The quantification of small degrees of disorder in lactose using solution calorimetry

There is a realisation that small quantities of amorphous material can have a significant impact on the properties of crystalline solids. Consequently there is a growing interest in quantifying the amount of amorphous material that is present in "crystalline powders". Success has been reported when using isothermal microcalorimetry and vapour sorption techniques, however, the use of solution calorimetry has largely been ignored. In this study the enthalpies of solution of mixtures of amorphous and crystalline lactose are reported concentrating on the range 0-10% w/w amorphous content. It was found that there was a possible error due to water vapour penetration into the ampoule, resulting in crystallisation of the amorphous content, however this was overcome by double sealing the ampoules with wax. Subsequently there was a good correlation between the enthalpy of solution and the amorphous content, which was not adversely affected by stirring rate used during the experiment. Over the range from 0 to 10% amorphous content, quantification of the amorphous content of an unknown would be good to +/-0.5%. The effects of residual moisture retained within a sample were also investigated. Storage at 33% or 43% RH resulted in a much reduced wetting (exothermic) response compared with that seen for completely dry samples, which in turn led to a higher net enthalpy of solution.

Near IR spectroscopy to quantify the silica content and difference between silicified microcrystalline cellulose and physical mixtures of microcrystalline cellulose and silica

Silicified microcrystalline cellulose (SMCC) has been shown to have advantages over conventional microcrystalline cellulose (MCC). These advantages are (i) improved tablet strength compared to that achieved with MCC, (ii) the retention of compressibility after wet granulation, whereas MCC produces weaker tablets after wet granulation, and (iii) superior flow properties than MCC. In this study near IR spectroscopy has been used to study MCC, SMCC (with different loadings of colloidal silicon dioxide, CSD) and physical mixtures of MCC and CSD. It was found that even though SMCC and MCC were very similar, there was a region of the near IR spectra (second derivative peak at 2194 nm) where a distinctive response was seen for SMCC. The size of the peak was proportional to the CSD content for the co-processed SMCC samples. The peak was not present to the same extent for physical mixtures. A combination of near IR and a test for total silica content would make it possible to discern whether microcrystalline cellulose samples were SMCC material or simple physical mixtures.

An exploration of inter-relationships between contact angle, inverse phase gas chromatography and triboelectric charging data

Surfaces of pharmaceutical powders have been assessed using contact angle, inverse phase gas chromatography (IGC) and triboelectric (electrostatic) charging techniques. The suitability of the Dynamic Angle Tester (DAT), an instrument based on the sessile drop technique, in determining contact angles and then the surface energy of pharmaceutical powders was assessed. The dispersive components of the surface energy of powders determined from the DAT and IGC method ranked the powders in the same order. The dispersive component values obtained by IGC were, as expected, higher than those from the DAT, due to IGC probing the highest energy sites on the powder surface. IGC and triboelectric studies allow materials to be characterised in terms of their electron donating-accepting tendencies, so inter-relationships between the data from the two techniques were explored. Although the data set was limited, there appeared to be a correlation between the charges developed by the powders on contact with stainless steel and the ratio of the electron-donating to electron-accepting tendencies of the materials as obtained from IGC.

Isothermal microcalorimetry and inverse phase gas chromatography to study small changes in powder surface properties

It is known that processing can alter the surface energetics of powders. In this study a sample of drug has been processed by use of different drying techniques. The samples were then assessed using inverse phase gas chromatography. It was seen that the original material had a highest surface energy and the tray-dried sample had the lowest energy surface, other samples were intermediate. The use of isothermal microcalorimetry to study water sorption to the powders revealed that the surface of the original material was unstable, as the water sorption response changed on repeat cycling. The tray-dried sample did have a stable surface which gave the same sorption response on repeat exposure to water vapour. It was concluded that the drug had minor variations in surface energy, with the as received material being in a high energy unstable state, which could be due to it being partially amorphous. The tray-dried sample had a lower energy stable surface. In certain applications differences in surface energetics could be expected to lead to changes in processing nature of the powder, so these vapour sorption techniques offer a good way of providing an assurance of the same surface energy between batches of nay material which may be at risk.

Differences in crystallization behavior between quenched and ground amorphous ursodeoxycholic acid

PURPOSE

To study the crystallization of ground and quenched ursodeoxycholic acid (UDCA) and to characterize their amorphous states.

