm	V₂ml	A₂	c₂gm/l	V₁=(10.c₂) ml	radius r cm
1.	1.0	10.8	10	0.1516	0.002121	0.02121	0.02499
2.	1.0	10.7	10	0.1496	0.002092	0.02092	0.02494
3.	1.0	10.5	10	0.1487	0.002080	0.02080	0.02511
Mean							0.0250cm

c₁=initial concentration of paracetamol solution = 1 gm/l.

l = total length of the paracetamol solution column in cm that was diluted to 10ml.

V₂ = 10ml, the final volume of the diluted solution.

A₂= Absorbance of diluted paracetamol solution.

c₂ = final concentration of paracetamolin the diluted solution in gm/l = A₂ /ab

a = absorptivity of paracetamol solution at 257nm = 71.5 and b = 1cm.

V₁=initial volume of paracetamol solution of concentration 1gm/l. = = = 10.c₂

r=radius of the capillary in cm. = ( )^1/2

Method I:

To determine the uniformity of the bore size (radius) of the capillary: - A dry and clean capillary was held in a vertical position. A test tube filled with liquid toluene was raised upward slowly just to touch the toluene surface to the capillary tip. The test tube was slowly raised vertically upward and again slowly brought down so that the tip of the capillary was about 1mm inside the liquid toluene. The height of the steady toluene column was measured with an accuracy of ±0.05cm using a ruler graduated in mm. The procedure was repeated five times and the constant column height (h_o) of the liquid column was recorded. The test tube was then further raised so that the tip of the capillary was 1cm below the toluene surface. The constant height (h₁) of the toluene column in the capillary above the toluene surface was measured. The height of the liquid column (toluene) was measured by raising the test tube such that the tip of the capillary was 1, 2, 3, 4, 5, 6, 7, 8cm below the surface of the liquid toluene. The corresponding heights h₁, h₂, h₃, h₄, h₅, h₆, h₇, h₈ were measured. It was found that the height of the liquid column (h) above the liquid surface remained constant. This observation indicated that the capillary was of uniform bore size (uniform radius). Although the organic liquid column in the capillary was visible to the naked eyes, it could be clearly seen using diffused white light, (from a tube light) passed through a pink coloured transparent gelatin paper.

Method II:

After measuring the height of the toluene column as given in the Method I, the test tube containing toluene was slowly lowered till the liquid surface detached from the capillary end. The height of the toluene column was measured and found to be the same as the column height observed before the capillary tip was removed from the toluene surface. A test tube filled with distilled water was slowly raised till the water surface just touched the tip of the capillary containing toluene and then further raised gradually (1mm/sec) till the tip of the capillary tube was about 8cm below the surface of the water in the test tube. Then the test tube was gradually lowered (1mm/sec) till the tip of the capillary was about 1mm below the surface of water. The height of the water column (lower column) h_win the capillary was measured and also the height of the toluene column h_twas measured. The toluene column appeared as shining column in the diffused light. The method was repeated three times and it was found that the column heights h_wand h_tremained constant.

The experiment was carried out at room temperature (30±1^oC)

Calculation for the interfacial tension (γ_int) between water and toluene:

The angle of contact between water and glass and between toluene and glass has been reported⁵to be zero degree and was taken as zero and hence the angle of contact between interface of water and toluene was also considered as zero. In the Method II the weight of the lower water column is balanced by the upward force due to interfacial tension and the weight of the toluene column that stands above the water column is balanced by the upward force due to surface tension of toluene. (Fig.1)

Fig 1: Capillary Rise Method to determine interfacial tension.

h_w = Height of the Water column

h_t= Height of the organic liquid column

Weight of the water column=Upward force due to interfacial tension

∴mgh = 2πrγ_intcosθ(General equation),

θ the angle of contact being 0^o, cosθ = 1,

∴mgh = 2πrγ_int

where, m = mass of the column

h = height of the column

g = acceleration due to gravity = 980 cm/s²

∴πr²(h_w)ρ_wg = 2πrγ_int (3)

where,

r = radius of the capillary in cm

h_w = height of the water column in cm

ρ_w= density of water = 1 gm/cc

γ_int = interfacial tension between water and toluene in dynes/cm

Similarly, for the column of toluene which is supported by surface tension of toluene and at equilibrium,

∴πr²(h_t)ρ_tg = 2πrγ_t

(4)

Where

h_t = height of the toluene column in cm

ρ_t= density of toluene in gm/cc.

