Xanthan adsorption

AMPS adsorption is found to be lower than HPAM, shown by data from Szabo (1979) in Table 5.10, where the polymer used is HPAM if not marked with AMPS. Broadly, xanthan adsorption in porous media is rather less than that of HPAM and also tends to show less sensitivity to the salinity/hardness conditions of the solvent (Sorbie, 1991 Green and Willhite, 1998). However, this conclusion is not supported by the data shown in Figures 5.38 and 5.39, which show that the median adsorption for synthetic polymers (24 pg/g) is lower than that for biopolymers (35 pg/g). 

Table III. Xanthan adsorption during the injection of successive slugs... 

In a study of the transport of xanthan in porous media, Kolodziej (1988) measured xanthan adsorption in dynamic core floods both at 100% water saturation and at residual oil in Berea cores. The isotherm which he derived is shown in Figure 5.20 (Kolodziej, 1988) and indicates adsorption levels of 751b/AF at 100% brine saturation and 381b/AF at residual oil. These... 

The adsorption density of polyacrylamides on quartz surfaces has been found by Lecourtier etal (1990) to be generally quite low (<500/xg/m ) which is attributed to the weak interactions with surface silanols. The adsorption density is found to be independent of pH and salinity for PAM, but increases with salinity and as pH decreases for HPAM (Lecourtier etal, 1990). The changes in xanthan adsorption with salinity and pH are found to be similar to HPAM since both of these are poly anionic species. 

The influence of calcium on the adsorption of high molecular weight EOR polymers such as flexible polyacrylamides and semi-rigid xanthans on siliceous minerals and kaolinite has been studied in the presence of different sodium concentrations. Three mechanisms explain the increase in polyacrylamide adsorption upon addition of calcium (i) reduction in electrostatic repulsion by charge screening,... 

The effects of calcium on polymer-solvent and polymer-surface interactions are dependent on polymer ionicity a maximum intrinsic viscosity and a minimum adsorption density as a function of polymer ionicity are obtained. For xanthan, on the other hand, no influence of specific polymer-calcium interaction is detected either on solution or on adsorption properties, and the increase in adsorption due to calcium addition is mainly due to reduction in electrostatic repulsion. The maximum adsorption density of xanthan is also found to be independent of the nature of the adsorbent surface, and the value is close to that calculated for a closely-packed monolayer of aligned molecules. 

This study aims at determining the effects of calcium on the adsorption of polyacrylamides and xanthans on siliceous minerals and kaolinite. 

Adsorption on Siliceous Minerals. All adsorption studies of xanthan (XCPS) in the presence of calcium are conducted at pH 6.5 to avoid precipitation which has been reported at pH>7 for xanthan solutions containing calcium (25). 

The adsorption results of both xanthan samples on sand at pH 6.5 in 20g/i NaCl are shown in Figure 12. The presence of calcium is seen to increase the adsorption of both xanthan samples and a maximum in adsorption is reached at high calcium concentrations. These variations are very similar to those observed for XCPS adsorption in the presence of NaCl (26). 

Adsorption on Kaolinite. As for polyacrylamides, adsorption of XCPS on kaolinite is conducted as a function of S/L and the results extrapolated to S/L=0. However, the S/L dependence of XCPS adsorption on kaolinite is considerably less than that for HPAM. This is due to the flat conformation of the adsorbed molecules of semirigid xanthan (25) compared to the more extended conformation of flexible HPAM (27). The absence of loops and tails in the adsorbed XCPS layer thus diminishes the probability of flocculation of particles by polymer bridging. The slight dependence in adsorption on S/L may therefore be attributed to coagulation of particles induced by Ca. ... 

The adsorption of xanthan is increased by calcium but to a less extent than that for HP AM. The increased adsorption seems to be due mainly to the effects of screening of polymer and surface charges by calcium, and the maximum adsorption density is equivalent to that induced by monovalent ions. 

