Phase behaviour and rheological properties of aqueous systems of industrial distilled monoglycerides

Chem. Phys. Lipids 2 (1968) 129-143 © North-Holland Publ. Co., Amsterdam

PHASE

BEHAVIOUR

AND

RHEOLOGICAL

PROPERTIES

OF

A Q U E O U S SYSTEMS OF I N D U S T R I A L D I S T I L L E D MONOGLYCERIDES NIELS KROG and KA,RE LARSSON Aktieselskabet Grindstedvaerket, Aarhus, Denmark and Crystallography Group, University of Gb'teborg, Sweden Received 17 July 1967 On the basis of earlier studies on a series of pure monoglyceride-water systems, the phase behaviour of industrial samples of distilled monoglycerides in aqueous systems is presented. The rheological properties have been examined and related to our knowledge of molecular arrangements in the phases. The effect of ionic strength and pH on the existence range of different phases was also studied, and it was found that drastic changes in phase equilibria could be obtained.

Introduction Monoglycerides in pure form are used increasingly within the food industry, mainly as emulsifiers and promotors for fat crystallization. Distilled monoglycerides are also used in fatfree foodstuffs and products with very low fat content. In these cases the monoglycerides cannot be dissolved in a fat phase and the best way to incorporate them is to add them in the form of monoglyceride-water mixtures. Detailed knowledge of the phase behaviour and structure of the actual monoglyceride-water systems is therefore very important from a technical point of view. Mesomorphism in a monoglyceride-water system was first described by Marsden and McBain 1), but their interpretations of the observations were not correct, as the organization of hydrocarbon chains in mesophases was not then known. Later Brokow and Lyman 2) reported some observations on mixtures of monoglyceride and water. They characterized the observed phases as emulsions and mixtures of crystals and water. No description of structures, however, was given. The liquid nature of the hydrocarbon chains in mesophases and the general structure principles in amphiphile-water systems have been established by the fundamental work of Luzzati and co-workers 3). The phase behaviour of a series of aqueous 1-monoglyceride systems ranging from C12 to C22 in acyl chain length has been studied by Lutton 4), who identified the phases with

130

NIELS KROG AND K,~RE LARSSON

those occurring in soap-water systems. The structure of micelles and mesophases in monoglyceride-water systems has been investigated in detail by Larsson 5). Very short members (down to C6) were then also examined. Lawrence and McDonald 6) have recently reported the phase diagram of the system monolaurin-water. The work reported here deals with industrial 1-monoglyceride samples, where one component dominates the composition although homologous members with longer and shorter chains are present as well as other glycerides. The main interest has been devoted to the behaviour in the water-rich region of the system, where the technical effects are optimal. Materials and methods

The industrial monoglycerides were produced by interesterification of fully-hardened lard and glycerol. The mixture of mono-, di- and triglycerides was then molecular distilled under high-vacuum, by which the content of 1-monoglyceride was concentrated to 92~o. The final product has the following composition : Monoglycerides 94.0 Diglycerides 3.5 Triglycerides -f 1 Free fatty acids -~. I Glycerol ~ 1 Fatty acid composition:

Myristic acid Palmitic acid Stearic acid Arachidic acid

3.2 32.5 62.0 2.3

It can be seen that l-monostearin dominates in this industrial product, which will be denoted G M S in the following text. The monoglycerides were mixed with water in concentration intervals of 5~o. The mixtures were placed in a thermostatically controlled water bath under stirring, and phase transitions were observed by microscopy and viscosity measurements. The viscosity in monoglyceride-water phases was measured on a Rotovisko coaxial-cylinder viscometer according to Haake 8). The X-ray scattering patterns of the monoglyceride-water mixtures were also recorded at different temperatures in a low-angle camera of the Guinier type equipped with a bent quartz monochromator for separation of CuK~Iradiation. The samples were kept in cells with mica windows and exposed in vacuum in order to reduce the background scattering. Results and discussion

The phase diagram of the system 1-monopalmitin-water is given in fig. 1.

