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1. Porous hydrophobic-hydrophilic composite membranes for direct contact membrane distillation

   https://doi.org/10.1016/j.memsci.2019.117225  

   Puranik, A.A.; Rodrigues, L.N.; Chau, J.; Li, L.; Sirkar, K.K. (July 2019, Journal of membrane science)

   Direct contact membrane distillation (DCMD) for desalination is attractive for high salt concentrations if low cost steam/waste heat is available and waste brine disposal cost for inland desalination is factored in. A number of innovations have taken place in DCMD in terms of the structure of the porous hydrophobic membrane. Composite membranes are of increasing interest. Composite membrane structures of great interest include a thin hydrophobic porous layer over a porous hydrophilic layer of polyvinylidene fluoride (PVDF) or a thin porous hydrophobic layer over a more conventional hydrophobic porous membrane. These membranes can be in the form of an integral composite or a stacked composite or a laminated composite. A facile method of fabricating such integral composite membranes is plasma polymerization under vacuum. A class of such membranes yielding quite high water vapor fluxes have been characterized using a variety of characterization techniques: Contact angle, liquid entry pressure (LEP), bubble-point pressure, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM). Stacked composites of a hydrophobic ePTFE membrane over a hydrophilic PVDF membrane or a hydrophobic PVDF membrane over another hydrophobic PVDF membrane were also studied. Novel conditions created lead to very high water vapor fluxes compared to those from conventional hydrophobic membranes supported on a mesh support. 

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2. Porous hollow fiber membrane-based continuous cross-flow anti-solvent crystallizer for production of API nanocrystals

   https://doi.org/10.1016/j.seppur.2025.134331  

   Song, Yufeng; Feng, Lixin; Zhong, Wei; Tripathi, Siddharth; Ananthoji, Padmini K; Armenante, Piero; Sirkar, Kamalesh K (July 2025, Separation and Purification Technology)

   Bruggen, Bart V (Ed.)

   In porous hollow fiber membrane (HFM) based anti-solvent crystallization (AsCr) technique, the anti-solvent is injected from HFM bore into the active pharmaceutical ingredient (API) containing solution flowing in the shell side. Conventionally, shell-side feed solution flows along the HFM. In our compact module design, the shell-side liquid flows perpendicular to the HFMs; the mean flow length is ~ 3 cm reducing considerably the residence time (tres) which strongly influences crystal growth. We investigated AsCr of griseofulvin (GF) from its acetone solution using anti-solvent water injected through HFM pores. The range of tres was 5–30 s; the volume flow rate ratio (FRR) of acetone/water ranged between 0.35 and 1.66; griseofulvin concentration was varied between saturation (3.73 g/100 g acetone) to 0.4 g GF in 100 g of solution having 50 wt% acetone in the total solution. Low tres yielded nanocrystals < 100 nm. The average API crystal size (Davg) remained around 230 nm in a three hour- long experiment. At a fixed mass FRR of 1.34 ±0.1, Davg increased linearly from 48 to 280 nm as tres increased from 5 to 28 s. For two HFM modules in series with the nanocrystal suspension from one module fed to the next module independently fed with anti-solvent water, GF crystallization yields as high as 98 % were achieved. AsCr of L-glutamic acid in water with anti-solvent acetone in the same device illustrated continuous nanocrystal production and similar behaviors vis-`a-vis variations of tres and FRR. The cross-flow HFM based continuous AsCr technique can continuously produce nanocrystals of APIs. 

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3. Porous hollow fiber membrane-based continuous cross-flow anti-solvent crystallizer for production of API nanocrystals

   Song, Y; Feng, L; Zhong, W; Tripathi, S; Ananthoji, Padmini K; Armenante, P; Sirkar, Kamalesh K (July 2025, Separation and purification technology)

   Bruggen, Bart V (Ed.)

   In porous hollow fiber membrane (HFM) based anti-solvent crystallization (AsCr) technique, the anti-solvent is injected from HFM bore into the active pharmaceutical ingredient (API) containing solution flowing in the shell side. Conventionally, shell-side feed solution flows along the HFM. In our compact module design, the shell-side liquid flows perpendicular to the HFMs; the mean flow length is ~ 3 cm reducing considerably the residence time (tres) which strongly influences crystal growth. We investigated AsCr of griseofulvin (GF) from its acetone solution using anti-solvent water injected through HFM pores. The range of tres was 5–30 s; the volume flow rate ratio (FRR) of acetone/water ranged between 0.35 and 1.66; griseofulvin concentration was varied between saturation (3.73 g/100 g acetone) to 0.4 g GF in 100 g of solution having 50 wt% acetone in the total solution. Low tres yielded nanocrystals < 100 nm. The average API crystal size (Davg) remained around 230 nm in a three hour- long experiment. At a fixed mass FRR of 1.34 ±0.1, Davg increased linearly from 48 to 280 nm as tres increased from 5 to 28 s. For two HFM modules in series with the nanocrystal suspension from one module fed to the next module independently fed with anti-solvent water, GF crystallization yields as high as 98 % were achieved. AsCr of L-glutamic acid in water with anti-solvent acetone in the same device illustrated continuous nanocrystal production and similar behaviors vis-`a-vis variations of tres and FRR. The cross-flow HFM based continuous AsCr technique can continuously produce nanocrystals of APIs. 

