Adsorption of Polyampholytes

Neyret S, Ouali L, Candau F, Pefferkorn E. Adsorption of polyampholytes on polystyrene latex effect on colloid stability. J Colloid Interface Sci 1995 176 86-94. 

A seeond example where the mean-field theory ean be used is the adsorption of polyampholytes on eharged surfaees [28]. Polyampholytes are polymers eonsist-ing of negatively and positively eharged monomers. In eases where the total eharge on sueh a polymer adds up to zero, it might seem that the interaetion with a eharged surfaee should vanish. Flowever, it turns out that loeal eharge fluetuations... 

Fig. 4. Influence of pH on the plateau-value /T of adsorption isotherms of polyampholytes. At either side of the isoelectric point, i.e.p., the polyampholyte attains a net charge causing intra- and intermolecular electrostatic repulsion. As a result, the mass of adsorbed polyampholyte, that can be accommodated per unit area of the sorbent surface, decreases. Electrostatic interactions are suppressed by increasing ionic strength, yielding /T less sensitive to pH.

While the bulk behavior of polyampholytes has been investigated for some time now, studies of interfacial performance of polyampholytes are still in their infancy. There are several reasons for the limited amount of experimental work the major one being the rather complex behavior of polyampholytes at interfaces. This complexity stems from a large array of system parameters governing the interaction between the polymer and the substrate. Nearly all interfacial studies on polyampholytes reported to-date involved their adsorption on solid interfaces. For example, Jerome and Stamm and coworkers studied the adsorption of poly(methacryhc acid)-block-poly(dimethyl aminoethyl methacrylate) (PMAA-fc-PDMAEMA) from aqueous solution on sihcon substrates [102,103]. The researchers found that the amount of PMAA-fo-PDMAEMA adsorbed at the solution/substrate interface depended on the solution pH. Specifically, the adsorption increased... 

Dependence of the adsorption behavior on the molecular weight has been observed only in the case of polyampholytes of small molecular weight [13]. These polymers show an increase in the adsorbed amount compared to larger polyampholytes, as shown in Fig. 17. This tendency is expected to result from the adsorption of whole micelles of high density. One generally has to consider micelle formation in solution and at the interface to obtain a better understanding of the complex adsorption behavior of polyampholytes. 

Other investigations have been performed where silicon substrates were modified from alkaline to acidic and hydrophobic nature [58], where salt concentration was varied during adsorption [55] and where the nature of one of the blocks was changed [59]. Monolayers of polyampholyte micelles were used to modify protein adsorption [60]. An example is shown in Fig. 18 where the adsorption of fibrinogen is controlled by preadsorption of polyampholyte micelles and the phosphate buffer concentration. 

Fig. 18 Adsorption of the protein fibrinogen as influenced by preadsorption of polyampholyte micelles (PMAA/PDAEMA 33/67 weight ratio, Mn 15,000) in presence of 0.01 mol L-1 (filled symbols) and 0.001 mol L-1 (empty symbols) phosphate buffer solution...

Alvarez-Lorenzo, C. Hiratani, H. Tanaka, K. Standi, K. Grosberg, A.Y. Tanaka, T. Simultaneous multiple-point adsorption of aluminum ions and charged molecules by a polyampholyte thermosensitive gel controlling frustrations in a heteropolymer gel. Langmuir 2001, 17, 3616-3622. 

Dobrynin AV, Rubinstein M, Joanny JF. Adsorption of a polyampholyte chain on a charged surface. Macromolecules 1997 30 4332-4341. 

Equation 9.5 represents a rough approximation because both the number of particles and the siun of surface should decrease in the course of adsorption leading to a decrease of kj. Nevertheless, it allows the calculation of the minimal time which is necessary for adsorption of part of the polyampholytes. From Table 13 it follows that the time taken to adsorb 90% of the polyampholyte reaches 32 s 50% and 10%... 

Degree of phase formation of polyampholytes before surfactant adsorption ... 

Following similar reasoning, the adsorption pattern observed for ampholytic polyelectrolytes Ocan be explained. As illustrated in Fig. 4, polyampholytes show maximum adsorption around their isoelectric point (i.e., the pH where the net charge of the polyampholyte is zero). 

The adsorption behavior of weak polyampholytes from aqueous solutions on to a solid wall is strongly determined by the pH of the polymer solution. So it is absolutely necessary to investigate the adsorption as function of pH. As an example the adsorbed amount of two polyampholytes with medium molecular weight of around 60,000 g mol-1 and different block sizes is shown in Fig. 14 [13]. In every case the isoelectric point IEP of the polymer, which is determined by the block ratio, is the prominent feature of adsorp-... 

Fig. 14 The adsorbed amount, A, of a polyampholyte as a function of the pH of the adsorption solution. The solid and the dashed lines are shown as a guide for the eye. The transmission, T, of the solutions (dotted line) is shown as a function of pH. The arrows below the graph indicate where the silicon surface, S, and the polyampholyte, P, are carrying a positive or negative net charge. Characteristics of polymers (Bl, B3, B4 - a, b, c) are Mn 68,000 g mol-1, 62,000 g mol-1, and 62,000 g mol-1, PMAA/PDMAEMA weight ratios 90/10, 55/45 and 19/81, pHIEP 3.8, 5.9 and 9.3, respectively...

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