METHODS

Amorphous UDCA was prepared by grinding and also by rapid cooling of the melt. These samples were characterized by powder X-ray diffraction (XRD), near IR spectra and dynamic water sorption. The heat associated with crystallization was measured in an isothermal microcalorimeter at 25 degrees C at various relative humidities (RH) (50%-100%) and, in the presence of the vapour from a mixed solvent of ethanol and water (ethanol conc. 10%-100%). The specific surface area was calculated from krypton adsorption. Contact angles were measured by using a Wilhelmy plate to calculate the surface energy of the samples.

RESULTS

Ground and quenched samples yielded amorphous XRD patterns. Differential scanning calorimetry thermographs of the milled sample revealed that crystallization occurred at around 80 degrees C, whereas the quenched sample did not crystallize. Exposure to humid air did not result in crystallization of either amorphous sample during the microcalorimetric experiments. In the presence of ethanol vapour, the ground sample did, but the quenched sample did not, crystallize. The amount of water sorption into the quenched sample was larger than that of the ground sample at low RH. The surface energy of the quenched material was different to that of the ground. Peak shifts were observed in the NIR spectra at around 1450, 2100 nm, allowing differentiation between the ground and quenched samples.

CONCLUSIONS

It can be concluded that different molecular states of amorphous UDCA were obtained depending on the preparation method. The crystallisation of amorphous UDCA was related to the molecular state of disorder.

A high sensitivity differential scanning calorimetry study of the interaction between poloxamers and dimyristoylphosphatidylcholine and dipalmitoylphosphatidylcholine liposomes

High sensitivity differential scanning calorimetry (HSDSC) has been used to measure the thermal behaviour of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) liposomes to which poloxamer surfactants P338 or P407 had been added during or after preparation. The phospholipid pre-transition was more sensitive than the main transition to the association of poloxamers with liposomal bilayers. Poloxamers reduced the enthalpy of the pre-transition of liquid-crystalline state DMPC and DPPC MLVs but not that of gel state DPPC MLVs. Freezing and thawing DMPC and DPPC liposomes in the presence of poloxamers was shown to increase their interaction with the liposomal bilayers.

Water sorption and near IR spectroscopy to study the differences between microcrystalline cellulose and silicified microcrystalline cellulose before and after wet granulation

Silicified microcrystalline cellulose (SMCC) has been shown to have advantages over conventional microcrystalline cellulose (MCC). These advantages are (i) improved tablet strength compared to that achieved with MCC, (ii) the retention of compressibility after wet granulation, whereas MCC produces weaker tablets after wet granulation, and (iii) superior flow properties than MCC. In this study gravimetric and calorimetric vapour sorption data and near IR spectroscopy have been used to study MCC and SMCC before and after wet granulation. It was found that MCC, SMCC and wet granulated SMCC had essentially identical physical structures (except for a size increase due to granulation). Wet granulated MCC had a different enthalpy of water sorption at low RH, and its near IR spectrum was different from the other samples in the region which relates to C-H bonding. It can be concluded that MCC and SMCC are of very similar structures, thus these analytical techniques cannot provide an explanation for the improvements in compressibility. However the change in compressibility in MCC after wet granulation may relate to the observed differences in internal bonding in this sample.

Assessment of disorder in crystalline powders--a review of analytical techniques and their application

The need to be able to measure amorphous contents in crystalline powders is now recognised. In this review, calorimetric and gravimetric methods are reviewed in a way that should alert workers in the field to the theoretical, and practical considerations which are important to understanding how best to study crystalline samples which contain low levels of amorphous material. It is shown that vapour sorption techniques are very powerful as long as serious consideration is given to the choice of environmental conditions and the exact experimental methodology. As the amount of published work in this field grows, it becomes increasingly necessary to describe experimental and data manipulation methods in great detail.

Crystallization of bulk samples of partially amorphous spray-dried lactose

The crystallization of partially amorphous spray-dried lactose was studied as a function of sample size. Crystallization occurred gradually over a period of 80 hr for a 95-g sample. The water content during crystallization was lower than that needed to cause crystallization if it had been distributed evenly throughout the bed, thus the absorbed water must have been unevenly distributed. The weight of the sample continued to change for days after crystallization was completed, because of the slow desorption of condensed water and the very slow formation of the hydrate form. Surprisingly, all samples with a weight between 42 and 95 g were found to take up the same mass (not percent) of water at the same time. This provides further evidence that the water was not evenly distributed throughout the sample. Water loss after this peak differed in the different weight samples, with the largest weights resulting in the lowest residual weight after 2 weeks. Only the sample of 22 g load had a different peak weight and a much lower weight loss after crystallization. This study provides detail of how partially amorphous bulk samples crystallize.