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4. Novel perfluorinated nanofiltration membranes for isolation of pharmaceutical compounds

   https://doi.org/10.1016/j.seppur.2020.117944  

   Chau, John; Sirkar, Kamalesh K; Pennisi, Kenneth J; Vaseghi, Gazelle; Derdour, Lotfi; Cohen, Benjamin (January 2021, Separation and purification technology)

   null (Ed.)

   Polymeric membranes for separation of pharmaceutical intermediates/products by organic solvent nanofiltration (OSN) have to be highly resistant to many organic solvents including high-boiling polar aprotic ones, e.g., N- methyl-2-pyrollidone (NMP), dimethylsulfoxide (DMSO), dimethylformamide (DMF). Unless cross-linked, few polymers resist swelling or dissolution in such solvents; however particular perfluoropolymers are resistant to almost all solvents except perfluorosolvents. One such polymer, designated AHP1, a glassy amorphous hydrophobic perfluorinated polymer, has been studied here. Additional perfluoropolymers studied here are hydrophilically modified (HMP2 and HMP3) versions to enhance the flux of polar aprotic solvents. OSN performances of three types of membranes including the hydrophilically modified ones were studied via solvent flux and solute rejection at pressures up to 5000 kPa. The solutes were four active pharmaceutical ingredients (APIs) or pharmaceutical intermediates having molecular weights (MWs) between 432 and 809 Da and three dyes, Oil Blue N (378 Da), Sudan Black B (456 Da), Brilliant Blue R (826 Da). Solvents used were: ethyl acetate, toluene, n- heptane, iso-octane, DMSO, tetrahydrofuran (THF), DMF, acetone, NMP, methanol. Test cells included stirred cells and tangential flow cells. Pure solvent fluxes through three membrane types were characterized using a particular parameter employing various solvent properties. All three membranes achieved high solute rejections around 91–98% at ambient temperatures. HMP2 membrane achieved 95% solute rejection for an API (809 Da) in DMSO at a high temperature, 75 ◦C. A two-stage simulated nanofiltration process achieved 99%+ rejection of a pharmaceutical intermediate (MW, 432 Da) in 75v% NMP-25v% ethyl acetate solution. 

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5. Hydrophobicity versus pore size: polymer coatings to improve membrane wetting resistance for membrane distillation

   https://doi.org/10.1021/acsapm.9b01133  

   McGaughey, Allyson L.; Karandikar, Prathamesh; Gupta, Malancha; Childress, Amy E. (February 2020, ACS applied polymer materials)

   null (Ed.)

   Initiated chemical vapor deposition (iCVD) was used to coat two porous substrates (i.e., hydrophilic cellulose acetate (CA) and hydrophobic polytetrafluoroethylene (PTFE)) with a crosslinked fluoropolymer to improve membrane wetting resistance. The coated CA membrane was superhydrophobic and symmetric. The coated PTFE membrane was hydrophobic and asymmetric, with smaller pore size and lower porosity on the top surface than on the bottom surface. Membrane performance was tested in membrane distillation experiments with (1) a high-salinity feed solution and (2) a surfactant-containing feed solution. In both cases, the coated membranes had higher wetting resistance than the uncoated membranes. Notably, wetting resistances were better predicted by LEP distributions than by minimum LEP values. When LEP distributions were skewed towards high LEP values (i.e., when small pores with high LEP were greater in number), significant (measurable) salt passage did not occur. For the high-salinity feed solution, the coated PTFE membrane had greater wetting resistance than the coated CA membrane; thus, reduced surface pore size/porosity (which may reduce intrapore scaling) was more effective than increased surface hydrophobicity (which may reduce surface nucleation) in preventing scaling-induced wetting. Reduced pore size/porosity was equally as effective as increased hydrophobicity in resisting surfactant-induced wetting. However, reduced porosity can negatively impact water flux; this represents a permeability/wetting resistance tradeoff in membrane distillation – especially for high-salinity applications. Membrane and/or membrane coating properties must be optimized to overcome this permeability/wetting resistance tradeoff and make MD viable for the treatment of challenging streams. Then, increasing hydrophobicity may not be necessary to impart high wetting resistance to porous membranes. These results are important for future membrane design, especially as manufacturers seek to replace perfluorinated materials with environmentally friendly alternatives. 

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