Thermodynamic Characteristics of Some Acacia gummiferae gums

Abstract

Physicochemical, thermodynamic and thermal characteristics of A.sieberana.var.sieberana, A.gerrardii.var.gerrardii and A.tortilis.var.raddiana were studied using different physicochemical and thermal techniques. The physicochemical characteristics are partly needed for authentication of samples and resources which include namely moisture content, ash content, nitrogen and protein contents. The percentages of moisture content, ash content, nitrogen and protein contents of A.sieberana.var.sieberana gum were found to be 8.28, 2.10, 0.63 and 4.15 respectively while for A.gerrardii.var.gerrardii gum they were found to be 8.4, 2.26, 2.8 and 19.6 and for A.tortilis.var.raddiana gum they were equal to 7.9, 3.4, 2.07 and 14 respectively. The specific optical rotation, intrinsic viscosity, equivalent weight and uronic acid results of A.sieberana.var.sieberana gum were found to be (+98)–(+104), 11.189 cmg1,1996 and 9.7 respectively while for A.gerrardii.var.gerrardii gum they were equal to (+38) to (+52), 51.706 cmg-1,1347 and14.4, and for A.tortilis.var.raddiana gum they were found to be (+77) to (+79), 15.67cmg-1, 2095 and 9.3 respectively. Cationic composition results of 16 different elements was determined using Atomic Absorption Technique show that Ca++ has the highest percent followed by Mg++ and K+ for A.sieberana gum and A.gerrardii gum while A.tortilis gum K+ has the highest value followed by Ca++ and then Mg++. The composition of sugars (arabinose, galactose, and rhamnose). found to be 36.58%,12.8%, and 0.62% for A.sieberana.var.sieberana gum, for A.gerrardii.var.gerradii gum 12.18%, 37.4% and 0.41% respectively and for A.tortilis.var.raddiana 9.53%, 39.67% and 0.77% respectively. The ratio of arabinose to galactose in all samples is > unity. The molecular weight of each sample was determined using GPC. MALL techniques, the values of Mw and Mn of A.sieberana.var.sieberana gum sample were found to be 1.959x106 and 1.918x106 gmol-1 and 1.605x106gmol-1 and 1.513x106 gmol-1 for A.gerrardii.var.gerrardii gum sample while for A.tortilis.var.raddiana they were found to be 1.687x10 6 and 1.664x106 gmol-1 respectively. The number average molecular weight for each sample was also calculated from the osmotic pressure measurements and was found to be 2.3x106gmol-1, 2.91x106gmol-1 and 9.0x106gmol-1 for A.sieberana.var.sieberana, A.gerrardii.var.gerrardii and A.tortilis.var.raddiana gum samples respectively. Thermodynamic measurements, the density of solid gum, the partial specific volume of gum (solute), the partial specific volume of water (solvent) and the volume fraction of gum and water were measured, for A.sieberana.var.sieberana, they were found to be 1.64gcm-3, 0.65cmg1, 1.004cm3g-1, 0.39 and 0.61 respectively while for A.gerrardii.var.gerrardii gum they were fund to be 1.21gcm-3, 0.59cmg1,1.004cm3/g, 0.37 and 0.63 respectively , for A.tortilis.var.raddiana gum sample they were 1.06gcm3,0.64, 1.003cm3/g, 0.39 and 0.61 respectively. The results of the density of solid gums show that A.sieberana.var.sieberana gum molecule is more compact than the other two gum samples. The chemical potential range of the gum samples has been calculated using the osmotic pressure measurements for different concentrations and was found to be -3.35x10-5 to -84.37x10-5, -43x10-5 to -254.38x10-5 and -2x10-5 to -31.33x10-5 Jg1 for A.sieberana.var.sieberana, A.gerrardii.var.gerrardii and A.tortilis.var.raddian gums respectively. The second virial coefficient, the free energy of mixing range and the gum – solvent (water) interaction parameter were calculated for all gum samples and were found to be 3.92x10-4, -20.61x10 -5 to -107.19x10-5 Joule g1 and 0.481 for A.sieberana.var.sieberana gum and 3 x 10-5, -98.43x10-5 to -254.08x10-5 and 0.499 for A.gerrardii.var.gerrardii gum while for A.tortilis.var.raddian gum they were 2.64x10-4, -14.67x10-5 to -20.64x10-5) and 0.495 respectively. The gum samples were subjected to DSC and TGA measurements. The results of DSC show that the glass transition temperatures of A.sieberana.var.sieberana and A.tortilis.var.raddiana gums were negative and fall in the range of – 32 to – 37 C while no glass transition temperature was observed for A.gerrardii.var.gerrardii gum. All gum samples show endothermic peaks at a temperature range of 101 – 105 C and exothermic peaks at temperature about 300 C. The enthalpy changes of A.sieberana.var.sieberana, A.gerrardii.var.gerrardii and A.tortilis.var.raddiana were found to be in the ranges of -399 to 250.6, -383 to 247.3 and -174.5 to 203.8 Jg1 respectively. The specific heat changes were in the ranges of - 3.19 to 6.08, - 4.18 to 6.95 and – 4.54 to 5.39 Jg1 C respectively.TGA and DTA results show that A.sieberana.var.sieberana and A.tortili.var.raddiana gums exhibit three stages of decomposition pattern while A.gerrardii.var.gerrardii gum show, only, two stages. The initial decomposition temperature (IDT) of A.sieberana.var.sieberana, A.gerrardii.var.gerrardii and A.tortilis. var.raddiana gums were found to be 289.21, 288.67 and 280.53 C respectively while the char yield % at 600 C were equal to 19.61, 24.41 and 24.88 respectively. The weight loss% at 180 to 200 C for A.sieberana.var.sieberana, A.gerrardii.var.gerrardii and A.tortilis.var.raddiana were found to be 13.49, 11.88 and 10.97 respectively.