Influence of Acacia gum molecular polydispersity on the complex coacervation with β-lactoglobulin

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Van der Waals interactions

For two surfaces separated by a small distance, electric and magnetic polarizations produce an electromagnetic field that fluctuates within the medium 32 and can be responsible for colloidal aggregation. The potential energy of this van der Waals interaction is a function of the separation distance, r, between dipoles (see Fig. I.2). van der Waals forces are always present and always attractive between particles of the same nature.
Two theoretical ways allow the evaluation of the attractive interaction: the microscopic and the macroscopic approach. The first approach is the classical or microscopic approach and is due mainly to Hamaker 33 and it is based on the assumption of additivity of London pair interaction energy. This predicts the van der Waals attraction with an accuracy of 80%–90%.
The macroscopic approach, that eliminates the additivity assumption of Hamaker approach has been suggested by Liftschitz 34. It gives a more accurate evaluation of the van der Waals attraction and it is based on the correlation between electric fluctuations of two macroscopic phases. However, this approach requires quantification of the dielectric dispersion data, which are available for only a limited number of systems. For this reason, most results on the van der Waals attraction are based on the microscopic approach that is described below.
For a one-component system, individual atoms or molecules attract each other at short distances due to van der Waals forces. The latter may be considered to consist of three contributions: dipole–dipole (Keesom), dipole–induced dipole (Debye) and London dispersion interactions. For not too large separation distances between atoms or molecules, the attractive energy Ga is short range in nature and it is inversely proportional to the sixth power of the interatomic distance r: 6 11 a r G β = − Eq. I.1.

Non DLVO interactions

Non DLVO forces such as steric interactions, hydration pressures, hydrogen bonding and hydrophobic effects, although rarely considered in quantitative attempts to model colloidal interactions, are recognized as important forces contributing to the aggregation of colloids 36.
For example, in aqueous systems, it is now clear that non DLVO hydration effects are possible due to the reorganization of surface bound water upon approach of the colloidal particle. Furthermore, hydrophobic surfaces have a greater tendency to aggregate than predicted by DLVO theory alone due to the migration of water from the solid- water interface to the bulk solution.

The hydration or solvation effect

Short-range deviations from the DLVO theory are attributed to solvation/hydration or structural forces. Many experimental studies have termed this interaction a “hydration pressure” or “solvation pressure” because the repulsion results from the partial ordering or polarization of water or solvent molecules by the hydrophilic bilayer surface 38,39,40.
At very short distances (1-2 nm) the structure of the liquid confined between two surfaces does not follow a continuum theory any longer. The overlap between the solvent layers on the surfaces results in changes in liquid density (oscillations), causing oscillating structural forces 41. The properties of the water molecules close to the surface of the particles can be very different from those in the solution. Frequently a monolayer of water is chemically adsorbed Interactions and dynamics in colloidal systems on the surface of particles (e.g. lipid membranes, proteins, metal oxides, clays). Such colloidwater interactions can retard the aggregation of colloidal systems, due to additional repulsive interaction that is distinct from the repulsion of the electric double layer. This repulsion comes from the reorganization of the surface hydration layers (several nm) so that the contact between the particles cannot take place. This induces an increase of the free energy of the system, unfavorable for colloid interactions. This hydration force is relatively important compared with the repulsion due to the electric double layer. It has an effect on colloidal stability, particularly for high ionic strengths and for small particles.
Short range repulsive interactions between electroneutral lipid bilayers were first measured 30 years ago 42. Similar interactions have been observed in systems such as DNA and polysaccharides indicating an ubiquitous nature of these forces 43. Israelachvilli 41 has described the importance of several non-DLVO forces (solvation, structural, hydration, steric, and fluctuation) in the interaction of surfaces separated by liquid. For example, in milk fat globules (MFG), steric repulsion caused by interpenetration and/or compression of hydrophilic chains that extend from the MFG surface has been implicated 44.
The origin of the hydration force remains the subject of intense study, both in experiment and theory. Unfortunately, there are no theoretical models allowing to calculate the magnitude and range of the hydration pressure. For example, the magnitude of this pressure in lipid bilayers has been shown to be proportional to the square of the dipole potential measured in monolayers in equilibrium with liposomes, suggesting that the “hydration” pressure results, at least in part, from the polarization of interbilayer water by electric fields arising from the lipid and water dipoles 42,43.

