Solution Properties of Combretum Glutinosum ( Habeil ) Gum

The solubility of Combretum glutinosum gum in distilled water ranged from 30 to 40% and gel fraction ranges between 60 to 70%. It was found that the gum solubility increased with increase the pH of solution. The reduced viscosity of Gum Habeil in distilled water was increased with decrease the concentration of gum solution, and this is the a behavior of ionic polysaccharide solutions, for that Huggins and Kraemer equations are incapable to derive the intrinsic viscosity value of aqueous gum solution. Some extrapolated equations such as Fouss, Schulz –Blaschke, Martin, Heller, and Fedors equations used to estimate the optimum model for measurement intrinsic viscosity of this type of gum. The influence of ion types and ionic strength on the viscosity of gum solutions were also determined. The effect of ionic strength on the intrinsic viscosity of Combretum glutinosum gum was used to determine the salt tolerance value and the chain stiffness parameters of this type of gum. This study exhibited that the values of chain stiffness parameter (B), and salt tolerance (S) of deacetylated gum in the range of 0.0574 to 0.0700, and0.083 to 0.101 dl× M / g respectively.


Introduction
Gums obtained from several genus of Acacia trees are extensively used in food industrial as emulsifiers, a thickeners, and adhesive agents.Combretum glutinosum gum in Sudan known as Gum Habeil, and this tree belongs to the Combretaceae family (Arbonnier, M., 2004, Devineau, J. L., 1999).The leaves of Combretum glutinosum trees have been used traditionally in Sudan for many years in the treatment of malaria, headache, bleeding, and utilized as diuretic (Jefferies, M., et al, 1981).Pousset 1993 publicized that, the hot aqueous extracted solution of Combretum glutinosum leaves has been used to therapy Hepatitis B virus (Jefferies, M., et al, 1978).The present work is an attempt to, determine the solubility of Combretum glutinosum gum in different solvents, measure the intrinsic viscosity and investigate the influence of ion types and ionic strength on the value of intrinsic viscosity of Combretum glutinosum gum.The work also aimed to determine the value of chain stiffness (B), and salt tolerance (S) parameters of deacetylated Combretum glutinosum gum.The author 2013 revealed that; the weight average molecular weight of AGP component of gum equal 8.611×10 5 , 4.456×10 6 , and 1.528 ×10 6 g / mol., and radius of gyration (Rg) equal 72.16, 30.03 , and 81.3 nm of the freeze dried deacetylated gum, freeze dried soluble part, and freeze dried calcium-free gum respectively.Freeze dried soluble part gum shown a highly branched compact chain backbone (low radius of gyration) with higher weight average molecular weight than the modified gum samples "deacetylated and Calcium-free gums" (Awad, M.H., et al, 2013).The author 2013 revealed that; the thermal analysis of gum samples exhibited endothermic peak ranged from 30 to 45 o C combined with heat content value ranged from 0.3 to 1.5 J/ g (Awad, M.H., et al, 2014).

Sample Collection and Preparation
Authenticated specimens of Combretum glutinosum gum collected from Blue Nile state -Sudan in seasons 2007 to 2009.The samples were cleaned from debris like sand and bark, then converted to Mechanical powder using an electrical grinder, and sieved using mesh size 250µm.

Solubility Measurement
1% w/w of gum in electrolyte (water) solution was stirred overnight.The dispersion solution was centrifuged at 3000 rpm for 45 min, then the soluble fraction was decanted and saved in 250 cm 3 volumetric flask.Insoluble fraction was dissolved in 20 cm 3 distilled water, then the mixture was stirred for 60 min and re-centrifuged at 3000 rpm for 10 min before the supernatant fraction transferred to the soluble part solution using a dropper.The last steps were repeated three times to remove electrolyte salts.Insoluble fraction was stored for 24 hrs. in oven at 105 o C and then weighed, while the assembled soluble part solution was completed to 250 cm 3 by adding distilled water, then 20 cm 3 of soluble part solution was dried for 24 hrs. in oven at 105 o C and weighed (Jefferies, M., et al., 1981;Jefferies, M., et al., 1978).

