Antibiotics in Action

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    Chemistry Activity
    Molecular Size
    Oleic Acid Monolayers

    Introduction

    One of the factors involved in the movement of substances across cell walls is the size of the molecules involved. This is true for most semipermeable membranes. In the Cell Activities the ability of water molecules or other molecules to pass through the cell wall depends in part on molecular size.
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    General Safety Guidelines
    Introduction
    Materials and Apparatus
    Warm-up: The Thickness
    of a Thin Layer
    Procedure
    Worksheet

    The difficulty, of course, is attempting to measure dimensions of objects that cannot be seen directly. Chemists can determine the size of molecules by various means. In this activity you will find the size of a single molecule of oleic acid or other carboxylic acid. A warm-up activity is included to familiarize you with the method used in the monolayer experiment itself.

    Materials and Apparatus

      Aluminum foil
      Ruler
      Balance
      Pizza pan
      Medicine dropper
      Beral® pipettes
      Oleic acid solution
      Powder

    Warm-up: The Thickness of a Thin Layer

    Begin by printing a copy of the worksheet that you will use in both the warm-up and the main activity. In this warm-up you determine the thickness of a sheet of aluminum foil. To understand the approach to this problem, think about a piece of aluminum foil as if it has a significant thickness as in the diagram below.

    diagram showing length, width, and height

    The volume (V) of the rectangular solid is:

    V = l x w x h

    where l = length, w = width, and h = height. In this equation we can measure the length and width and we can find the volume using the equation:

    d = m/V or V = m/d

    where d = density and m = mass. The density of aluminum is 2.7 g/cm3. You now have all the information you need to carry out the warm-up by following the instructions below:

    1. Obtain a piece of aluminum foil.

    2. Measure its length and width and record both on your worksheet.

    3. Measure the mass of the foil and record on your worksheet.

    4. Calculate the volume of the foil. Show your calucations on your worksheet.

    5. Calculate the thickness of the foil. Show your calucations on your worksheet.

    In this part of the activity you should focus on the method used to find the very small thickness. The thickness is actually made up of a large number of aluminum atoms. However, a method similar to the one used here will be used to find the thickness of a layer of oleic acid, which is a liquid. As the liquid spreads out on a water surface, we will assume that it forms a monolayer, a layer that is a single molecule thick. By finding the area of the layer and the volume of the layer we can perform a simple calculation to find its thickness.

    Procedure

    1. Obtain a pizza pan or other similar shallow container capable of holding water.

    2. Fill the container with water and place it on a level surface.

    3. Obtain a medicine dropper or plastic Beral® pipette. Squeeze water into the dropper or pipette. Count the number of drops of water needed to deliver 1.0 ml with the dropper held vertically. Record this number on your worksheet. Squeeze out all the water and squeeze in ethyl alcohol to clean the dropper or pipette. Remove all the alcohol from the dropper or pipette.

    4. Obtain a solution of oleic acid in the dropper or pipette.

    5. Carefully shake a powder onto the surface of the water in the container. Your teacher will provide the powder and instruct you on how to apply it.

    6. Hold the dropper or pipette vertically, bring the tip close to (but not touching) the water surface, and drop on to the surface one drop of the oleic acid solution. Observe for at least 30 seconds. Add another drop and wait. Add drops of the oleic acid solution until the clear surface covers one half to two thirds of the water surface. Keep track of the number of drops used. Record on your worksheet.

    7. Measure the diameter of the clear surface. If it is irregular in shape, make measurements along several diameters and average these. Record on your worksheet.

    For more information, at other Web sites...

      Gilbert Newton Lewis and Irving Langmuir — biographical sketches, part of Chemical Achievers from the Chemical Heritage Foundation. Irving Langmuir carried out pioneering investigations of monolayers.

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