MICROBIO: A NOVEL MICROEMULSION AS AN EFFICIENT

TANK-MIX ADJUVANT FOR HERBICIDE TREATMENTS

Alberto Colombo, Gianfranco Paganini and Peter Bohus

Agro-Chemistry Laboratory; Cesalpinia Chemicals (Lamberti Group);

Via Piave, 18 - 21041 Albizzate (VA), Italy

SUMMARY

A new microemulsion adjuvant for tank mix application containing: fatty acid esters

from transesterification of vegetable oils, anionic ester of alkylpolyglucoside and

ethoxylated fatty alcohols was developed. Physico-chemical properties were optimized

to improve stability, synergism amongst components and compatibility with various

pesticide formulations.

Surface tension and particle size were measured; rheological characterisation of the

adjuvant was carried out to verify microemulsion physical status and stability.

This adjuvant mixture was designed to quickly wet and spread droplets on the leaf in

order to get a uniform spray deposit over foliage surface. Wettability and spreading on

specific surfaces were also determined.

Biological efficacy in weed control of various tank-mix blends of the adjuvant with

commercial herbicide formulations was studied. Three herbicide/weed combinations of

Microbio as tank mix adjuvant were examined: one water soluble herbicide (Bentazone

Na salt) applied on black nightshade; two water insoluble actives (Nicosulfuron and

Quizalofop-P-ethyl) applied on wild oat.

Key words: microemulsion, alkylpolyglucoside, tank-mix.

INTRODUCTION

Fatty acid esters from transesterification of vegetable oils, alkylpolyglucosides and

ethoxylated fatty alcohols have individually been widely used as tank-mix or built-in

adjuvants for herbicides. They are valuable because of the following properties:

improvement of biological efficacy of active ingredients, excellent versatility and ease

of use in combination with various formulations of actives, application technologies and

crop varieties. In addition, they can replace adjuvants which are harmful to the

environment, such as nonylphenol ethoxylates or fatty amine ethoxylates.

Recently, a new class of anionic esters of alkylpolyglucoside have been described as

novel tank-mix adjuvants (1,2), which appear to have good potential as adjuvants in

agro-chemistry.

Microemulsion is a highly effective physical form of agrochemical delivery system,

which combines stability of complex formulations, performance and easy handling. We

have developed a stable microemulsion (Microbio® from Cesalpinia Chemicals SpA,

Italy) containing a citric ester of alkylpolyglucoside, fatty acid esters from

transesterification of a vegetable oil and an ethoxylated fatty alcohol, which proved to

be a higly effective adjuvant with different herbicide actives.

METHODS AND MATERIALS

Wettability and Surface Tension

Analysis of static surface tension (EST) was performed using a Sigma 70 tensiometer

(KSV Instrument Ltd) on the adjuvant mix diluted in water at different concentrations at

25°C (ISO 304, ISO 4311).

Wetting power of aqueous spray was evaluated at 25°C by determining the sinking time

of a cotton diskette, using an internal method based on DIN 53901.

Spreading

Dynamic contact angle, as a function of time, was studied using a Rycobel PG-X

goniometer at different concentrations of Microbio on a glass plate (ASTM D-724).

Particle Size Analysis

Was carried out using a N4 Plus Nanosizer (Coulter) at 20°C with the following

parameters: refractometric index (n=1.433), density (d=0.98 kg/dm3) and viscosity

(η=40 mPa.s), which are characteristic of Microbio. Scanning was performed at

different angles with 360 sec run time.

Rheological Characterisation

Rheological profile was studied with Advanced Rheometer 2000 (TA Instruments),

using a double gap geometry. Viscosity of Microbio was measured as a function of

temperature (from 0° to 70°C) and shear rate (from 1 s-1

to 400 s-1

). Rheograms of shear

stress vs shear rate (under steady state flow) and of ln η vs T-1

were obtained.

Biological Efficacy Trials (carried out by SurfaPlus bv, Wageningen, NL)

Plant material

Black nightshade (Solanum nigrum L.; Bayer code SOLNI) and wild oat (Avena fatua

L.; Bayer code AVEFA) were grown in a growth chamber under 14 hrs of light, at

18/12 (±0.5)°C (day/night) temperature, and in 70/80 (±5)% (day/night) relative

humidity. Light was provided by high-pressure mercury lamps and fluorescent tubes to

give 70 W/m2 (PAR) at leaf level. The plants were grown in 11 cm diameter plastic pots

filled with a mixture of sand and humic potting soil (3) (1:2 by volume). The pots were

placed on a sub-irrigation matting, which was wetted daily with a half-strength nutrient

solution. After emergence, the black nightshade and wild oat plants were thinned to one

and five plant(s) per pot, respectively. Black nightshade was treated at the four-leaf

stage and wild oat was treated at the 2-3 leaf stage.

