In Vitro Test for Effects of Surfactants and Formulations on

Chapter 3

In Vitro Test for Effects of Surfactants and Formulations on Permeability of Plant Cuticles

Downloaded by CORNELL UNIV on October 11, 2016 | http://pubs.acs.org Publication Date: June 24, 1988 | doi: 10.1021/bk-1988-0371.ch003

U. Geyer and J. Schönherr Institut für Botanik und Mikrobiologie, Technische Universität München, Arcisstrasse 21, D-8000 München 2, Federal Republic of Germany The effects of surfactants on water permeability of i s o l a t e d astomatous c u t i c l e s from C i t r u s leaves were measured. Rates of water l o s s across c u t i c l e s were determined g r a v i m e t r i c a l l y p r i o r t o and a f t e r a p p l i c a t i o n o f surfactants using coverages ranging from 0.015 t o 25 g/m2. Sodium dodecylsulfate d i d not affect water permeability. Polyoxyethylene p-t-octylphenol and polyoxyethylene nonylphenol in the HLB range of 4 t o 16 increased water permeability by up t o 2.1 f o l d . Polyoxyethylene a l k y l e t h e r in the HLB range of 5 t o 13 increased water permeability by seven- t o e i g h t f o l d . The cationic surfactant dodecyltrimethylammonium c h l o r i d e (HLB 18.5) increased water permeability very e f f e c t i v e l y by up t o twentytwofold. Most formulations of p e s t i c i d e s contain surfactants as e m u l s i f i e r s and as wetting agents. I n a d d i t i o n , surfactants may a f f e c t the b i o l o g i c a l a c t i v i t y of the a c t i v e ingredient o f a f o r n u l a t i o n (1). This e f f e c t i s not w e l l understood (2). Better wetting and greater r e t e n t i o n may be responsible i n p a r t , but e f f e c t s on permeability of cuticles have a l s o been considered t o e x p l a i n improved h e r b i c i d a l a c t i v i t y i n the presence o f surfactants (3, 4 ) . Generally, permeation o f a solute molecule across a c u t i c u l a r membrane depends on the concentration of that molecule i n the membrane (which i s a f u n c t i o n of the concentration gradient i n the adjacent solutions and the p a r t i t i o n c o e f f i c i e n t ) and i t s m o b i l i t y (J> ,M. When analysing the e f f e c t s o f surfactants on penetration of a c t i v e ingredients i t i s therefore necessary t o i n v e s t i g a t e both the e f f e c t s o f surfactants on the concentration and on m o b i l i t y o f the solute i n the c u t i c l e . To date, systematic studies o f t h i s type do not appear t o have been conducted. Plant c u t i c l e s are l i p i d membranes. They have a high sorption capacity f o r l i p o p h i l i c solutes. Cutin i s the main sorbent (7, 8 ) . Transport resistance i s due t o soluble l i p i d s associated w i t h the c u t i n (9, 10, K e r l e r , F.; Schonherr, J . Arch. Environ. Contam. T o x i c o l . , i n press). The permeability of c u t i c l e s increases w i t h 0097-6156/88/0371-O022$O6.00/0 ° 1988 American Chemical Society

Cross and Scher; Pesticide Formulations ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


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Permeability of Plant Cuticles

