Pesticide Formulations - American Chemical Society

Chapter 4

Studies on Octylphenoxy

Surfactants

Effects of Concentration and Mixtures on 2-(1-Naphthyl)acetic Acid Sorption by Tomato Fruit Cuticles 1

Warren E. Shafer and Martin J. Bukovac

Downloaded by UNIV OF ARIZONA on October 11, 2016 | http://pubs.acs.org Publication Date: June 24, 1988 | doi: 10.1021/bk-1988-0371.ch004

Department of Horticulture, Michigan State University, East Lansing, MI 48824 E f f e c t s of polyethoxy (EO) derivatives of octylphenol (OP) with 5 (OP+5EO), 7.5 (OP+7.5EO), 9.5 (OP+9.5EO) and 40 (OP+40EO) ethyleneoxy groups on sorption of 2-(1-naphthyl)acetic a c i d (NAA) by c u t i c l e s enzymatic a l l y i s o l a t e d from mature tomato (Lycopersicon esculentum Mill. cv. Sprinter) fruit were studied at pH 3.2 and 25°C. Below the critical micelle concen t r a t i o n (CMC), surfactants had little e f f e c t on sorption by c u t i c u l a r membranes (CM) and dewaxed CM (DCM). Above the CMC, sorption decreased with an increase in surfactant concentration, except f o r OP+5EO and OP+7.5EO where sorption by CM, but not DCM, increased at concentrations (0.1%) j u s t above the CMC Surfactant mixtures (1:1) of OP+9.5EO and OP+40EO, OP+7.5EO and OP+40EO, and OP+5EO and OP+7.5EO all yielded NAA sorption values s i g n i f i c a n t l y lower than control or component surfactant treatments. Surfactants are commonly used i n agrochemical formulations to improve the c h a r a c t e r i s t i c s of the spray s o l u t i o n and to increase absorption of the active ingredient (1-3). However, there i s evidence suggesting that surfactants do not always enhance f o l i a r absorption of an applied compound (4-6). A better understanding of surfactant/active ingredient/plant surface interactions may provide i n s i g h t into the mechanism(s) of surfactant action, thereby leading to more e f f e c t i v e use of surfactants and improved agrochemical formulations. The c u t i c l e covering plant surfaces i s the i n i t i a l (7) and primary (8) b a r r i e r to the penetration of f o l i a r - a p p l i e d chemicals. U t i l i z a t i o n of c u t i c l e s i s o l a t e d from the underlying c e l l s allows for e x p l i c i t examination of surfactant/active i n g r e d i e n t / c u t i c l e 1

Current address: Agricultural Research Center, Abbott Laboratories, Oakwood Road, Long Grove, IL 60047 NOTE: This is the sixth in a series of studies on octylphenoxy surfactants. 0097-6156/88/0371-0034$06.00/0 ° 1988 American Chemical Society

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

4.

SHAFERANDBUKOVAC

Octylphenoxy Surfactants

35

interactions. Although e a r l i e r studies (9, 10) have documented s i g n i f i c a n t surfactant e f f e c t s , an adequate data base i s lacking. To t h i s end, we have focused on the e f f e c t s of selected polyethoxylated derivatives of octylphenol (OP) on 2-(1-naphthyl)acetic acid (NAA) sorption by plant c u t i c l e s . Sorption was selected f o r study because i t i s an important component of membrane ( c u t i c l e ) permeability (11) and can be viewed as an e a r l y phase i n the f o l i a r penetration process.