Hydrophobic interactions

Longer-range attractive deviations to the DLVO theory are attributed to hydrophobic forces between hydrophobic surfaces, as well as steric and fluctuation forces between diffuse interfaces, when polymer chains are present at the surface 45.
The nor affinity between hydrophobic particles and water may be due to the absence of ionic or polar groups and sites for hydrogen bonds. The properties of the water molecules in contact with the surfaces are thus different from those in the solution. Indeed, hydrogen bonds among water molecules structure ordinary water. The presence of a hydrophobic surface can restrict the natural tendency of water to structure by imposing a barrier, which prevents the organization of aggregates in a direction. If the interstice between particles is narrow, the limitation is important and there is an increase in the free energy of the water compared to that of the solution. In other words, there is an attraction of hydrophobic surfaces due to the migration of water molecules from the interstice to the solution, where the possibilities of creating hydrogen bonds are not restricted.
Accurate measurement of interaction forces between different surfaces is important for a variety of natural phenomena and industrial processes 46,47. From there, hydrophobic force is among the most important nonspecific interactions in biological systems and plays a central role in many surface phenomena. Hydrophobic interactions have been largely investigated to describe a variety of biochemical processes such as the conformational changes of biopolymers, the substrate/enzyme binding, and the association of subunits to form a multisubunit enzyme 48. Also, they are relevant in formation of detergent micelles, proteinamphiphile complexes, and biological membranes 49.

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Thermodynamics of polymer-polymer miscibility

When a solution of polymer A is added to a solution of polymer B, the main characteristic of the blend will be the thermodynamic compatibility or incompatibility of the components. Thus, two different events might occur: the two solutions will mix or the solutions will not mix and the system separates into two phases. Complete miscibility or stability in a mixture of two polymers requires that the change in Gibbs energy upon mixing (ΔGmix) must be lower than zero 50. However, phase separation take place in the case of ΔGmix > 0.
The thermodynamic properties of a macromolecular blend, giving rise to miscibility or phase separation, depends on many factors, of which the most important are the chemical structure of the polymers, their molecular mass, temperature (T) and blend composition φ. For example, heating or cooling the sample shifts the blend from the mixed state to a completely demixed one. Therefore, a critical temperature and a critical composition define the boundary between the mixed state and the demixed state. The phase diagram of a polymer-polymer system is a representation of the critical volume fraction of one component (for example φA) as a function of T (see Fig. I.8).

Thermodynamic incompatibility: segregative phase separation

As assessed above, the phase behavior of a binary blend is reflected in its phase diagram (Fig. I.12.a). In totally miscible binary blends, the phase diagram is a continuous 1-phase region (Fig. I.8). In polymer blends that segregate, the composition of the phases depends on the spinodal and the temperature. The binodal being the border between the 1- and 2-phase regions. When segregation occurs at a definite temperature, the equilibrium composition of the segregated phases, φA and φB, is defined by the binodal 50,70. It must be noticed that the temperature-dependence of phase behavior is established for polymer blends and it is not well investigated for biopolymeric systems.
The segregative phase separation phenomenon is the general main consequence of the thermodynamically unfavorable interactions between polymers with high concentrations, which mainly arise from the excluded volume effects and the electrostatic repulsions between the like charged polymers. The difference in the thermodynamic affinities of the polymers for an aqueous medium is the second most important parameter that determines the water partitioning between the coexisting phases 71. It is well established that the segregative phase separation is often due to changes in the solvent-polymer affinity.

Mixtures of β-lactoglobulin and Acacia gum

In aqueous solution at peculiar pH and ionic strength conditions, attractive interactions between BLG and AG lead to complex coacervation. First kinetic studies of complex coacervation appeared on the system composed of β-lactoglobulin and Acacia gum (AG) using diffusing wave spectroscopy (DWS) and confocal scanning laser microscopy (CSLM) 135. The DWS patterns were difficult to interpret because coalescence and sedimentation of the coacervate droplets occurred at the same time. Further kinetic studies and great interest were carried out in complex coacervation of 􀈕-lactoglobulin/Acacia gum/water system because of its implication in many food processes 3,57,61,62,63,135,136,140. Using Small Angle Static Light Scattering (SALS), it was not possible to conclude on the occurrence of spinodal decomposition or nucleation and growth phenomena. This is because mixing the two biopolymers at pH and Pr:Pol ratio where maximum electrostatic interactions took place, induced an instantaneous phase separation 136. An interesting approach is to acidify mixtures in situ using glucono-delta-lactone (GDL) in order to slow down the demixing rate. Thus, the pH can be decreased slowly from a value at which no interaction takes place to a value at which phase separation occurs 137. Recently, Sanchez et al. (2006) get a better understand about the mechanism of complex coacervation in BLG/AG system. The authors suggested that complex coacervation between BLG and AG follows a Nucleation and Growth mechanism.

