Use of Solubility Parameters for Predicting the Separation

Chapter 14

Use of Solubility Parameters for Predicting the Separation Characteristics of Poly(dimethylsiloxane) and Siloxane-Containing Membranes

Downloaded by UNIV LAVAL on January 26, 2015 | http://pubs.acs.org Publication Date: August 2, 2007 | doi: 10.1021/bk-2007-0964.ch014

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Alexander R. Anim-Mensah , JamesE.Mark , and William B. Krantz 3

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Departments of Chemical and Materials Engineering and Chemistry, University of Cincinnati, Cincinnati, OH 45221 Department of Chemical and Biomolecular Engineering, National University of Singapore, Republic of Singapore 117576 3

The separation characteristics of poly(dimethylsiloxane) (PDMS) and siloxane-containing membranes can be predicted by the use of the solubility parameter (δ) differences between the membranes and solvents and/or the solutes. The membranes considered were PDMS and siloxane derivatives such as polysiloxaneimide (PSI) and poly[l-(trimethylsilyl)-lpropyne] (PTMSP), respectively. PSI is a copolymer made of PDMS and polyimide (in this case using 3,3,4,4benzophenonetetracarboxylic dianhydride (BTDA) while silicone is a substituent in PTMSP. The system considered was the separation of PDMS and PSI using different aqueous alcohol feed solutions through pervaporation. The separation characteristics of PSI membranes having varying PDMS content were also investigated using an aqueous ethanol feed solution. PDMS was investigated for the different alcohol separations from water while PSI was investigated for the recovery of ethanol or water from different aqueous ethanol solutions. It was observed that an increase in the δ-difference between the membrane and solvent resulted in a general decrease in the separation factor (SF). In all the cases investigated, the closeness in the δ-difference between the membrane and solvent compared to that of the membrane and solute indicated good membrane performance. However, an © 2007 American Chemical Society

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In Science and Technology of Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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increasing concentration of ethanol in an aqueous ethanol feed resulted in membrane swelling that caused an increased flux and decreased SF of ethanol relative to water. It could be concluded from the systems considered here; that a small difference between the δ of the membrane and solvent compared with that of the membrane and solute corresponds to an improved SF. However if this difference is too small, it might cause excessive membrane swelling. Thus, this approach provides a quick way of selecting membranes including PDMS and siloxane-containing polymers for specific membrane applications.

Introduction This chapter will provide an overview of how the solubility parameter (δ) can be used as a guide for selecting membranes, especially PDMS and siloxanecontaining membranes for a specific separation task. Emphasis is placed on membranes where the separation mechanism involves solution-diffusion. Hence, the focus is on improving nanofiltration (NF) , reverse osmosis (RO) and pervaporation membrane processes. It is well known that PDMS is the most studied siloxane and has been used in addition to other siloxane-containing polymers for gas separations " . The siloxanes have also been used for the recovery of organics from aqueous systems " . In membrane separations involving liquids, three components are encountered: the solvent, the solute, and the membrane. In solution-diffusion membrane processes, the solute or solvent undergoes four sequential steps: (1) adsorption on the membrane surface at the high pressure side; (2) solubilization in the membrane; (3) diffusion through the membrane under the driving force; and (4) desorption at the low-pressure side of the membrane. In most cases, the extent of solubilization of the solute in the membrane is dictated by the partition or distribution coefficient of the solute between the solvent and membrane at a particular temperature . The solubility parameter has been extensively used for identifying suitable solvents, swelling agents, plasticizers and non-solvents for polymers. It provides useful semi-quantitative information that can be used as a rule-of-thumb when experiments are unavailable for predicting miscibility of polymer solutions. The foundation for using δ is deeply rooted in macroscopic thermodynamics . The theory behind δ is based on the principles of solubility or miscibility, that is, "like dissolves like". For two or more components to mix at constant temperature and pressure, the Gibb'sfree-energy-of-mixing(AG ) should be negative. The change in AG is defined as the difference between the Gibb's free energy (AG) of the fully mixed solution and the weighted sum of the individual Gs of the pure components as given by Equation (1) : 1

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10

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12

m

m

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The change in the Gibb'sfreeenergy of-mixing (AG ) is related to the enthalpyand entropy-of mixing as given by Equation (2) : m

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AG =AH -TAS m

m

(2)

m

The change in the entropy-of-mixing AS is always positive since mixing implies increased disorder; hence, for AG to be negative requires that the enthalpy-ofmixing AH which is usually positive owing to intermolecular attraction, not be too large. AH is related to the internal-energy-of-mixing AU and volume-ofmixing AV at constant pressure by Equation (3). m

m


m

m

m

m

AH = AU -APV = AU -PAV m

m

m

m

tn

(3)

m

m

τη

Although, AV is generally negative for polymers solutions, its effect on AH is generally less than that of AU and ignored in the δ-approach to assess the miscibility of polymer solutions; that is, the following assumption is made in Equation (2): m

m

m

AG

« AU

m

(4)

m

m

m

Hildebrand in his work related AU to the change in the internal-energy-ofvaporization Aw/, the partial molar volumes v and the volume fractions $ of the individual components in the mixture, as well as the total volume of the mixture V by using a 6-n intermolecular potential energy function that can be shown to result in Equation (5) below : m

h

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AU

Λ AU:

=V m

V

)

Λ/2

1 / 2

Au,

κ

J

v

(5)

fir j J

v

The solubility parameter is related to the energy-of-vaporization (Au, ) and the partial molar volumes (ν,·) as shown in Equation (6) : 14

4=

(6)


206 In the absence of experimental data, group contribution methods can be used to estimate δ for polymers in which case it is given by the following : 15

15

where the group contribution Fj can be obtained from the standard references , Pi is the density of the polymer and M is the molecular weight of the repeat group in the polymer. The solubility parameter can be calculated from knowledge of the polar and hydrogen-bonding (h) contributions. The polar contribution is subdivided into dispersive (d) and permanent dipole-dipole (p) components. The overall δ is related to the polar and hydrogen-bond contributions as follows :


r

15

2

2

2

δ =δ/+δ +δ„

(8)

ρ

The Hanson radius (Ry) defined by Equation (9) can be used to predict the miscibility or compatibility between two substances [15], particularly that of a polymer and a liquid component.

,

,

*.-k».-*j;*fe-* );*fe-'A J ° J

«>

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The (δ δβ terms represent only the difference in the interactions between the molecules of different polarities. Smaller Ry values for a polymer and possible solvent suggest that stronger interaction that can result in swelling or miscibility. Smaller Ry values for a polymer and a solute again suggest stronger interactions that can be manifested by fouling of a polymer membrane by the solute. Smaller Ry values for any two (2) components generally correspond to an increased flux of either the solute or solvent a polymeric membrane whose transport is governed by a solution-diffusion mechanism. Conversely, larger Ry values generally imply less interaction and corresponding diminished swelling or immiscibility, reduced fouling, and lower flux. A quick approach of estimating δ from knowledge of only the overall δ is to use the difference between the

Use of Solubility Parameters for Predicting the Separation

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