Experimental Plant Material/Cuticle Isolation. L o c a l l y field-grown mature tomato (Lvcopersicon esculentum M i l l . cv. Sprinter) f r u i t free of v i s u a l defects were selected for reasons previously discussed (12). Discs, 20 mm i n diameter, were punched from the f r u i t and incubated at 35±1°C i n an aqueous mixture of pectinase (4%, w/v; ICN N u t r i t i o n a l Biochemicals), c e l l u l a s e (0.4%, w/v; Sigma) and NaN3 (1 mM) i n 50 mM sodium c i t r a t e buffer at pH 4.0 (13). A f t e r two days and two changes of enzyme solution, the c u t i c l e was separated from the c e l l walls of the epidermis. Adhering c e l l u l a r debris was removed with a j e t of d i s t i l l e d water and the c u t i c l e s were a i r dried and stored at 23°C u n t i l used. C u t i c l e s i s o l a t e d by t h i s procedure w i l l be referred to as c u t i c u l a r membranes (CM). CM extracted for 3 d with at l e a s t 10 changes of chloroform:methanol (1:1, v/v) at 50°C to remove the e p i c u t i c u l a r and c u t i c u l a r waxes, i . e . soluble c u t i c u l a r l i p i d s (SCL), w i l l be termed DCM. Radioisotope. Radioactive 2-(1-naphthyl[1-^C])acetic a c i d (sp. act. 2.3 GBq mmol"^; Amersham) with a p u r i t y of 98%, as determined by radio-TLC, was used i n t h i s study. Surfactants. 4-(l,l,3,3-tetramethyl)butylphenol (OP) condensed with e i t h e r 5 (0P+5E0), 7.5 (0P+7.5E0), 9.5 (0P+9.5E0) or 40 (OP+40EO) moles ethylene oxide (EO) was used. Trade names (registered trademarks, Rohm and Haas Co.) f o r these four nonionic surfactants are T r i t o n X-45, T r i t o n X-114, T r i t o n X-100 and T r i t o n X-405, respectively. Selected properties of these surfactants relevant to f o l i a r penetration have been reported (14). The octylphenoxy surfactants were representative of commercial preparations. The EO number l i s t e d i s an average value, with the ethoxymer mole r a t i o d i s t r i b u t i o n s following a Poisson d i s t r i b u t i o n (15). No attempt was made to chemically p u r i f y any of the surfactants. The CMC values for 0P+5E0, OP+7.5EO, OP+9.5EO and OP+40EO were 0.005, 0.012, 0.019 and 0.16% (w/v), respectively. Concentrations used were on a w/v basis. Measurement of Sorption. Sorption was measured f o r the systems CM/buffer and DCM/buffer using the procedure of Riederer and Schdnherr (16). Sodium c i t r a t e buffer (20 mM) at pH 3.2, containing 1 mM NaN3 to prevent b a c t e r i a l and fungal growth, was used i n a l l experiments. The pK of NAA i s 4.2. Random samples (20 to 50) of CM or DCM discs were selected and s l i c e d into small (approx. 1 mm x 10 mm) s t r i p s (preliminary r e s u l t s showed no s i g n i f i c a n t e f f e c t of s t r i p s i z e ) . Weighed suba


36

PESTICIDE FORMULATIONS: INNOVATIONS AND DEVELOPMENTS

samples (approx. 5 mg) were placed into 5 ml glass v i a l s and 1.5 ml of C - l a b e l e d NAA (300-500 nM) buffered solution was pipetted into each v i a l . V i a l s were closed with t e f l o n - l i n e d screw caps and shaken h o r i z o n t a l l y i n a water bath at 25±0.5°C. At designated time i n t e r v a l s , 100 / i l aliquots were removed and r a d i o a c t i v i t y determined by l i q u i d s c i n t i l l a t i o n spectrometry (LKBWallac LSC, Model 1211). S c i n t i l l a t i o n c o c k t a i l was composed of 1,4-dioxane (10 ml), containing 100 g naphthalene and 5 g diphenyloxazole (PPO) l i t e r " - . A l l samples were counted to a 2a error of approximately 1.0% and corrected f o r background. Since quenching was constant throughout the course of these experiments, a l l calculations were performed with CPM values. The amount of l^C- labeled NAA sorbed by the CM or DCM was determined by subtracting the quantity of C-NAA i n the dosing (bulk) solution from the amount o r i g i n a l l y present (12). Radioassay of the bulk solution i n control v i a l s , containing only C-NAA treatment solution (no CM or DCM), indicated there was n e g l i g i b l e loss i n ^ C-NAA over the experimental periods f o r the following treatments: buffer-only control, 0P+7.5E0, 0P+9.5E0, OP+40EO and the various surfactant mixtures used. These r e s u l t s were f o r a l l concentrations examined. Therefore, the assumption was made that the t o t a l amount of l^C-NAA l o s t from the bulk s o l u t i o n was sorbed (or associated) by CM or DCM. For the C-NAA treatment solutions containing 0P+5E0, there was a consistent 3 to 5% decrease i n l^C-NAA i n the bulk solution i n control v i a l s . This decrease i n l^C-NAA from the bulk solution of control v i a l s may be associated with low 0P+5E0 water s o l u b i l i t y and/or 0P+5E0 adsorbing to glass walls of the v i a l s . Equilibrium i n the control v i a l s was reached rapidly (unpublished r e s u l t s ) , demonstrating that v i a l leakage was not a factor. The assumption was made that the loss i n ^^C-NAA observed i n 0P+5E0 control v i a l s would not be affected by the presence of c u t i c l e . Therefore, a correction factor was developed on t h i s basis and used i n calcu l a t i n g the quantity sorbed. I f t h i s loss was not independent of the presence of the CM or DCM, our sorption values would be s l i g h t l y underestimated. l4

