The molecules at the surface of a liquid are subjected to an unbalanced force of molecular attraction as the molecules of the liquid tend to pull those at the surface inward while the vapor does not have as strong an attraction. This unbalance causes liquids to tend to maintain a minimum surface area. The magnitude of this force is called the surface tension. The symbol for surface tension is γ. Conventionally, the tension between the liquid and the atmosphere is called surface tension while the tension between one liquid and another is called interfacial tension.
The surface tension is depend in the ideal solution on the concentration and temperature which shown by Williard Gibbs thus the following equation is named Gibbs Isotherm (constant temperature)
where γ is the surface tension in dynes/cm, C is the bulk solute concentration moles/cm³, and u is the surface concentration moles/cm², R is gas constant and T is constant or particularly as the room temperature.
The method used in the experiment is static method wherein the use of equipment called Du Nouy Tensiometer was shown, which consists of a platinum-iridium ring supported by a stirrup attached to the beam of a torsion balance. The ring is placed at the interface of two liquids or at the surface of a liquid with air. It is then pulled upward until it breaks free of the liquid and moves into the second liquid or into the air. The force that is just requiring breaking the ring free of the liquid/liquid or liquid/air interface is proportional to the surface tension.
Another property of the liquid or solution is the viscosity. The general concept of viscosity is a familiar one. Less mobile liquids such as tar and lubricating oils are said to have greater viscosity than the more mobile liquids such as water and benzene. In general, it may be stated that the flow rate of a liquid is determined by its viscosity. More precisely, the resistance experienced by one layer of liquid in moving past another layer is called the viscosity. A column of liquid in a circular tube can be considered to be made up of concentric layers, or cylinders of liquid. In moving through the tube, the layer nearest the wall remains stationary if wetting of the surface takes place. Each successive inner layer moves past, on the inside, with a velocity that increases as the centre of the tube is approached. It is known as streamline flow and is generally characterised by the absence of eddies and turbulence.
Hagen and Poiseuille obtained the following equation applicable to the Ostwald procedure,
where p is hydrostatic pressure on the liquid (proportional to the density ρ), t is the time of flow in seconds, r is the radius of the tube, l is the length of the capillary in centimetres, and V is the volume of the liquid in cubic centimetres.
The equation may be determined experimentally for a given viscometer so that the absolute viscosity can be calculated. The usual procedure is to determine the viscosity relative to a reference substance at a selected temperature. This is determined as the relative viscosity. Water at 20oC is usually used as the reference liquid.
Delta P can be expressed as the product of density of the liquid, gravity and height which varies during measurement yielding to equation 1.10 and manipulating and simplifying it will yield to equation 1.11.
where A is a constant for a given viscometer which can be determined through calibration with a liquid of known viscosity and density at a given temperature.
Methodology
The experiment was conducted to determine the properties of Cyclohexane-benzene binary solution. Properties include measurement of density and determination of partial molar volume, determination of surface tension, and measurement of viscosity by capillary flow method. The methods will be discussed separately to understand simultaneously such properties of the liquid mixture: cyclohexane and benzene.
The most common method of measuring partial molar volumes is to measure the dependence of the volume of a solution upon its composition. The observed volume can then be fitted to a function of the composition using MS Excel application and the slope of this function can be determined at any composition of interest by differentiation.
This experiment used equipment and apparatus such as pycnometer, 250-mL beakers, 25-mL graduated cylinders, 10-mL pipet, 2-mL pipet, top-loading balance and aspirator. The pycnometer (see Appendix D, Figure 1) is a glass flask with a close-fitting ground glass stopper with a capillary hole through it. This fine hole releases a spare liquid after closing a top-filled pycnometer and allows for obtaining a given volume of measured and/or working liquid with a high accuracy.
Calibration of the pycnometer to determine its weight and volume was first performed. It was done by filling the pycnometer with water thoroughly without the presence of tiny bubbles. In the calibration, the group acquired the volume of the pycnometer by the difference of mass of pycnometer with and pycnometer without water resulting to the mass of water divided by the density of water with respect to its temperature.
