Salt concentrations temperature, phase-transition

FIGURE 5.6 Logarithmic plot of the salt concentration c against the phase-transition temperature Tc. The gradient of the plot is equal to -0.077 K 1. 

The phase diagram was mapped out on the LOQ instrument at ISIS, Didcot, U.K. [16], just as for the butylammonium vermiculite system [6]. Typical scattering patterns from an r = 0.01, c = 0.25 M propylammonium vermiculite gel are shown at T = 36, 38, 40 and 42°C in Figure 9.6. The story is by now a familiar one a gel with a well-defined d-value at low temperatures (in this case d = 60 A) collapses as the temperature increases at a well-defined phase transition temperature (in this case Tc = 39°C). Note that the butylammonium system will not swell at c > 0.2 M, whereas here we have colloidal swelling at c = 0.25 M. The complete c, T phase diagrams at low r for both the propylammonium and butylammonium systems, taken in contiguous experiments, are shown in Figure 9.7. It is clear that the propylammonium vermiculites will swell in salt concentrations up to about 0.5 M in cold water. In these circumstances, their -values decrease below 50 A, and so could be measured on LAD. 

Before moving on to our attempt to measure the complete double layer in a swollen propylammonium vermiculite with d = 43.6 A [18], we pause to note that (a) at ionic strengths relevant to cell fluids, namely c 0.12 M [19], the phase-transition temperature in the propylammonium vermiculite system is not so far away from our body temperature and (b) similar temperature-induced gel-crystal transitions are observed in many biochemical systems. An example is the deoxyhemoglobin molecule that causes sickle cell anemia [33], We also note that with both counterions, Tc decreases linearly with the logarithm of the salt concentration. 

Based on this measurement strategy, the phase transition temperature of a responsive system can be determined as a function of the liquid environment or the material parameters. This is demonstrated by Jhon et al. (2(X)6) for grafted PNiPAAm films and different salt concentrations in the medium. For an increasing NaCl concentration up to 1 M, a depression of the phase transition temperature of several K is observed. The study of Kauftnann et al. (2010) investigates copolymers of NiPAAm and carboxyal-kylacrylamide. QCM-D reveals the shift of the phase transition temperature depending on different alkyl chain length and comonomer fractions. 

It is well known that polymer chains exhibit transition between the elongated coiled state and the collapsed-globule state depending on the parameters in the environment, i.e., temperature, salt concentration, dielectric constant, etc. [1,2]. From theoretical considerations [2, 3], it has been expected that the coil-globule transition can be discrete for stiff polymers, i.e., first-order phase transition. However, previous experimental studies have indicated that the transition is always continuous [4, 5] in other words, there is no phase transition on conforma-... 

At higher salt concentration (c>0.5 M) the screening exceeds to an even greater extent that predicted by Gouy-Chapman theory, and the transition temperature flattens off to a plateau value at salt concentrations of c —1.5 - 2 M. Independent measurements using charged partitioning spin labels indicate that the surface potential of the bilayers is completely screened at this salt concentration. Thus from Fig. 2.7 it appears that the electrostatic phase transition temperature shift is effectively screened in >2 M... 

In a subsequent study, the effect of reducing the ELP molecular weight on the expression and purification of a fusion protein was investigated. Two ELPs, ELP [V-20] and ELP[VsA2G3-90], both with a transition temperature at 40°C in phosphate-buffered saline (PBS) containing 1 M NaCl, were applied for the purification of thioredoxin. Similar yields were observed for both fusion proteins, resulting in a higher thioredoxin yield for the ELP[V-20] fusion, since the ELP fraction was smaller. However, a more complex phase transition behavior was observed for this ELP and therefore a selection of an appropriate combination of salt concentration and solution temperature was required [39]. 

Fig. 24. Parallel effects of salts on the molar reduction in the precipitation temperature of polyethylene oxide in the salt concentration of I mol-1-1, ATm and the transition molality of AMsopropylacrylamide gel, CT. In the case of continuous phase change, Cr was defined as the point of inflexion in the volume-molality curve...

There is a large body of experimental work on ternary systems of the type salt + water + organic cosolvent. In many cases the binary water + organic solvent subsystems show reentrant phase transitions, which means that there is more than one critical point. Well-known examples are closed miscibility loops that possess both a LCST and a UCST. Addition of salts may lead to an expansion or shrinking of these loops, or may even generate a loop in a completely miscible binary mixture. By judicious choice of the salt concentration, one can then achieve very special critical states, where two or even more critical points coincide [90, 160,161]. This leads to very peculiar critical behavior—for example, a doubling of the critical exponent y. We shall not discuss these aspects here in detail, but refer to a comprehensive review of reentrant phase transitions [90], We note, however, that for reentrant phase transitions one has to redefine the reduced temperature T, because near a given critical point the system s behavior is also affected by the existence of the second critical point. An improper treatment of these issues will obscure results on criticality. 

It is possible to control the pressures at which the phase transitions occur by fine tuning the strength of intermolecular interactions between the amphiphilic molecules. The interactions between the hydrophobic tails depend on temperature [37], while the interactions between the hydrophilic heads depend on the chemical composition of the subphase, namely its pH and ionic strength [4], For example, the fatty acid molecules in films prepared on subphase with high pH and high concentration of divalent salt, such as CaCl2 or CdCl2, are normal to the surface, i.e. are in solid state, even at low pressures. Pressure-area isotherms of such films are featureless compressed films are stable and easy to transfer [38]. 

FIGURE 1.15 Temperature-induced phase transition between the gel and crystalline phases of n-butylammonium vermiculite at an external salt concentration of c = 0.01 M A = 17 A. 

At ambient pressure, at an effective salt concentration slightly greater than 0.1 M and at 9°C, the sample was in the swollen gel state. At this temperature, three orders of the 00Z reflection were observed corresponding to a d-value of 120 A. Upon warming the sample to 11°C, the gel underwent the phase transition to the crystalline phase having a (i-value of 19.4 A. This transition was monitored, as before, by the growth of the first-order reflection of the crystalline phase at a value of the momentum transfer Q of 0.325 A. The momentum transfer is defined as... 

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