Tween partitioning

The objectives of this work were to (1) study the equilibrium sorption characteristics of an anionic surfactant (SDS) and a nonionic surfactant (Tween 80) to kaolinite, a common soil mineral, as a function of solution chemistry (2) examine the equilibrium partitioning of two HOCs (phenanthrene and naphthalene) to the surfactant micelles and sorbed... 

Figure 4. Time to LNAPL conversion of chlorobenzene vs. partitioning plateau (SN120, Brij 35, Tween 80 with n-butanol).

Fig. 4.5 Absorption-partition correlations obtained in rat stomach for a series of acid xenobiotics in absence of any additive (ka) and in presence of sodium taurocholate (NaT) and Tween 80 at a concentration below its CMC. (Adapted from Garrigues et al. [8]).

Tween 80 (also known as At-Plus 109 and Polysorbate 80) is a polyoxyethylene sorbitan monooleate and was obtained from Atlas Chemical Industries, Inc. This nonionic surfactant has an HLB (hydrophile-lipophile balance) of 15.0 and is used as an emulsifier, solubilizer, and dispersant. It was used without further purification at the 0.1% (w/v) level in partitioning and greenhouse herbicidal evaluations as described below. 

Partitioning in the Presence of Surfactant. Partitioning experiments in the presence of Tween 80 were carried out in a manner identical to that discussed above except that 0.1% (w/v) of the surfactant was thoroughly mixed with the water phase before partitioning. Octanol/water partitioning studies carried out with only 0.1% Tween 80 (and no TFMS compound) present initially indicated that at equilibrium, the surfactant partitions approximately 60/40 in favor of the octanol phase. 

In a typical octanol/water partitioning experiment carried out in the presence of Tween 80, the TFMS compound was first dissolved in 50 ml octanol and then partitioned against an equal volume of pH 1.0 water containing 0.1% Tween 80 as described above. All samples were centrifuged to ensure the best possible octanol/water phase separation and optical clarity. 

Effect of Tween 80 on TFMS tt Values. Table VI lists octanol/water partitioning data for the 15 substituted TFMS compounds used in this study. Logarithms of the partition coefficients obtained both in the absence (log Px) and presence (log Px) of 0.1% (w/v) Tween 80 are given. (Procedures for determining the partition coefficients Px and Px have been outlined in the Experimental section.) tt, tt, and w" values in Table VI were calculated according to the following relationships ... 

In Equation 6, w and tt values are calculated relative to log Ph — 3.05 for the parent compound (H-substituted TFMS) determined in the absence of surfactant. Thus, the v value determined for the parent compound in the presence of 0.1% Tween 80 differs from zero (wH = —0.45). The 7r" substituent constants in Table VI and Equation 6 are calculated relative to the logarithm of the partition coefficient of the parent TFMS compound determined in the presence of 0.1% Tween 80 (log PHf = 2.60). 

Since Tween 80 itself partitions 60/40 in favor of the octanol phase, a significant effect of this surfactant on the partitioning properties of the TFMS series members might be expected. Table VI indicates this to be the case. Comparing log Px with log Px values, it is evident that the... 

Table VI. Effect of 0.1% Tween 80 on Octanol/Water Partitioning Characteristics of TFMS Herbicides ...

The tr" values in the far right column of Table VI bring to light another interesting fact. They indicate that Tween 80, in addition to lowering the octanol/water partition coefficients of all TFMS series members in a general fashion, appears to alter the order of lipophilicity of certain individual series members. In the presence of surfactant, for example, the 4-C1, 4-CH3, and 4-F TFMS derivatives all exhibit equal or... 

For all regression analyses, herbicidal activity data were taken from the appropriate column in Table VII. Corresponding tt, tt or tt" values were selected from Table VI. Hammett sigma constants (a) were taken from the compilation of Jaffe (13) and correspond to those in Table IV. Since it was assumed throughout that <7 would be relatively unaffected by the presence of surfactant, the <7 values in Table IV were used to correlate data obtained in the presence and absence of Tween 80 for all three weed types. This assumption is reasonable since the surfactant was used at a low 0.1% level in all herbicidal and partitioning tests. Furthermore, surfactant effects would be expected to manifest themselves primarily in the partitioning behavior (tt values) of the TFMS compounds... 

C1-TFMS data omitted from fits of 3-series in presence of Tween 80 because of experimental difficulties in measuring its partition coefficient see Table VI). 

