Richard England

Performance of PDMS and organofunctionalised PDMS membranes for the pervaporative recovery of organics from aqueous streams

Abstract Modified polysiloxane membranes containing different organofunctional side chains were produced and tested for the recovery of various organic contaminants from aqueous sterams, using the process of pervaporation. Four separate organic components, phenol, chloroform, pyridine and methylisobutylketone (MIBK), each representative of an industrially significant family of chemicals, were chosen for evaluation. In each case significant performance enhancements, over that achieved with an unfunctionalised poly(dimethylsiloxane) membrane, were realise. Phenol transport is significantly facilitated by the incorporation of basic groups into the membrane structure. This is thought to be due to a weak acid-base interaction, increasing phenol sorption into the membrane. For pyridine, chloroform and MIBK separations from water, selectivity towards the organic component is greatly enhanced by the incorporation of long chain alkyl groups. A dual effect of the increased organic content of the membrane leading to increased organic component sorption and reduced water sorption is thought to be responsible. The effect of varying the three process parameters, feed concentration, membrane cross-linking density and membrane functional loading, were investigated. Some flux coupling effects were apparent, particularly affecting the transport of water with changing organic component concentration. Changing the cross-linking density of the membrane was found to have negligible effect across the range studied. An optimal functional loading generally close to 10% exists.

A fundamental study of organofunctionalised PDMS membranes for the pervaporative recovery of phenolic compounds from aqueous streams

Abstract Modified polydimethylsiloxane (PDMS) composite membranes in which a known mol% of the SiMe groups were replaced by a short methylene spacer group terminated by one of the four side-arm functional groups, acetate, CO 2 Me; ethylether, OEt; dimethylamino, NMe 2 ; and pyridyl, py; were fabricated and tested in short term trials for the pervaporative recovery of cresols from aqueous solution. The influences of functional group type, functional group loading, pH, temperature and purity of the feed solution were investigated. Three commercially available hydrophobic membranes were also examined for comparison. It was found that significant performance enhancements, compared with an unfunctionalised PDMS membrane, were realised for all functionalised PDMS membranes, with dimethylamino and ethylether functionalities proving the most effective. A functional group loading level close to 20% was found to be optimal for this application, The influence of feed pH in the range of 5–8.5 was small on total flux but significant on selectivity for amine loaded membranes. By contrast, the effect of feed temperature was significant on total flux but negligible on selectivity. Low levels of phenolic impurities in a p -cresol feed solution were found to have negligible effect on the separation. Compared with commercially available zeolite filled PDMS and PEBA membranes, the functionalised PDMS membranes showed a better overall performance. The PV performance of all new membranes followed the trend: o -cresol> p -cresol>phenol.

Tapered fibers embedded in silica aerogel

We embedded thin (down to 2 µm diameter) tapered fibres in silica aerogel with low loss. The aerogel is rigid but behaves refractively like air, protecting the tapered fibre without disturbing light propagation along it.

THE PERMSELECTIVITY OF POLYORGANOSILOXANES CONTAINING ESTER FUNCTIONALITIES

Abstract The permeabilities of carbon dioxide, methane, oxygen and nitrogen in a range of polyorganosiloxane membranes containing a side-chain ester functionality of 0-21.4 mol% of Si atoms, have been determined by a continuous flow method over the temperature range 35-100°C. The ratio of the CO 2 and CH 4 permeabilities, or permselectivity, increases with increase in ester functionality. As measurement of the diffusivities at 35°C shows that there is little change in the relative diffusivity of CO 2 to CH 4 with increase in ester functionality, the greater permselectivity results from an increase in the relative solubility of CO 2 to CH 4 . This is confirmed by measurements of the relative solubility using a vacuum microbalance. Moreover, since the activation energy for CO 2 permeation is much lower than that for CH 4 , the permselectivity becomes greater as the temperature is lowered. In contrast, the ratio of the oxygen/nitrogen permeabilities decreases slightly with increase in ester functionality as a result of a decreasing relative solubility, the relative diffusivity again remaining virtually unchanged.

Stable low-loss optical nanofibres embedded in hydrophobic aerogel

Nanofibres, optical fibres narrower than the wavelength of light, degrade in hours on exposure to air. We show that encapsulation in hydrophobic silica aerogel (refractive index 1.05) provides protection and stability (over 2 months) without sacrificing low attenuation, strong confinement and accessible evanescent field. The measured attenuation was <0.03 dB/mm, over 10 × lower than reported with other encapsulants. This enables many nanofibre applications based on their extreme small size and strong external evanescent field, such as optical sensors, nonlinear optics, nanofibre circuits and high-Q resonators. The aerogel is more than a waterproof box, it is a completely-compatible gas-permeable material in intimate contact with the nanofibre and hydrophobic on both the macroscopic and molecular scales. Its benefits are illustrated by experiments on gas sensing (exploiting the aerogels porosity) and supercontinuum generation (exploiting its ultra-low index).

