Steric stabilization temperature-dependency

One of the characteristic features of sterically stabilized dispersionsis the temperature dependence of stability(18, 19). 

Emulsion stability is required in many dairy applications, but not all. In products like whipped cream and ice cream, the emulsion must be stable in the liquid form but must partially coalesce readily upon foaming and the application of shear. The structure and physical properties of whipped cream and ice cream depend on the establishment of a fat-globule network. In cream whipped to maximum stability, partially coalesced fat covers the air interface. In ice cream, partially coalesced fat exists both in the serum phase and at the air interface also, there is more globular fat at the air interface with increasing fat destabilization. Partial coalescence occurs due to the collisions in a shear field of partially crystalline fat-emulsion droplets with sufficiently-weak steric stabilization (low level of surface adsoiption of amphiphilic material to the interface per unit area). To achieve optimal fat crystallinity, the process is very dependent on the composition of the triglycerides and the temperature. It is also possible to manipulate the adsorbed layer to reduce steric stabilization to an optimal level for emulsion stability and rapid partial coalescence upon the application of shear. This can be done either by addition of a small-molecule surfactant to a protein-stabilized emulsion or by a reduction of protein adsorption to a minimal level through selective homogenization. 

In the case of the bulkiest substituents, the equilibrium actually favors the monomer (such silenes are thermodynamically stabilized with respect to dimerization). An example of a silene indefinitely stable in neat form is l,l-bis(trimethylsilyl)-2-(trimethylsiloxy)-2-adamantylsilene (42)105. In the case of the slightly less hindered 1,1 -bis(trimethyIsilyl)-2-tert-butyl-2-(trimethylsiloxy)silene, the monomer and the head-to-head dimer were observed to coexist in a temperature-dependent equilibrium in solution, but only the dimer was isolated103. An X-ray structure determination107 of this head-to-head dimer revealed a remarkably long C-C bond distance of 166 pm, probably caused by steric repulsion of the adjacent bulky substituents. 

Dilatant Flows Krieger and Choi [1984] smdied the viscosity behavior of sterically stabilized PMMA spheres in silicone oil. In high viscosity oils, thixotropy and yield stress was observed. The former was well described by Eq 7.41. The magnimde of Oy was found to depend on ( ), the oil viscosity, and temperature. In most systems, the lower Newtonian plateau was observed for the reduced shear stress value = Oj d / RT > 3 (d is the... 

The foregoing results may be contrasted with those of Croucher and Hair (1981) and Feigin et al. (1981). The former measured the particle concentration dependence of the UCFT of polyacrylonitrile latices sterically stabilized by polyisobutylene in 2-methylbutane (see Fig. 5.7). The UCFT was found to be insensitive to the variation of the latex volume fraction, within the limits of experimental error, over the range studied (2 x 10 -2 x 10 ). Note that in these experiments, the molecular weight of the stabilizing moieties (7-6 X 10 ) was relatively high. Moreover, flocculation did not occur in better than 0-solvents. Rather, there was close agreement between the UCFT (327 K) and the 0i-temperature (325 K). 

It should first be stressed that the foregoing classification of sterically stabilized dispersions, based on whether they flocculate on heating or cooling, is quite rigorous near to the CFT. It is independent of any temperature dependence of ASp and AHp because it is always possible to write the temperature dependence of the free energy of flocculation as... 

Fig. 8.4. The dependence of the CFT ( ) of polyfmethyl methaciylate) latex paitides sterically stabilized by polyfdimethylsiloxane) in n-alkanes of different chain lengths. AJso shown are the critical temperatures of the liquids (O) and the corresponding 0-temperatures (x) (after Everett and Stageman, 1978a).

The temperature dependence of the steric interaction between sterically stabilized particles mirrors the temperature dependence of [t x(7)]-Croucher and Hair (1978) have calculated the temperature dependence of the steric repulsion between two sterically stabilized particles of poly(acrylonitrile) stabilized by poly(a-methylstyrene) in n-butyl chloride. The particle radius was taken as 100 nm and each steric layer, assumed to be of constant segment density, was taken to be 12 nm thick. The parabolic nature of the repulsion as a function of temperature for two different distances of particle separation is obvious from Fig. 12.10. 

Fig. 12.10. The theoretical temperature dependence of the steric free energy of interaction for two spheres stabilized by poly(o-methylstyrene) in n-butyl chloride for a particle radius of 100 nm and a barrier layer thickness of 12 nm (after Croucher and Hair, 1978).

For sterically stabilized suspensions, one can measure the incipient flocculation when the medium for the chains becomes a 0-solvent. This occurs, for example, on heating an aqueous suspension stabilized with poly(ethylene oxide) (PEO) or poly(vinyl alcohol) chains. Above a certain temperature (the 0-temperature) that depends on electrolyte concentration, flocculation of the suspension occurs. The temperature at which this occurs is defined as the critical flocculation temperature (CFT). This process of incipient flocculation can be followed by measuring the turbidity of the suspension as a function of temperature. Above the CFT, the turbidity of the suspension rises very sharply. For this purpose, the cell in the spectrophotometer that is used to measure the turbidity is placed in a metal block that is connected to a temperature programming unit (which allows one to increase the temperature raise at a controlled rate). 

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