J. Phys. Chem. B 2008, 112, 15943
Reply to “Comment on Osmotic Pressure beyond Concentration Restrictions’” Alessandro Grattoni† and Mauro Ferrari* DiVision of Nanomedicine, Department of Biomedical Engineering, The UniVersity of Texas Health Science Center at Houston, Suite 537, 1825 Pressler Street, Houston, Texas 77030 ReceiVed: August 14, 2008 In the manuscript “Comment on Osmotic Pressure beyond Concentration Restrictions”, Andriy Yaroshchuck presents an interesting analytical exercise on molecular diffusion in porous media. Unfortunately, the manuscript, which relies on noncomplete information, does not apply to the case of the work done by A. Grattoni, M. Merlo, and M. Ferrari in the article, “Osmotic Pressure beyond Concentration Restrictions”.1 Yaroshchuck bases his analysis on the erroneous hypothesis of sucrose permeation during the osmotic pressure measurements performed with the employment of polymeric semipermeable membranes. Aware of the fact that the permeation of sucrose would have nullified the osmotic pressure measurements, A. Grattoni, M. Merlo, and M. Ferrari (G.M.F.) addressed the problem in the development of the membrane osmometer.1,2 Sucrose permeation would have caused a concentration gradient across the supporting porous disk and a local reduction of the concentration difference. As a result, a decrease in the osmotic flow of the solvent toward the solution would have occurred. First, G.M.F. verified the technical features of the semipermeable membranes (the distributor is Sterlitech Corporation, and the producer is GE Osmonics) through personal communication with the companies. It was guaranteed that the membranes presented a 99.5% rejection of NaCl. Moreover, the companies affirmed that the actual membrane cutoff ranges between 150 and 250 Dalton, hence presenting a nominal 100% cutoff for molecules larger than 250 Dalton. Since the membrane cutoff is significantly smaller than the sucrose molecular weight (342.3 Dalton), no sucrose should have diffused throughout the membrane. Nevertheless, by considering the random structure of the polymeric membrane and the possibility of the occurrence of defects, G.M.F. did not neglect a priori the permeation of sucrose. Second, prior to performing the osmotic pressure experimentation, G.M.F. performed several experimental tests to ensure that sucrose could not permeate the membrane. The tests were performed by separating sucrose solution and bidistilled water * To whom correspondence should be addressed. Tel: (713)-500-2444. Fax: (713)-500-2462. E-mail:
[email protected]. † E-mail:
[email protected].
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with the membrane. No pressure was applied, and no supporting disk was employed. This set of tests showed that, generally, no sucrose diffusion occurred over a period of 12 h. In few cases, a tiny flow of sucrose was observed as reported in the Supporting Information of the manuscript.1 In order to verify if sucrose diffused through the membranes during the osmotic pressure experimentation, the grain pore size of the porous support disk was properly selected. G.M.F. chose the grain size to homogeneously support the membrane under pressure and to allow the collection of the solvent from its pores at the end of each measurement. Thus, at the end of each osmotic pressure test, the liquid from the solvent chamber and from the porous disk was collected. This operation was easily performed by extracting the liquid from the porous disk with a compressed air flow. The samples were analyzed to verify that no sucrose had crossed the membrane. In few cases, a tiny amount of sucrose was detected. As reported in the Supporting Information of the manuscript,1 the few cases in which the sucrose diffusion occurred were used to calculate the maximum measurement error due to membrane nonideality. However, the few tests that presented even slight sucrose permeation were nullified,1,2 and the corresponding osmotic pressure data were not considered in the experimental analysis.1,2 This information is also reported by A. Grattoni, G. Canavese, F. M. Montevecchi, M. Ferrari in “Fast Membrane Osmometer as Alternative to Freezing Point and Vapor Pressure Osmometry”.2 As communicated to Dr. Yaroshchuk,3 no sucrose permeation and, consequently, no gradient of concentration affected the osmotic pressure data shown in the manuscript.1,2 In order to complete the experimental analysis, GMF performed an error analysis on the experimental results.1,2 According to the analysis, there are no errors related to solute permeation, and the individual experimental values present a deviation from the novel theory developed by Granik and Ferrari4 by no more than 17%. In conclusion, the study conducted by A. Yaroshchuk is based on an erroneous hypothesis which leads to results with no scientific relevance due to the inapplicability of the analysis to the work of G.M.F.1,2 References and Notes (1) Grattoni, A.; Merlo, M.; Ferrari, M. Osmotic Pressure beyond Concentration Restrictions. J. Phys. Chem. B 2007, 111, 11770–11775. (2) Grattoni, A.; Canavese, G.; Montevecchi, F. M.; Ferrari, M. Fast Membrane Osmometer as Alternative to Freezing Point and Vapor Pressure Osmometry. Anal. Chem. 2008, 80, 2617–2622. (3) Yaroshchuk, A. 2008, personal communication. (4) Granik, V. T.; Smith, B. R.; Lee, S. C.; Ferrari, M. Osmotic Pressures for Binary Solutions of Non-electrolytes. Biomed. MicrodeV. 2002, 4, 309–321.
JP807289X
10.1021/jp807289x CCC: $40.75 2008 American Chemical Society Published on Web 11/14/2008
