Electrolyte concentration, zwitterionic

FIG. 9 Simulated electrical potential and space charge density profiles at the water-1,2-DCE interface polarized at/= 5 in the absence (a) and in the presence (b) of zwitterionic phospholipids. The supporting electrolyte concentrations are c° = 20 mM and c = 1000 mM. The molecular area of the phospholipids is 150 A, and the corresponding surface charge density is a = 10.7 xC/cm. The distance between the planes of charge associated with the headgroups is d = 3 A. 

FIG. 10 Simulated enhancement factor for monolayers of zwitterionic phospholipids with different molecular areas (shown on the curves) at the polarized water-1,2-DCE interface. The supporting electrolyte concentrations are c° = 20 mM and c" = 1000 mM. 

The course of h(Cci) dependence indicating the decrease in equilibrium thickness up to the transition to NBF as well as the course of n(Ii) isotherm with a distinct barrier transition, reveal the electrostatic character of the forces acting in the film. Thus, double electric layer can be estimated, knowing that n / = pc+T vw The capillary pressure pa was measured experimentally while Tlvw was calculated from Eq. (3.89). The potential was determined within the electrolyte concentration range of 5-10 4 to 10 3 mol dm 3 (Fig. 3.48) in which the films were relatively thick, yielding a value of (po = 36 3 mV. In this respect films stabilised with the zwitterionic lipid DMPC are very similar to those stabilised with non-ionic surfactants [e.g. 100,186,189] (see also Section 3.4.1.1). The low ( -potential leads to the low barrier in the FI(Ii) isotherm which can easily be overcome at relatively low electrolyte concentrations and low pressure values. 

Lyso PC and Lyso PE films. The knowledge in the field of interaction forces in foam films stabilised with soluble zwitterionic phospholipids lyso PC (lysophosphatidylcholine) and lyso PE (lysophosphatidylethanolamine) has improved due to the studies of microscopic foam films [e.g. 191,192,292], The main dependences studied were of film thickness vs. electrolyte concentration and disjoining pressure vs. thickness, under specially chosen conditions in the presence of Na+ and Ca2+. The /i(pH) dependence proved to be very informative for understanding the charge origin in films from the neutral phosopholipids lyso PC and lyso PE (see Section 3.3.2). 

The aforementioned studies indicate that hydration repulsion is observed in (at least) two types of systems. (1) charged interfaces at relatively high electrolyte concentrations, where electrostatic and osmotic effects related to the presence of bound and mobile counterions are expected to play an essential role and (2) electroneutral surfaces with zwitterionic surface groups, like phospholipid bilayers, where the water structuring near the polar surface and surface charge discreteness could be the main sources of the observed repulsion. Correspondingly, for the theoretical explanation of the hydration repulsion different models have been proposed, which could be adequate for different systems. The most important theoretical models are as follows ... 

The use of surfactants (ionic, nonionic and zwitterionic) and polymers to control the stability behavior of suspensions is of considerable technological importance. Surfactants and polymers are used in die formulation of dyestuffs, paints, paper coatings, agrochemicals, pharmaceuticals, ceramics, printing inks, etc. They are particularly robust form of stabilization which is useful at high disperse volume fractions and high electrolyte concentrations, as well as under extreme conditions of high temperature, pressure, and flow. In particular, surfactants and polymers are essential for the stabilization of suspensions in nonaqueous media, where electrostatic stabilization is less successful. 

Other common practices include the use of well-degassed solvents, low concentrations of electrolytes, a relatively large amount of the organic component in the mobile phase, working at reduced temperatures (e.g., 15°C) when possible, and the use of low conductivity electrolytes (i.e., zwitterionic buffers). The addition of sodium dodecyl sulfate (SDS) into the mobile phase at low concentrations has also been used to minimize bubble formation [67]. 

FIGURE 3.9 Scaled surface potential i/ o of a plate immersed in a mixed solution of a monovalent electrolyte of concentration rii and rod-like zwitterions of length a and concentration as a function of reduced rod length ku for several values of r=njni. 

This means that for ka 3> 1 zwitterions behave like univalent electrolytes of concentration nJ2 instead of That is, the probability of finding of one of the rod ends near the particle surface is half that for the case of point charges due to the presence of the other end. 

We again see that in the limit of small Ka, the effective Debye-Hiickel parameter is given by Eq. (3.52) and that in the limit of large Ka, rod-like zwitterions behave like monovalent electrolytes of concentration nJ2 and the effective Debye-Hiickel parameter is given by Eq. (3.55). 

However, surfactants incorporated into the electrolyte solution at concentrations below their critical micelle concentration (CMC) may act as hydrophobic selectors to modulate the electrophoretic selectivity of hydrophobic peptides and proteins. The binding of ionic or zwitterionic surfactant molecules to peptides and proteins alters both the hydrodynamic (Stokes) radius and the effective charges of these analytes. This causes a variation in the electrophoretic mobility, which is directly proportional to the effective charge and inversely proportional to the Stokes radius. Variations of the charge-to-hydrodynamic radius ratios are also induced by the binding of nonionic surfactants to peptide or protein molecules. The binding of the surfactant molecules to peptides and proteins may vary with the surfactant species and its concentration, and it is influenced by the experimental conditions such as pH, ionic strength, and temperature of the electrolyte solution. Surfactants may bind to samples, either to the... 

For aqueous electrolyte solutions, both electrostatic interaction between dissociated ions and ionic hydration induce the deviation of freezing-point depression from that of the ideal solution at high concentrations. In the case of aqueous zwitterion solutions where each ion within a molecule carmot... 

For zwitterionics of the betaine and sulfobetaine types, Ci2H25N+(CH3)2(CH2)m COO- and C12H25N+(CH3)2(CH2)3S03-, respectively, the micellar aggregation number varies very little with change in surfactant concentration or electrolyte content (Kamenka, 1995a). 

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