Pulsed-gradient spin-echo (PGSE) 1 H NMR experiments were carried out on swollen poly-(N,N-dimethylacrylamide) (PDMAA) gels with various degrees of swelling (Q) under a state of equilibrium with deuterated water (D2O) as solvent and poly(ethylene glycol) (PEG) as a probe polymer, of which the molecular weight ranged from 4250 to 20000. Self-diffusion coefficients of PEG and HDO in D2O solvent (containing a small amount of HDO) in the gels were determined. From these results, the diffusion coefficient for HDO in the gel (DHDO) was found to increase with an increase in Q and asymptotically approach the diffusion coefficient for HDO in neat D2O. This behavior was reasonably explained by the modified free volume theory. Further, it was found that the diffusion coefficient for PEG (DPEG) in the gel is followed by DPEG/D PEG 0 ) exp(-κR), where D PEG 0 is the diffusion coefficient for an isolated PEG, κ -1 the dynamical screening length, and R the hydrodynamic radius of PEG, and that κ -1 is proportional to c -0.71 , where c is the polymer concentration.
Asphaltene aggregation causes several problems in the petroleum industry, and an understanding of the aggregation behavior is needed to solve the problems. In this study, the effects of asphaltene concentration on aggregate size were investigated to elucidate the aggregation mechanism of asphaltene. Diffusion coefficients (D) of three asphaltenes from the vacuum residue (VR) of Khafji, Iranian Light, and Maya crude oils and one resin were determined in deuterated chloroform solution using pulsed-field gradient spin-echo 1 H NMR. The pulsed-field gradient spin-echo 1 H NMR of asphaltenes and resin showed different diffusion behaviors from well-characterized reference compounds such as polystyrene because of structural and compositional irregularities; that is, they are a complex mixture of molecules of various molecular weights and structures. From the D values, their average hydrodynamic radii were estimated. The concentration dependency (0.1-30 g/L) of the D values and the hydrodynamic radii support the widely accepted stepwise aggregation mechanism (i.e., monomer < small aggregates < medium-size aggregates < large aggregates (precipitate)). At low concentrations (0.1 and 1 g/L), the D values corresponding to the range of small to medium aggregates were observed, and at higher concentrations (10 and 30 g/L), only medium aggregates were detected. Similar D values were obtained for the three asphaltenes, although Maya asphaltene gave slightly lower D values. For the resin, higher D values (lower hydrodynamic radii) were obtained than the asphaltenes.
To better understand the relation between recrystallization rate and water mobility in freezeconcentrated matrix, isothermal ice recrystallization rates in several sugar aqueous solutions and selfdiffusion coefficients of water component in corresponding freeze-concentrated matrix were measured. The sugars used were fructose, glucose, maltose, and sucrose. The sugar concentrations and temperature were varied so that ice contents for all samples were almost equal. Neither recrystallization rates nor diffusion coefficients depended uniformly on temperature. The recrystallization rates increased with increasing the diffusion coefficients, and a direct relationship was found between recrystallization rate and diffusion coefficient. This indicated that self-diffusion coefficient of water component in freeze-concentrated matrix is a useful parameter for predicting and controlling recrystallization rate in sugar solutions relevant to frozen desserts.
The effects of droplet size and emulsifiers on oxidative stability of polyunsaturated TAG in oil-in-water (o/w) emulsions with droplet sizes of 0.806 +/- 0.0690, 3.28 +/- 0.0660, or 10.7 +/- 0.106 microm (mean +/- SD) were investigated. Hydroperoxide contents in the emulsion with a mean droplet size of 0.831 microm were significantly lower than those in the emulsion with a mean droplet size of 12.8 microm for up to 120 h of oxidation time. Residual oxygen contents in the headspace air of the vials containing an o/w emulsion with a mean droplet size of 0.831 microm were lower compared with those of the emulsion with a mean droplet size of 12.8 microm. Hexanal developed from soybean oil TAG o/w emulsions with smaller droplet size showed significantly lower residual oxygen contents than those of the larger droplet size emulsions. Consequently, oxidative stability of TAG in o/w emulsions could be controlled by the size of oil droplet even though the origins of TAG were different. Spin-spin relaxation time of protons of acyl residues on TAG in o/w emulsions measured by H NMR suggested that motional frequency of some acyl residues was shorter in o/w emulsions with a smaller droplet size. The effect of the wedge associated with hydrophobic acyl residues of emulsifiers was proposed as a possible mechanism to explain differences in oxidative stability between o/w emulsions with different droplet sizes.
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