4. Measurements of contact angles for several liquids on a poly(tetrafluoroethylene) surface \begin{tabular}{lcc} \hline Liquid & Contact angle \( \left({ }^{\circ}\right) \) & Surface tension \( \left(\mathrm{mN} \mathrm{m}^{-1}\right) \) \\ \hline Water & 107 & 72.8 \\ Propylene carbonate & 76 & 40.7 \\ Hexachloro butadiene & 59 & 35.3 \\ Di(2-ethyl hexyl) adipate & 63 & 30.6 \\ Dibutyl ether & 35 & 22.8 \\ \( n \)-Octane & 33 & 21.8 \\ \hline \end{tabular} (Teflon) at \( 20^{\circ} \mathrm{C} \) gave the following results: Estimate the critical surface tension of Teflon using the Zisman Plot. Is Teflon an easy material to wet? How can we improve the wetting of Teflon surfaces?
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The Zisman Plot is used to estimate the critical surface tension of a solid by plotting the cosine of the contact angle (θ) against the surface tension of various liquids. The critical surface tension is the value at which the plot intercepts the cosine θ = 1 Show more…
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Most liquids follow Trouton's rule, which states that the molar entropy of vaporization lies in the range of $88 \pm 5 \mathrm{~J} / \mathrm{mol}-\mathrm{K}$. The normal boiling points and enthalpies of vaporization of several organic liquids are as follows: $$ \begin{array}{lrl} \hline \text { Substance } & \begin{array}{l} \text { Normal Boiling } \\ \text { Point }\left({ }^{\circ} \mathrm{C}\right) \end{array} & \begin{array}{l} \Delta H_{\text {vap }} \\ \text { (kJ/mol) } \end{array} \\ \hline \text { Acetone, }\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO} & 56.1 & 29.1 \\ \text { Dimethyl ether, }\left(\mathrm{CH}_{3}\right)_{2} \mathrm{O} & -24.8 & 21.5 \\ \text { Ethanol } \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} & 78.4 & 38.6 \\ \text { Octane, } \mathrm{C}_{8} \mathrm{H}_{18} & 125.6 & 34.4 \\ \text { Pyridine, } \mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N} & 115.3 & 35.1 \\ \hline \end{array} $$ (a) Calculate $\Delta \mathrm{S}_{\mathrm{vap}}$ for each of the liquids. Do all of the liquids obey Trouton's rule? (b) With reference to intermolecular forces (Section 11.2), can you explain any exceptions to the rule? (c) Would you expect water to obey Trouton's rule? By using data in Appendix $\mathrm{B}$, check the accuracy of your conclusion. (d) Chlorobenzene $\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\right)$ boils at $131.8^{\circ} \mathrm{C}$. Use Trouton's rule to estimate $\Delta H_{\text {vap }}$ for this substance.
You are investigating the properties of an unknown fluid. You create a flat film using the apparatus below, which has a rigid U-shaped frame and a light, movable pin. The frame is oriented vertically as shown in the figure. You find that if the pin has a mass of 30 mg and a length of 2 cm, it experiences an upward acceleration of 26.3 m/s^2. What is the cohesive surface tension of the fluid? Enter your answer in N/m to three decimal places. (Hint: It will probably help to draw a free-body diagram for the pin first.) A droplet of this fluid with a radius of 2 mm is dropped from a pipette at sea level. What would the gauge pressure inside the droplet be? The droplet falls on a horizontal sheet of plastic, making a contact angle of 133°. What is the adhesive surface tension? Suppose you wanted to increase the contact angle. Could you do so by changing the type of plastic in the sheet, changing the adhesive surface tension? If so, should you increase or decrease adhesion? Explain your reasoning.
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