**4. Conclusions**

12 Herbicides – Properties, Synthesis and Control of Weeds

of the active principle, and dissolution and erosion of the matrix or the polymeric wall

A number of mathematical models have been extensively used to analyze the characteristics of the release of substances from polymeric systems (Costa & Lobo 2001). Here, the results of the release experiments (Figure 5) were analyzed using the zero order, first order, Higuchi and Korsmeyer-Peppas models (Table 4). For the formulations investigated, the Korsmeyer-Peppas model provided the best explanation of the ametryn release mechanism, according to the correlation coefficient obtained. The curves obtained for each formulation

**Release constant (k)** 4.59184 min-1 0.00667 min-1 4.49925 min-1/2 0.00628min-n

**coefficient (r)** 0.92307 0.98452 0.97721 0.99364

**Release constant (k)** 0.20767 min-1 0.00624 min-1 4.35701 min-1/2 0.0072min-n

**coefficient (r)** 0.89455 0.96782 0.98115 0.9879

**Release constant (k)** 0.07283 min-1 0.00581 min-1 1.52624 min-1/2 0.0162 min-n

**coefficient (r)** 0.86545 0.97701 0.9893 0.9929

**Release constant (k)** 0.048 min-1 0.00495 min-1 1.00913 min-1/2 0.0194min-n

**coefficient (r)** 0.90337 0.96059 0.97587 0.98839

**Release constant (k)** 0.0983 min-1 0.00441 min-1 2.17047 min-1/2 0.0429min-n

**coefficient (r)** 0.79093 0.92828 0.99035 0.9927

The Korsmeyer-Peppas model is based on a semi-empirical equation (Korsmeyer & Peppas, 1991; Korsmeyer et al., 1983) that is widely used when the release mechanism is unknown. When the release exponent (n) is equal to 0.43 the mechanism involved is diffusion. When the value of the exponent is greater than 0.43 but smaller than 0.85, the release occurs due to anomalous transport that does not obey Fick's Law. Values less than 0.43 are indicative of porous systems in which transport occurs by a combination of diffusion through the polymeric matrix and diffusion through the pores. The values obtained (Table 4) differed

Table 4. Results of the application of four mathematical models to the release curves of

**Zero order First order Higuchi Korsmeyer-Peppas** 

n = 0.79373

n = 0.62532

n = 0,82641

n = 0.5671

n = 0.42726

(Polakovic et al., 1999; Schaffazick et al., 2003).

using this model are illustrated in Figure 6.

**Formulation A** 

**Correlation** 

**Formulation B** 

**Correlation** 

**Formulation C** 

**Correlation** 

**Formulation D** 

**Correlation** 

**Formulation E** 

**Correlation** 

ametryn associated with different microparticles.

Ametryn herbicide was efficiently encapsulated in microparticles composed of PHB, PHBV and mixtures of the two polymers. The highest encapsulation efficiencies were achieved when higher proportions of PHBV were used. SEM analysis showed that the microparticles were spherical, although with different surface features (either smooth or rough with pores). The release profile of ametryn was modified when it was encapsulated, with slower and more sustained release compared to the free herbicide. This finding suggests that the use of encapsulated ametryn could help to mitigate adverse impacts on ecosystems and human health. This is particularly important given the increasingly widespread and intensive use of agents such as ametryn in modern agriculture.
