(222r) Experimental Characterization of the Solubility of High Molecular Weight Liquids in Polydimethylsiloxane (PDMS) in Presence of a Low Molecular Weight Solvent

Experimental
characterization of the solubility of high molecular weight liquids in polydimethylsiloxane (PDMS) in
presence of a low molecular weight solvent 

Giovanni Cocchi,  Maria
Grazia De Angelis*, Ferruccio Doghieri

Department of Civil, Chemical, Environmental
and Material Engineering - University of Bologna, Via Terracini
28, 40131 Bologna, Italy

* Corresponding Author Email Address: grazia.deangelis@unibo.it

In order to characterize the separation
performances of polymeric membranes that could be applied in separation
processes relevant to food and pharmaceutical industry, experimental values of
the solubility of low molecular weight species in the polymer that forms the
membrane are required [1]. In this work, the experimental characterization of
the solubility of liquids of interest to the food and pharmaceutical industry
in a crosslinked PDMS film is discussed, considering
the effect of the presence of a low molecular weight solvent that swells the
matrix, affecting the transport [2,3]. Crosslinked PDMS was selected for its wide use in
applications, especially in membrane separation processes and for its
resistance to solvents [4]. The measurement of liquid solubility in PDMS was
performed gravimetrically, according to the blot and weight method: samples
were immersed in vials filled with liquid, placed in a thermostatic bath and
weighted at regular intervals with an analytical balance. In this way the mass
uptake as a function of time was obtained. In the case of mixed liquids
solubility, we considered solutions formed by a volatile component and a second
one with negligible vapor pressure. Firstly the measurement of the total mass
uptake of the liquid mixture was performed, according to the previously
described procedure. Then, the samples are left under hood to let the volatile
component evaporate, until constant weight was obtained.  Mass balance calculations provided the
separate mass uptake of the volatile and of the non volatile component. It was
found that the solubility of the less soluble component is deeply enhanced by
the presence of the other, more soluble component, that
usually is the low molecular weight solvent. For example, as shown in the
figure below, the swelling induced by n-pentane promotes the dissolution of eicosane in PDMS, which shows a maximum at an intermediate
mass fraction of n-C₅ in the mixture. This behavior has been
observed in many other mixtures such as the one formed by:   acetone and squalene,
pentane and oleic acid, pentane and olive oil, acetone and oleic acid, acetone
and olive oil and tertbutanol and PEG 400. The Figure
1 depicts the solubility of pentane and eicosane in
PDMS, as a function of pentane concentration. For the case of pentane and eicosane, an approximate thermodynamic analysis has been
conducted in order to compare the infinity dilution activity coefficient of
pentane in eicosane in this ternary mixture, to the
value available in the literature.

Figure 1: Solubility
of Pentane and Eicosane in Crosslinked
PDMS at 308 K.

[1]  S.P. Nunes,
K.V. Peinemann, Membrane Technology in the Chemical
Industry, 2^nd Edition, 2006, WILEY-VCH Verlag
GmbH & Co. KGaA, Weinheim,
Germany.

[2] R. Subramanian, M. Nakajima, T. Kawakatsu, Processing
of Vegetable Oils Using Polymeric Composite Membranes, Journal of Food
Engineering, 38, 1998, 41-56. 

[3] R. Subramanian, K. Raghavarao,
M. Nakajima, H. Nabetani, T, Yamaghuchi,
T. Kimura, Application of Dense Membrane
Theory for Differential Permeation of Vegetable Oil Constituents, Journal
of Food Engineering, 60, 2003, 249-256. 

[4] M.G. DeAngelis,
T. C. Merkel, V.I. Bondar, B.D. Freeman, F. Doghieri, G.C. Sarti, Hydrocarbon and Fluorocarbon Solubility and
Dilation in Poly(DimethylSiloxane): Comparison of
Experimental Data with Predictions of the Sanchez ? Lacombe Equation of State,
Journal of Polymer Science Part B: Polymer Physics, 37, 1999, 3011-3026.