Table of contents :

Chapter I Interactions and dynamics in colloidal systems
I.1. Introduction
I.2. Theoretical description of interaction forces
I.2.1. DLVO interactions
I.2.1.1. Electrostatic interactions
I.2.1.2. Van der Waals interactions
I.2.2. Non DLVO interactions
I.2.2.1. The hydration or solvation effect
I.2.2.2. Hydrophobic interactions
I.3. Interactions in macromolecular solutions
I.3.1. Thermodynamics of polymer-polymer miscibility
I.3.1.1. The Flory-Huggins theory
I.3.1.2. Dynamics of phase separation
I.3.2. Main demixing mechanisms in mixed macromolecular solutions
I.3.2.1. Depletion flocculation
I.3.2.2. Thermodynamic incompatibility: segregative phase separation
I.3.2.3. Thermodynamic compatibility: associative phase separation
I.4. Complex coacervation between β-lactoglobulin and Acacia gum
I.4.1. Characteristics of 􀈕-lactoglobulin
I.4.2. Particularity of Acacia gum
I.4.3. Mixtures of 􀈕-lactoglobulin and Acacia gum
I.5. Objectives of the thesis
I.6. Outline of the thesis
I.7. References
Chapter II Thermodynamic characterization of interactions between β-lactoglobulin and major molecular fractions of Acacia gum
II.1. Introduction
II.2. Experimental Section
II.2.1. Materials
II.2.2. Preparation of β-Lactoglobulin (BLG) and Acacia gum AG) stock dispersions
II.2.3. Isothermal Titration Calorimetry
II.2.4. Optical density
II.3. Results and Discussion
II.3.1. Thermodynamic characteristics of BLG/AG interactions
II.3.2. Ratio-induced structural transitions during complexation between BLG and AG
II.3.3. Thermodynamic contribution of individual molecular fraction of TAG in interactions with BLG
II.3.4. Parameters influencing the thermodynamic characteristics of the different BLG/AG systems
II.3.4.1. Initial concentration of AG
II.3.4.2. pH
II.3.4.3. Temperature
II.4. Conclusions
II.5. References
Chapter III Influence of Acacia gum molecular polydispersity on the complex coacervation with β-lactoglobulin
III.1. Introduction
III.2. Experimental section
III.2.1. Materials
III.2.2. Samples preparation
III.2.2.1. Stock solutions of β-lactoglobulin (BLG) and Acacia Gum (AG)
III.2.2.2. Mixed dispersions
III.2.3. Dynamic light scattering
III.2.4. Electrophoretic mobility measurements (μE)
III.2.5. Small Angle Static Light Scattering (SALS)
III.3. Results and Discussion
III.3.1. Dynamic light scattering (DLS)
III.3.2. Electrophoretic mobility (μE)
III.3.3. Small angle static light scattering (SALS)
III.4. Conclusions
III.5. References
Chapter IV Control of the strength of interactions between β-lactoglobulin and Acacia gum molecular fractions
IV.1. Introduction
IV.2. Experimental section
IV.2.1. Materials
IV.2.2. Preparation of β-Lactoglobulin (BLG), Acacia gum (AG) stock dispersions and BLG/AG mixed dispersisons
IV.2.3. Dynamic light scattering
IV.2.4. Electrophoretic mobility measurements (μE)
IV.2.5. Small Angle Static Light Scattering (SALS)
IV.2.6. Granulo-Polarimetry
IV.2.7. Epi-fluorescence Microscopy
IV.3. Results and Discussion
IV.3.1. Effect of the Pr:Pol ratio on complexation/coacervation mechanism
IV.3.1.1. Dynamic light scattering
IV.3.1.2. Electrophoretic mobility
IV.3.1.3. Small angle static light scattering
IV.3.2. Effect of the total biopolymer concentration (Cp)
IV.3.2.1. Granulo-Polarimetry
IV.3.2.2. Optical microscopy
IV.4. Conclusions
IV.5. References

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