Optimisation of organofunction PDMS membranes for the pervaporative recovery of phenolic compounds from aqueous streams

Experimental studies for optimising organofunctionalised PDMS membranes for the pervaporative recovery of phenolic compounds from water are presented. Four functional groups i.e. acetate, ethylether, dimethylamino and pyridyl were loaded into pure PDMS membranes with the functional loading ranging from 10 to 25%. Composite membranes were fabricated and tested in short term trials with 5% (w/w) phenol/water and 2% (w/w) p-cresol/water binary systems. The influences of the pH and temperature of the feed solution were also examined. It was found that significant performance enhancements were achieved for functionalised PDMS membranes, especially those loaded with either the dimethylamino or the ethylether group. The optimal loading level for the dimethylamino group is about 20%. For this membrane, increasing the pH of the feed solution within the range 5–8.5 would double the selectivity for p-cresol/water separation. For the phenol/water separation the effect is more modest. The results also showed that an increasing feed temperature from 50 to 90°C resulted in an increase of 300% in flux, but had a negligible effect on selectivity.

Analysis of carrier-mediated transport through supported liquid membranes using functionalized polyorganosiloxanes as integrated mobile/fixed-site carrier systems

Abstract A new quantitative model is developed which describes the kinetics and mechanism of transport of a weak organic acid derivative through an integrated solvent/carrier supported liquid membrane (SLM). The transport model is based upon simultaneous occurrence of both mobile and fixed-site jumping carrier mechanisms, and incorporates the assumption of diffusion-limited transport. The theoretical model is verified by comparison with experimental data obtained from a facilitated transport study of ethyl lactate through a supported amine functionalized polyorganosiloxane SLM. The transport parameters, which include the product of solute partition coefficient and diffusion coefficient ( K p D s ), the equilibrium extraction constant ( K ex ), and the effective diffusivity of solute–carrier complex due to both mobile and jumping carrier mechanisms ( D m + D j ), have been determined, and excellent agreement is achieved between predicted and experimental data in most of the studies.

Monofunctional chiral crowns. Part 1

Abstract A siloxane oligomer bearing a chiral crown with high affinity for α-amino acids has been synthesised. The host is a modified form of a coronand first prepared by Cram, in which two 1,1′-binaphthol systems are linked through the oxygen atoms at the 2,2′-positions to form a 22-membered ring system containing six ether oxygen atoms attached to each other by four ethylene units. This was selectively mono-alkenylated to form an undec-10-en-1-yl derivative, which was bonded to linear siloxanes [HSiMe 2 O(SiMe 2 O) n SiMe 2 H] and [Me 3 SiO(Me 2 SiO) x (MeHSiO) y SiMe 3 ] which total chain lengths of ca 4 and 200 Si atoms respectively, via a Pt catalysed hydrosilylation reaction.

Preparation of modified difunctional PDMS membranes and a comparative evaluation of their performance for the pervaporative recovery of p-cresol from aqueous solution

Abstract Cross-linked polydimethylsiloxane (PDMS) membranes modified by the sequential introduction of two different side-arm functional groups, (CH2)3OC2H5 and (CH2)3NMe2, have been prepared and evaluated for the pervaporative recovery of p-cresol from aqueous streams. The loading of the ether group was varied from 5 to 20% and that of the amino group from 5 to 15%. These loadings are expressed in terms of the mole percentages of Si atoms within the membrane containing the identified functional group. The performances of these difunctional membranes are compared with that of pure PDMS and with membranes modified with a single functional group. It was found that various combinations of ether and amino groups improved p-cresol flux while maintaining the high separation factor exhibited by PDMS modified by a high loading of the amino group. These new materials also facilitated the preparation of defect-free films. Membranes containing AEE (10%) +AMI (5%) gave a very similar performance to monofunctional membranes containing 20% amino, but unlike those membranes the new difunctional membranes possess good film forming properties. Another combination of note is the AEE (20%) + AMI (10%) membrane that produced both an excellent cresol flux (that was 30% higher than that achievable using either AEE or AMI modified membranes) and a high separation factor ca. 50.

Silica aerogel in optical fibre devices

Silica aerogel is a very light and highly porous form of silica glass, with densities and refractive indices much lower than those of ordinary solids. We describe optical interactions with aerogel surrounding tapered fibres or inside the holes of photonic crystal fibres. This enables a new class of fibre devices exploiting the properties of the aerogel as a rigid yet porous replacement for air, as a nonlinear medium and as a host for dopants. Examples include a stable package for fused couplers, a gas sensor and a nonlinear light source.