Hansch correlations as described above. In performing the correlations, appropriate w and t/ values were selected from Table VI the latter values were used for data gathered in the presence of 0.1% Tween 80. Both tr and t/ are calculated relative to log PH = 3.05 for the parent compound in the absence of surfactant (see Equation 6). Thus, a given TFMS compound in the presence of Tween 80 might be thought of as a new member of the similarly substituted TFMS series characterized by somewhat different partitioning properties than it originally exhibited in the... 

TFMS derivative in the presence of surfactant (cf. also Figures 2a and 2c). Thus, the loss in Foxtail activity which results from the deleterious effect of surfactant on the pa term in the Hansch equation is compensated by the improved partitioning characteristics (large Bn term) of the 4-methylsulfonyl derivative in the presence of surfactant. The net result is that the overall pre-emergence Foxtail activity is not greatly affected by Tween 80. As illustrated in Table XIII, however, this effect is only apparent since significant changes in both ir and a dependence have taken place. 

Wild Mustard activity of the 4-Cl-TFMS derivative in both the presence and absence of Tween 80 is higher than that observed for Foxtail because both r = +0.91 and v = —0.45 for this compound lie closer to their appropriate tr optima (tt0 = +0.36 (no surfactant) tr0 = —0.59 (with surfactant)) than was the case for Foxtail. Hence, the Air2 + Bw contribution for the broadleaf in both the presence and absence of surfactant is more favorable than it is for the grass. The pa contribution is also more favorable for Wild Mustard because of the high value of p for 4-substituted TFMS compounds in the presence and absence of Tween 80 (see Table XI). Thus, for the 4-Cl-TFMS derivative, both enhanced partitioning characteristics and a more favorable electronic situation (more positive pa) combine to give higher Wild Mustard than Foxtail activity. 

The 2 r values in Table XIV were calculated by subtracting log PH = 3.05 for the original parent compound (in the absence of Tween 80) from the measured log Px values for the disubstituted compounds. The 2 r values tabulated above for the 2,4-di-F- and 2,4-di-Cl-TFMS derivatives thus represent the actual experimental effects on partitioning that 2,4-di-F or 2,4-di-Cl substitution have on the parent molecule. Comparing the above experimental 2tt values with 2tt(4-C1) = +1.82 and 2 r(4-F) = +0.40 used in the calculations of Equations 32-37, we note that the experimentally measured 2tt is considerably lower than the estimate in both cases. For 2,4-di-Cl substitution, for example, the addition of the second Cl in the ortho position renders the molecule more hydrophilic (i.e., tt(4-C1)---1-0.91, 2tt(2-C1 + 4-C1) = +0.83). It would... 

Item d implies that in terms of the Hansch model of Equation 1, partitioning STEP 1 is the primary rate-controlling process characterizing the herbicidal action of the 3-TFMS compounds on Wild Mustard in the presence of Tween 80. Since all the other Hansch relationships in Table XI include fairly significant pa contributions, partitioning as well as other rate processes (possibly more intimately connected with the receptor site within the plant or seed) must be involved in determining overall herbicidal activity in these latter cases. One may speculate that the anomalous observations (a-c) above are the direct consequence of (d)—the lack of Hammett a dependence. If the pa term in the Hansch equation does indeed reflect rate or equilibrium events occurring at or near the herbicidal site of action within the plant or seed (as is often assumed but not... 

S02CH3-TFMS and 3-S02CH3-TFMS derivatives as they do in the Foxtail surfactant case (see Table XV). Although methylthio-TFMS derivatives might possibly be protected from in vivo oxidation by Tween 80 absorbed by the Wild Mustard seeds (micelle formation), a process of this type would not be observable experimentally since slower ratedetermining partitioning events control the overall herbicidal activity as previously explained. 

Surfactant. Polyoxyethylene (20) sorbitan monooleate (also known as Tween 80, At-Plus 109, or Polysorbate 80) was obtained from Atlas Chemical Industries and used as received. It was used at the 0.1% (w/v) concentration level in all herbicidal activity evaluations and partitioning studies requiring the use of surfactant. 

Partition Coefficient. Partition coefficients of the TFMS herbicides in both the presence and absence of surfactant were determined between 1-octanol and pH 1.0 water (made acid by addition of HC104) by ultraviolet spectroscopy. The absorption spectrum of Tween 80 did not interfere with the spectra of the sulfonanilides (6). 

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