1


14

14

4

14

S t a t i s t i c s . A l l measurements were made using f i v e r e p l i c a t i o n s per treatment. For the time-course measurements, the same f i v e r e p l i c a t e s were repeatedly sampled. The results are presented as means with t h e i r respective 95% confidence i n t e r v a l s (Figures 1-6) or c o e f f i c i e n t s of v a r i a t i o n (Table I ) . Results and Discussion Sorption of NAA by tomato f r u i t CM and DCM was markedly affected by the OP derivatives. Factors having the greatest e f f e c t on surfactant i n t e r a c t i o n with the NAA/CM or DCM systems were: surfactant concentration, EO chain length and presence of mixed micelles. Concentration E f f e c t s : Pre-CMC. Surfactant concentrations below the CMC had l i t t l e or no e f f e c t on NAA sorption by tomato CM and DCM, compared to controls, f o r 0P+5E0, 0P+7.5E0 (Figures 1 and 2,



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SHAFERANDBUKOVAC


37

r e s p e c t i v e l y ) , OP+9.5EO (Figure 4) and OP+40EO (Figures 3, 4, 5). Levy e t a l . (18) suggested that polysorbate 80 may form a surfactant monomer/drug complex which increased rates of drug absorption. The observation that NAA sorption was independent of surfactant concentrations below the CMC (Figures 1, 2, 3) suggests that a surfactant monomer/NAA complex was not formed i n our system. The s l i g h t increase i n NAA sorption at concentrations below the CMC may be r e l a t e d to improved wetting of the tomato CM and DCM by the sorbate solution, thereby increasing the a c c e s s i b i l i t y of sorbing s i t e s within the c u t i n matrix. Thus, our data support the hypothesis that octylphenoxy surfactant monomers and NAA molecules interacted with CM or DCM independently of one another. Withington and C o l l e t t (19) also concluded that transfer of s a l i c y l i c a c i d across cellophane membranes occurred independent of surfactant monomers. Concentration E f f e c t s : Post-CMC. At surfactant concentrations above the CMC, s i g n i f i c a n t concentration-dependent enhancement or depression of NAA sorption was observed (Figures 1, 2, 3). Both 0P+5E0 and 0P+7.5E0 caused s i g n i f i c a n t increases i n NAA sorption by CM at 0.1% (Figures 1, 2), while OP+40EO d i d not (Figure 3). This enhancement of NAA sorption was only observed over a r e l a t i v e l y narrow concentration range and when the SCL were present. With a further increase i n surfactant concentration (above 0.1%) NAA sorption by CM was dramatically suppressed to where at 4% and higher no sorption was detectable (Table I ) .

Table I. Sorption o f NAA by Tomato F r u i t Cuticular Membrane, as Affected by 0P+5E0 Concentration Surfactant Concn Amount Sorbed (%. w/v) umol-kg" 0 59 ( 2 ) 0.5 65 (8) 1.0 23 (12) 2.0 6(20) 4.0 0 0b 8.0 determined at equilibrium (48 h), pH 3.2 and 25°C. Mean of f i v e r e p l i cations with c o e f f i c i e n t of v a r i a t i o n i n parenthesis. None detected. 1

a

b

b

These results (Figures 1, 2; Table I) suggest that 0P+5E0 and 0P+7.5E0 micelles influenced NAA sorption by CM. However, the nature of the response (enhancement or depression) was dependent on surfactant concentration. For OP+40EO, NAA sorption by CM was inversely r e l a t e d to post-CMC surfactant concentration (Figure 3). These r e s u l t s were s i m i l a r to those obtained with 0P+9.5E0 and CM (unpublished data).