γ_t= the surface tension of toluene in dynes/cm

∴Upward force of surface tension due to toluene F₁+Upward force due to water/ toluene interfacial tension F₂=weight of water column W₁ + weight of toluene column W₂

Substituting the values from equation (3) and (4), we get,

i.e., F_{1 +}F₂ = W₁ + W₂

2πrγ_t+ 2πrγ_int= πr²(h_t)ρ_tg + πr²(h_w)ρ_wg

∴2πr (γ_t+ γ_int) = πr²g ( h_tρ_t+ h_wρ_w)

∴ - (5)

Similarly, the interfacial tension between water/benzene, water/p-xylene and water/ n-hexane was determined by the Method II. The values of surface tension of organic liquids used to determine the interfacial tension are given in the Table 2. The values of interfacial tension obtained by the capillary rise method are reported in the Table 3.

RESULTS AND DISCUSSION:

The interfacial tension between water and immiscible organic liquids having densities less than the density of the water using capillary rise method has not been reported in the literature. Mention has been made of the difficulties encountered in the measurement of interfacial tension between two immiscible liquids using capillary rise method.

The interfacial tension between water and the immiscible organic liquid lighter than water was successfully determined using capillary rise method at room temperature (30^oC). The interfacial tension between water/benzene, water/toluene, water/p-xylene and water/n-hexane determined are given in Table 3. The values of interfacial tension determined by the capillary rise method using capillary of radius 0.025cm are in close agreement with the reported values of interfacial tension. Reported values are at 25⁰C and the experimentally determine values by the present author are at 30^oC±1^oC. Considering the fact that the interfacial tension decreases with the temperature, the values of the interfacial tension determined by the authors of this article are lower (except for the water/benzene) than the values reported in the literature which are at lower temperature.

CONCLUSION:

Although the capillary rise method for the determination of the interfacial tension appears to be a simple technique there are few difficulties that had to be overcome to get reliable results. The major difficulty was the determination of the uniformity of the capillary radius. The difficulty was overcome successfully as discussed in Method I. The other difficulty was to view the liquid column with naked eyes or magnifying glass. The liquid column in the capillary was made visible by using tube light as the source of light and passing it through a pink coloured gelatin paper. The results obtained were within 3% of the reported values of interfacial tension.

ACKNOWLEDGEMENT:

The authors of the article wish to thank the Trustees of Mumbai Educational Trust for the laboratory facilities provided to carry out the research work. We also like to thank Mr. SudhirAyare for making the capillaries required for the experiment.

REFERENCES:

1. A. Bahl, B. BahlG.Tuli. Essentials of Physical Chemistry, S. Chand and Company Ltd. 2009, pp.423.

2. Martin’s Physical Pharmacy and Pharmaceutical Sciences. 2006. 5^th Ed, pp. 438.

3. S.B. Reddy Karri, V.K. Mathur. Measurement of Interfacial Tension of Immiscible Liquids of Equal Density.American Institute of Chemical Engineers Journal, Vol 34, Issue 1, January 1988; pp.155-157.

4. L.A Grifalco and R.J. Good. A theory for the estimation of interfacial energies. Derivation and application of interfacial tension, J.Phys. Chem.1957;61, pp. 904.

5. Martin’s Physical Pharmacy and Pharmaceutical Sciences. 2006. 5^th Ed, pp. 442.

6. Lange’s Handbook of Chemistry, Editor: John A. Dean, McGraw Hill Book Company, 12^th Ed. pp.10-103,10-115.

7. Leon Lachman, Herbert A Lieberman, Joseph. L. Kanig. The Theory and Practice of Industrial Pharmacy, 3^rdEd, 1987. pp.103.

8. T. Fraser Young and William D. Hawkins. Interfacial Tension for solid-solid and liquid-liquid interfaces. International Critical Tables of Numerical Data, Physics, Chemistry and Technology, 1928. Vol4, 1^stEd, pp.436.

Received on 10.12.2018 Accepted on 24.01.2019

Asian J. Pharm. Tech. 2019; 9(1):27-30.

DOI: 10.5958/2231-5713.2019.00006.0

Sr. No	Organic liquid	Temperature ^oC	Density gm/cc	Surface Tension dynes/cm	Ref
1	Benzene	20	0.873	28.88	6
2	Toluene	25	0.862	30.90	6
3	p-Xylene	20	0.861	30.69	6
4	n- Hexane	20	0.659	20.44	6

Sr. No	Interface	Height of water column (h_w) cm	Height of organic liquid column (h_org) cm	Interfacial Tension dynes/cm by Capillary rise method	Standard deviation	Interfacial tension dynes/ cm (literature values) by Drop weight method
1	Water / Benzene	2.8	2.8	36.2	± 0.45	35⁷ at 25^oC
2	Water / Toluene	2.9	3	34.93	± 0.5	36.1⁸at 20^oC
3	Water / p-Xylene	2.9	2.9	36.65	± 0.55	37.77⁸at 20^oC
4	Water / n- Hexane	4.0	2.3	47.57	± 0.55	50.8⁷at 25^oC