Molecular weight of a solution

Thus we have finally established how light scattering can be used to measure the molecular weight of a solute. The concentration dependence of r enters Eq. (10.54) through an expression for osmotic pressure, and this surprising connection deserves some additional comments ... 

It is seen that the molecular weight of a solute having a density between 0.85 and 1.25 can be estimated experimentally from peak width measurements for 90% of the compounds within an error of 13% (80% of the samples gave an error of less than 10%). Again, depending on the field of application, it should be noted that such data for a given substance can be obtained in addition to its separation from... 

The nominal molecular weight cut-off is normally defined as the molecular weight of a solute for which R = 0.95. Values of MWCO typically lie in the range 2000-100,000, with values of the order of 10,000 being most common. Figure 16.10 shows an AFM scan of 30,000 MWCO membrane. 

How does one use osmotic pressure measurements to determine the molecular weight of a solute How does polydispersity in molecular weight affect such a measurment ... 

EXAMPLE 7.1 Determination of the Molecular Weight of a Solute from it Versus A Isotherms. The molecular weight of a gas may be determined by measuring the mass of the sample as well as a set of p, V, and T values for the same sample. Use this fact to criticize or defend the... 

Likewise, if surface concentrations (in weight per area) are measured as a function of it, the molecular weight of a solute that forms an insoluble monolayer may be determined. Romeo and Rosano obtained the following data for a monolayer of acetyl lipopolysac-charide on 0.2 M NaCl at 20° C ... 

Molecular weight of a solute from tt versus A isotherms Use of the van t Hoff equation for monolayers Suppression of evaporation by monolayers Surface excess concentration from surface tension data... 

The ultracentrifuge can be used to separate fractions of different molecular weights in a mixture of solutes and to determine the molecular weight of a solute. Two different approaches can be taken. In the sedimentation velocity approach, the change in the concentration profile with time is determined during the centrifugation process. This nonequilibrium process requires a knowledge of diffusion rates and is not based directly on thermodynamics. We will leave a discussion of this process to other texts. 

The device is often operated with a refractive index detector in series in order to coincidentally measure the refractive index of the eluent. This is necessary to calculate (K) from the refractive index as given in equation (5). A common refractive index detector used for this purpose is that manufactured by the Wyatt Technology Corporation and it is described as the interferometer detector in chapter 11. As discussed above the molecular weight of a solute is determined from the intercept of the graph relating... 

Usefulness of partition coefficients derives from the empirical observation (Figure 5-3) that they may be related to the molecular weight of a solute. Although these observations have been analyzed extensively it must be conceded that as yet they cannot be totally accounted theoretically. When a plot such as that in Figure 5-3 is used to make an estimate of molecular weight, it is advisable to use only the linear region of the plot and to include standard proteins which have molecular weights above and below that of the unknown. 

Membrane manufacturers prefer to use a parameter called the nominal molecular weight cut-off (NMWCO) or simply the molecular weight cut-off (MWCO) to specify the sieving properties of their membranes. For a given membrane this is defined as the molecular weight of a solute that has an apparent sieving coefficient of 0.1. Polymers, such as dextran or polyethylene glycol, are usually used to determine MWCO. 

There are cases in which the molecular weight of the povidone grade used does not change, as it has been found that certain substances such as iodine or iodides can prevent this undesirable effect. Further, no increase in the molecular weight of a solution of povidone K 30 in a mixture of 90% polyethylene glycol and 10% water was observed after gamma irradiation. 

The shapes of solutes are also important in their retention behavior it has been shown that DNA restriction fragments (rod-shaped) have Km values that are more sensitive to molecular weight than those obtained with denatured (random-coil) proteins. In fact, the SEC parameter governing the retention is the hydrodynamic volume of the solute, which is related to its radius of gyration, Rg. The molecular weight of a solute is related to its radius of gyration by Eq. 14.12 ... 

The connection between turbidity and the molecular weight of a solute may be obtained by substitution of dH/dcir into Eq. (4.86). Representing the osmotic pressure for a monodisperse (i.e., single molecular weight) solute by a virial equation [cf. Eq. (4.52)] in the form... 

The First Method of obtaining the molecular weight of a solute is to measure directly, as Pfeffer did, the osmotic pressure P when a mass x is dissolved in a volume V at temperature T The numerical value to be... 

Dew-point Method of Determining the Molecular Weight of a Solute... 

The elevation of the boiling point is used to determine the molecular weight of a solute in the same way as is the freezing-point depression. It is desirable to use a solvent that has a large value of K. In Eq. (13.30) if is replaced by Tq then... 

The authors collected the test data shown in Table 9.10. From these data it can be noted that the number-average molecular weight, or Af , of a commercial emulsion SBR such as IISRP 1500 or 1712 is typically 90,000-175,000. The primary molecular weight of a solution-polymerized polymer produced with an anionic lithium catalyst can, in contrast, be increased toward 250,000 without gelation. In addition, emulsion-polymerized SBR contains only about 92% rubber hydrocarbon as a result of the presence of residues from the production process solution polymers tend to be near 100% hydrocarbon. As a consequence, the authors concluded that the number-average molecular weight... 

Sedimentation equilibrium experiments may be used to determine the molecular weight of a solute. For molecules that self-associate or bind, the stoichiometry of the complex and a measure of the kinetics of association may also be determined. 

Practically, membranes should have a high permeate flux, high contanunant rejection, great durability, good chemical resistance and low cost (Zhou Smith, 2002). The other property that is also important in the selection and/or classification of a membrane process is pore size or molecular weight cutoff (MWCO) (Zhou Smith, 2002). The MWCO expresses the retention characteristics of the membrane in terms of molecules of known sizes (Brock, 1983) and defines the maximum molecular weight of a solute to be rejected (Zhou Smith, 2002). 

The value of N is a function of the retention volume or the molecular weight of a solute and it decreases with a decrease in the retention volume or with an Increase in the molecular weight [ref. 22]. 

The knowledge of isotonic solutions helps us in determining the molecular weight of a solute. If the molecular weight of one of the substances forming isotonic solutions is known, then that of the other can be calculated with the help of equation (1). 

Determining solute retention is a straightforward way to characterize the selectivity of a membrane as retention is directly related to membrane performance. Molecular weight cut-off (MWCO) is the most commonly used term to express the retention of a membrane, particularly for UF membranes. MWCO is defined as the molecular weight of a solute where the retention is 90%. Both linear (PEG) and branched (dex-tran) molecules have been frequently used as the solute to determine MWCO of membranes, and the result is usually different for the different solutes used (Table 15.3J). 

---