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increasing l i p i d s o l u b i l i t y o f the solutes. For solutes having the same s i z e (molar volume) permeability i s proportional t o the cuticle/water p a r t i t i o n c o e f f i c i e n t (11). Equilibrium d i s t r i b u t i o n o f a solute between the c u t i c l e and an aqueous s o l u t i o n i s g r e a t l y affected by surfactants when present i n concentrations above the c r i t i c a l m i c e l l e concentration (cmc) (11). L i p o p h i l i c solutes tend t o be s o l u b i l i z e d i n the m i c e l l e s and cuticle/water partition c o e f f i c i e n t s decrease rapidly with increasing surfactant concentration. As a consequence, t h e concentration o f a solute i n the c u t i c l e i s much lower i n presence of surfactants i n the aqueous solutions and the flow across the cuticle i s slowed i n proportion. Surfactant concentration increases during droplet drying. A c t i v e ingredients that d i d not enter t h e c u t i c l e during droplet drying w i l l eventually be dissolved i n pure hydrated surfactant o r formulation. In this simplest case, surfactants reduce rates o f penetration, because they reduce the d r i v i n g f o r c e , i . e . the concentration gradient o f t h e solute i n the c u t i c l e . I t i s important t o r e a l i z e that transport properties o f the c u t i c l e (that i s the m o b i l i t y o f the solute i n the c u t i c l e ) are not affected by the surfactant. I f permeability c o e f f i c i e n t s o r permeances were calculated using the correct driving force, namely t h e concentration gradient o f the solute i n the c u t i c l e , rather than the concentration gradient i n the adjacent bulk s o l u t i o n s , the c o e f f i c i e n t s would be independent o f t h e concentration o f the surfactants. Of course, permeation o f p o l a r solutes, which are not s o l u b i l i z e d i n surfactant m i c e l l e s , w i l l not be slowed, because t h e i r concentration gradient i n the c u t i c l e w i l l be unaffected by the presence o f surfactants i n s o l u t i o n . If permeability c o e f f i c i e n t s f o r solutes depend on t h e concentration o f surfactants, even though they were calculated using t h e concentration gradient i n t h e c u t i c l e , i t must be concluded that transport properties o f the c u t i c l e s were changed by the surfactants. The b a r r i e r l i m i t i n g transport across c u t i c l e s consists o f soluble l i p i d s associated w i t h the c u t i n ( c u t i c u l a r waxes). I f permeability i s increased by surfactants, i t i s l i k e l y that structure and/or composition o f soluble c u t i c u l a r l i p i d s have been a l t e r e d . These considerations show that surfactants can have two opposing e f f e c t s on the flow o f a chemical across the c u t i c l e : (1) They slow i t by decreasing the d r i v i n g f o r c e and (2) they may increase permeability by a l t e r i n g t h e properties o f the wax b a r r i e r . Both e f f e c t s a c t simultaneously and they depend on surfactant concentration and l i p o p h i l i c i t y o f solutes. I n simple experiments they cannot be separated and depending on experimental conditions and type o f solute, surfactants may enhance, depress o r have no e f f e c t on herbicide a c t i v i t y (1). I t f o l l o w s , that e f f e c t s of surfactants on permeation can only be understood i f the above e f f e c t s a r e studied separately. The t e s t we are presenting here measures e f f e c t s o f surfactants on the transport properties o f the b a r r i e r . The p a r t i t i o n c o e f f i c i e n t e f f e c t i s absent, because water permeability i s studied. The d r i v i n g force o f water across c u t i c l e s (the gradient o f water a c t i v i t y ) i s p r a c t i c a l l y unaffected by the presence o f surfactants. Soluble c u t i c u l a r l i p i d s are the l i m i t i n g b a r r i e r f o r water


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just as f o r solutes and a good c o r r e l a t i o n between water permeability and solute permeability o f c u t i c l e s has been reported (Schonherr, J . ; Riederer, M. Rev. Environ. Contam. T o x i c o l . , i n press). I t i s , therefore, meaningful t o study the e f f e c t s o f surfactants on transport properties o f c u t i c l e s by measuring the e f f e c t s o f surfactants on water permeability.