INDUSTRIALDISTILLEDMONOGLYCERIDES Neat 100

+ viscous

~ f Fluid isot

131

lsotropic

~

.

.

.

.

.

.

Viscous i s o t r o p i c

90

Viscous ist)tropic + tl20

70

+.o

Dispersion

,,..~~

i

i ..........

40

Crystals + I!20 or metastable

10,

%

;o

Crystals + lt2C) or n + e t a s t a b l e g e l + 1120

gel

40, ~ 50 - - - I P ~ ,m,I12(~

i

60

70, .....

80 ~

/o

100

Fig. 1. Phase diagram of l-monopalmitin-water. This example of a pure monoglyceride will first be discussed briefly since the same phases occur as in samples of industrial monoglycerides. Even the phase diagram is quite similar to that of the monopalmitin-water system, although the sample consists of a complex mixture. This fact can be correlated with earlier observations on the solid state behaviour. Although complex lipids, such as fats, consists of many components, they show the same type of phase behaviour as a single component. If a sample of 1-monopalmitin crystals in 20-30~o water is heated, nothing happens until the temperature reaches about 60°C, when a mesophase is formed. This is the neat phase (the nomenclature from soap-water systems a) is used). The well-known structure consists of bimolecular lipid layers separated by water, and is indicated in fig. 2a. The chains are in a disordered state similar to that of liquid n-paraffins. If this phase is cooled, a gel is formed, the structure of which is shown in fig. 2b. The structure is still lamellar, but the crystallization temperature of the hydrocarbon chains has been reached. The chains are therefore extended and arranged in parallel in the same way as in the solid state. The chains are tilted about 54 ° towards the water layers, and the lateral packing of the chains can be described by a hexagonal subcell,

132

NIELS KROG AND KARE LARSSON

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INDUSTRIAL DISTILLED MONOGLYCERIDES

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which indicates that the chains possess rotational freedom (cf. ref.7)). The gel is metastable, and transforms into anhydrous crystals+water. This is the reason for two lower boundaries of the neat phase (the upper one corresponds to transformation of fl-crystals+water to neat phase on heating, and the lower one corresponds to transformation of neat to the metastable gel on cooling). A further complication of the phase diagram is the polymorphism of the crystals, a-Crystals+water are first formed from the gel, and then transformation into the stable crystal form fl, takes place. Another phase transition occurs when the neat phase is heated. A very stiff cubic phase - viscous isotropic - is then formed. The structure is shown in fig. 2c. It consists of small water spheres arranged in a face-centred lattice and separated by monopalmitin molecules, which expose their polar groups towards the water. An isotropicfluid is obtained on further heating, and an earlier analysis of its structure 5) indicated building units of micellar size with lamellar structure within these units. The polar groups and the water in-between form discs with a diameter of about 300 A, and the proposed arrangement is indicated in fig. 2d. The water layer of the neat phase does not exceed about 20 A in thickness. With excess of water a dispersion is formed, which is not neat phase particles +water. The structure consists of concentric bimolecular shells of monoglyceride molecules alternating with water shells as indicated in fig. 2e. The particles are spherical or cylindrical, and the cylindrical particles can be obtained from the spherical ones by mechanical treatment, e.g. when the spherical particles are forced to flow rapidly. The sperical lipid layers will then deform into cylindrical layers along the path of the particles, and an initial stage in this process is also indicated in the figure. This transformation process leads ultimately to a transparent gel-like state which consists of a three-dimensional network of cylindrical threads. A consequence of this successive change in structure is that the dispersion exhibits negative thixotropy. The spherical particles represent the stable state of the dispersion, and the cylindrical threads will thus change into spherical units. The transition time increases with chain length, and the gel-like state of the dispersion (cylindrical particles) can exist for several days when the acyl chains are longer than C16. This state will be discussed in detail later, as it exhibits optimal emulsification properties. The effect of chain length will also be briefly described. At very short chain length (C6) an ordinary micellar solution is formed in the region where the dispersion exists for longer members. The micelles are spherical with a radius of 12 A. No mesophase exists there; crystals + water goes over directly to an isotropic fluid. For chain lengths C8 to C12 a neat phase is formed, and

