Shahriar Sajjadi

Dynamic behaviour of drops in oil/water/oil dispersions

Abstract The dynamic behaviour of drops of oil/water/oil (O/W/O) and water/oil (W/O) in abnormal polymer/water/surfactant systems was investigated. The size of internal oil droplets continuously decreased with time until it reached a steady-state value. Whereas the size of multiple water drops showed a minimum. After the minimum, the size of multiple water drops either reached a steady-state value or continued enlarging until phase inversion occurred. The phase inversion occurred because inclusion of oil droplets into water drops resulted in a continuous increase in effective volume fraction of dispersed phase. The time evolution of the size of multiple drops was described in terms of a balance between (a) drop break-up and escape and (b) drop coalescence and inclusion. The inclusion events retarded the initial decrease in the size of multiple water drops with time and increased the drop size after the minimum. By reducing the surfactant concentration, the ability of the dispersed phase to entrain the continuous phase decreased so that no minimum was achieved for the size of multiple drops with time, similar to conventional systems with simple drops. The size distribution of the multiple water drops initially narrowed and then widened again, whereas the size distribution of internal oil droplets continuously narrowed with time until it reached a constant value. Generally, the size distribution of drops narrowed as the average size of drops decreased. The possible mechanisms for complex drop formation were discussed and drop deformation was suggested as the main cause for inclusion at a low dispersed phase ratio.

On the growth mechanisms of nanoemulsions

The shelf stability of nanoemulsions made by ultrasound, phase inversion composition, and the Ouzo effect was studied using a range of hydrocarbons, as the model oils, and surfactants. The cube of the average drop radius of the nanoemulsions displayed a linear increase with time. Both Ostwald ripening and coalescence can exhibit such behaviour. A new approach, based on the time evolution of drop size distribution, is proposed for unravelling the aging mechanism of nanoemulsions. Sequences of fall and rise in the average drop size of nanoemulsions were clearly observed. The decrease in the drop size could unambiguously be attributed to Ostwald ripening, but the increase could be due to either Ostwald ripening or coalescence/flocculation. Coalescence was identified as the dominant growth mechanism at low surfactant concentrations evidenced by drop size distribution broadening with time associated with the rise in the average drop size. Ostwald ripening was the dominant mechanism at higher surfactant concentrations where the drop size distributions broadened with time during the falls and narrowed with time during the rises of the average drop size. The nanoemulsions produced via the Ouzo process, displayed a coalescence-dependent transient stage and an Ostwald ripening dominated asymptotic regime in the absence of surfactant. The nanoemulsion produced via phase inversion was found to be the most stable one, however, still showed vulnerability to Ostwald ripening and flocculation in the long term.

Unseeded semibatch emulsion polymerization of butyl acrylate: Bimodal particle size distribution

Unseeded semibatch emulsion polymerization of butyl acrylate (BA) using sodium lauryl sulfate as emulsifier and potassium persulfate as initiator was carried out at the conditions where secondary nucleation was probable. This was achieved by using no emulsifier in the initial reactor charge. The effects of changes in monomer emulsion feed rate, initiator concentration and distribution, emulsifier concentration in the feed, and temperature on the evolution of particle size averages and distribution were investigated. Bimodal particle size distributions (PSD) were obtained for most of the latexes. Inhibition effects were found to be important in the development of PSD. Primary particle formation occurred through micellar nucleation, whereas secondary nucleation probably occurred through homogenous nucleation. The polydispersity index (PDI) of the latexes increased with the decreasing monomer emulsion feed rate. The application of a larger amount of initiator to the reactor charge or using a higher temperature, reduced the formation of secondary particles and resulted in a formation of an unimodal PSD. The overall steady-state rate of polymerization was found to approach the rate of monomer addition (Rp ≈ Ra ), if the emulsifier concentration in the aqueous phase was appreciable. This is different from the correlation 1/Rp = 1/K + 1/Ra obtained for the BA semibatch process with neat monomer feed. This suggests that different rate expressions can be used for BA semibatch emulsion polymerization at different conditions.

