CHM170L - Final Report 3

CHM170L Physical Chemistry 1 Laboratory 1st Quarter SY 2015-2016

Experiment No. 3: Surface Tension of Liquids Soriano, Allan N.1, Authors: Bagumba, Ivan Bilog, Jhaselyn Klevy Bucud, Katrina Cruz, Patricia Anne Domingo, Angelo2 CHM170L / A11 Professor, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology; 2Student (s), Subject/Section, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology 1

ABSTRACT Surface tension is defined as the force (acting perpendicular to the surface) per unit length. The force generated acts mostly upon surface molecules since the forces balance within bulk molecules of a liquid, resulting to zero net force. In order to change the shape of a liquid, the required surface free energy must be applied on the substance as dictated by surface tension for pure solvents and by surface concentration for solutions of liquid-liquid mixture and/or partly miscible interface. The experiment utilizes the Tensiometer -ring method or the Du Nuoy method in determining the surface tension of 0.1 M to 0.8 M nbutanol solutions. Distilled water was used for calibration procedures. The Du Nuoy method uses a platinum-iridium alloy ring in order to measure the maximum force generated upon submersion on the surface of the liquid samples. The obtained surface tension of water was relatively high, The plot of concentration versus surface tension for n-butanol showed a decreasing trend; increasing the concentration of n-butanol decreases the solution surface tension due to non-uniformity of solute upon the surface of the solution. Using the Gibbs isotherm as well as the slope of the plot generated, the surface concentration of the n-butanol set was found to be 2.497 x 10 -12 moles/cm2 at 28O C. A strong correlation coefficient of 0.9697 was noted for the data points collected. The objectives of the experiment were met with a few recommendations. Keywords: surface tension, du nouy ring, surface concentration, gibbs isotherm

INTRODUCTION

Two forces are dominant upon liquid systems. Cohesive forces are responsible for holding atoms and/or molecules together; intermolecular in a sense. Adhesive forces are accountable for the adherence of the mixtures within containers or for the tension between two systems (liquid to any other phase) at the surface. Surface tension is the amount of surface energy per unit area required to alter the shape of a substance. Surface free energy refers to the amount of work needed to increase the area occupied by the molecules of a substance. Upon physical aspects, surface tension is the liquid property which causes the substance to behave as if a thin sheet of film is covering its surface. This behaviour is caused by the

Experiment 03│ Group No. 3│ Date July 27, 2015

tendency of the liquid molecules to assume the lowest possible free energy to attain equilibrium. Liquid molecules tend to contract to an ideal shape at a given temperature and pressure. Among liquids, two types of molecules are considered: molecules within the bulk of the liquid and molecules at the surface. The difference is the direction as well as magnitude of the force attraction; the molecules within the liquid bulk is surrounded by other molecules on all sides, allowing for attraction in all directions. Hence, zero net force is applied for bulk molecules. However, molecules on the liquid surface are attracted by a force directed inward to the molecule. In order to change the shape of a liquid, work must be applied on both the bulk molecules and

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the surface molecules so as to extend the surface area occupied by the substance. Among solutions, the surface tension corresponding to the force attraction on the interface of two different substances is known as interfacial tension. The factors that affect surface tension (interfacial) are concentration and type of the solute dissolved as well as the presence of any surfactants. Gibbs isotherm represents the relationship between the factors of surface tension and surface concentration. The experiment utilizes static method of determination of the surface tension of the interface between two liquids in an aqueous solution (n-butanolwater mixture), specifically, the tensiometer-ring method. The objectives were to measure the surface tension of pure liquids and of an aqueous solution, obtain the effect of bulk solute concentration for aqueous solution surface tension and to evaluate the variables affecting Gibbs isotherm using graphical method.

trials were performed. The Du Nuoy ring method utilizes a platinum-iridium alloy ring which hangs parallel to the surface of the liquid. Upon operation, the ring is sunk into a designated level into the liquid. In a vertical direction, the ring is gradually drawn apart. Throughout this process, the surface tension of the liquid generates a force upon the ring. The surface tension is measured using the changes with the force generated, wherein the maximum force value and the properties of the platinum ring were related by a correction factor. The Gibbs adsorption isotherm was also utilized in the experiment:

where: u = surface concentration (moles/cm2) = surface tension (dynes/cm) C = bulk concentration (moles/cm3) T= temperature, K

METHODOLOGY

The experimental procedures made use of the Du Nuoy method which utilizes the following apparatus: 8 - 50 mL volumetric flask, 2 - 250 mL beaker, 3 - 5mL pipet, 2 - suction bulb and Du Nuoy Tensiometer. The Du Nuoy instrument calibration was performed by measuring the surface tension of distilled water for three trials. A correction factor ( ) was obtained and was utilized for the succeeding measurements of n-butanol. * 8 different concentrations of n-butanol were prepared as test samples. For each concentration, five


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RESULTS and DISCUSSIONS In determining the surface tension of various n-butanol solutions, the tensiometer was first calibrated. Upon calibration of the instrument, the surface tension of distilled water was taken at 28 O C. Data obtained (see Table 1) during calibration were treated and used as the correction factor for the nbutanol solutions. The correction factor takes into account the shape of the liquid which is held up by the platinum-iridium ring. The ring is slightly bent and circular in shape, allowing for a few liquid molecules to flow downward. As a consequence, the surface tension is not perfectly even on the surface. The use of the correction factor accounts for the irregularities with the ring method.