Materials and Methods The astomatous adaxial l e a f c u t i c l e s from healthy and vigorously growing C i t r u s aurantiun L. trees were used. I n one experiment the astomatous c u t i c l e s from commercially supplied green pepper f r u i t s (Capsicum annuum L.) were used. The c u t i c l e s were i s o l g t e d enzymatically (12), a i r d r i e d and stored i n the r e f r i g e r a t o r (4 C) f o r f u r t h e r use. These i s o l a t e d c u t i c l e s w i l l be r e f e r r e d t o as c u t i c u l a r membranes (CM). Mature leaves were taken from |rees grown i n growth chambers (16 h l i g h t a t 500 t o 800 uE m s PAR, 25° C, 50 % r e l a t i v e humidity; 15° C and 9o % r e l a t i v e humidity during the dark period). Leaves were never treated w i t h any p e s t i c i d e t o avoid contact w i t h surfactants and/or formulations p r i o r t o experimentation. Table I . Surfactants used f o r experimentation Trade name BRU 52 BRIJ 30 BRU 56 BRIJ 58 BRIJ 35 RENEX 36 TRITON X-15 TRITON X-35 TRITON X-45 TRITON X-100 TRITON N-57 TRITON N-101

chemical name POE c e t y l ether POE l a u r y l ether POE c e t y l ether POE c e t y l ether POE l a u r y l ether POE t r i d e c y l ether POE p-t-octylphenol POE p-t-octylphenol POE p-t-octylphenol POE p-t-octylphenol POE nonylphenol POE nonylphenol Na-dodecylsulfate Dodecyl trimethyl anmonium c h l o r i d e

HLB

EO 1 3 9 20 20 6 1 3 5 9-10 5 9-10

-

5.2 9.8 12.9 15.8 16.9 11.6 3.6 7.8 10.4 13.5 10.0 13.5 40.0 18.5

Surfactants were purchased from Serva (Heidelberg, FRG) and were used without f u r t h e r p u r i f i c a t i o n . They are l i s t e d i n Table I . The ethylene oxide contents (EO) are averages. The hydrophile l i p o p h i l e balance (HLB) f i g u r e s were taken from the l i t e r a t u r e (13, 14). The t e s t procedure i s very simple. Water permeability of each c u t i c u l a r membrane i s measured g r a v i m e t r i c a l l y (15) p r i o r t o and a f t e r a p p l i c a t i o n o f predetermined amounts o f surfactant. The t r a n s p i r a t i o n chambers depicted i n Figure 1 were used. They are made o f brass and are f i t t e d w i t h a rubber 0-ring t o a t t a i n a good s e a l . Each chamber i s f i l l e d w i t h 0.5 ml deionized water and a CM i s positioned on top o f i t , the morphological inner side f a c i n g the —




GEYER AND SCHONHERR

Figure 1: Drawing o f a brass chamber used i n the t e s t .