Different concentrations of 60cm3 Cyclohexane-benzene mixture were prepared in separate beakers. The pycnometer was filled with the mixture one at a time then covered with the thermometer and glass stopper. Elimination of the overflowed mixture by wiping the surface of the pycnometer was meticulously finished. The temperature of the weighed mixture was recorded. Using the same principle, the density of the mixture is the change in mass of the pycnometer after sample is contained in it per computed volume of the pycnometer. Same method is applied to all mixture. After weighing a mixture using top-loading balance, the pycnometer was cleaned by pouring out its content, washing, drying in the oven with minute amount of ethanol. The group was cautious in washing the pycnometer and kept it uncontaminated before drying since flammable compounds were used. Before the repetition of the steps, the experimenters waited until the high temperature of the pycnometer decreases to room temperature.
For the partial molar volume of ethanol-water system, the values for the partial molar volume of benzene, C6H6 vs. mole fraction of cyclohexane, C6H12 was plotted. The data was expected to have a parabolic path. MS Excel was used in determining the most fixed quadratic equation on the data. Tangent lines to the curve through the plotted points were positioned. A measurement of distance of the y-value by means of horizontal lines to the intersection of the tangent line at molar fraction points was obtained. The partial molar volume was then determined by the addition of the initial molar volume of the mixture.
Surface tension is a measurement of the cohesive energy present at an interface. The molecules of a liquid attract each other. The interactions of a molecule in the bulk of a liquid are balanced by an equal attractive force in all directions.
Equipment and glassware used in the experiments were Du Noüy tensiometer (see Appendix D, Figure 2), 50-mL volumetric flasks, 1-mL pipets, rubber aspirators. Du Noüy ring method is one technique by which the surface tension of a liquid can be measured. The method involves slowly lifting a ring, made of platinum iridium alloy, from the surface of a liquid. The force required to raise the ring from the liquid's surface is measured and related to the liquid's surface tension.
The group performed three trials of pure water for calibration then obtained its mean value. Calculation for the surface tension correction factor is equal to the difference of the mean value and literature value of the surface tension. If the difference is positive, it would be subtracted to the measured surface tension of binary solution; otherwise, it would be added.
Concentrations were prepared with fixed increments of cyclohexane-benzene solution. The concentration of the solution was obtained by getting the product of the density and volume of benzene divided by the molecular mass of benzene multiplied by volume of solution. To clearly understand, see Appendix A: Sample computation.
Surface tension of cyclohexane-benzene solution was determined by pouring two-thirds of the mixture in the sample flask. The Du Noüy tensiometer automatically measure surface tension in clicking buttons in five steps: Up, Stop, Peak, Down, Stop. Up button was first clicked so the vessel will be raised until the Du Noüy ring break the surface tension of the liquid then stopped followed by Peak then Down until it break again the tension then stopped.
The group plotted the natural logarithm of concentration and the surface tension to observe their relationship. Then, the computation for surface tension using the slope of the line was done.
The group last performed the measurement of viscosity of cyclohexane-benzene solution by capillary flow method or Ostwald-Fenske method to be specific. Ostwald-Fenske method made use of the Hagen-Poiseuille equation for the determination of liquid viscosities by measuring the time flow of a given volume liquid through a vertical capillary tube under the influence of gravity.
The experimenters performed the measurement of viscosity using Ostwald-Fenske viscometer (see Appendix D, Figure 2), pipet, rubber suction bulb, volumetric flasks and beakers. The viscometer was filled with 10mL of the pure benzene. The viscometer was fractionally immersed in the water bath at room temperature. Using suction bulb, the liquid was drawn into the viscometer up to the capillary arm until the feed bulb was filled and the meniscus was above the upper mark. When the liquid was in the upper meniscus, the group recorded the time elapsed from the upper mark to the lower mark. The method was repeated for pure cyclohexane.
Mixtures with different concentrations of cyclohexane and benzene were also measured. Fixed increments in addition to the volume were performed to have different concentrations. The mixtures’ concentrations were determined by dividing the volume of cyclohexane and benzene. Ostwald Fenske method was also applied. For the calculation of viscosity, it is important to compute for the area of contact, A, which is constant. It can be determined through calibration with a liquid of known viscosity and density like pure water.
In performing these experiments, the group was cautious with every procedure since benzene and cyclohexane are flammable and benzene is carcinogenic. It is important to be familiar with the compounds first before using the binary compounds like the experimenters did.
