Determination of critical micelle concentration of sodium dodecyl sulfate in butyl acrylate emulsions

Abstract

A series of monodispersed (PDI <0.05) butyl acrylate emulsions with different sizes (100–500 nm) and solid contents (22, 17, 12 and 7 wt%) were prepared using a semibatch seeded emulsion polymerization method. Sodium dodecyl sulfate (SDS) was used as an emulsifier and potassium persulfate as an initiator. Surface tensions of the emulsions were investigated and the relationship of the emulsion stability, particle size and solid content was examined. The critical micelle concentration (CMC) of the emulsifier SDS was determined by the intersection of two tangents to surface tension curves. We found that the surface tension at CMC as high as 65.99 mN/m for the emulsion with a solid content of 22 % and a particle size of 115 nm, very close to the value of it for pure water. The surface tension decreased from 65.99 to 54.74 mN/m with an increase in particle size from 115 to 494 nm concomitantly with a decrease in the CMC of SDS from 56.10 to 19.83 mmol/l for the emulsion with a solid content of 22 wt%. As to the emulsions with the same particle size, CMC value was positively correlated with solid content. The minimum value of CMC was 9.12 mmol/l for the emulsion with a solid content of 7 % and a particle size of 494 nm, whereas the maximum CMC was 56.10 mmol/l for the emulsion with a solid content of 22 wt% and a particle size of 115 nm. As a result, an equation describing the relationship between CMC and solid content was deduced. This model yielded the same trends as the experimental results.

Appendix

When the concentration of emulsifier reaches CMC equilibrium, the total number of emulsifier molecules is the sum of the number of free molecules and adsorbed molecules, that is:

$$\frac{m}{{M_{0} }} \times N_{\text{A}} = a + b$$

(7)

where m is the mass of emulsifier, M ₀ is molar mass of emulsifier, a is the number of free molecules, b is the number of adsorbed molecules.

We can solve the value of a and b, respectively, are

$$b = a_{0} \times A$$

(8)

where a ₀ is the adsorption density, A is the total surface area of particles, $A = \alpha .\pi .D^{2}$, where α is the number of particles and D is the effective grain size.

Assumes that the experiment uses 100 ml of the emulsion, at CMC point, C = C ₀, thus,

$$a = C_{0} \times V_{\text{water}} \times N_{\text{A}} = C_{0} \frac{{\left( {100 - \frac{1}{6}\alpha \cdot \pi \cdot D^{3} } \right)}}{{M_{ 0} }}N_{\text{A}} .$$

(9)

So, substituting Eqs. (8) and (9) into (7), we get

$$nN_{\text{A}} = C_{0} \frac{{\left( {100 - \frac{1}{6}\alpha \cdot \pi \cdot D^{3} } \right)}}{{M_{0} }} \times N_{\text{A}} + a_{0} \cdot \alpha \cdot \pi \cdot D^{2} .$$

(10)