Differences between dynamic and equilibrium surface tension of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) block copolymer surfactants (poloxamers P407, P237, and P338) in aqueous solution

Poloxamer surfactants are macromolecules with complex interfacial behavior. Although a number of studies of equilibrium surface tension have been published recently, there is little information on the diffusion of these large molecules to the air-liquid interface. Because most surfactants are used in dynamic systems, the diffusion to the surface can be critical in controlling performance. In this study a maximum bubble pressure method was used to study dynamic surface tension (DST) of Poloxamer P407, P237, and P338, at a range of bubble rates (surface age) and concentrations, at either 25 or 35 degrees C. The DST did not change at the critical micelle concentration and also did not vary in the same manner as the equilibrium surface tension (EST) with respect to temperature. It was concluded that DST behavior of the surfactants was most closely related to the poly(oxyethylene) content and/or total molecular weight of the surfactants, whereas the micellization and hence the EST were more closely related to the poly(oxypropylene) content.

The effect of temperature on the surface nature of an adsorbed layer of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) block copolymers

PURPOSE

To investigate the influence of the temperature at which adsorption takes place and the temperature at which the adsorbed surface is studied on the polarity of Poloxamer adsorbed to a hydrophobic surface. The implication is that changes in surface nature of adsorbed Poloxamer may subsequently be related to functionality, such as changes in opsonisation of Poloxamer coated latex in animals.

METHODS

The surface energies of Poloxamer surfactant have been calculated following adsorption to silanised glass plates. The adsorption to the plates was undertaken at a range of concentrations and at different controlled temperatures. The contact angles were measured using three different liquids on each surface, at a range of controlled temperatures. The surface energies were calculated using the harmonic mean and the acid-base models, via Wilhelmy plate contact angle measurements. These data were compared with previously published adsorption and hydrophobic interaction chromatography studies.

RESULTS

The apolar surface energy term remained consistent, but the polar contribution (which was totally of the electron donor type) changed depending upon the temperature of adsorption (and to a lesser extent the temperature at which the surface energy was measured). The polar nature was most elevated at the critical micelle concentration/temperature. The data are consistent with estimates of surface hydrophobicity made using hydrophobic interaction chromatography.

CONCLUSIONS

It is argued that the changes in surface energy, which result from the different adsorption conditions, can be expected to influence the functionality of the adsorbed coat, especially for application such as drug targeting.

Dissolution behaviour of sulphonamides into sodium dodecyl sulfate micelles: a thermodynamic approach

The surface energies of four sulfonamides have been assessed from contact angle data, using the Lewis acid-base approach. From these data the free energy of adhesion between the drugs and sodium dodecyl sulfate (SDS) head groups and tails has been calculated. The most favored interaction was for adhesion to the SDS tails, rather than the head groups. The initial rotating disk dissolution rate (hereafter termed dissolution rate) of drug compacts has been measured in water and water with SDS micelles at a range of temperatures. The thermodynamic parameters of activation have been calculated from the rate data. Linear relationships exist between the enthalpy of transfer between water and SDS micelles and the free energy of adhesion between the drugs and both SDS head groups and SDS tails. The most nonpolar drugs had the most favored free energy of adhesion and the most favored enthalpy of transfer. The most polar drug had a disfavoured free energy of adhesion to the SDS head and a disfavoured enthalpy of transfer. This response demonstrates that the most important barrier to the passage from the aqueous fluid to the hydrophobic core of the micelle is the monopolar repulsion between the polar forces of the drug and head group surface energies. This provides a new insight into a possible mechanism of solubilization and offers the prospect of understanding even more complex partitioning behavior.

A microcalorimetric investigation of the interaction of surfactants with crystalline and partially crystalline salbutamol sulphate in a model inhalation aerosol system

PURPOSE

The purpose of the work is to study the adsorption of Oleic acid and Span 85 (materials frequently used in aerosols as surfactants) onto partially amorphous and essentially crystalline salbutamol sulphate, attempting to understand the behaviour of metered dose inhalers (MDIs) and observing whether there were any differences in adsorption behaviour and if this could be related to the surface properties of the powder.