134

NIELS K R O G A N D K A R E L A R S S O N

consequently also a dispersion in the water-rich region of the phase diagrams. A hexagonal mesophase is formed when the acyl chains are very long (C21), with water cylinders arranged in a hexagonal lattice surrounded by the monoglyceride molecules. The phase diagram of the system GMS-water is shown in fig. 3. It can be seen that viscous isotropic occupies a larger proportion than in the phase diagram monopalmitin-water, whereas the neat phase occupies a smaller area. The dotted line under the region of the neat phase (or the dispersion) shows the phase transition into gel (or gel +water) by cooling, and the corresponding solid line shows the transition into neat (or dispersion) when crystals (fl-form) + water is heated. Optical textures When examined in polarized light the dispersion shows characteristic spherical or cylindrical aggregates like myelin figures. A photomicrograph is shown in fig. 4. When the dispersion is cooled the hydrocarbon chains will "crystallize" as mentioned earlier, and the inner structure of the particles will then be the Neat + viscous

izotropic

100 t,'luid isotropic

90

Viscous

i~otropic Viscous

lsotropic

+ I120

80

70

~C

Neat /

Dispersion

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50 ~

~

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Gel or + IIEO

G e l + t120 o r ÷ fig()

crystals

crystals I L

30

I ~__ 10

__J__ 20

I 30

A 40

1 q0

I

60

_.A

70

% ti20

Fig. 3. Phasediagram of GMS-water.

1

a

80

90

100

INDUSTRIAL DISTILLED MONOGLYCERIDES

135

same as in the gel (fig. 2b). The optical texture after this transition is shown in fig. 5, where the structure is a three-dimensional network of long aggregates with gel structure. At high water content ( > 8 5 ~ H 2 0 ) the dispersion is transparent after it has been brought into the metastable gel-like state and behaves like a single phase. It should also be mentioned that the dispersion shows all the optical textures that characterize the nematic state. Very little was known earlier about the structure in the classical nematic state, and it has been proposed that the structure of the dispersion described above represents a general model of the nematic state ~).

Fig. 4. Optical texture of a GMS-water 10:90 dispersion. Magnification 875 × (linear).

Viscosity The viscosity curves of fluid isotropic as a function of water content for members Ca to C22 are shown in fig. 6. The most striking feature is the tendency of the curves; the viscosity of long members increases with water content whereas it decreases with short members. Rheograms were recorded

136

N1ELS KROG AND K/~RE LARSSON

Fig. 5. Optical texture of a GMS-water 10:90 sample which consists of gel g H20. Magnification 875 × (linear). at different rates of shear, and it was found that fluid isotropic show the behaviour of a newtonian liquid. In the concentration region which corresponds to neat and dispersion there is a remarkable change in viscosity from up to 10000 cP in the neat phase to values in the range 1-100 cP in the dispersion region. Both the neat phase and the dispersion show plasticity. The viscosity curve shown in fig. 7 demonstrates the decrease in relative viscosity from the region of neat phase into the dispersion. It can be seen that the logarithmic viscosity values decreases almost linearly with increasing water content in the dispersion region. When the water content is increased by 20~o, there is a decrease in viscosity with a factor of about 10. The variations in relative viscosity of the dispersion indicated in fig. 7 correspond to differences in shape and size of the particles due to the mechanical treatment of the sample before the recording.

Effect of electrolyte concentrations It was found that the ionic strength influences the phase transition from

INDUSTRIAL

DISTILLED

137

MONOGLYCERIDES

fl-crystals + H 2 0 to neat phase or to dispersion strongly. The effect of sodium chloride on phase transition temperatures from crystals (fl-form)+ water to the neat phase is shown in fig. 8. Addition of sodium chloride gives a higher transition temperature. If 2 ~ NaCI is added (all concentrations of sodium chloride are expressed in per cent of the water phase) the phase transition I4 e l a t i v e

viscosity cP

C 20

/

40

t.