Herbicide application

Treatment solutions were applied with an air-pressured laboratory track sprayer having

1.2 mm nozzles fitted with a perforated (0.6 mm) whirling pin and delivering 200 L/ha

at 303 kPa. The adjuvants Microbio and Hasten® (emulsifiable esterified rapeseed oil

from Victorian Chemicals) were added at 0.25% (v/v).

Herbicide formulations used were: Bentazone (Basagran® from BASF); Nicosulfuron

(Milagro® from Syngenta); Quizalofop-p-ethyl (Targa Prestige® from Bayer).

Measurement of herbicide performance

The fresh weight of the aerial parts of black nightshade and wild oat was determined as

a parameter of herbicide performance.

Experimental design and data analysis

Experiments were conducted with four replicates per experiment, according to a

completely randomized design. The data of the adjuvant tests were subjected to analysis

of variance using the Genstat statistical package (Release 6.1; Rothamsted Experimental

Station). The means of each treatment were compared according to Fisher’s LSD (0.05).

LSD (0.05) is the Least Significant Difference between mean values at the 5% level.

RESULTS AND DISCUSSION

Wettability and Surface Tension

Aqueous spray of Microbio show better wetting properties than a corresponding

emulsifiable vegetable oil. A lower surface tension and sinking time with the

microemulsion system were obtained (Table 1 and 2).

Table 1: Static surface tension of aqueous solutions at 25°C

Surface tension (mN/m)

0.2%

Surface tension (mN/m)

0.5%

Microbio 31.1 29.3

Emulsifiable vegetable oil 33.5 31.5

Ethoxylated Alcohol 27.0 26.7

Alkylpoly Glucoside Ester 25.8 25.5

Table 2: Sinking time of a cotton diskette in 0.25% w/w of aqueous spray at 25°C

Water Microbio Ethoxylated

alcohol

Emulsifiable

vegetable oil

Alkylpoly

Glucoside Ester

Ethox.

NF

Time (sec) > 300 110 5 182 25 5

Spreading

Spreading of aqueous dilutions of Microbio was determined in comparison to a solution

of ethoxylated fatty alcohol, as a function of time (Table 3).

Fig. 1: Contact angle determination vs time

In agreement with the effect of surfactants on surface tension and contact angle reported

in the literature (4), it can be seen in Fig. 1 that, within the range considered, the higher

the concentration of Microbio (curves group marked A), the lower the contact angle

measured. This finding is analogous to the behaviour typical spreading adjuvants

(curves group marked B).

Particle Size Analysis

Particle size analysis (Table 3) suggests a droplets distribution of Microbio (detected

only at high deviation angles) that falls within the microemulsion range (5).

Table 3: Particle size of Microbio measured at 25°C (expressed in nm)

Angle Medium diameter Standard deviation

21.3° Not detectable ---

64.6° 31.0 13.2

90.0° 25.7 11.7

Rheological Characterisation

The flow curve of Microbio (Fig. 2) indicates a Newtonian behaviour (a general feature

of low viscosity microemulsions) (6). From the linear curve of Fig. 2, viscosity was

found to be independent of shear rate and equal to 44.8 mPa.s.

Fig. 2: Flow curve of Microbio at 25°C

Thermodynamical parameters were calculated from a rheogram of ln η vs T-1

(Fig. 3)

using equation 1 (6):

(eq. 1)

With eq. 1, values of ∆H and ∆S were calculated, from which ∆G was obtained, as

reported in Table 4. The thermodynamic parameters derived from rheological studies

suggest an intrinsic stability of Microbio, as reported in literature for similar system (6).

y = 0,0438x + 0,0113

y = -3E-06x + 0,0448

1,00E-03

1,00E-02

1,00E-01

1,00E+00

1,00E+01

1,00E+02

0,1 1 10 100 1000

Shear rate (1/s)

Sh

ear

Str

ess (

Pa)

shear stress

viscosity

shear stress interpolation

viscosity interpolation

( )RT

H

R

S

VhN

hNR

S

RT

H

hNhNRTG

∆+

∆−=

=

∆−

∆

=∆

=∆

lnln

ln

lnRT

G ln

η

ηυ

ηυηυ

Fig 3: rheogram of ln η vs T-1

Tab. 4

Biological Efficacy Tests

As detailed in Methods (see above), Microbio was tested on Black nightshade (Solanum

nigrum L.) and Wild oat (Avena fatua) in combination with three herbicide active

ingredients: Bentazone (Basagran from BASF); Nicosulfuron (Milagro from Syngenta);

Quizalofop-P-ethyl (Targa Prestige from Bayer). Our new adjuvant was compared at the

same concentration of 0.25% (v/v) in water to a well known commercial adjuvant

(Hasten, from Victorian Chemicals, Australia).

Results are reported in Fig. 4 (A, B, C).

