Wetting and Spreading A Laboratory Experiment LOUIS J . BIRCHER Vanderbilt University, Nashville, Tennessee
1935 the development of new wetting and SINCE spreading agents has been very rapid, and the industrial use of these agents has spread into many fields (1). In 1941 Van Antwerpen (2) listed over 250 trade preparations known as "surface active agents." These preparations include substances classed as detergents, emulsifiers, penetrants, levelers, and so forth; and are used in such industrial operations as metal cleaning, dying, electroplating, and air conditioning. They are used in the textile, paper, steel, paint, and leather industries, among others. They appear in the formulas of insecticides, embalming fluids, fruit washing compounds, shampoos, and the newer dentifrices. More than a score of the large chemical manufacturing corporations of America are engaged in the production of these spreading agents. A paper by H. L. Cupples (3) of the Bureau of Entomology and Plant Quarantine of the United States Department of Agriculture has suggested a laboratory experiment which will not only teach the theory and technique of measuring surface and interfacial tensions, but will also emphasize the practical importance of surface tension studies. The data obtained in this experiment give opportunity for the application of theories of adsorption and the molecular orientation of solutes a t the surface and interface of solutions. The spreading coefficients calculated from the data of the experiment can he linked with the theory of wetting and spreading and with the effectiveness of detergents, fungicides, and insecticides. THEORY OF WETTING
There are several theories (4, 5) concerning the factors which decide whether one liquid will wet and spread out over the surface of another liquid or not. One widely accepted theory is that one liquid will wet a second liquid only if the work of adhesion of that liquid for the second liquid exceeds the work of cohesion which holds the molecules of the first liquid together as a drop. If a drop of liquid b (see Figure 1) is placed on liquid a, the work of adhesion of liquid b for liquid a is W A = r.
+ Yb -
^/oh
where y. and y, are the surface energies or surface tensions of liquids a and b, and y., is the interfacial energy or interfacial tension between the two liquids. The work of cohesion of liquid b, Wc,is
wo = 2rb
and represents the work necessary to break the liquid b and form two square centimeters of new surface of that liquid.
The sfireading coeficient, S,is a measure of the tendency for one liquid to spread on the second, and is the difference between the work of adhesion, W,, and the work of cohesion, W,. For the spreading of liquid b on liquid a S = W A wc = r. - (Y* + Y,$
-
If the surfac'e tension of the liquid of the drop is large, i t has little tendency to spread on the second liquid. It should be noted that if liquid b will spread on liquid a, it does not follow that liquid a will spread on liquid b. It has been pointed out that a great many organic liquids (liquids with low surface tensions) will spread on water, but that water spreads on only a few organic liquids. WETTING AGENTS
To make water "wetter," that is, to promote spreading (6) "surface active" substances, such as the sulfonated higher alcohols, are added to water to lower the surface tension of water and to lessen the work of cohesion of water. Solutes dissolved in water which are less polar than water tend to be adsorbed a t the surface of the solution and lower the surface tension. In aqueous solutions of soap-forminp solutes the ratio
of the amount of base to the amount of fatty acid has a marked effect on the surface tension of the solution. EXPERIMENTAL STUDY O F SPREADING
Cupples studied the effect on surface tensions of the mole ratio of sodium hydroxide to fatty acid in aqueous soap solutions; the effect of these mole ratios on the interfacial tensions of these solutions against paraffin oil; and the effect of these ratios on the tendencies of these solutions to spread on oil or oily surfaces.
With lauric acid and sodium hydroxide the results shown in Figures 2, 3, and 4 were obtained. With an excess of fatty acid in the solution the surface tension and interfacial tension of the solution against oil are low, and the spreading coefficient on oil is high. If sodium hydroxide is added until the ratio of base to acid is unity, the surface and interfacial tensions rise rapidly, and the spreading coefficient drops to a negative value. The base, in forming the
soap, renders the fatty acid more soluble, that is, lessens adsorption. Adding considerable excess of base causes the surface tension and interfacial tension of the solutions to decrease again. The salting-out effect of the excess sodium hydroxide on the soap may be a factor a t this stage. These decreases in surface and interfacial tensions due to a large excess of base are not sufficient to bring the spreading coefficient to a positive value again. Cupples points out that the ratio of base to fatty acid and the pH value of a soap solution has a marked effect upon the effectiveness of the solution as an insecticide and must have a similar effect when these solutions are used as detergents.
