S. Ablett

Differential scanning calorimetric study of frozen sucrose and glycerol solutions

Differential scanning calorimetry (DSC) thermograms have been recorded for sucrose and glycerol solutions as a function of moisture content. Simple second-order transitions were observed at the glass transition for the higher concentration samples which did not form ice. More complicated thermograms were observed from the lower solution concentrations which formed ice. The origin of these transitions in the DSC thermograms from frozen solutions is discussed, together with the methods used to calculate the amount of ice in the freeze-concentrated solutions. The glass-transition temperatures (Tg) and the ice-melting temperatures (Tm) were used to construct the supplemented phase diagrams for both of these solutes. The maximum freeze concentration (C′g) for sucrose is determined to be 81.2% sucrose, and is shown by a novel experimental approach to occur at a temperature (T′g) of –40 °C.

A calorimetric, NMR and X-ray diffraction study of the melting behavior of tripalmitin and tristearin and their mixing behavior with triolein

X-ray diffraction, differential scanning calorimetry and NMR have been used to give information on the molecular order and melting of tripalmitin and tristearin crystals. They also have been used to study the melting and solubility of these pure triglycerides when mixed with triolein. From these studies we propose demixing in the liquid state and observe modification of the crystallization kinetics of these saturated triglycerides such that the highest melting (β-polymorphic) form is observed within a minute at 298K for cooling rates up to 120K/min. In the mixtures the long d-spacing, corresponding to the distance between layers in the crystals, was observed to be approximately 0.5 nm greater after cooling in excess of 60K per min than for close packed layers in the β form crystals. A study carried out after rapid cooling showed that two processes occurred. Initially, the α polymorphic form is produced; this transformed to the β-like crystals in a matter of a few minutes. There is then a much more gradual decrease in the long d-spacing as a slow annealing into the tightly packed β polymorphic form occurs.

Nuclear magnetic resonance investigations of polysaccharide films, sols, and gels: I. Agarose

Spin-spin relaxation measurements on hydrated films of agarose show that it is necessary to invoke a minimum of four proton species to explain the 1H relaxation of agarose gels: These are bulk water, bound water, a very tightly bound proton species, and nonexchangeable polysaccharide protons. Using this model previously published data which had been interpreted as implying widely different hydration levels can be reconciled. Combination of the film and bulk water relaxation allows a satisfactory prediction of the concentration and temperature dependences of the gel NMR relaxation. A previous anomaly in the gel concentration dependence of T2 is also explained. A distribution of environments is indicated for the bound species; at a certain critical water content ≈0.2 g of water/g of agarose (three to four water molecules per disaccharide repeat unit), the entire bound water species increases its mobility. The assignment of the very tightly bound proton species is still undetermined; circumstantial evidence favors the agarose hydroxyl groups, but the absence of a pH dependence in the exchange rates casts a doubt upon this assignment.

Calorimetric study of the glass transition occuring in fructose solutions

Abstract The glass transition temperatures for dilute and concentrated fructose-water solutions have been determined by differential scanning calorimetry. The supplemented phase diagram is presented, and from this, the glass transition temperature (T′ g and concentration (C′ g of the maximally freeze-concentrated glass have been determined and found to differ significantly for the latter and slightly for the former, from recently published values. Reasons for this discrepancy are discussed. Annealing vitrified fructose-water glasses above their T g allows the fructose concentration in the amorphous phase to approach C ′ g as a result of ice formation.

Rheometry and detection of apparent wall slip for Poiseuille flow of polymer solutions and particulate dispersions by nuclear magnetic resonance velocimetry

A flexible and robust approach to nuclear magnetic resonance (NMR) based capillary rheometry has been developed. The precision of the technique has been explored, with particular regard to the capability for measurement of apparent wall slip velocities. The practical implications of alternative NMR rheometry protocols are discussed with regard to robustness and speed. NMR rheometry results are presented for a Newtonian fluid (aqueous 50.4% sucrose), a shear thinning solution (0.2% aqueous xanthan gum), and a particulate system composed of 5–50 μm irregular, soft agar gel particles. In all cases, fully developed Poiseuille flow was studied in a 4‐mm‐i.d., glass capillary. Radial velocity profiles were measured by NMR velocimetry; radial differentiation provides shear rate values, which have been scaled by the associated radial positions and measured pressure drops to determine viscosity as a function of shear rate. Agreement with cone‐and‐plate or parallel plate rheometry has been established for each syst...

