Multicomponent Diffusion of Distearyldimethylammonium Polyelectrolyte Solutions in the Presence of Salt:  Coupled Transport of Sodium Chloride

Abstract

A sensitive conductance method has been used to determine binary and ternary diffusion coefficients of distearyldimethylammonium chloride, bromide, and hydroxide (DSDMACl, DSDMABr, and DSDMAOH) in aqueous solutions in the absence and presence of salt. Experiments were conducted above and below the critical micelle concentration (CMC) at 25°C, and the concentration of salt, when present, was 0.015 M NaX (X ) Cl, OH, Br). The experiments reveal that diffusion of DSDMA chloride, bromide, and especially hydroxide induces concurrent flow of NaCl, NaBr, and NaOH, respectively. Also, diffusion of DSDMAX in the presence of NaCl drives countertransport of DSDMACl or DSDMAOH. At pH 7.5 and an ionic strength of 0.015 M, each mole of diffusing DSDMACl (N agg ) 72, R ) 0.15) cotransports 9.0 mol of NaCl. However, coupled flow of NaCl decreases as the solution pH approaches 5.2 and is almost zero at pH 4.5. The corresponding value for diffusing DSDMAOH at pH 7.5 is about 120 mol of NaCl. At pH 6.5 only 10 mol of NaCl are cotransported, and the value is almost zero at pH 5.5. However, at pH 4.5, diffusing DSDMAOH generates a counterflow of NaCl due to bound protons and a release of H 2 O (DSDMAOH + H + T DSDMA + + H 2 O), yielding a net positive charge; hence, the D 21 value is negative. The cross-diffusion coefficient D 21 of diffusing DSDMACl in the presence of 0.015 M (0.095-0.009) NaOH and 0.015 M NaCl yields a flux density of NaOH and NaCl, which is generated by the DSDMAX (X ) Cl + OH) gradient. A similar cross-diffusion coefficient, D 21 , is revealed when DSDMAOH, is used, instead of DSDMACl, in the presence of 0.015 M NaCl through induced coupled transport of salt. The coupled transport of NaCl or of NaCl and NaOH is primarily driven by the diffusion-induced electric field along the DSDMAOH and DSDMACl concentration gradients. For the DSDMAOH system, the direction and magnitude of the coupled flow of either NaCl or NaOH can be explained through the ionic mobility of DSDMA + and OH -, in addition to the charge and degree of dissociation (N agg ) 6400, R ) 0.33-0.35) at 25°C. The tracer and mutual diffusion coefficients for the DSDMAX system were compared with those from light-scattering measurements of polyelectrolyte solutions of DSDMAX.

Cross-diffusion and pattern formation in reaction–diffusion systems

“…These equations are often used, even at large concentrations of charged species, to provide at least a qualitative description of diffusion processes. 27,29,98,99 The Nernst-Hartley equations are based on the hypothesis of electroneutrality: charged particles cannot move independently, since each volume element in the solution must be neutral. For a binary mixture with a univalent single electrolyte, for example, the Nernst-Hartley equation takes the form…”

“…Models of predator-prey systems with cross-diffusion have been extensively analyzed in the literature, though often with respect to their mathematical properties rather than to provide insight into the kinds of patterns that can emerge. del-Castillo-Negrete et al 135,152 examined a two-variable model with cross-diffusion relevant to both plasma physics and predator-prey population dynamics, qe/qt = ee 2 -s 2 e + r(d + De)re (27) qs/qt = -ms + ase + r(d + De)rs + rDsre (28)…”

“…Such processes can be characterized by the Fickian diffusivity matrix D or by the Maxwell-Stefan diffusion matrix D, where the diffusion coefficient D ij has the physical meaning of an inverse drag coefficient in the Maxwell-Stefan equations, 22 rather than by a vector of coefficients D ii . Examples of systems that exhibit cross-diffusion include strong electrolytes, [23][24][25] micelles, 26,27 or microemulsions, 28 and systems containing molecules of significantly different sizes, for example protein-salt. [29][30][31] Similar phenomena also occur in biological systems, but since the transport process is driven by an input of energy, as, for example, in bacterial chemotaxis [32][33][34] or in predator-prey systems, [35][36][37][38] these are not true diffusion processes.…”

Structure and Properties of Calcium Pyruvate in Aqueous Solutions

“…Crystalline (Ca(pyr) 2 ) exhibits at least two different structures depending on the degree of hydration [4]. However, in aqueous solutions an unexpected solution behaviour of Ca(pyr) 2 has been noticed during crystallisation studies, investigations on excitable membranes [5], and self diffusion measurements including coupled transport of salts [6]. Here, transparent aqueous solutions of Ca(pyr) 2 exhibit a concentration dependent aggregation up to concentrations of 8.5% (g/g), where a non-crystalline gel is formed as determined by X-ray diffraction techniques.…”

COPE elimination reaction observed in the biodegradation of quaternary ammonium surfactants

“…The biotransformation of DADMA X is strongly dependent on the critical micelle concentration (CMC) pH, temperature and ionic strength. 6 The initial rate constants for the anion exchange step where DSDMA X -? DSDMA OH (X = Cl, Furthermore, the fraction of exchange of Cl 2 (a) by OH 2 at pH 6.0 (25 °C) at equilibrium is 1.56%, and at pH 7.5 it is 30.6%.…”