FIG-
4.-MOLE RATIOOP NAOHTO LAURICAcm
The writer has found that students in studying surface tension and the theories of adsorption and orientation, and in learning to measure surface tension, can easily duplicate the work of Cupples on lauric acid solutions using the Du Noiiy tensiometer (7). Correlating the measurement of surface tension with a practical problem lends interest to the study. (The special Du Nouy interfacial tensiometer is not required.) Other forms of surface tension measuring apparatus, such as the drop weight apparatus, could be used. (Surface tension data obtained by using a dynamic method may differ from those obtained by using a static method of measuring.) APPARATUS
A Du Noiiy tensiometer Analytical weights 12 crystallizing dishes, 6 to 8 cm. diameter and 2 to 4 cm. deep 5 volumetric flasks of 100-ml. size. CHEMICALS
Lauric add A gwd grade of white paraf3b oil A stock solution of 0.50 normal sodium hydroxide (20 g. per liter), standardized to two places
DmECTIONS To clean the aystallizing dishes, allow them to stand
three hours in a hot chromic acid solution, and then rinse thorouahlv - . with distilled water before usine them. Weigh four l-g. (0.005 mole) samples of lauric acid. Shortly before the surface tension measurements are to be made, make up four 100-ml. solutions as follows: u
Solution No. 1. 1 gram lauric acid (0.005mole), 8 ml. of 0.5 N NaOH, sufficientto neutralize 0.8 of the acid, and distilled water to make 100 ml. of solution. Solution No. 2. Same as solution No. 1, except that 10 ml. of NaOH solution is used to give a 1: 1 ratio of base to acid. Solution No. 3. Same as No. 1. except that 12 ml. of base is used to give a 1:2 ratio of base to acid. Solution No. 4 . Same as No. 1, except that 14 ml. of base is used to give a 1 :4 ratio of base to acid. Solution No. 5, etc., can he aqueous solutions of some of the newer wetting a m t s , such as "Aerosol" or "Dreft." using 1 drop or 0.5 g. of the agent per 100 ml. of solution.
Heat each of the lauric acid solutions to boiling, shake, and allow to stand one hour before using. Assemble the tensiometer according to the directions given by the manufacturer of the instrument and calibrate it, using weights from a set of analytical weights. The ring of the tensiometer must be deaned before each measurement is made by heating to dull red heat in a Bunsen flame. Measure the surface tension of distilled water, each of the lauric acid solutions, the wetter water solution, and the paraffin oil. Rinse the crystallizing dish with a particular solution before putting the sample to be measured in the dish. Record the temperature of the liquids measured. In making the measurements, approach the point a t which the tensiometer ring breaks away from the surface of the liquid or from the interface v n y slmly. Measure the interfacial tension of pure water and of each of the solutions against layers of parafEn oil. The diameter of the crystallizing dishes used should be large enough to prevent excessive m a t u r e of the surfaces of the liquids. Measure the interfacial tension of each of the solutions against the oil about 10 minutes after the interface has been formed. Using the surface tension of oil, the surface tension of water or the several solutions, and the interfacial tension of each liquid against oil, calculate the spreading coefficient of each liquid on oil. Plot on cross-section paper the surface tension of the several lauric add solutions against the mole ratio of sodium hydroxide to lauric acid in the solutions. Make plots for the interfacial tensions and the spreading coefficients of these solutions against the mole ratio of base to acid. How do the curves obtained compare with those obtained by Cupples? Prepare an oil film on a piece of celluloid or some other receptive surface and put drops of water and drops of the several solutions on the oil film. Note whether water, the several lauric acid solutions, and the wetter water will spread on the oil film. How do
these observations agree with the spreadmg cwfficients calculated from the surface tension data obtained with these liquids? DATA SualAca Teasrows
TEXP.
Lipid
-
Trio1 I
-
Pure water Pe%ffinoil Lauric acid solution No. 1 L e e " acid solution NO.2 Laurie add solution N o . 3 La""" acid ao1vtion NO.4 Solution of wetting agent
-
--
INTBRPICIAL TBNSION Liquids on PoroBin Oil
- - -
Pure water Lauric add solvtian NO.I Lauric add solution NO.2 L a e c acid solution No. 3 Laurie add .01ution NO.4 Solution of wetting- agent
- -
- --- -
SPRBADINO COBI~CKBNT. A N D OBBBRVBD SPPBADINO Liquids on Oil
Calcrrlolad
Obreraed
- -
Pure mate Laurie add solution NO.1 Laurie acid solution NO.2 Laurie add solution No. 3 Laurie add solution No. 4 so1vtion of wetting agent
- -
- - -QUESTIONS
To correlate the data obtained in the experiment vith the theories of surface tension, adsorption and orientation of solutes, and of spreading, the student may be asked to answer questions such as the following at the conclusion of the experiment.
1. Why do aqueous solutions usually have surface tensions lower than that of pure water? What type of substances are positively adsorbed a t the surface of solutions? Explain. 2. (a) Why do the surface tensions of the lauric acid solutions increase as the ratio of base to acid approaches and passes through the equivalence point? (b) How would a molecule of sodium laurate orient itself in the interface between water and oil? Explain. (c) Why does a large excess of sodium hydroxide in the lauric acid solutions depress the surface tension again? 3. What factors determine whether a liquid will wet or spread on a second liquid? What industrial problems are better understood in terms of quantitative data on surface energies and spreading coefficients? See reference no. 1. REVERENCES
(1) "Surfaceactive agents- symposium,"Ind. Eng. Chem., 31, 31-69 (1939). (2) VANANTWERPEN, Ind. Eng. Chcm., 33,16 (1941). (3) CWPLES, Ind. Eng. Chcm., 20,924 (1937). (4) B o c u ~ "Colloidal , Behavior," Vol. 1, pp. 169-78, McGrawHill Book Company, New York, 1924. (5) GLASSTONE, "Physical Chemistry." p. 476, D. Van Nostrand Company. Inc.. New York. 1940. (6) CAWL AND ERICKS, Ind. Eng. Chem., 31, 44 (1939). (7) "Cenco-du Nouy tensiometers" Bulletin 101, Central Scientific Company. Chicago.
