Factors Affecting Phase Inversion

It is apparent from the literaturethat the phase inversion in nSOW systems has several controlling factors. Among these factors the following can be identified  

The first five factors affect the surfactant s affinity while the last two factors are dynamic variables. Various techniques and concepts have been used to correlate surfactant affinity variables and hence the emulsion type these are described in chronological order below. 

Full descriptions of the hydrophile-lipophile balance (HLB) concept are given by Becher and Becher and Schick. Griffin first defined the affinity of a nonionic surfactant in terms of an empirical quantity, the HLB. Surfactants are assigned an HLB number at 25 °C on a scale of 1 to 20, where low HLB numbers represent lipophilic surfactants and high HLB numbers represent hydrophilic surfactants. Generally, the application of a surfactant can be derived from its HLB number in accordance with Table 6.1.  

HLB numbers are calculated for a surfactant from simple formulae based either on analytical or composition data. For polyoxyethylene nonylphenyl ethers (NPE), HLB = E/S where E = the weight % of polyoxyethylene in the surfactant. For example, for NPE 12 (12 oxyethylene groups in the hydrophilic chain), HLB = 14.2. For a polyhydric fatty acid ester, HLB = 20(1 - S/A), where S — saponification number of the ester and A — acid number of the fatty acid. For example, for polyoxyethylene(20) sodium monolaurate (trade name Tween 20), S = 45.5 and A = 276 hence HLB = 16.7. 

Attempts have been made to assign HLB numbers to various oils to predict which surfactants will produce the most stable emulsion these are called the required HLB of the oil (see Table 6.2). 

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Factors affecting the inversion locus. The central A /A boundary depends on the position of the optimum formulation transition. However, it does not always correspond to a straight line crossing the three-phase region. In systems which exhibit wide three-phase regions (vertically) and a narrow A region (horizontally) there is no neat plateau. The position of the lateral branches of the inversion locus may depend on surfactant type and alcohol formulation and the oil viscosity may also alter and shift the locus.""... 

It has been shown (, , 2.) that a membrane casting dope is a strongly structurlzed polymer solution, and that the morphology of the membrane surface layer can be correlated to the structure of the casting solution. The latter parameter affects the nature and details of the phase inversion process occuring in the upper part of the cast solution, in an incipient skin. Thus the solution structure is one of the factors responsible for the skin properties, and consequently for the performance of the ultimately formed asymmetric membrane. 

Coalescence, not dispersion, dominates as the controlling mechanism in phase inversion. Factors affecting film drainage rates, such as agitation rate. 

Phase inversion in emulsions can be one of two types (i) a transitional inversion, induced by changing factors that affect the HLB of the system, such as temperature and/or electrolyte concentration and/or (ii) a catastrophic inversion, which is induced by increasing the volume fraction of the disperse phase. 

In summary, the results show that a small variation in the nature (i.e., polarity and functionality) of the diamine or the epoxidized triglyceride oil leads to a big difference in thermoset morphology in terms of particle-size distribution (Table V) and phase inversion (Table VI). In addition to the nature of the diamines and the oils, other factors, such as their reactivity, are expected to influence the phase-separation process. Although we do not have data, a small difference in the cure rates of the DDM formulations at 75 °C and the DDS formulations at 150 °C might affect both the particle-size distribution and phase inversion. 

It was a century ago that researchers started to study the factors affecting the behaviour of water-oil-surfactant systems but it is only with the introduction of Winsor s R theory (1954) that the formulation effects could be interpreted. Winsor s R theory was the first qualitative description of the formulation, paving the way to an understanding of how intermolecular interactions among the different chemical species present in a system are related to its behaviour. Throughout the following decades, several empirical experimental correlations such as the phase inversion temperature (PIT), semiempirical ones such as the cohesive energy ratio (CER), and models based on thermodynamics such as the surfactant affinity difference (SAD) or the hydrophilic-lipophilic deviation (HLD) [15, 143, 144] led... 

As can be seen from the above description, there are many variables involved in the phase-inversion technique. Among others the composition of the polymer solution, the solvent evaporation temperature and evaporation period, the nature and the temperature of the gelation media, and the heat treatment temperature are the primary factors affecting the reverse osmosis performance of the membrane. When polymers other than cellulose acetate are used, solvents and nonsolvent additives appropriate to prepare membranes from the particular polymer must be found. Depending on the combination of variables, membranes of different polymeric materials with different pore sizes can be prepared. 

Despite the fact that there has been extensive study on the preparation and characterizations of flat-sheet PVDF membranes [16-20] fabricated by NIPS, limited studies have been devoted to the fabrication and characterization of PVDF hollow-fiber membranes [21-24]. It is well accepted that the phase inversion process of hollow-fiber membranes is much more complex than that of flat-sheet membranes, and the controlling factors for the former during membrane formation are distinctly different from those for the latter. One essential difference is the dope formulation that affects the dope viscosity. Usually, a polymer dope possessing a viscosity of... 

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