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S Downloaded by UNIV OF ARIZONA on October 11, 2016 | http://pubs.acs.org Publication Date: June 24, 1988 | doi: 10.1021/bk-1988-0371.ch004

CO

3 o

-3.0

-2.0

-1.0

0.0

LOG CONCENTRATION (%)

Figure 1. E f f e c t of selected 0P+5E0 concentrations on sorption of NAA by tomato f r u i t c u t i c u l a r membranes (CM) and dewaxed c u t i c u l a r membranes (DCM).

PXJJNAA

LOG CONCENTRATION (%)

Figure 2. E f f e c t of selected OP+7.5EO concentrations on sorption of NAA by tomato f r u i t c u t i c u l a r membranes (CM) and dewaxed c u t i c u l a r membranes (DCM).



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SHAFERANDBUKOVAC


39

The e f f e c t s o f 0P+5E0 and OP+7.5EO on sorption of NAA by DCM were q u a l i t a t i v e l y s i m i l a r to the e f f e c t s obtained with OP+9.5EO (unpublished data) and OP+40EO (Figure 3). That i s , no s i g n i f i c a n t e f f e c t of surfactant concentration below the CMC, but above the CMC, NAA sorption was inversely r e l a t e d to surfactant concentration. These data provide strong evidence that the 0P+5E0 and OP+7.5EO surfactants i n t e r a c t with the SCL, r e s u l t i n g i n increased NAA sorption by CM. There may be two explanations for the suppression of NAA sorp t i o n observed i n our study. F i r s t l y , the surfactant may p h y s i c a l l y block sorption s i t e s and secondly, micelles may compete with the CM or DCM f o r NAA thereby reducing the amount o f NAA available f o r sorption (20). Based on data obtained i n our studies (unpublished data), NAA p a r t i t i o n i n g into micelles was the more l i k e l y mechanism responsible f o r decreased NAA sorption by tomato CM and DCM a t postCMC concentrations. However, adsorption of 0P+9.5E0 by c u t i c l e s was shown to occur at high concentrations (21). Therefore, one cannot preclude the p o s s i b i l i t y that at high surfactant concentrations physical blocking of NAA sorption s i t e s also occurred. The mechanism o f enhanced NAA sorption by 0P+5E0 and 0P+7.5E0 over a r e l a t i v e l y narrow concentration range i s not c l e a r . Octyl phenoxy surfactants are mixtures of various ethoxymers (15, 22). The enhancement of NAA sorption with 0P+5E0 and OP+7.5EO at 0.1% may r e f l e c t a s p e c i f i c ethoxymer mole d i s t r i b u t i o n pattern which enables higher NAA sorption (or association) by CM to occur. The s p e c i f i c ethoxymer d i s t r i b u t i o n at 0.1% may have l e d to a softening or swelling of the CM, leading to increased a c c e s s i b i l i t y of NAA sorption s i t e s . Since there was no s p e c i f i c increase i n the sorption of 0P+5E0 by CM at 0.1%, as compared to 0.5 or 1.0% (21), i t i s u n l i k e l y that t h i s enhancement was r e l a t e d to NAA associated with the surfactant (e.g. i n micelles) sorbed by the CM. I t i s clear that the SCL of the CM play an important r o l e , since no surfactant-enhancement of NAA sorption was observed when they were removed (DCM). Elucidation of the mechanism(s) responsible for the enhancement e f f e c t s remains to be documented. Further, i t should be noted that 0P+5E0 and 0P+7.5E0 enhancement of NAA sorption has been observed also with CM i s o l a t e d from pepper f r u i t and Ficus leaves (Shafer, W. E.; Bukovac, M. J . ; Fader, R. G. Proc. Adjuvants and Agrochemicals. Vol. 1, CRC Press, i n press). EO Chain Length E f f e c t s . At 0.1%, enhanced NAA sorption by CM was greatest with 0P+5E0, s l i g h t l y less with OP+7.5EO, and absent with OP+40EO (Figures 1, 2, 3). In a r e l a t e d study (unpublished data), i t was determined that the enhancement e f f e c t was l o s t when the average EO chain length was 9.5EO or greater. With an EO chain length of 9.5EO or greater with CM, or for any octylphenoxy surfactant studied with DCM, NAA sorption was, i n general, d i r e c t l y r e l a t e d to EO chain length. This response was s i m i l a r for plant c u t i c l e s from several genera and also held f o r a surfactant series where the hydrophobe was a l i n e a r alcohol (Shafer, W. E.; Bukovac, M. J . ; Fader, R. G. Proc. Adjuvants and Agrochemicals. Vol. 1, CRC Press, i n press). These r e s u l t s c l e a r l y demonstrate that small s h i f t s i n the ethoxymer mole d i s t r i b u t i o n can profoundly a f f e c t NAA sorption.



