July 1,1925 Spreaders and Spreading Qualities of Spray Materials 73 
spray does not possess these requirements it may be modified by the 
addition of some substance that reduces the tension values. The 
question then arises, what substances will reduce the surface tension 
of a solution. It is well known that adsorption of a solution at the 
surface layer will cause a reduction in the surface tension. The 
same is true regarding adsorption at the interface of a liquid and 
solid. It then becomes a search for those substances that are 
adsorbed at the liquid-air interface and the liquid-solid interface. 
Moore (7), in answer to the question regarding those substances 
that are positively adsorbed in the surface layer or at the interface 
' of a liquid and solid, applies the theory of orientation of molecules 
at the surface or interface. From the work of Harkins, Brown, and 
Davies (S), of Harkins, Davies, and Clark ( 4 ), and of Langmuir ( 6 ), 
Moore suggests that those substances that contain active polar 
groups which render them soluble in water and inactive groups that 
cause adsorption at the surface and thus reduce the surface tension 
should augment the spreading properties of the solute. Then if the 
inactive group is similar to the surface (of the plant) to be covered 
or soluble in it, spreading should result. 
Since the amount of adsorption in the surface layer of a solute 
and at the liquid-solid interface indicates, to a certain extent, the 
spreading power of the solution, a measurement of the surface and 
the interfacial tension values should disclose its possibilities as a 
spreader. 
EXPERIMENTAL DATA 
In order to obtain more complete data on surface adsorption of 
various materials, surface-tension determinations of numerous 
substances were made. The method employed was mechanical 
and rapid. The Du Nous instrument was used at first, but a new 
apparatus devised by Fahrenwald ( 2 ) was found more adaptable 
for the work at hand. This apparatus measures the surface tension 
of a solution by the film method. An accuracy of 0.1 dyne-cm. is 
claimed for the instrument. 
All measurements were made at 25° C. The time necessary for 
static equilibrium to be established was found to vary appreciably 
with the different substances. In order to proximate the time it 
takes for a drop of spray to leave the nozzle of the spray gun and to 
settle on the surface of the plant, about three minutes were allowed 
for the development of partial static tension before the measurement 
was recorded. The results given in Table I report the observations 
made on part of the miscellaneous substances studied. 
In order to correlate the surface-tension results with the actual 
spreading qualities of a substance, several kinds of leaves and half- 
grown apples were sprayed with similar solutions on which surface- 
tension observation had been made. The leaves and fruit were 
sprayed with an atomizer, and the degree of spreading as observed 
was recorded. The degree of spreading designated by the term 
“good” indicates the formation of an even film covering the entire 
surface, “medium” signifies that small portions of the surface drained 
dry, “poor” indicates very slight spreading, and “none” means 
spreading on the surface similar to distilled water. The degree of 
spreading of various substances dissolved or suspended in distilled 
water at the concentration indicated is given in Table II. 
59836-25f—6 