Results and Discussion
The first part of the experiment dealt with the measurement of the density, molar and partial molar volume of cyclohexane-benzene binary system. Density is physically defined as the mass per unit volume. In hydromechanics, it is an indication of whether a substance floats or sinks in a given fluid. A dense substance sinks while a less dense substance floats. Also, density is an extensive property since it depends on the amount of the substance and in addition, it depends on temperature. Another property is the molar volume. This property is defined as the ratio of volume to the number of moles, making it an extensive property unlike volume which is an intensive property. Molar volume is exceptionally useful in simplifying thermodynamic and material balance equations like the Van Der Waals Equation of State. The last expression considered is the partial molar volume. Unlike molar volume, partial molar volume is a better expression since it considers intermolecular attractions between two different molecules. Partial molar volumes are applicable to real mixtures, including solutions, in which the volumes of the separate, initial components are not additive. This is generally the case, in distinction to the paradigm of ideal mixtures. For real mixtures, there is usually a contraction or expansion on mixing due to changes in interstitial packing and differing molecular interactions. Even so, the total volume is the sum over the partial molar volumes times the numbers of moles, because the volume is a homogeneous function of degree one in the amounts of the various chemical species present. Partial molar quantities can be defined for all the extensive thermodynamic variables in a system.
In the experiment, initial calibration of the pycnometer is done. Based on the result (See Appendix A, Table 1.1), pycnometers used have different weight and volume capacity as well. The volume of the pycnometer was derived from the mass of the water that was fully contained in the instrument through its density. Since the density of liquid water is dependent with temperature due to thermal expansion, it is important also to record the temperature of the water.
Table 1.2 shows the experimental molar volume of the cyclohexane-benzene mixture at varying concentrations of the two components. It also shows the amount of moles of each component, their respective volumes, density of mixture and the molar fraction of water. As observed, when the concentration of cyclohexane is increased in the mixture, its resulting density (ρ mix) decreases.
Since in the beginning of the mixture, the benzene is the predominating species, the density of the mixture is just the density of benzene. However, as the cyclohexane is gradually and continually added in the mixture and the concentration of benzene (in the mixture) is reduced, the density of the mixture becomes closer to the density of the cyclohexane.
The graph of the mole fraction of cyclohexane against the partial molar volume is a parabola. The graph indicates that when present alone, any of the two substances exhibit maximum and minimum extrema of partial molar volumes compared when they are in some proportion. Thus, the changing molecular environment, and the consequential modification of the forces acting between the molecules results in the variation of the thermodynamic properties of the substances primarily in this case, the partial molar volume.
All- in- all, the experiment has encountered little errors since the predicted values and trends regarding the properties in consideration were met. This was greatly exemplified by the graph of the partial molar volume against the mole fraction of water, which assumes a nearly perfect parabola.
Based on study by Smith et al. the pycnometer method is not a good technique for measuring partial molar volumes based on their experiment. However it is used in the experiment because it is the easiest way among the introduced methods. By this way there are several methods to use for the evaluation of data.
The method to use is the comparison test by percentage error. The data is shown on table 1.5 on Appendix A. Based on this, the data shows a very small percentage error that they show a very high accuracy and precision, however the values for the pure substance shows a very great difference between the trends of the pure substance that it deviated from the true value using the equation given by Weeks and Benson, and Tokes et al. as described in a report by Kiyohara et al. This is because of the tool used; the true value compared used dilatometric and magnetic float technique while the experimental method used by the students is the pycnometer method.
After performing the experiment, data were gathered and recorded. Temperature of water was noted at 26.9°C. From the three trials performed to determine the surface tension of the binary, it can be seen that the data were close to each other.
In line with the determination of surface tension of cyclohexane-benzene binary in different concentrations, it can be seen that the surface tension of a 1.82747 M concentration of the binary obtained was the highest among all. This means that the solution with a higher volume of benzene has a higher surface tension than that of the other solutions. The data demonstrated the dependency of surface tension in the concentration of the liquids or solutions. Having water as the reference liquid, it can be seen that the surface tension of the succeeding solutions with increasing concentration decreases. This made them inversely proportional to each other. One mere fact about this scenario was different solutes gave off different effects on the surface tension of solutions. It was found out that surface tension of a solution was affected depending on the structure of the solute. A correlation value of 0.98 was obtained which means that there is a very high correlation between the data of the experiment.
