Effects of Surfactant Concentration on Stability of Dispersion

Chapter 25

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 17, 2016 | http://pubs.acs.org Publication Date: March 8, 2005 | doi: 10.1021/bk-2005-0899.ch025

Effects of Surfactant Concentration on Stability of Dispersion Tatsuo Sato Monsanto, Midorigaoka, Chofu, Tokyo 182-0001, Japan

The effect of concentration of a cationic surfactant (polyoxyethylene diethylenetriamine dialkylamide) on the stability of aqueous titanium dioxide (TiO ) dispersion was studied by measuring adsorption and zeta potential. It was found that as the surfactant concentration increases, stability decreases initially and then increases, showing a minimum. As the concentration increases further, stability decreases, showing a maximum. The zeta potential changes from negative to positive as the adsorption increases and reaches a maximum when the adsorption reaches a plateau value. The dispersion flocculated when the absolute value of the zeta potential dropped below 10 mV, indicating that the destabilization by the surfactant in the low surfactant concentration is due to reduction of the electrical repulsion. Destabilization at the higher surfactant concentration is likely caused by the depletion effect since there is no change in adsorption and zeta potential in the surfactant concentration range, and the surfactant molecules are not sufficiently large to form bridges between particles (1). 2

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© 2005 American Chemical Society Clark and Ohkawa; Environmental Fate and Safety Management of Agrochemicals ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction Aqueous dispersion formulations such as concentrated suspensions (SC) and concentrated emulsions (EW) are becoming more and more important as regulation of the safety of pesticides becomes more and more strict. The most important physical property required for these formulations is dispersion stability. Dispersion stability is important not only for storage stability but also for biological activity. In general, the biological activity decreases remarkably when the active compound particles flocculate (2,3). Therefore, biologically active compounds must be dispersed as fine particles for stability and bioactivity. Dispersion is usually stabilized by the addition of surfactants. Therefore, it is essential for formulation chemists to know how to use surfactants efficiently in developing dispersion formulations. Particularly, selection of appropriate type of surfactants and optimization of the concentration are very important. In this work, the effect of surfactant concentration on the stability of aqueous suspensions was studied by measuring surfactant adsorption and zetapotential of dispersed particles (1). Aqueous T i 0 suspensions were used as a model of SC and a cationic surfactant was used as an example of surfactants. 2

Experiments Materials Surfactant The surfactant used in this work was a cationic surfactant of the following structure supplied by Takemoto Oil and Fat Co. in Japan. Polyoxyethylene(6moles)diethylenetriaminedialkylamide (C OCNHCH CH NHCH CH NHCOC (CH H40) ), Average Mw: 744 12

2

2

2

2

12

2

6

Titanium Dioxide Titanium dioxide used in this work was rutile-type Tipaque R-680 supplied by Ishihara Industrial Company in Japan. Surface area: 8 m /g; Average diameter: 0.21 microns; Specific gravity: 4.2. 2

Clark and Ohkawa; Environmental Fate and Safety Management of Agrochemicals ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Procedure The stability of dispersion was determined by a sedimentation method. The suspensions were prepared by dispersing titanium dioxide pigments by a paint shaker with glass beads in the surfactant solutions. The solution was then poured into a sedimentation test tube. Stability was determined by measuring the settling rate and the sedimentation volume. As the stability increases, the settling rate decreases and the sedimentation volume formed in long-term storage (about one month) decreases. The stability was graded 0 (very unstable) to 5 (very stable) by an overall evaluation.

Results and discussion Change of dispersion stability with surfactant concentration Fig. 1 shows the change in the stability of the T i 0 suspension and the zeta potential with surfactant concentration. The stability is initially very good in low surfactant concentration up to 0.05%. As the surfactant concentration increases from 0.05 %, the stability decreases and reaches a minimum (very poor) at about 0.2 %. As the concentration increases further, the stability increases rapidly and then decreases, showing a maximum at about 1.0 %. The zeta-potential of T i 0 is initially negative. As the surfactant concentration increases, the potential changes from negative to positive, passing the zero point of charge (zpc). The stability decreases as the absolute value of the zeta-potential decreases and reaches a minimum at the zpc. These results indicate that the decrease of the stability with the increase of the surfactant concentration in the low surfactant concentration range (0 0.2 %) is caused by the reduction of electrical repulsion between negatively charged particles and the increase of the stability in the higher surfactant concentration range (0.2 - 1.0 %) is caused by the increase of the electrical repulsion between positively charged particles. The adsorption isotherm is shown in Fig. 2. Fig. 2 shows that as the surfactant concentration increases, the adsorption increases and reaches a plateau at about 1.0 %, indicating that the adsorption follows the Langmuir isotherm. It can be seen by comparing Fig. 1 with Fig. 2 that the dispersion stability reaches a maximum at the concentration where the adsorption reaches a plateau. 2

