Hydrophilic porous polyvinylidene fluoride (PVDF) membranes (MILLIPORE, DURAPORE® MEMBRANE FILTERS, Millipore Corporation, Billerica, MA, USA, average micropore diameter 0.22 μm) were used as the support. CYPHOS IL-101, CYPHOS IL-102, CYPHOS IL-104, CYPHOS IL-109, CYPHOS IL-111, Aliquat 336 were the ionic liquids used. All the CYPHOS ionic liquids were purchased from STREM CHEMICALS, Newburyport, MA, USA. The molecular structure and properties of all these ionic liquids have been tabulated in Table 1. Concentrated hydrochloric acid and concentrated sodium hydroxide (Wako Pure Chemicals, Ltd., Osaka, Japan) were used for preparing the permeate solution and for adjusting pH of feed phase, respectively.

A PVDF membrane was used as the support to maintain the ionic liquid in the membrane. A hydrophilic PVDF membrane was soaked in a particular ionic liquid in a petri dish. These membranes were left in soaked state for 24 h. The membrane was then wiped and was dried in the vacuum desiccator for 24 h. After that the membrane was weighed. These membranes were used as the supported liquid membrane based on ionic liquid for the transport experiment.

The apparatus consisted of two cells made of glass clamped together and placed on a magnetic stirrer as shown in Figure 1. The supported liquid membrane was put in between the two cells using vacuum grease (silicon grease) to avoid slipping and/or leakage.

The effective area (A) of this device is 12.5 cm². The feed solution consisted of 0.01 M lactic acid with sodium hydroxide to maintain the initial pH approximately equal to 5.5. The permeate (receiving) phase was a 0.1 M aqueous solution of hydrochloric acid. The transport experiment for the SLM, based on the ionic liquid, was started by pouring 100 mL of feed and 100 mL of permeate solutions into their respective glass cells. A magnetic stir bar was used to constantly stir the feed and receiving solutions at 300 rpm throughout the experiment. The pH of feed side was checked using a pH meter (HORIBA) at intervals of 1 h. For the normal experiments (without pH control), the experiment was allowed to proceed freely without any intervention. For the pH control experiments, dilute sodium hydroxide solution was added to maintain the desired pH level, i.e., 5.5. Thus, pH was monitored throughout the experiment at intervals of 2 h for transport experiments without pH control and at intervals of 1 h for transport experiments with pH control. The permeation experiment was stopped after 24 h and the membrane was weighed again.

The concentration of lactic acid in the feed and permeate phases were measured by drawing 0.5 mL samples at intervals of two h and using HPLC (Shimadzu LC-10ADvp) with an RI detector (Shimadzu RID-10A) to analyze the samples. Analysis was performed using a Shodex SUGAR SH-1011 (Showa Denko) column and 5 mM H₂SO₄ solution as the mobile phase.

Because the viscosities of CYPHOS IL-101, CYPHOS IL-102 and Aliquat 336 when exposed to water were required, water-saturated ionic liquids were prepared. Their viscosities and water contents were measured by an Ostwald viscometer and Karl Fisher titration, respectively. Their measured results were also listed in Table 1.

Liquid–liquid extraction experiments were performed to determine the distribution of lactic acid in ionic liquid phase. 100 mL of 0.01 M lactic acid solution was prepared and its pH was adjusted close to 5.5 by adding NaOH. 0.5 mL of CYPHOS IL-101 and 0.5 mL of the lactic acid solution prepared as mentioned above was poured into the same safe lock tube. These safe lock tubes were shaken in a thermomixer (Eppendorf Thermomixer Comfort) for 2 h followed by centrifuging at 4000 rpm for ten minutes in a centrifuge (Eppendorf Centrifuge 5415 D). The mixture had been separated into two distinct phases. The aqueous phase in each safe lock tube was carefully transferred using a micropipette to a separate vial for HPLC analysis.

The membrane weights were recorded before and after the transport experiment for the ionic liquids under study and tabulated in Table 2. The data from Table 2 indicates that Aliquat 336, CYPHOS IL-101 and CYPHOS IL-102 show hardly any loss of ionic liquid from the membrane pores over a period of 24 h. This is in agreement with the established fact that ionic liquids are not easily displaced from the pores under a cross-membrane pressure difference as they are immobilized by large van der Waals forces and due to their high viscosities. However, CYPHOS IL-104-based SLM showed a slight decrease in membrane weight and white streaks appeared on the membrane at the end of 24 h. CYPHOS IL-109 and CYPHOS IL-111 based SLMs showed a considerable loss of ionic liquid from the membrane and this was reflected in the decrease of membrane weight up to 30%. Since CYPHOS IL-104, -109 and -111 are less viscous, it is conceivable that the displacement of these ionic liquids from the membranes could be higher. Furthermore, they have more hydrophobic anion than the other ionic liquids. As is reported previously [20], the stability of SILM has been related with the hydrophobic/hydrophilic character of the ionic liquid, concluding that a better stabilization is obtained by using hydrophilic membranes support due to the hydrophilic nature of ionic liquids.

The permeation rate (P [%]) was calculated by the following formula: (1) P [ % ] = C R , 24 h C F , 0 × 100where C_{R,24 h} is concentration of lactic acid in receiving phase after 24 h and C_F,0 is concentration of lactic acid in feed phase initially.

The typical time courses of concentrations of lactic acid and pH in feed and permeate phases in the absence pH control are shown in Figure 2 using Aliquat 336 as an ionic liquid. A general observation is that the concentration of lactic acid in the feed phase decreases while that in the receiving phase increases with time. This is because lactic acid permeates through the membrane from the feed side into the permeate side. Another observation in the absence of pH control is that the pH of feed phase tends to decrease whereas the pH of the receiving phase does not change significantly over a period of 24 h. This decrease of pH on feed side is because hydrochloric acid permeates through the membrane from the permeate side into the feed side.

It is known that lactic acid, being a weak acid, dissociates to permeate species, lactate anion, at higher pH level. Therefore, to maintain the high pH of the feed phase, the experiments under the pH control of the feed phase was conducted and the result was also shown in Figure 2. The permeation rate was found to have improved as expected. The permeation rates of lactate through the SLM based on the ionic liquids in the absence and presence of pH control were summarized in Table 3, along with the pH value of feed phase after 24 h.

In the cases of transport experiment with SILM based on CYPHOS IL-101 and -102 without pH control, the pH remained above 4.1 in feed phase as such. In other words, a higher concentration of lactate ions was already available in the feed phase for diffusion across the membrane (pK_a of lactic acid = 3.86). Thus, even under pH control conditions, a significant increase in the permeation rate was not observed. CYPHOS IL-102-based SILM gave the highest permeation rate among all the ionic liquids studied. Lactate scarcely permeated through pure CYPHOS IL-104 membrane because CYPHOS IL-104 cannot extract lactate [21]. As shown later, it may be difficult to form CYPHOS IL-104–H₂O complex as the acid carrier due to the hydrophobicity of the anion moiety. In the cases of CYPHOS IL-109 and -111, the experiments with pH control failed due to the leak of ionic liquids from the membrane.