8/3/2019 Wetting&Spreading 1943

1/3

Wetting and SpreadingA Laboratory Experiment

LOUIS J . BIRCHERVanderbilt University, Nashville, Tennessee

SNCE 1935 the development of new wetting andspreading agents has been very rapid, and theindustrial use of these agents has spread into manyfields (1). In 1941 Van Antwerpen (2) isted over 250trade preparations known as "surface active agents."These preparations include substances classed as deter-gents, emulsifiers, penetrants, levelers, and so forth;and are used in such industrial operations as metalcleaning, dying, electroplating, and air conditioning.They are used in the textile, paper, steel, paint, andleather industries, among others. They appear in theformulas of insecticides, embalming fluids, fruit wash-ing compounds, shampoos, and the newer dentifrices.More than a score of the large chemical manufacturingcorporations of America are engaged in the productionof these spreading agents.A paper by H. L. Cupples (3) of the Bureau ofEntomology and Plant Quarantine of the United Sta tesDepartment of Agriculture has suggested a laboratoryexperiment which will not only teach the theory andtechnique of measuring surface and interfacial tensions,but will also emphasize the practical importance ofsurface tension studies. The data obtained in thisexperiment give opportunity fo r the application oftheories of adsorption and the molecular orientationof solutes a t the surface and interface of solutions.The spreading coefficients calculated from the dataof the experiment can he linked with th e theory ofwetting and spreading and with the effectiveness ofdetergents, fungicides, and insecticides.

THEORY OF WETTINGThere are several theories (4, 5) concerning thefactors 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 willwet a second liquid only if t he work of adhesion of thatliquid for the second liquid exceeds the work of cohesionwhich holds the molecules of the first liquid togetheras a drop.If a drop of liquid b (see Figure 1) is placed on liquida , the work of adhesion of liquidb for liquid a is

WA= r. + Y b - /o hwhere y. and y, are the surface energies or surfacetensions of liquids a and b, and y., is the interfacialenergy or interfacial tension between the two liquids.Th e work of cohesion of liquid b, Wc, s

wo= 2rb

and represents the work necessary to break the liquidb and form two square centimeters of new surface ofthat liquid.

The sfireading coeficient, S, s a measure of the ten-dency for one liquid to spread on the second, and is thedifference between the work of adhesion, W,, and thework of cohesion, W,. For the spreading of liquid bon liquid a

S = W A- wc = r. - Y*+ Y,$If the surfac'e tension of the liquid of the drop islarge, it has little tendency to spread on the secondliquid. It should be noted that if liquid b will spreadon liquid a, it does not follow tha t liquid a will spreadon liquid b. It has been pointed out th at a great manyorganic liquids (liquids with low surface tensions) willspread on water, but that water spreads on only a feworganic liquids.

WETTING AGENTSTo make water "wetter," th at is, to promote spread-ing (6) "surface active" substances, such as the sul-fonated higher alcohols, are added to water to lowerthe surface tension of water and t o lessen the work of

cohesion of water. Solutes dissolved in water which areless polar than water tend t o be adsorbed a t the surfaceof th e solution and lower the surface tension. Inaqueous solutions of soap-forminp solutes the ratio

8/3/2019 Wetting&Spreading 1943

2/3

of the amount of base to the amount of fat ty acid has amarked effect on the surface tension of the solution.EXPERIMENTAL STUDY O F SPREADING

Cupples studied the effect on surface tensions of themole ratio of sodium hydroxide to fatty acid in aqueoussoap solutions; the effect of these mole ratios on t heinterfacial tensions of these solutions against paraffinoil; and the effect of these ratios on the tendenciesof these solutions to spread on oil or oily surfaces.

With lauric acid and sodium hydroxide the resultsshown in Figures 2, 3, and 4 were obtained. Withan excess of fat ty acid in the solution the surfacetension and interfacial tension of the solution againstoil are low, and the spreading coefficient on oil ishigh. If sodium hydroxide is added until the ratioof base to acid is unity, the surface and interfacialtensions rise rapidly, and the spreading coefficientdrops to a negative value. The base, in forming the

soap, renders the fatty acid more soluble, that is,lessens adsorption. Adding considerable excess ofbase causes the surface tension and interfacial tensionof the solutions to decrease again. The salting-outeffect of the excess sodium hydroxide on the soap maybe a factor a t this stage. These decreases in surfaceand interfacial tensions due to a large excess of baseare not sufficient to bring the spreading coefficient to apositive value again. Cupples points out that theratio of base to fatty acid and the pH value of a soapsolution has a marked effect upon the effectiveness ofthe solution as an insecticide and must have a similareffect when these solutions are used as detergents.

FIG- 4.-MOLE RATIO P NAOHTO LAURIC AcmThe writer has found that students in studying sur-face tension and the theories of adsorption and orienta-tion, and in learning to measure surface tension, caneasily duplicate th e work of Cupples on lauric acid

solutions using the Du Noiiy tensiometer (7). Cor-relating the measurement of surface tension with apractical problem lends interest to the study. (Thespecial Du Nouy interfacial tensiometer is not re-quired.) Other forms of surface tension measuringapparatus, such as the drop weight apparatus, could beused. (Surface tension data obtained by using a dy-namic method may differ from those obtained by usinga sta tic method of measuring.)