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water. The brass r i n g i s coated l i g h t l y w i t h h i g h vacuum s i l i c o n grease on the surface f a c i n g the CM (to minimize creeping o f surfactant s o l u t i o n between c u t i c l e and metal r i n g ) and i s secured w i t h w i t h 3 screws. These chambers are then inverted and placed i n a desiccator h a l f f i l l e d w i t h d r i e d s i l i c a g e l . To prevent damaging the c u t i c u l a r membranes a coarse metal screen was positioned between the s i l i c a g e l and chambers. The desiccators were kept a t 24.5 t o 25.5 C and the water l o s s from the chambers was followed by p e r i o d i c a l l y weighing them u n t i l about 10 t o 20 mg o f water had been l o s t . Surfactants were dissolved i n deionized water and 50 p i o f these t e s t solutions were applied t o the outer surfaces o f the c u t i c l e s a f t e r p l a c i n g the chambers up r i g h t . The desiccators were closed and a f t e r the water had evaporated, the chambers were inverted again and weighing was resumed as before. A Satorius 1702 MP8 e l e c t r o n i c balance (0.1 mg accuracy) o n l i n e w i t h an O l i v e t t i M24-SP computer was used. At the end o f an experiment the rates o f water l o s s p r i o r t o and a f t e r treatment w i t h surfactant were determined by l i n e a r regression a n a l y s i s . The e f f e c t o f the treatment was judged from the r a t i o o f the slopes a f t e r and before treatment. The r a t i o i s u n i t y i n case o f an i n e f f e c t i v e surfactant and i t i s greater than u n i t y i f water permeability was increased. For each surfactant and coverage 12 t o 15 membranes were used as r e p l i c a t e s . Data are reported as means together w i t h a 95 % confidence i n t e r v a l (p = 95 Z ) . During the second weighing period, the outer surfaces o f the CM were coated w i t h a t h i n l a y e r o f surfactant, rather than w i t h an aqueous s o l u t i o n o f the surfactant. This was intended t o simulate the s i t u a t i o n i n the f i e l d , a f t e r the water o f the spray has evaporated. Initially, a l l surfactants were tested using a r e l a t i v e l y h i g h coverage (mass per area o f 5 t o 25 g/m ) o f surfactant i n order t o screen out the most e f f e c t i v e ones. Later some o f the surfactants were tested using a wide range o f surface coverages (0.015 t o 25 g/m ). During the drying o f the t e s t solutions the surfactant contained i n the droplets were not deposited q u a n t i t a t i v e l y on the surfaces o f the c u t i c l e s as explained below. The y i e l d o f deposition was determined using (phenyl-3H(N))-labelled T r i t o n X100 (NEN, s p e c i f i c a c t i v i t y 48.1 MBq/nfc). A 50 p i droplet containing l a b e l l e d T r i t o n X-100 was placed on each o f 30 CM mounted as usual on top o f the chambers. A f t e r the droplets had d r i e d i n the desiccator the area o f the c u t i c l e s exposed t o the solutions was excised and the r a d i o a c t i v i t y was determined by l i q u i d s c i n t i l l a t i o n counting. Recovery was 46.2 Z (standard d e v i a t i o n 8.5 Z) and 53.4 Z (standard d e v i a t i o n 14.8 Z) f o r t h e o r e t i c a l coverages o f 5 and 10 g/m , r e s p e c t i v e l y . The average recovery f o r both coverages was 49 Z (standard d e v i a t i o n 11 Z). The e f f e c t i v e surface coverages were c a l c u l a t e d from the t h e o r e t i c a l values assuming that only 49 Z were deposited on the surfaces o f the c u t i c u l a r membranes. The remaining 51 Z o f r a d i o a c t i v i t y were probably deposited on the surfaces o f the top metal r i n g s used t o prevent spreading o f the droplets. Some may a l s o have been l o s t due to l a t e r a l d i f f u s i o n between r i n g and c u t i c l e . Since a l l t e s t solutions were applied a t concentrations f a r above the cmc and since the concentration e f f e c t on recovery was small i t was assumed


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that wetting o f and sorption on the metal r i n g s (and thus recovery) would be the same (within experimental error) f o r a l l surfactants and concentrations.