In Table 1.10, measurements of viscosity of benzene and cyclohexane are shown. From the data, it can be seen that cyclohexane has higher value of viscosity compared to benzene. This difference in viscosity is due to the intermolecular forces (can be attractive or repulsive) acting between neighboring particles of each liquid. Regarding these forces, if each molecule has strong cohesion with each other, it will allow them to be more intact, thus increasing the resistance of the liquid to flow. It is known that all molecules exhibit London dispersion because everything has an electron cloud, however in our case; cyclohexane exhibits a greater force since it has a higher molar mass of 84.16 g/mol. Aside from cohesive forces, viscosity of liquid is also affected by the size/shape of the molecule. The longer/bulkier the molecules of a fluid is, the greater is the tendency for them to be tangled, providing greater resistance to the fluid’s flow. This factor is held responsible for the difference in the values of viscosity obtained for the samples wherein both of them have comparable composition and orientation. Furthermore, by analyzing the structures of the samples, it can be noticed that benzene is smaller/ less bulky due to the presence of double bonds, hence, has lower value of viscosity.
For the effect of varying concentration on viscosities of the binary system listed on Table 1.11, it is shown that when the amount of cyclohexane is increased, the resulting viscosity also increases proving that the viscosity of the system is approximately a linear function of the concentration. Thus, the viscosity of solution compared to a pure solvent is greater.
Based from the data and results of the experiment, the hypotheses given by the students were all proven null and thus, it was disproven. Therefore, there is significant difference between the changes in concentration and the viscosity of the system, there is also significant difference between the changes in concentration and the surface tension of the liquid and the data of the partial molar volume of the system is acceptable that there is a significant difference between the observed and expected value of the partial molar volumes of the system.
References
1. Atkins, P. W.; Physical Chemistry, 8th ed., W. H. Freeman, New York, 1994
2. Castellan, Gilbert W. (1983). “Physical Chemistry,” 3rd Ed. Addison-Wesley.
3. Levine, Ira N. (2009) “Physical Chemistry,” 6th Ed. McGraw-Hill, Boston.
4. Alvin R. Caparanga, John Ysrael G.
Baluyut, Allan N. Soriano; Physical Chemistry Laboratory Manual, Part 1., Philippines, 2006, pp. 4-8
5. Moore, Walter J. (1972). “Physical
Chemistry,” 4th Ed. Prentice-Hall, New Jersey.
6.
Bufford D. Smith, O. M. (1982). Evaluation of Binary Excess Data for C6 Hydrocarbons Benzene + Cyclohexane.
7. Osamu Kiyohara, G. C. (1973). Determination of Excess Volumes of Cyclohexane + Benzene Mixtures with a Mechanical Oscillator Densimeter.
Acknowledgement
The student researchers would like to express their sincerest gratitude and appreciation to the following people who have helped them in their research project. This project would not be completed without their support and guidance.
First of all, the student researchers would like to thank the staff of the libraries which they visited for gathering data and other relevant information about the topic: The Mapua Insititute of Technology Library, Caloocan City Public Library;
To the student researchers’ very own professor Mrs. Ariziel Ruth Marquez, for being supportive in their activities, for being patient and hardworking, for her good guidance and suggestions, and for sharing with them his knowledge in the field of Research;
To the student researchers’ classmates that have helped them in some way.
Mr. Allenwer Capati and other staffs of Mapua Institute of Technology Chemistry Laboratory for allowing the student researchers to conduct the research activities safely and for being considerate and helpful to them and for providing the students the materials and resources they need;
For the student researchers’ parents who supported them in this research project, for always being there whenever the student researchers need their help and guidance; for providing financial support and sharing their time; and for their encouragement, guidance, support, prayers and advice;
Above all these the student researchers thank God Almighty for strengthening them along the way, for giving hope and inspiration to persevere, for giving them the gift of knowledge, wisdom and patience to complete this research project.
THANK YOU VERY MUCH!!
by
rj22 (student)