METHODS

Isothermal titration microcalorimetry was the principal technique used to measure the adsorption behaviour of surfactants to salbutamol sulphate. A Malvern particle size analyzer was also employed to provide size data on the interactions between the surfactant and powder suspensions.

RESULTS

The calorimetric data revealed that surfactant adsorption to the crystalline micronised powder (78% RH and aged dry sample) produced significant exotherms, whereas adsorption to the partially amorphous micronised powder resulted in small heat responses. The differences in adsorption behaviour to the partially crystalline and crystalline surfaces resulted in changes in aggregation behaviour.

CONCLUSIONS

The stability of MDIs varies depending on the water content, crystallinity and surface composition of the powder. The advantages of using isothermal titration microcalorimetry to evaluate this surface behaviour in such difficult systems was demonstrated.

Development of a flow microcalorimetry method for the assessment of surface properties of powders

PURPOSE

This study describes the development of a microcalorimetric flow cell which allows powder surface energetics to be probed my means of measuring their interaction with water vapour.

METHODS

A flow cell has been constructed and tested in an isothermal microcalorimeter to assess the interaction between water vapour and powder surfaces. The cell was constructed to mix two separate air streams (0% and 100% relative humidity respectively) to create any humidity at a standard flow rate. The powder sample was equilibrated in dry air and then exposed to sequential increments in humidity.

RESULTS

Adsorption isotherms were constructed from the cumulative heat as a function of humidity. It was possible to differentiate between different samples of alpha-lactose monohydrate (which appeared identical by contact angle determination). It was also possible to measure adsorption to two different alkyl p-hydroxybenzoates which were hydrophobic and of low surface area.

CONCLUSIONS

This technique offers a very sensitive and versatile method of obtaining a reliable indication of powder surface energetics and as such is a major advance in the field.

Modelling mucoadhesion by use of surface energy terms obtained from the Lewis acid-Lewis base approach. II. Studies on anionic, cationic, and unionisable polymers

Surface energies of carbopol, chitosan, hydroxypropyl cellulose (HPC) and poly(HEMA) were assessed from contact angle and surface tension experiments. The surface energy was considered in terms of an apolar Lifshitz-van der Waals term and a polar acid-base term, which in turn is divided into electron donor and electron receptor (Lewis acid-Lewis base) contributions. Using these surface energy terms the interaction of dry and hydrated polymer with mucin in the presence of either artificial gastric or intestinal fluid, or saline was predicted. The predictions were related to measured forces of detachment. There was a significant difference between the surface energy on dry and hydrated HPC and also for carbopol; for the other polymers either the surface energy of the hydrated material was not detectable, or the effect of hydration was minimal. There were good correlations between mucoadhesive strength and the calculated free energies of interaction between mucin and polymer in the presence of each of the fluids, for each individual polymer. Thus, two trends were observed, one for unionisable and the other for ionisable polymers. It is argued that the increased mucoadhesion seen with ionisable polymers (compared with the predicted value based on results of unionisable polymers) is a direct result of the ionic interaction. No attempt has been made to correct for the ionisation effect, but the surface energy predictions provide insight into the mechanism of the mucoadhesion process. This approach is useful for understanding and predicting interactions between different materials and biological components.

The interaction of albumin and drugs with two haemofiltration membranes

The sorption of phenobarbitone sodium, barbitone sodium and fluconazole onto haemofiltration membranes made from polysulphone or a co-polymer of polyamide and polyvinylpyrrolidone was investigated in the presence and absence of albumin. The sorption of albumin was also followed in the presence of phosphate-buffered saline. Drug binding to the membrane was found to be reversible. Knowledge of the lipophilicity of the drug and hydrophobic/hydrophilic nature of the membrane did not allow successful prediction of the extent of binding of all the drugs; nor did knowledge of the extent of ionization of the drug and the charge of the membrane. Albumin bound to the polysulphone membrane in a manner that suggested the surface area to which it was binding was around 10 times greater than reported. In the presence of albumin there was a larger coefficient of variation in the binding of drugs to both membranes. The presence of albumin significantly decreased the binding of fluconazole, but not the other drugs, to the polysulphone membrane; however, albumin had no effect on the binding of any of these drugs to the polyamide membrane. We conclude that the binding of drugs to haemofiltration membranes cannot be simply predicted from knowledge of the hydrophilic/hydrophobic nature or charge of the drug and membrane, nor from the protein binding of the drug.