Surface Tension (dynes/cm)

Concentration (mol / L)

Corrected Surface Tension (dynes/cm)

0.10

73.57

0.20

64.33

0.30

56.73

0.40

52.96

0.50

49.59

0.60

45.41

0.70

43.06

0.80

40.73

72.6

Trial 1 Trial 2

72.5

Trial 3

65.1

Avg. Surface Tension

70.07

Literature value

71.501

Correction factor

It is found, however, that the surface tension of n-butanol solutions are different from the surface tension of water. The expected literature value for the surface tension of pure n-butanol is only 24.2 dynes/cm. Thus, the surface tension for the dilute nbutanol solutions is expected to range from 35 to 50 dynes/cm. The high discrepancy of the data obtained (Table 2) from the expected values can be accounted for the calibration procedure performed as well as the sensitivity of the tensiometer. Table 2. Surface Tension of n-butanol solution at varying concentrations

Table 1. Surface Tension of Water

Measurement

other pure solvents. An example 4 of the tendency of water molecules to contract to the smallest possible area is the formation of water droplets and the appearance of meniscus when water is held in containers.

1.0204

2

It is observable (Table 1) that water has a relatively high surface tension. Again, surface tension is defined as the force acting over the surface of a solution per unit length of the surface perpendicular to the force. Lower area occupied by the molecules mean higher surface tension. Hence, surface tension is a result of the intermolecular forces that cause the molecules to occupy the smallest possible area in order to achieve equilibrium 3. The intermolecular forces acting among water molecules are strong due to hydrogen bonding, thus, the molecules occupy a small area resulting to higher surface tension than


A plot of the corrected surface tension of nbutanol as a function of the natural logarithm of the various concentrations (Figure 1) show a decreasing trend. The dependence of surface tension on concentration can be explained using the Gibbs

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adsorption isotherm5 (see Methodology) which provides for the relationship between surface tension, solution concentration and temperature. Based on the Gibbs isotherm, the slope of the plot of concentration versus surface tension of the n-butanol solutions is equal to the negative reciprocal of the product of the surface concentration, temperature and the ideal gas constant. Figure 1. Concentration versus corrected surface tension of nbutanol solutions

Using the Gibbs isotherm the slope of the plot, the surface concentration7 for the set of n-butanol solutions at 28OC was found to be 2.497 x 10 -12 moles/cm2. This surface concentration yield is conducive with the theories since a decrease in surface tension must result to a positive surface concentration based on the Gibbs isotherm. Surface concentration may also be used to determine the amount of surface area occupied of each molecule using Avogadro's number8. The abovementioned data gives insight on the amount of surface energy required for n-butanol and may be combined with results from pressure tensiometers (and other types) for adsorption coefficient determination.

CONCLUSIONS RECOMMENDATIONS

Interfacial tension is applicable when two phases are present6. In this case, a two-liquid interface was applied in the experiment. Due to the adhesive forces caused by interfacial tension, an interface, or an interaction at the surface, occurs between the water and butanol molecules. Increasing the concentration of n-butanol adds more butanol molecules to the solution, thereby increasing the non-uniformity of solute concentration near the surface. This increase along the surface can be classified as positive surface concentration; when free energy is increased by this surface concentration along with the decreasing surface tension, equilibrium is reached.


AND

The surface tension of substances shows how much surface free energy is needed to bring the bulk molecules to the surface in order to break or change the area of the substance's surface. Smaller area translates to higher surface tension and is usually dictated by the intermolecular forces; water has a high surface tension due to hydrogen bonding. The surface tension of nbutanol decreases as the concentration increases due to interfacial tension which then results to an increase in free energy. Liquids, whether present as pure solvents or solutions, will always assume the smallest possible area in order to reach state equilibrium. The Gibbs isotherm relates temperature, surface concentration and surface tension wherein a positive surface concentration leads to reduced surface tension while a negative surface concentration gives an increase on surface tension.

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Despite the errors noted in the experiment, a correlation of 0.9697 for the plot of concentration versus surface tension of n-butanol serves as a strong indication of the relationship between the data points. The objectives of the experiment were met although a few recommendations were noted. Due to the sensitivity of the platinum-iridum alloy ring used, the method itself is not perfectly reliable for determination of the surface tension at the equilibrium state. Further use of the Du Nouy method requires careful handling of the instrument.

REFERENCES 1. Physical Chemistry, 3rd ed. Mortimer, R. (Data interpolated from source) 2. Surface Tension. Patel, I. (pdf version) 3. Physical Chemistry of Surfaces, 6th ed. Adamson, A., and Gast, A. John Wiley, New York. 1997 4. Fluids: Surface Tension. Ducree, J. (pdf version) 5. Physical Chemistry I Laboratory Manual, Part 1. Caparanga, A., Baluyut, J., Soriano, A. Manila. 2006 6. Interfacial Tension. Non-Newtonian Fluid Dynamics Research Group. MIT (retrieved from database, 7/24/15) 7. IUPAC: Compendium of Chemical Terminology, 2nd ed. ("The Gold Book"). Compiled by McNaught, A. and Wilkinson, A. Blackwell Scientific Publications, Oxford. 1997 8. Surface excess concentration. Kruss Surface Science Group. Hamburg, Germany. 2015 (retrieved from database, 7/24/15)


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CHM170L - Final Report 3

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