Stability of the Organosilicone Surfactant Silwet L-77 in Growth Regulator Sprays

Abstract

The stability of the organosilicone surfactant Silwet L-77 (L-77) was investigated in growth regulator solutions representative of those frequently used as foliar sprays in horticultural research and production. Surfactant stability was followed by measurement of surface tension at selected time intervals after addition of the surfactant. Time-course studies revealed that surface tension of Ethrel (2.08 ml·liter) and Pro-Gibb (3.13 ml·liter) solutions made up with deionized water with 0.01% L-77 increased immediately after preparation. A lag phase of constant surface tension was observed at a higher (0.1'%) L-77 concentration. The rate of increase in surface tension was higher for the more acidic Ethrel solution (pH 2.5) than for the Pro-Gibb solution (pH 3.8). The water source used for preparation of the spray solution significantly affected surfactant stability. Surface tension of Ethrel solutions (2.08 ml·liter 0.1% L-77) prepared with well water remained constant over 4 hours, whereas surface tension increased when rain or deionized water was used. Surface tension increased in Pro-Gibb spray solutions prepared with deionized water at concentrations typical for low- (1/10 x), but not high- (1x), volume spray applications or when solutions were prepared with well water. In all experiments, pH of the spray solution was the only factor that affected L-77 stability. Chemical names used: (2-chloroethyl) phosphoric acid (Ethrel); gibberellin A (Pro-Gibb); polyalkyleneoxide modified polydimethylsiloxane copolymers (Silwet L-77).