2



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Equilibrium concentration, %

Log C (100%) Fig. 2. Adsorption isotherm of POE-DETADAA on Ti0 at 20 % (w/w) 2

Computation of total potential energy According to the DLVO theory, total potential energy between two particles is expressed by the sum of van der Waals attractive potential energy and electrical repulsive potential energy and the dispersion is stable when the total potential energy is higher than 15 kT (4). The electrical repulsive potential energy due to the electrical charge of the particles in aqueous solution can be approximated by; E = (εΓζ /2) 1η{1 + exp(-KH)} [1] el

2

R


294 whereeis the dielectric constant of medium, r is the radius of particles^is zeta potential,Kis the Debye-Huechel reciprocal length parameter, and H is the shortest distance between two particles. The van der Waals attractive potential energy between particles at small distance can be approximated by; E = -Ar/12H [2] where A is the Hamaker constant. The potential energy between T i 0 particles calculatedfromEq. [1] and Eq. [2] is shown in Fig. 3. Curve 3 is the potential energy when the dispersion is stable and Curve 4 is the potential energy when the dispersion is not stable. The maximum height of Curve 3 and Curve 4 are 67 kT (higher than 15 kT) and 8 kT (lower than 15 kT), respectively as shown in Fig. 3. The results are well accounted for by the DLVO theory. A


2

Fig. 3. Potential energies between Ti0 particles. (a)E with no adsorbed layer (b) E with adsorbed layer of thickness 50 A (1) E at 15 mV (2) E at 10 mV. (3) Total potential energy at 15 mV(l + b) (4) Total potential energy atlOmV (2 + b) 2

A

el

A

R

el

R

Destabilization in High-Surfactant Concentration Solutions. Fig.l shows that the stability decreases when the surfactant concentration becomes higher than the concentration where the adsorption



295 reaches a maximum, although there is no change in the zeta potential in this surfactant concentration range. Thisflocculationcan not be explained by the DLVO theory (4 - 6) which states that the dispersion stability depends on electrical repulsion. It was found fairly recently thatfree(non-adsorbing) polymer can affect colloid stability (4-11). Flocculation caused byfreepolymers is called "depletion flocculation" (5,6). The first theory for the depletion flocculation was the theory proposed by Asakura and Oosawa (10, 11) of Nagoya University in Japan. According to them, when two particles approach each other in a polymer solution to a distance of separation that is less than the diameter of polymer molecules, polymer may be extrudedfromthe inter-particle space. This leads to a polymer-depleted-free zone between two particles. An osmotic force is then exerted from the polymer solution outside the particles and this results in flocculation. It is likely that the flocculation of Ti0 dispersion observed in the high surfactant concentration is caused by the depletion effect offreesurfactant molecules in this solution. A number of theories derived to explain the mechanism of depletionflocculationwere reviewed by Napper in his textbook (6). 2

Fig. 4. Model for calculation of osmotic attraction between two particles containing adsorbed layer in a solution of spherical surfactant molecules.


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Conclusions


The following conclusions were obtained in this work. 1.

Destabilization of TiO in low-surfactant-concentration solutions (0.05 - 0.2 %) is due to the reduction of electrical repulsion between the particles caused by the reduction of negative charge on the particles.

2.

Stabilization in medium-surfactant-concentration solutions (0.2 1.0 %) is due to the increase in electrical repulsion caused by the increase in positive charge on the particles. The results are well accounted for by the DLVO theory.

3.

Destabilization in high-surfactant-concentration solutions (1.03.0 %) is likely due to the depletion effect caused by free (nonadsorbing) surfactant molecules.

2

References 1. 2.

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

Sato, T; Kohnosu, S.; J. Colloid Interface Sci., 1991, 143, 434. Sato, T.; Application in Agriculture in "Electrical Phenomena at Interface" Ed. Ohshima, H.; Furusawa, K.; Marcel Dekker, Ν. Y. 1998. Sato, T.: News Letter (Chemical Soc. Japan) 1996, 21(4), 8. Sato, T.; Ruch, R.; " Stabilization of Colloidal Dispersions by Polymer Adsorption", Marcel Dekker, Ν. Y., 1980. Sato, T.; J. Coatings Technology, 1993, 65, 113. Napper, D. H.; "Polymeric Stabilization of Colloidal Dispersions ", Academic Press, London, 1983. Sato, T.; J. Applied Polymer Sc., 1971, 15, 1053. Sato, T.; J. Applied PolymerSci.,1979, 23, 1693. Sato, T.; Sieglaff, C. F.; J. Applied Polymer Sci., 1980, 25, 1781. Oosawa, F.; Asakura, S.; J. Chemical Physics; 1954, 22, 1255. Asakura, S.; Oosawa, F.; J. Polymer Sci., 1958, 33, 183.


Effects of Surfactant Concentration on Stability of Dispersion

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