Results I n Figure 2 t y p i c a l r e s u l t s obtained w i t h a c o n t r o l (A) and w i t h a treated c u t i c u l a r membrane (B) are shown. The c o n t r o l membrane was treated w i t h 50 p i water, whereas the CM i n Figure 2 B was treated w i t h gn aqueous Renex 36 s o l u t i o n a t a concentration equivalent t o 5 g/m . Slopes 1 and 2 o f the c o n t r o l membrane do not d i f f e r s i g n i f i c a n t l y , as t h e 95 % confidence i n t e r v a l s o f the slopes overlap. I n experiments w i t h fewer data points than shown i n Figure 2 confidence i n t e r v a l s are l a r g e r , such that slope r a t i o s ranging from 0.7 t o 1.3 are s t a t i s t i c a l l y not s i g n i f i c a n t l y d i f f e r e n t from u n i t y . Treatment w i t h Renex 36 r e s u l t e d i n a 7.4 f o l d increase i n water permeability, which i s s t a t i s t i c a l l y h i g h l y s i g n i f i c a n t (p > 99.9 % ) . Surfactants from the T r i t o n s e r i e s had a very small e f f e c t on water permeability o f CitruSpCM (Figure 3 ) , even a t the rather h i g h surface coverage o f 15.5 g/m . The e f f e c t s o f T r i t o n surfactants on water permeability ranged from 1.6 t o 2.1 and there were no s i g n i f i c a n t (p = 95Z) differences between d i f f e r e n t types o f T r i t o n surfactants. The e f f e c t o f the surface coverage on water permeability was studied using T r i t o n N-101 and T r i t o n X-35 and was found t o be rather small (Figure 4 ) . A very h i g h increase i n water permeability by about a f a c t o r of seven was observed w i t h B r i j 52, B r i j 30 and B r i j 56, while the other two members o f the s e r i e s w i t h HLB values around 16 t o 17 had very l i t t l e e f f e c t (Figure 3 ) . Note that the surface coverage was only 5 g/m , which i s about 1/3 that used w i t h the T r i t o n s e r i e s . Renex 36 increased water permeability very e f f e c t i v e l y . A small, but s t a t i s t i c a l l y s i g n i f i c a n t increase i n water permeability was observed even a t very low coverages o f 15 mg/m (Figure 4 ) . An e f f e c t comparable t o that observed w i t h Renex 36 and the more e f f e c t i v e members o f t h e B r i j s e r i e s was found f o r the c a t i o n i c surfactant dodecyltrimethylammonium c h l o r i d e , whereas the anionic surfactant sodium dodecylsulfate was i n e f f e c t i v e and even reduced water permeability a t higher surface coverages (Figure 4 ) . The e f f e c t o f Renex 36 on water permeability o f green pepper f r u i t CM was much smaller than the e f f e c t observed w i t h C i t r u s CM. At a coverage o f 1.5 g/m water permeability o f pepper CM was only increased by a f a c t o r o f 1.4 (confidence i n t e r v a l 1.3-1.5), whereas w i t h C i t r u s a 3.5 f o l d increase was observed a t the same coverage (Figure 4 ) . Renex 36 increased water permeability o f C i t r u s polymer matrix membranes (CM from which soluble l i p i d s wgre removed w i t h chloroform). A t a surface coverage o f 2.4 g/m an increase by a f a c t o r o f 2.0 (confidence i n t e r v a l 1.8 - 2.1) was observed. A t t h i s coverage the water permeability o f CM would have been increased by a f a c t o r o f about 4.2 (Figure 4 ) .




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TIME

Figure 2: T y p i c a l examples o f computer printouts (redrawn) from a c o n t r o l experiment (A) and a c u t i c l e treated w i t h Renex 36 (B). Arrows i n d i c a t e time o f treatment. The slopes have the dimension g/min.



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i

1

1

i

i

0

2

U

6

8 10 HLB

i

i

12

29

i

i

1

U

16

18

Figure 3: The e f f e c t s o f T r i t o n (X o r N) and B r i j surfactants on water permeability o f C i t r u s c u t i c u l a r membranes. The coverage was 15.5 g nf and 5 g m f o r the T r i t o n and B r i j surfactants, respectively. HLB values are given i n parentheses. The e f f e c t i s given as t h e r a t i o o f t h e permeances (F) a f t e r and p r i o r t o treatment. Bars represent h a l f a confidence i n t e r v a l (p = 95 1).




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Figure 4: The e f f e c t o f coverage (mass per area) o f selected surfactants on water permeability o f C i t r u s c u t i c u l a r membranes. Bars represent the confidence i n t e r v a l s (p = 95 Z). The e f f e c t i s given as the r a t i o o f the permeances (P) a f t e r and p r i o r t o treatment.


