TH E ROLE OF MOLECULAR W EIGHT IN TH E FLOCCULATION PROCESS.

The molecular weight distribution of ’Polymin P’ PEI was found to be so wide that it became a variable in the flocculation process. Here, model systems were arranged to investigate how such a wide molecular weight distribution is affected by the flocculation process ie:- whether the whole molecular weight distribution is involved in the flocculation process or only a section of it. Model systems were chosen because they could be separated from solution after flocculation so that the supernatant could be assayed. Bentonite or latex were flocculated with ’Polymin P ’ PEI; the samples were then centrifuged and the molecular weight distribution of the PEI remaining in solution was determined (by UV absorption). Under optimum flocculation conditions there was no detectable PEI remaining in solution, however ultra-violet absorbance is not a sensitive technique in this case (see Appendix 3, figure A3.1). Overdosing the system caused increasing amounts of PEI to remain in the supernatant. The molecular weight distribution of the unbound PEI was found to be a function of the strength of interaction between the polymer and the oppositely charged moiety. Figure 3.1.9a shows the flocculation of bentonite under conditions of increasing ionic strength; increasing the polymer concentration caused the weight average molecular weight to approach that of the native PEI (0.48 x 10® Da, Table 1). Figure 3.1.9b shows that bentonite is strongly electro-negative in solution and the addition of increasing quantities of PEI causes charge neutralisation, followed by restabilisation at high polyelectrolyte concentrations. Increasing the ionic strength reduces this interaction due to charge screening. Appendix 5, figures A5.1.1/2 show the effect of the flocculation of bentonite on the molecular weight distribution of PEI; the distribution is skewed towards the higher molecular weights. U nder increasingly overdosing conditions the molecular weight distribution becomes increasingly similar to that of native PEI (see fig. 3.1.5). Increasing the ionic strength reduces the interaction between bentonite and PEI, hence requiring increased amounts of polymer to achieve a similar zeta potential (figure 3.1.9b). Flocculation of latex with ’Polymin P’ PEI showed that the higher molecular weight fractions were preferentially removed (figure 3.1.9c). As the dosage increased, the weight average molecular weight increased toward that of the native PEI. Figure 3.1.9d shows that

the anionic latex was weakly electronegative and required only low doses of PEI to become electropositive. Appendix 5, figures A5.2.1/2 show that after flocculation the molecular weight distribution is skewed toward the lower weights; under overdosing conditions the weight average molecular weights increase.

Since the molecular weight distribution of PEI is polydisperse, it follows that the range of diffusion coefficients must be equally wide. The species with the highest diffusion coefficients will arrive at the surface first; if the interaction is strong then there is little chance of that molecule becoming detached from the surface. However if the interaction is weak, then other molecules with greater charge densities are capable of displacing them from the surface. For the bentonite/’Polymin P’ PEI system, since the surface is strongly electro-negative (due to the silanol groups) the binding affinity is strong, hence the molecules that attach to the surface first remain bound. Since the lowest molecular weight molecules have the highest diffusion coefficients it is probable that they will attach to the surface first and then remain bound (due to the high charge difference). This is reflected in the resultant molecular weight profiles which show that the lowest molecular weights are removed first. When the interaction between polymer and surface is weak, as in the latex/Tolymin P ’ PEI system (surface carboxyl groups), the low molecular weight species will arrive first but are subsequently displaced by molecules with greater charge densities {ie:-

higher molecular weights). The resultant molecular weight distribution shows that the higher molecular weights have interacted with the latex and been removed; overdosing gradually restores the molecular weight distribution. A nother important factor affecting the flocculation of both latex and bentonite is the charge density on the surface ie:- the number of charged groups per unit area.

The washed cell debris (figure 3.1.9e) showed a similar pattern on the molecular weight distribution of PEI as the latex, with the high molecular weight species being removed under optimum flocculation conditions. Under over-dosing conditions the trend is upward toward the average molecular weight of the native PEI. The type and number of charged groups on the cell debris will determine its charge density. Phosphate and carboxyl groups are the main groups on the cell debris surface which will determine the j mobilities (which was found to be -2.45 pm/sec/V/cm hence weakly electronegative). From the residual PEI molecular weight distributions

(Appendix 5, figure A5.3.1/2), it appears that the interaction is weak, with the high molecular weight molecules displacing the lower ones. The implications are that considerable ion exchange can occur on the cell debris during flocculation. It also implies that the low molecular weight PEI molecules remain in the supernatant and can interact with other charged species with different charge densities {eg> nucleic acids). These results imply that methods to remove residual PEI must take into account these changes in the molecular weight distribution especially if size exclusion chromatography is to be used.

Felter and Ray (1970) studied the adsorption of polyvinyl chloride (PVC) onto non- porous calcium carbonate (CaCOj) whereby the molecular weight distribution of the non-adsorbed PVC was determined by size exclusion chromatography. It was shown that the high molecular weights were preferentially adsorbed. Below the isotherm limiting plateau {ie> below surface saturation concentration), the molecular weight of the adsorbed species was independent of the polymer concentration. It was thought that the high molecular weight species displaced the lower ones at the surface. Further work (Felter, 1974) using polycaprolactone (PCL) adsorbed onto CaCOj showed that the low molecular weights were preferentially adsorbed. The author points out the importance of kinetic factors in the preferential adsorption of the low molecular weight fraction. The adsorption mechanism was thought to be responsible for controlling the resultant molecular weight distribution. PCL is more flexible and polar than PVC, which allowed it to uncoil on the CaCO^ surface leading to irreversible adsorption of the low molecular weight fraction.

3.1.6.2 PEI - SDS BINDING RATIOS. /

M ilbum et al., (1990) noted that PEI removed lipids from cell homogenates; however the nature of the interaction was not known, since PEI is hydrophilic and the lipids are essentially hydrophobic. To investigate the interaction between Tolymin f*’ PEI and hydrophobic molecules, it was added to solutions of sodium dodecyl sulphate (SDS); the aggregation was followed by absorbance readings (at 360 nm). Figure 3.1.10a. shows that Ig of PEI will bind approx. 3.3g of SDS. Tanford and Reynolds (1970) have reported that Ig of protein which is fully denatured, will bind 1.4g of