The Principles of Film-Forming Sprays 61 



Just as there are substances which tend to concentrate in the 

 interior there are other substances which tend to concentrate in the 

 surface layer. These materials are considered as being positively 

 adsorbed in the surface as distinguished from the above negatively 

 adsorbed substances. Having molecules with a tendency to move to 

 the surface, less force will be necessary to bring them to the surface 

 and the surface tension is reduced. Soap, proteins, and various or- 

 ganic svibstances exhibit this character of positive adsorption. A 

 saturated solution of soap (sodium oleate) at 20 degrees C. has a sur- 

 face tension of 25 dynes as compared with water with 72.8 dynes. 



It is not always practical to produce spreading of a spray over 

 the surface of a leaf by lowering the surface tension alone. The in- 

 terfacial tension must also be lowered. To solve this problem sub- 

 stances must be selected which will be positively adsorbed at the leaf- 

 spray interface. 



It has been shown that when one liquid, such as oleic acid, is 

 placed on the surface of another liquid, such as water, the active 

 portion (COOH group) of the oleic acid molecule will be attracted 

 by the water molecules while the portion rich in carbon groups will 

 be turned away from the water. When three materials are used, 

 such as oleic acid dissolved in a mineral oil and the oil placed on the 

 surface of the water, the oleic acid will be adsorbed at the oil-water 

 interface, the portion of the molecule similar to the oil being found in 

 the oil while the active COOH group is attracted to the water. This 

 arrangement results in the lowering of the interfacial tension, and the 

 mineral oil will spread over the surface of the water. Applying this 

 to a spray on a waxy surface such as that of cabbage, it would appear 

 that a substance should be dissolved in the water which will have 

 active groups such as COOH, CO, CN, OH, CONH= to render it 

 soluble in the water, and carbon groups similar to the molecule of 

 the wax. Such a su])stance would be attracted to or positively adsorbed 

 at the leaf-spray interface thus lowering the interfacial tension. 

 Such organic substances not only lower the interfacial tension but 

 also the surface tension of the spray to a certain extent, hence they 

 maintain the spray in the form of thin film on the leaf surface. Such 

 was found to be the case. Saturated aqueous solutions of amyl alco- 

 hol, benzyl alcohol, beechwood, creosote (cresol and guaiacol) carva- 

 crol, and eugenol gave spreading over cabbage leaves. 



Leaves with a heavy cuticle, such as citrus or pear, consist largely 

 of celluloses and not wax. It is therefore not surprising that these 

 aqueous solutions or organic compounds do not cause spreading on 