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Discussion The gravimetric method of measuring water permeability i s simple and inexpensive. The t e s t i s very s e n s i t i v e , because i t uses the method o f paired observations. Each membrane serves as c o n t r o l and as treatment and b i o l o g i c a l v a r i a b i l i t y o f water permeability o f CM i s thus eliminated. Furthermore, there was no c o r r e l a t i o n between the e f f e c t o f a surfactant and the water permeability o f the CM before treatment. The s e n s i t i v i t y o f the t e s t can be adjusted a p r i o r i t o t h e needs of the experimenter by using the appropriate number o f data points. There i s a l i m i t t o t h i s , however, because the t o t a l water l o s s from a chamber during the experiment should not exceed 50 tqg. The l o s s o f water i s accompanied by a decrease i n pressure i n t h e chamber. The CM assumes a concave curvature and could rapture. When the gravimetric method was introduced (15), chambers made of p l e x i g l a s s were used. These had a small but f i n i t e water permeability, such that t o t a l water l o s s from the chambers was the sum o f two p a r a l l e l flows, namely across the c u t i c l e s and across the w a l l s o f t h e chambers. The w a l l s o f the brass chambers are impermeable t o water and t h e permeance o f a membrane i s e a s i l y calculated from the slope (g/nrin), the membrane area (7.85 x 10" m ) and t h e d r i v i n g force. The gradient o f water a c t i v i t y i s p r a c t i c a l l y u n i t y , because t h e a c t i v i t y o f pure water i n the chamber i s 1 and the a c t i v i t y over d r i e d s i l i c a g e l i s p r a c t i c a l l y zero. The permeance (P) i s therefore P = slope/ (area x d r i v i n g force) (1) 2 This permeance has the dimension g/min m which can be converted t o the usual u n i t s (m/s) by d i v i d i n g by the s p e c i f i c g r a v i t y o f l i q u i d water and by 60 t o obtain seconds instead o f minutes. Since area and d r i v i n g f o r c e a r e the same before and a f t e r treatment, they cancel and the e f f e c t of a surfactant on permeance ( i . e . the r a t i o of t h e permeance o f the CM treated w i t h surfactant over the permeance p r i o r t o treatment; P /P) i s numerically i d e n t i c a l t o the r a t i o o f the slopes a f t e r aSSFbefore treatment (Figures 3 and 4). There i s no clear-cut dependence o f water permeability on e i t h e r HLB o r surfactant structure. Maximum e f f e c t s were found i n the HLB range 5 t o 13 ( B r i j 52, B r i j 30, B r i j 56, Renex 36). The nonionic surfactants w i t h HLB values above 13 have l i t t l e e f f e c t , but the c a t i o n i c surfactant dodecyltoimethylanmonium c h l o r i d e which has a HLB o f 18.5 very e f f e c t i v e l y increases water permeability. The anionic surfactant sodium dodecylsulfate (HLB about 40) d i d not increase water permeability. The t e s t was designed t o measure the e f f e c t s o f surfactants on water permeability o f c u t i c l e s rather than t o r e v e a l the cause(s) of effectiveness o r ineffectiveness. At t h i s p o i n t , we can only speculate. The b a r r i e r that i s r a t e l i m i t i n g i n both transport o f water and solutes across c u t i c l e s i s made up o f soluble c u t i c u l a r l i p i d s , probably i n a s s o c i a t i o n w i t h c u t i n (10, 11, 160. E x t r a c t i n g soluble c u t i c u l a r l i p i d s w i t h chloroform increases permeability of C i t r u s CM much more than that of pepper f r u i t CM (16). Water permeability