40


Mixed Surfactant E f f e c t s . Results obtained with selected octylphenoxy surfactant mixtures provide i n t e r e s t i n g data regarding surfactant/NAA/CM interactions (Figures 4, 5, and 6). Addition of OP+40EO (0.1%) to 0P+9.5E0 or 0P+7.5E0 (0.1%) resulted i n a s i g n i f i c a n t decrease i n NAA sorption (Figures 4 and 5, respec t i v e l y ) . Considering these experiments, three factors should be noted: (a) OP+40EO was present at a concentration below i t s CMC, (b) 0P+7.5E0 resulted i n s i g n i f i c a n t l y higher NAA sorption at 0.1% i n the absence of OP+40EO and (c) t o t a l surfactant concentration i n the mixtures was 0.2%. Thus, differences i n EO chain length of octylphenoxy surfactants had a profound impact on NAA sorption by CM, and, i n a l l cases, mixtures resulted i n less NAA sorption than component surfactants. Perhaps the most i n t r i g u i n g mixture e f f e c t was observed with 0P+5E0 and OP+7.5EO (Figure 6). Both surfactants, at 0.1%, yielded high (60 to 70 /imol'kg"*) NAA sorption values. However, when 0P+5E0 and OP+7.5EO were combined at equal concentrations ( t o t a l surfactant concentration of 0.1 or 0.2%), the NAA-enhancement e f f e c t was l o s t . The mixture treatments yielded sorption values (29 to 39 /imol'kg"!) s i g n i f i c a n t l y lower than sorption values obtained with controls (45 /imol-kg"l). I t should be mentioned that for the two concentrations examined, both surfactants were above t h e i r CMC. Two explanations f o r the mixture e f f e c t s may be proposed: (a) the t o t a l number of micelles increased on mixing, thereby increasing the t o t a l quantity of NAA p a r t i t i o n i n g into micelles and/or (b) mixed micelles were formed with greater NAA s o l u b i l i z i n g capacity. However, since a d i r e c t micellar/CM (or s p e c i f i c a l l y SCL) i n t e r a c t i o n has already been established, perhaps the focus should be on decreased micellar/SCL interactions, rather than increased micellar/NAA interactions. These suggestions do not preclude the p o s s i b i l i t y that some other mechanism(s) may be involved. Our r e s u l t s with surfactant mixtures suggest that the blending of d i f f e r e n t surfactants for various purposes (1) should be c a r r i e d out with the recognition that some negative consequences may occur. Conclusions Based on the data obtained herein, the following conclusions regarding octylphenoxy surfactant/NAA/cuticle interactions can be made. 1. A surfactant monomer/NAA complex was not involved i n NAA sorption by CM or DCM. 2. Surfactant micelles competed with the CM/DCM f o r NAA molecules, thereby decreasing NAA sorption. 3. 0P+5E0 and OP+7.5EO surfactant micelles at 0.1% interacted with the SCL, leading to a s i g n i f i c a n t increase i n NAA sorption by, or association with, the CM. 4. Mixed surfactant systems ( t o t a l concentration greater than CMCs) resulted i n decreased NAA sorption by CM.


SHAFER AND BUKOVAC


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Pesticide Formulations - American Chemical Society

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