Fig. 4, A

Fig. 4, A shows the effect on Black nightshade at two dosages of Bentazone. (20

and 40 g/ha). Microbio and Hasten have a dramatic effect on herbicide efficacy at the

lower sub-optimal dosage of 20 g/ha.

Fig. 4, B shows the effect on Wild oat at two dosages of Quizalofop-P-ethyl (2

and 6 g/ha). Microbio shows a slight better effect on herbicide efficacy than Hasten at

the lower sub-optimal dosage of 2 g/ha.

Infuence of adjuvants on bentazone efficacy (SOLNI)

0

5

10

15

20

25

Un

trea

ted

Be

nta

zo

ne

20

g/h

a (B

E-

20

)

BE

-20

+

Mic

rob

io

BE

-20

+

Ha

ste

n

Be

nta

zo

ne

40

/ha

(BE

-

40

)

BE

-40

+

Mic

rob

io

BE

-40

+

Ha

ste

n

FW 16

DAT (g)

LSD(0.05)=2.31

Sample controlled variable ∆∆∆∆H ∆∆∆∆ S ∆∆∆∆G (298K)

(kJ/mol) (J/K*mol) (kJ/mol)Microbio Shear Stress 35.6 -61.5 53.9

Evaluation of ∆∆∆∆ H e ∆∆∆∆ S

y = 4281,1x - 17,414

- 4 ,5

-4

-3,5

-3

-2,5

-2

- 1,5

-1

-0,5

0

0,00305 0,0031 0,00315 0,0032 0,00325 0,0033 0,00335 0,0034 0,00345 0,0035 0,00355

1/T (K-1

)

ln (

vis

co

sit

y)

viscos ity measures

interpolation

Fig. 4, B

Fig. 4, C

Fig. 4, C shows the effect on wild oat at two dosages of Nicosulfuron (5 and 15 g/ha).

Microbio shows a similar effect on herbicide efficacy as Hasten at the lower sub-

optimal dosage of 5 g/ha.

CONCLUSIONS

We have developed a mixture containing three components, each one displaying

individual adjuvant activity in combination with herbicides. Therefore, a carefully

balanced microemulsion of the three ingredients shows excellent stability, synergism

amongst components and compatibility with various pesticide formulations.

Rheological profile and thermodynamic parameters confirm the microemulsion physical

status of the mixture, which appears to also have good spreading and wetting properties.

These properties should translate in ease of use and effectiveness in actual field

applications,

Biological efficacy trials in weed control with various tank-mix combinations of the

adjuvant with different herbicide active ingredients applied on Black nightshade or Wild

oat, at a constant concentration of 0.25% (w/w) in water, showed that at all hebicide

dosages tested, Microbio showed either an equal or better efficacy than a reference

commercial adjuvant. Our novel microemulsion adjuvant combines the advantages of

eco-friendly components with long term storage stability, easy handling and good

biological efficacy.

Infuence of adjuvants on nicosulfuron efficacy (AVEFA)

0

5

10

15

20

25

Un

trea

ted

Nic

os

ulfu

ron

5 g

/ha

(NIC

-

5)

NIC

-5 +

Mic

rob

io

NIC

-5 +

Ha

ste

n

Nic

os

ulfu

ron

15

g/h

a (N

IC-

15

)

NIC

-15

+

Mic

rob

io

NIC

-15

+

Ha

ste

n

FW 21

DAT (g)

LSD(0.05)=2.58

Infuence of adjuvants on quizalofop-P-ethyl efficacy (AVEFA)

0

5

10

15

20

25

Un

trea

ted

Qu

iza

lofo

p-P

-eth

yl 2

g/h

a(Q

UIZ

-2)

QU

IZ-2

+ M

icro

bio

QU

IZ-2

+ H

as

ten

Qu

iza

lofo

p-P

-eth

yl 6

g/h

a(Q

UIZ

-6)

QU

IZ-6

+ M

icro

bio

QU

IZ-6

+ H

as

ten

FW 21

DAT (g)

LSD(0.05)=2.78

REFERENCES

1. Bohus, P., Colombo, A.; 2004. Proceedings of 7th

International Symposium on

Adjuvants for Agrochemicals, Cape Town, SA pp 301 – 306

2. European Patents n° EP 510564

3. Soil no. 12, Colent b.v. , Lent, The Netherlands

4. Singh, M., Orsenigo, J.R., Shah, D.O.; 1984. JAOCS vol 61 n°3 pp 596 - 599

5. Tadros, T. F.; 2005. Applied Surfactants; Wiley-VCH; Weinheim pp 309 – 311

6. Acharya, A., Ghosal, S.K., Moulik, S.P., Subramanian, N. et al.; 2005. Chem.

Pharm. Bull. vol 53(12) pp 1530 –1535.

Acknowledgements

We wish to thank Dr. Hans de Ruiter for very helpful suggestions and collaboration.