## 2 Results and Discussion
### Surface Tension  
Figure 8 shows the variation of latex surface tension (\(\gamma\)) with \(t_{r}\). The behavior of \(\gamma\) in both polymerizations is similar, that is, values close to 35 mN/m up to \(t_{r}\approx 0.8\), followed by an increase up to \(\approx 45\) mN/m at \(t_{r}=1\), attaining a value close to 55 mN/m at the end of the polymerization.  
The \(\gamma\) value close to 35 mN/m suggests the presence of micelles [24] up to at least \(t_{r}\approx 0.8\), while the higher values indicate that micelles have disappeared and all surfactant in the latex is on the nanoparticles' surface. In accordance with Figure 3, particle nucleation occurs up to approximately 80% in global conversion for the polymerizations at lower dosing rate and up to 90%, for the other one. Undoubtedly, the presence of micelles up to at least \(t_{r}\approx 0.8\), which corresponds to 60%-65% global conversion, is the determinant factor for the particle formation. Therefore, it would be interesting to know why particle formation occurs at higher conversions. In fact, according to Figure 8, the  
Figure 8: Surface tension of latex as a function of relative time in polymerizations at 0.05 (**a**) and 0.15 (**b**) g/min in organic dosing rate. Runs M (\(\blacklozenge\)); runs MR (\(\blacktriangle\)).  
disappearance of micelles not necessarily takes place at \(t_{r}\approx 0.8\) for both polymerizations, but this event occurs in a point between \(t_{r}\approx 0.8\) and \(t_{r}=1\), so most probably the micelles are present up to near \(t_{r}=1\), allowing the particle formation up to that point. To explain why particle coalescence occurs at the end of the polymerizations the surface coverage ratio, \(r\), defined as the total surface of particles that the available surfactant would cover forming a saturated monolayer divided by the total surface area of the particles at \(t_{r}=1\)was calculated. For this task data from the recipe and the values of \(D_{n}\), \(N_{p}\), \(X\), density of the porous particle and the area per SDS molecule in a saturated monolayer on particle surface (0.5 nm\({}^{2}\)/molecule) [25] were used. The \(r\) values for M5 and M5R were 0.56 and 0.54, respectively, for the polymerizations carried out at the higher dosing rate, while the polymerizations at the lower dosing rate showed the same value of 0.55 for M5 and M5R runs. The magnitude of these \(r\) values indicates that the particles are covered by a layer of surfactant with approximately half of the number of molecules required for saturation. This implies a certain degree of particle instability, which would result in the particle coalescence observed at the end of the polymerization. The low \(r\) values and the fact that they are practically the same for both dosing rates could also explain why a smaller particle size at the lower dosing rate was not achieved. What probably occurs is that a value close to 0.55 is the lowest value that \(r\) can achieve under the conditions at which the polymerizations were carried out and since smaller particle sizes require lower \(r\) values, an average particle close to 28 nm is the minimum particle size that can be attained. In fact, the \(r\) values for the particles obtained at the end of the polymerizations increase up to 0.6-0.7, which would result from particles that coalesce trying to reduce the area to volume ratio and thereby increase their stability.