30

C

16

20

C 12

10

L____ C8

Fig. 6.

I

I

I

I

I

Z

4

6

S

10

-~ % [-t20

Relative viscosity in fluid isotropic of the Cs-Cz0 1-monoglyceride series as a function of water content. Shear rate 1310 sec -1, temperature 80 °C.

138

NIELS KROG AND K/~RE LARSSON

Relative viscosity

cP

iO00

500 400 300

\ 200

\ 100

' \

50 40 30

20

10

i 40

Fig. 7.

50

60

70

80

90

lOO

R e l a t i v e v i s c o s i t y in G M S - w a t e r m i x t u r e s in t h e r e g i o n of t h e n e a t p h a s e a n d d i s p e r s i o n . S h e a r r a t e 437 sec -1, t e m p e r a t u r e 60 °C.

INDUSTRIAL DISTILLEDMONOGLYCERIDES

139

Temperature °C

v i s c o u s isotropic

Neat~ C r y s t a l s + H20

60

50

I 0

I 2

I

I 4

I

I 6

I

I 8

I

I i0

~ % NaC1

Fig. 8. The effect of the addition of sodium chloride to a G M S - w a t e r 50:50 mixture. (The concentrations of sodium chloride are given in per cent of the water phase.) Temperature °C

70 i

D r sion i s ~pViscous e

sotropic

65

60

y s t a l s + H20

55

50

Fig. 9.

I 0.2

I 0.6

1 1.0

I 1.4

I 1,8

I 2.0

~-~ % N a C I

The effect of the addition of sodium chloride to a G M S - w a t e r 10:90 mixture.

140

NIELS KROG AND KARE LARSSON

temperature increases from 57 °C to 65 °C, and 4 ~ increases the temperature to 69 °C. At higher salt concentrations a direct transition from crystals + H20 to viscous isotropic is obtained. This effect is more pronounced at low concentrations of monoglycerides in water (the dispersion region). In fig. 9 the influence of sodium chloride concentration on the phase transition temperature of a mixture of 10~o GMS in water is shown. The system was buffered to a pH of 7.0. In this case a dispersion is normally formed at 52 °C. 0.3~o sodium chloride increases the transition temperature from//-crystals+ water to dispersion from 52°C to 60°C. At higher concentrations (>l'y,,) no dispersion was formed, and viscous isotropic was obtained directly from crystals and water. It was also found that more than 0.2~o sodium chloride decreases the transition temperature from dispersion to viscous isotropic from 70 °C to 65 °C. The same effect was also observed in aqueous systems of pure 1-monopalmitin and l-monostearin. In the case of shorter members (C8-Cx2), where no cubic phase exists, a direct transition from crystals + water to fluid isotropic+water is obtained. The changes in phase transition by the addition of sodium chloride to a l-monolaurin-water 10:90 mixture is shown in fig. 10. A dispersion can not be formed if the salt concentration is higher than 3~o. The particle size in the dispersion region depends on the °C

7

\

0

~

i

d

isc Fluid isotropic + II20

0!; j

1

2

3

4

5

6

k. 7

--

1 8

----II1~ % ~Na(~l

Fig. 10. The effect of the addition of sodium chloride to a 1-monolaurin-water 10:90 mixture. The shaded region corresponds to a transparent dispersion.

INDUSTRIAL

DISTILLED

MONOGLYCERIDES

141

temperature, and when a diluted dispersion of monolaurin is heated to about 50°C, the milky appearance disappears and a transparent phase is formed. Such a clear dispersion is still optically birefringent, and the birefringence increases strongly when the dispersion is streaming. The particle size in the dispersion is also related to the salt concentration as shown by the shaded area in fig. 10.