The maximum adsorption density of semi-rigid xanthan is not very sensitive to the nature of the adsorbent surface provided that the surface has a homogeneous adsorption site density. This maximum level is close to the value calculated for a closely-packed monolayer of xanthan molecules. 

Xanthates have been used for flotation of lead and copper. In these examples, it is the adsorption of xanthane that dominates the flotation. 

Table 7.1 shows that rather similar results were also found by Makri et al. (2005) for samples of coarse emulsions containing thermodynamically incompatible mixtures of legume seed protein + xanthan gum. The protein surface load was found to be enhanced in the presence of xanthan gum, especially at elevated ionic strengths. That is, there was observed to be an increase in the adsorption of legume seed proteins at the surface of the emulsion droplets which could be attributed to an increase in the thermodynamic activity of the proteins in the system in the presence of the incompatible polysaccharide (see Table 7.1). Associated with the greater extent of protein adsorption, the authors reported an enhancement in the emulsion stability.

Chemicals Primary surfactant (e.g., petroleum sulfonate) Co-surfactant/co-solvent (e.g., C3 to C5 alcohol) Polymer (e.g., xanthan) Alkaline agents (e.g., sodium carbonate) Bactericides (e.g., formaldehyde) Sacrificial adsorption agents... 

Fig. 34.—Purification of anti-xanthan antibodies (Ab) by adsorption on Xan-Sepharose 4B and elution with NH4SCN. Inset shows agar-diffusion pattern Pr = serum protein, Xa = xanthan. (Reprinted from Carbohydrate Polymers, Volume 27, J. H. Pazur, F. J. Miskiel, and N. T. Marchetti, pp. 85-91, copyright 1995, with permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington, 0X5 1GB, UK.)...

COO" in hydrophilic group Good viscosifier, iess adsorption on sandstones due to the repuision between chain iinks, but precipitation with and Mg, iess chemicai stabiiity Sodium alginate, sodium carboxymethyl cellulose, HPAM, xanthan gum... 

The adsorption kinetics are slow in a surfactant concentration range around CAC and become fast again close to the CMC of the pure surfactant, where the surface layers with and without polymer become similar. Below CAC, they behave as incompressible layers. When subjected to small compression-expansion cycles, the layers exhibit a viscoelastic response, similar with the different polymers. Appreciable differences are seen only when the compression is more important (decrease of the surface area by a factor up to five) the layers with xanthan still behave as insoluble layers (even above CAC), whereas those with PAMPS appear as partially soluble. 

These experiments show that it is possible to achieve positive results using EOR after a thorough investigation of the nature of mineral rock constituents of the oil reservoir and the choice of the surfactant delivery method. The dynamic interfacial tension is crucial in EOR. Using a model acidic oil, alkali solutions and surfactants at an optimum ratio, ionised water and surfactant adsorb simultaneous onto the interface, resulting in low dynamic interfacial tension [229]. Combined adsorption of surfactant (alkyl propoxyethoxy sulphate) and polymer (xanthan) was studied in [230]. 

Most polysaccharides are not very surface active. For instance, starch and dextrane display very weak surface activities in various tests (for instance, in the classical Gold number tests). Xanthane has been shown to create depletion flocculation in several types of experiment, as shown by an unpredicted fast creaming. However, other polysaccharides, for instance, gum arabic and modified cellulose, do display surface activity. The latter is defined here as the ability to adsorb to surfaces. The ability to reduce the interfacial tension is associated with the ability to adsorb, as the surface tension represents the strength of the molecule-surface interactions. However, for large molecules, where the mixing entropy is only a minor contribution to the free energy, adsorption can also be achieved also when the adsorption energy is small. The surface activity of gum arabic is explained by the proteinaceous components associated with the molecule (22). When these parts are eliminated, the surface activity is lost. With modified cellulose, the surface activity is more related to... 

BIOPOLYMERS Xanthan Scleroglucan - high productivity in brine - resistant at high shear rates and temperature - adsorption (salt) - problematic injec-tability - sensitive to microbial degradation - sensitive to oxygen... 

---