Semibatch Emulsion Polymerization of Methyl Methacrylate with a Neat Monomer Feed

Semibatch emulsion polymerization of methyl methacrylate (MMA) with neat monomer feed in the presence of sodium lauryl sulphate as emulsifier and potassium persulphate as initiator was investigated. The effects of the monomer feed rate (R a ), the emulsifier concentration, and the monomer distribution ratio between the initial charge and feed on the kinetic features of MMA emulsion polymerization in a semibatch reactor were studied. Under monomer‐starved conditions particles did not undergo an appreciable growth during polymerization and as a result a large number of particles were formed. The number of particles increased significantly as R a decreased. The number of polymer particles formed under starved conditions showed an exponent of − 1.98 against R a , which is larger than the value of − 0.67 that is theoretically obtained for the styrene monomer. The average molecular weights decreased and the molecular weight distribution became narrower with decreasing R a . The steady state rate of polymerization (R pss ) was confined to the correlation of the type R pss ≈ R a when a high concentration of the emulsifier was used. The rate of polymerization under monomer‐starved conditions was found to depend on the size of polymer particles formed. The experimental results suggest that models, which can take account of more specified size‐dependent kinetic parameters, should be developed for prediction of kinetic behavior of MMA in semibatch reactors.

Phase inversion in p-xylene/water emulsions with the non-ionic surfactant pair sorbitan monolaurate/polyoxyethylene sorbitan monolaurate (Span 20/Tween 20)

Abstract Phase inversion behaviour of a model oil–water emulsion, p-xylene–water, with Span 20/Tween 20 as a surfactant set was investigated. The surfactant hydrophilic-lipophilic balance (HLB), p-xylene/water ratio, and the addition policy were varied for different surfactant concentrations. The morphology of the resulting emulsions was recorded, and maps for different surfactant concentrations were developed. The ‘transitional’ inversion was irreversible and the inversion boundary was found to be discontinuous and was only observed at a high dispersed-phase ratio. No transitional inversion was recorded for the middle range of dispersed phase ratio whatever the type of emulsion was. The transitional inversion boundaries became wider with increasing surfactant concentration. An interaction between catastrophic and transitional inversion boundaries was found for the Span 20/Tween 20 system. The transitional inversion occurred only if, according to the catastrophic inversion boundaries, there is only one morphology possible for the inverted emulsion. The discontinuity in the transitional inversion boundary and the fact that its existence depended on the direction of HLB change and water/oil ratio are uncharacteristic of conventional transitional inversions. However, the drop size and interfacial-tension varied with the surfactant HLB and both decreased to a minimum value at the potential inversion boundary, even though the inversion did not occur, similar to the trend generally observed for a conventional transitional phase inversion. The results suggest that a minimum in drop size, together with a minimum in the interfacial tension, does not necessarily lead to a phase inversion.

Butyl acrylate batch emulsion polymerization in the presence of sodium lauryl sulphate and potassium persulphate

The batch emulsion polymerization of butyl acrylate in the presence of sodium lauryl sulphate as emulsifier and potassium persulphate as initiator was investigated. The effects of emulsifier concentration, initiator concentration, and monomer/water ratio on the kinetic features were studied. The kinetic data showed that at the conditions studied, the number of particles is proportional to [KPS]0.39 and [SLS]0.54. The number of particles did not practically vary with monomer concentration at the high range of monomer and emulsifier concentrations. At low emulsifier concentration, particle coagulation occurred in the course of reaction, which increased with monomer concentration. Particle nucleation was found to occur during Interval III of the batch process if undissociated micelles exist. It was also confirmed that the zero-one kinetics system can better fit the experimental results, compared to the pseudobulk kinetics.