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of C i t r u s CM i s much more affected by Renex 36 than permeability of pepper f r u i t CM i n d i c a t i n g that surfactants i n t e r a c t w i t h the soluble c u t i c u l a r l i p i d s , leading t o an increased m o b i l i t y of water i n the c u t i c l e . Surfactants a l s o i n t e r a c t w i t h the c u t i n , because water permeability of polymer matrix membranes (free of soluble c u t i c u l a r l i p i d s ) was a l s o increased by Renex 36, even though t o a lower extent than permeability of CM. There are b a s i c a l l y two ways, how water permeability of c u t i c l e s could be increased by surfactants. They could swell the polymer matrix and they may s o l u b i l i z e c u t i c u l a r waxes. Swelling of the polymer matrix would increase the water content of c u t i c l e s . As a consequence of s w e l l i n g , the d i f f u s i o n c o e f f i c i e n t of water i n the c u t i c l e might be increased. Both e f f e c t s could e x p l a i n the increase i n water permeability of polymer matrix membranes. Swelling of the polymer matrix of c u t i c u l a r membranes could lead t o defects between wax c r y s t a l l i t e s and thus increase permeability. Surfactants may a l s o p a r t i a l l y s o l u b i l i z e c u t i c u l a r waxes. These hypotheses are currenty under i n v e s t i g a t i o n . F a i r l y h i g h coverages have been used i n most of our screening t e s t s . The C i t r u s CM used had a mass per area of about 2.5 g/m and the soluble c u t i c u l a r l i p i d s amount t o only about 0.1 g/m , which i s 4 Z by weight (17). Only Renex 36 was tested a t coverages equivalent t o the amounts o f soluble c u t i c u l a r l i p i d s (Figure 4 ) . A l l other surfactants were tested using coverages i n excess of the amounts of soluble c u t i c u l a r l i p i d s . This appears t o be necessary f o r large e f f e c t s on water permeability. Sodium dodecylsulfate a c t u a l l y decreased water permeability when applied a t high surface coverages. This surfactant i s i n the s o l i d s t a t e a t 25 C. At a coverage of 25 g/m the l a y e r of surfactant on top of the CM was 10 times t h i c k e r than the c u t i c l e i t s e l f and i t probably acted as resistance i n s e r i e s . Even though i t i s not c l e a r how surfactants increase water permeability of c u t i c l e s , i t i s a f a c t that some of them s i g n i f i c a n t l y do so. Since soluble c u t i c u l a r l i p i d s are the main barrier not only f o r water, but a l s o f o r solutes, solute permeability i s most l i k e l y increased by surfactants that increase water permeability (Schonherr, J . ; Riederer, M. Rev. Environ. Contam. T o x i c o l , i n press). However, as pointed out e a r l i e r , w i t h l i p o p h i l i c solutes t h i s e f f e c t w i l l be confounded w i t h the e f f e c t of surfactants on the p a r t i t i o n c o e f f i c i e n t . Both e f f e c t s act i n opposite d i r e c t i o n . The p a r t i t i o n i n g e f f e c t w i l l decrease permeability, whereas an increase i n m o b i l i t y of solutes i n the c u t i c l e w i l l increase the permeability. I n an experiment comparing the uptake of a p e s t i c i d e w i t h and without a surfactant (or a formulation) the two e f f e c t s could cancel and i t would be concluded that the surfactant had no e f f e c t on the c u t i c l e . The advantage of our t e s t i s , that when measuring water permeability the p a r t i t i o n i n g e f f e c t i s absent and the e f f e c t of surfactants on m o b i l i t y of water (and solutes) can be measured unobscured by other e f f e c t s . The t e s t may a l s o be used f o r mixtures of surfactants, complete formulations containing solvents or f o r other adjuvants. Surfactants that increase water permeability of c u t i c l e s , w i l l probably also increase permeability f o r surfactants. Since


3.

GEYER A N D SCHONHERR


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surfactants a r e t o x i c t o c e l l s (18) they can increase t o x i c i t y o f herbicides applied t o the f o l i a g e . Acknowledgment This work was supported by a grant from the Deutsche Forschungsgemeinschaft. L i t e r a t u r e Cited 1.


2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15. 16. 17. 18.

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RECEIVED December 28, 1987


In Vitro Test for Effects of Surfactants and Formulations on

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