## 3 Results and Discussion  
### Qualitative Liquid-Liquid Equilibria  
Due to the presence of both carboxylic and alcohol functional groups in the lactic acid molecule, it is anticipated that the ternary systems considered in this study will behave differently compared to the linear aliphatic short chain carboxylic acids. Thus, the initial phase of the study focused on observing the behavior (i.e., ability to form two-phase region) of the ternary systems under study through visual observation. This initial assessment determines the suitability of the non-ionic surfactant for the targeted extractive separation process. The qualitative phase equilibrium observations were conducted for at least a month. Based on the observations, the two-phase region or phase envelope was estimated for each ternary system and the results are presented in Figure 1. The suitability of the surfactant as solvent for lactic acid extraction can be initially assessed by the type of the ternary system. The results indicated that all the systems studied belong to the Type 1 Treybal classification of ternary systems [35], with the water-surfactant binary pair exhibiting partial immiscibility. It is important to note that all the surfactants used in this study are mixtures containing different lengths of the alcohol component. However, the surfactants were treated as a single compound and thus, all the systems studied can be considered as pseudo-ternary systems. The results showed that AE and AA1 (Figure 1a, b) are the two surfactants that could possibly be used for the extraction of lactic acid from aqueous mixtures based on the size of the estimated phase envelope. The size of this two-phase region dictates the region where the liquid-liquid extraction can be strategized and consequently determines operational parameters (e.g., amount of solvent needed for extraction and the number of extraction stages needed). As Figure 1c shows, AA2 is not applicable for the targeted lactic acid separation process due to a very small phase envelope. It is well established that water-nonionic surfactant binary systems at temperatures between 273.15-373.15 K exhibit a closed loop miscibility gap (that typically has a vertical oblong shape) with a lower and upper critical point [36]. The lower critical point is also known as the cloud point which is the temperature at which the water-miscible surfactant comes out of the solution as a result of decreased H-bonding between water and surfactant molecules [37]. The size of this miscibility gap with respect to concentration is represented by the length of the water-surfactant binary tie-lines in Figure 1. According to the manufacturers, the surfactants used in this study have cloud points of 327.15 K, 288.15 K, and 290.15 K for AE, AA1, and AA2, respectively. The differences can be attributed to the alkyl chain length of the surfactants, which has direct correlation with cloud point (i.e., cloud point increases with the alkyl chain length) [38]. Accordingly, these suggest that at 308.15 K, the phase envelope of the AE ternary system can be attributed mainly to HLB. Conversely, the phase envelope for the AA ternary systems was due to both the HLB and cloud point of the surfactants. The shorter length of the miscibility gap (thus smaller phase envelope) for AA2 could indicate that the system has either an intrinsically narrower loop compared to the other surfactants or the temperature of 308.15 K is close to the upper critical temperature of the loop. For the latter case, the length of the miscibility gap could be increased by decreasing the extraction temperature. Consequently, this could increase the size of the phase envelope of the system that could lead to a better extraction process. However, since the focus of this work is on the extractive fermentation of lactic acid at 308.15 K, AA2 cannot be used as solvent due to the small phase envelope of its ternary system.