APPARATUSA D u Noiiy tensiometerAnalytical weights12 crystallizing dishes, 6 t o 8 cm. diameter and 2 to 4 cm. deep5 volumetric flasks of 100-ml. size.

CHEMICALSLauric addA g w d grade of w hite paraf3b oilA stock solution of 0.50 normal sodium hydroxide (20 g. perliter), standardized to tw o placesDmECTIONS

To clean the aystallizing dishes, allow them to stand

8/3/2019 Wetting&Spreading 1943

3/3

three hours in a hot chromic acid solution, and thenrinse thorouahlv with distilled water before usine- . uthem.Weigh four l-g. (0.005 mole) samples of lauric acid.Shortly before the surface tension measurements areto be made, make up four 100-ml. solutions as

follows:Solution No. 1. 1 gram lauric acid (0.005 mole), 8 ml. of 0.5 NNaOH, sufficient to neutralize 0.8 of the acid, and distilled waterto make 100ml. of solution.Solution No . 2. Same as solution No. 1, except that 10ml. ofNaOH 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 isused to give a 1 :2 ratio of base to acid.Solution No . 4 . Same as No. 1, except that 14 ml. of base isused to give a 1 :4 ratio of base to acid.Solution No . 5, etc., can he aqueous solutions of some of thenewer wetting am t s , such as "Aerosol" or "Dreft." using 1 dropor 0.5 g. of the agent per 100ml.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 directionsgiven by the manufacturer of the instrument andcalibrate it , using weights from a set of analyticalweights. The ring of the tensiometer must bedeaned before each measurement is made by heatingto dull red heat in a Bunsen flame.Measure the surface tension of distilled water, eachof the lauric acid solutions, th e wetter water solution,and the paraffin oil. Rinse the crystallizing dishwith a particular solution before putting the sampleto be measured in the dish. Record the temperatureof the liquids measured. In making th e measure-ments, approach the point a t which the tensiometerring breaks away from the surface of the liquid or

from the interface v n y s lm l y .Measure the interfacial tension of pure water and ofeach of th e solutions against layers of parafEn oil.The diameter of the crystallizing dishes used shouldbe large enough to prevent excessive m a t u r e ofthe surfaces of the liquids. Measure the interfacialtension of each of the solutions against the oil about10 minutes after the interface has been formed.Using the surface tension of oil, t he surface tension ofwater or the several solutions, and the interfacialtension of each liquid against oil, calculate thespreading coefficient of each liquid on oil. Plot oncross-section paper the surface tension of the severallauric ad d solutions against the mole ratio of sodiumhydroxide to lauric acid in the solutions. Makeplots for the interfacial tensions and the spreadingcoefficients of these solutions against the mole ratioof base to acid. How do the curves obtained com-pare with those obtained by Cupples?Prepare a n oilfilm on a piece of celluloid or some otherreceptive surface and put drops of water and dropsof the several solutions on the oil film. Notewhether water, the several lauric acid solutions, andthe wetter water will spread on the oilfilm. How do

these observations agree with the spreadmg cwffi-cients calculated from the surface tension data ob-tained with these liquids?

SualAca TeasrowsL ip idPure waterPe%ffin ilLauric acid solution No. 1L e e " acid solution NO .2Laurie add solution No . 3La""" acid ao1vtion NO.Solution of wetting agent

DATATEXP. -Trio1 I--

INTBRPICIALBNSIONLiquids on PoroBin Oil

Pure water --auric add solvtian NO . --auric add solution NO.2 --a e c acid solution No. 3 --aurie add .01ution NO.4 -- -olution of wetting- agent -- -

PRBADINOOBI~CKBNT.N D OBBBRVBDPPBADINOLiquids on Oil Calcrrlolad ObreraedPure mate --aurie add solution NO.1 --aurie acid solution NO. --aurie add solution No. 3 --aurie add solution No. 4 - -o1vtion of wetting agent - -QUESTIONS

To correlate the data obtained in the experiment vith thetheories of surface tension, adsorption and orientation of solutes,and of spreading, the student may be asked to answer questionssuch as the following at the conclusion of the experiment.1. Why do aqueous solutions usually have surfacetensions lower than that of pure water? What typeof substances are positively adsorbed a t the surface of

solutions? Explain.2. (a) Why do the surface tensions of the lauricacid solutions increase as the ratio of base to acidapproaches and passes through the equivalence point?(b ) How would a molecule of sodium laurate orientitself in the interface between water and oil? Explain.(c ) Why does a large excess of sodium hydroxidein the lauric acid solutions depress the surface tensionagain?3. What factors determine whether a liquid willwet or spread on a second liquid? What industrialproblems are better understood in terms of quantitativedata on surface energies and spreading coefficients?See reference no. 1.

REVERENCES(1) "Surface active agents- symposium,"Ind. Eng. Chem., 31,31-69 (1939).(2) VANANTWERPEN,nd. Eng. Chcm., 33,16 (1941).( 3 ) CWPLES,nd. Eng. Chcm., 20,924 (1937).(4) B o c u ~ , Colloidal Behavior," Vol. 1, pp. 169-78, McGraw-Hill Book Company, New York, 1924.(5) GLASSTONE,Physical Chemistry." p. 476, D. Van NostrandCompany. Inc.. New York. 1940.(6) CAWLAND ERICKS,nd.Eng. Chem., 31, 44 (1939).( 7 ) "Cenco-du Nouy tensiometers" Bulletin 101, CentralScientific Company. Chicago.