Modelling of heat capacity–temperature data for sucrose–water systems

The significance of the point of intersection of the ‘ice-melting curve’ and glass-transition curve (measured by coordinates T′g and C′g on a ‘state’ diagram) as a characteristic feature of the behaviour of aqueous solutions of small sugar (and other) molecules on freezing, has been much emphasised in recent years. In consequence, accurate measurement of T′g and C′g for a solute–water system is important, and at present much reliance is placed upon differential scanning calorimetry as a means of achieving this. Recently, however, controversy has arisen regarding the validity of this approach, particularly where studies are restricted to the freezing of dilute solutions. In an attempt to clarify this matter, the present paper focuses on the sucrose–water system, and describes simulation of experimental heat capacity–temperature scan data for a range of sucrose concentrations. The simulations suggest that a true value for C′g can be obtained from DSC scans performed on dilute solutions only if a correction is made for the temperature dependence of the latent heat of melting of ice and, more importantly, the correct heat capacity ‘background’ is known. The simulations also suggest that, in the ideally reversible situation in which the state diagram is followed precisely, the heat capacity step occurring at T′g will contain a substantial extra contribution from initiation of ice dissolution. In real situations, however, as a result of imperfect annealing of samples during preparation, and irreversibility arising during DSC experiments, a separation and broadening of these events occurs, with the practical consequence that the measured temperatures corresponding to them bracket the true T′g value.

Rapid MRI and velocimetry of cylindrical couette flow

A narrow-gap, temperature-controlled Couette flow rheometer has been developed to study fluid velocities within the annular gap between two concentric cylinders by nuclear magnetic resonance (NMR) imaging and velocimetry. Alternative pulsed-field-gradient-based nuclear magnetic resonance imaging strategies which may be used for measurement of velocity within the Couette flow device have been evaluated. These include two-dimensional (2-D) imaging techniques with acquisition times of several minutes and a one-dimensional (1-D) projection method which exploits the symmetry of the device to reduce overall measurement time to less than 1 min. Velocity measurements made using each technique are presented for a Newtonian fluid undergoing Couette flow at shear rates of approximately 20 and 60 s(-1).

Investigations of the n.m.r. relaxation of aqueous gels of the carrageenan family and of the effect of ionic content and character

Abstract Measurements of the temperature dependences of the spin-spin relaxations of the water protons have been made in gels of kappa- and iota-carrageenan and of mixtures thereof as functions of gel concentration, and ionic content and character. A maximum and subsequent minimum are observed in the temperature dependence of the spin-spin relaxation time T 2 , with the minimum generally being related to the onset of gel melting. The temperature and the depth of the T 2 minimum are enhanced by a decrease in the sulphate content of the polysaccharide and an increase in ionic content, with differential cationic and anionic effects being observed. A model is applied in which it is assumed that, in the temperature region between the T 2 maximum and minimum, the relaxation is dominated by an increasingly effective contribution by a highly immobile proton species associated with the polysaccharide. The determination of the relaxation of the water protons by the mobility and rate of exchange of this immobile species allows the water relaxation to act as a sensitive probe of the polysaccharide dynamics and aggregation.

NMR flow imaging of aqueous polysaccharide solutions

Nuclear‐magnetic‐resonance (NMR) flow imaging with spatial resolution of the order of 200 μm is used to measure the velocity fields in aqueous solutions of 0.2% and 1% xanthan gum and 1% guar gum for steady flow in a 1.2 cm internal diameter cylindrical polymethylmethacrylate pipe. The velocity fields show little evidence of apparent wall slip and are differentiated to obtain relationships between shear stress and shear rate which span over four decades in shear rate for the xanthan solutions and approximately three decades for the guar solution; these data agree well with those obtained by cone‐and‐plate viscometry. The behavior of the xanthan solutions is well described by a power‐law dependence of shear stress on shear rate, and the guar solution is better described by a Cross‐type relationship. The implications of these studies for future NMR flow imaging studies of more complex systems are discussed.

A13C-NMR study of the crystal polymorphism and internal mobilities of the triglycerides, tripalmitin and tristearin

The combined NMR technique of cross-polarization, high power decoupling and magic angle spinning (CP/MAS) has been used in the study of the polymorphic forms of the two triglycerides, tripalmitin and tristearin. Variations in the13C chemical shift values for the different polymorphic forms have been found and used to obtain molecular information, and to distinguish between chain orientations. Information on the relative mobilities of particular regions within the molecule for the different polymorphs also is presented.