Semibatch emulsion polymerization of butyl acrylate. I. Effect of monomer distribution

The effects of initial monomer charge on the particle formation and on the rate of polymerization were investigated for semibatch emulsion polymerization of butyl acrylate using sodium lauryl sulfate (SLS) as a surfactant and potassium persulfate (KPS) as an initiator. For the semibatch process with monomer (M) feed, it was found that by varying the monomer distribution ratio between the initial reactor charge and the feed it is possible to alter the contribution of monomer-flooded and monomer-starved nucleation mechanisms to the whole nucleation process. The number of particles increases as the initial monomer charge decreases, if the monomer concentration is below a critical value for any fixed system. The increase in number of particles is associated with a broad particle-size distribution which might depict an emerging second peak on the particle-size distribution curve. For low emulsifier concentration systems, a larger number of particles was obtained for a lower amount of monomer charge. Particle coagulation and emulsifier adsorption on the monomer droplets were counted as the main reasons for such behavior. For a semibatch process with monomer emulsion (ME) feed, the larger number of particles was formed at a lower initial monomer charge, similar to an M-add semibatch process. However, the application of monomer charge to an ME-add process was found to increase the possibility of secondary nucleation and led to the occurrence of a bimodal particle-size distribution.

Phase behaviour of polymer emulsions during the phase inversion process in the presence of non-ionic surfactants

Abstract A phase inversion map for polyisobutene–water–nonionic surfactant system showing the behaviour of the emulsion system as a function of the hydrophobic–lipophilic balance (HLB) and water volume fraction is developed. The apparatus and the different experimental techniques used to characterise the emulsion system is described. It was found that, using information from the phase inversion map, it is possible to produce stable polymer emulsions with sub-micron polymer drops via the transitional inversion route. The drop size distribution before transitional inversion was wide, while after transitional inversion it was much narrower. An average drop size of ∼200 nm was obtained for a mixture of polyoxyethylene nonylphenyl ether (NPE) surfactants (5% overall concentration by weight). It is, also, possible to produce small polymer drops via stable catastrophic inversion as the emulsion becomes more concentrated. The unstable multiple emulsion drops were found to decrease in size as we get closer to the transitional inversion line.

Phase behaviour of non-ionic surfactant–p-xylene–water systems during the phase inversion process

Abstract The present paper describes briefly the apparatus and the different experimental techniques used to characterise the p -xylene–water–surfactant dispersion system. Phase inversion maps for the p -xylene–water–non-ionic surfactant system were developed. It was found that, contrary to what was previously thought, the reduction in drop size could occur during both ‘stable catastrophic’ and ‘transitional’ inversions. The presence of multiple emulsions was also investigated. It was found that multiple emulsions present before phase inversion could, after phase inversion, form single emulsions and vice versa. For the polyoxyethylene sorbitan monolaurate (SML) surfactants used (Tween20 and Span20), transitional inversion could occur either at low volume fraction or high volume fraction, leaving a gap in the middle range of water volume fraction where inversion did not occur. The gap reduced with increasing SML concentration. It was also found that the transitional inversion is not reversible when the blend Tween20/Span20 is used. For the polyoxyethylene nonylphenyl ether (NPE) surfactant system (Igepal co720/Igepal co520), transitional inversion occurred in the range of medium to high water volume fraction. The range where transitional inversion occurred, broadened when NPE concentration was increased. The application of a mixed non-ionic surfactant model to the NPE and SML systems was also investigated.

Semicontinuous monomer-starved emulsion polymerization as a means to produce nanolatexes: analysis of nucleation stage.

In this research, particle nucleation was decoupled from particle growth in the monomer-starved semicontinuous (micro)emulsion polymerization of styrene by close monitoring of the end of nucleation. This enabled us to exclude the effects of particle growth on nucleation and therefore unravel inherent features of nucleation in this process. Nanolatexes with average particle sizes as small as 15 nm were obtained. The average size of particles at the end of nucleation was found to be almost independent of surfactant concentration ([S]) but varied with the rate of monomer addition (Ra) to the 1/3 power. Nucleation time varied almost proportionally with [S](1.0). The sharpest particle size distribution was obtained at the lowest monomer feed rate used. The weight-average molecular weights (M̅w) of the polymer produced decreased with decreasing Ra. A simple correlation was developed which shows that the number-average molecular weight (M̅n) is proportional to Ra(1.0) but independent of [S] (i.e., [S](0.0)), which was in fair agreement with the experimental results. It is also shown that the polymer molecular weight is proportional to the average volume of particles; the smaller the particle, the lower the molecular weight.