Determination of Gel Volume
50 cm 3 of 2 % w /v of gum in electrolyte solution was stirred overnight.The dispersion solution was centrifuged at 3000 rpm for 45 min, then the supernatant was decanted.50 cm 3 of electrolyte solution was added to the insoluble fraction, then the solution was stirred for 60 min and re-centrifuged at 3000 rpm for 10 min and the supernatant was decanted.The last step was repeated three times before the final centrifugation.Lastly 50 cm 3 of electrolyte solution was added to the gel fraction and the mixture was stirred for 10 min, and allowed to stand for 20 hrs., then the gel volume was measured.The above experiment was carried using 0 to 0.3 ionic strength solutions of magnesium chloride, and calcium chloride, and sodium chloride (Brito, A. C. F., et al., 2004).

Intrinsic Viscosity Measurement
0.875 g /dl of deacetylated gum, and soluble part solutions in different ionic strengths were prepared.Ubbelohde viscometer (75 / N94 or 75/ N 104) immersed in a viscometer bath (C.T-500 Series II, USA) was used to determine the flow time of the gum solutions flowing through the Ubbelohde capillary tube.The relative viscosity was measured by comparing the flow time of 3cm 3 of gum solution to the flow time of solvent passage through the Ubbelohde capillary tube, then the relative viscosity was converted to inherent, specific, and reduced viscosity (Le Cerf, D., et al., 1990).

Salt Tolerance and Chain Stiffness Parameter
The value of salt tolerance (S) was obtained from the following equation (Smidsrod, O., et al., 1971): where [η] ∞ is the intrinsic viscosity at infinite ionic strength (μ).The relative stiffness parameter (B) was determined from the below equation: where the υ constant used for this study equal 1.3 (Lapasin, R., et al., 1995).

Solubility and Gel Volume
Table 1 shows the solubility of Gum Habeil increased with the solvent basicity directly, the high solubility of gum solution in sodium carbonate due to calcium ion depletion.The weight of gel was calculated by discounting the weight of insoluble materials (sand and bark) from the total weight of the insoluble fraction.

Influence of Alkali Salts on [η] of Deacetylated Gum Solution
The hydrolysis of hydrophilic and hydrophobic functional groups performs a significant role on the polymer backbone conformation stability.The physicochemical study of the relation between solvent influence, and the hydrolysis properties of hydrophilic, and hydrophobic groups should provide insights into the role of solventsolute interactions associated with fundamental biopolymers phenomena (Wang, J. J., et al., 2000).The relation between relative viscosity of electrolytes solutions and solvent -solute interactions could be described by Jones and Dole below (Jones, G., & Dole, M., 1925): where η s is viscosity of gum solution , ηo represent viscosities of solvent, A and B are constants, and C is the molar concentration of gum solution.A -Falkenhagen coefficient following the value of electrostatic forces related to ion-ion interactions, and B -coefficient exhibit the influence of solvent -solute interactions on the value of solution viscosity.Viscosity of neutral polysaccharides solution could be represented by the following equation: where η mix represented viscosity of gum dissolved in mixture of two solvents with a fixed molar concentration, η o is viscosities of mixture solvent.Table 3 showed that the B-coefficients value was decreased with the alkali salts concentration increased, in contrast A-Falkenhagen coefficient value was increased with the alkali salts concentration decreased.Also Table3 exhibited that the B-coefficients values of Gum Habeil in alkali salts concentration are positive and higher than A-coefficients, and this emphasizes that the increases of alkali salts concentration lead to decrease the B-coefficients values, increases the interaction between gum and alkali salt solvent, and this decreases the gum viscosity.

Influence of Alkali Salts on Viscosity of Deacetylated Gum
Fig. 5 to 7 showed that with alkali salts addition, the repulsion forces between the opposite functional group in chain segments was eliminated.Results exhibited that internet, and reduced viscosity versus gum concentration are given linear relationship in high concentration of alkali salt, and experimentally viscosity data obey Huggins and Kraemer equations with the lower standard deviation errors.Thus, this observation may be due to shielding the charge on the side chain of the polymer.

Figure
Figure 7.A

Table 3 .
The values of A-Falkenhagen coefficient and B-coefficients of deacetylated gum in deionized water and different salts at 25 o C