Effect of pH The value of p H appeared to be another important factor equilibria in monoglyceride-water systems. G M S - w a t e r mixtures were prepared and buffered to a p H from 4.0 to 8.0 with intervals of 0.5. The amount of buffer solutions in relation to the total water content was 10~. The result is shown in fig. 11. When the G M S - w a t e r °G

Viecous

iaotropic

65

60

55

Gryst

50

I

I 5

r

l 6

I

I 7

I

I 8

~-- pH

Fig. 11. The effect ofpH on a GMS-water 10:90 mixture. The region where a transparent dispersion exists is shaded. mixture is heated from room temperature to about 50°C a change of state from crystals + water to the dispersion takes place in samples with a p H of 8. As p H is lowered, a higher temperature is needed before any phase change occurs. At a p H of 7 the dispersion is formed at 53 °C, and at a p H of 6 the

142

NIELS KROG AND K,g,RE LARSSON

formation of dispersion does not take place until 60 °C. I f p H is below 5 the dispersion cannot be obtained, and crystals and water transform directly to the viscous isotropic phase. Just above the temperature limit where the dispersion is formed this has a translucent appearance, but a pH of 7 or higher the dispersion is completely clear in the temperature range above 60-65 °C. Similar effects were also confirmed in systems of pure 1-monoglycerides. For shorter members (Cs-C12), where a viscous isotropic phase does not exist, crystals + water are transformed directly into fluid isotropic and water. The relation between pH and phase transition temperatures in the case of 10% 1-monolaurin in water is shown in fig. 12. As can be expected the diagram is very similar to the corresponding one of a GMS-water mixture (fig. 11). The dispersion is completely clear in the actual temp. and pH range, which shows that the particle dimensions are less than about 500 A. These changes in phase equilibria are important from a theoretical point of view. A considerable body of literature concerns the effect of ions on the °C

75

Fluid isotropic

+ H20

70

)ispe r sion/ 65

// // // // //

60

Crystals

+ H20

55

50

I

I

5

Fig. 12.

[

I

6

I

I

7

I

i

~

pH

8

The effect of pH on a 1-monolaurin-water 10:90 mixture. The dispersion is transparent (cf. fig. 10) in the shaded region.

INDUSTRIAL DISTILLED MONOGLYCERIDES

143

water structure. It seems to be generally accepted t h a t the i n t r o d u c t i o n o f ions changes the structure in the direction o f a smaller degree o f ice-likeness. Such s t r u c t u r e - b r e a k i n g properties should have a similar effect as an increase in t e m p e r a t u r e . The observations r e p o r t e d here indicate t h a t much m o r e c o m p l e x m e c h a n i s m s are involved. The salt c o n c e n t r a t i o n is a p p a r e n t l y critical for the existence o f the neat phase and the closely related dispersion, whereas the phases viscous i s o t r o p i c and fluid isotropic are a l m o s t uninfluenced. The effect o f p H on the phase equilibria m a y reflect differences in the effect o f anions a n d cations on the water structure. The presence o f a t m o s p h e r i c c a r b o n dioxide should, however, also be k e p t in m i n d when the p H effects are considered.

References 1) 2) 3) 4) 5) 6) 7) 8)

S. S. Marsden, Jr. and J. W. McBain, J. Phys. Colloid Chem. 52 (1948) 110 G. Y. Brokow and W. C. Lyman, J. Am. Oil Chemists' Soc. 35 (1958) 49 V. Luzzati, H. Mustacchi, A. Skoulios and F. Husson, Acta Cryst. 13 (1960) 660 E. S. Lutton, J. Am. Oil Chemists' Soc. 42 (1965) 1068 K. Larsson, Z. phys. Chem. (Frankfurt) (1967) in press A. S. C. Lawrence and M. P. McDonald, Mol. Cryst. 1 (1966) 205 K. Larsson, Nature 213 (1967) 383 Viscosity and flow measurements. A laboratory handbook of rheology. Interscience Publ., London (1963) p. 103