"Lignosulfonat som ytaktivt mne"
Lignosulfonat som ytaktivt ämne Krister Holmberg, Chalmers Föreläsning FPIRC Augusti 2008 Schematic illustration of a surfactant A good surfactant has a strong tendency to go to interfaces (surfaces), where it should pack densely. A good surfactant should have low solubility in the bulk phases. Some surfactants – and some surface active macromolecules – are only soluble at the interface, i.e., in the presence of both bulk phases. Typical values of surface and interfacial tensions: air – water 72-73 mN/m air – aqueous surfactant solution 28-32 mN/m hydrocarbon – water 20-40 mN/m hydrocarbon – aqueous surfactant solution 1-10 mN/m At a certain surfactant concentration micelles start to form. This concentration is called the critical micelle concentration (CMC). Many physical-chemical properties change at the CMC In solution: The physical-chemical properties of a solution at the CMC: Osmotic pressure Surface tension Turbidity CMC 0 0,01 0,02 At the air/water interface: Concentration (mol/cm3) air water Surfactants are used … • as dispersants • as emulsifiers • as foaming agents • as wetting agents • to make microemulsions • as anti-corrosion agents • for impregnation • to solubilize drugs, dyes, etc • for soil removal • as hydrophobizing agents • as lubricants • etc Polymers can be surface active Amphiphilic polymers are used… • as dispersants • as stabilizer of emulsions • as stabilizer of foams • to control the rheology of a solution • as antiredeposition agents • etc General • Small amphiphiles, surfactants, are needed in the dynamic processes, such as - foaming - wetting - emulsification • Larger amphiphiles, surface active polymers, are good at stabilizing dispersed systems • The two are often used together. (Nature often uses a combination of a polar lipid and a surface active macromolecule to make stable emulsions and foams.) The structure of lignin (according to Adler) Is lignin surface active? Surface activity of lignins from black liquors Lignin Derivatives 70 Surface Tension, mN/m Typical Surfactant UNICAL 60 50 INDULIN C 40 ¿CMC? 30 CMC ¿CMC ? 20 0,0001 0,001 0,01 0,1 1 10 100 Concentration, w% But … • the pH-sensitivity is a big problem • only the dissociated form of Kraft lignin can be regarded to be surface active • the pKa of guaiacol is around 10 Formation of lignosulfonates in the sulfite pulping process Carbocation Lignin Lignosulfonate intermediate Lignosulfonate The lignosulfonate obtained from the sulfite process is not pure Softwood Hardwood Spent sulfite liquor Lignosulfonate 58 % 55 % Lignosulfonate intermediate Hexoses 20 % 7% Pentoses 8% 20 % Organic acids 6% 10 % Ash 8% 8% Difference in: Softwood and hardwood - chemical composition lignosulfonate can be processed to - molecular weight have the same performance - degree of sulphonation Lignosulfonates are mainly used as dispersants • Plasticizer in making concrete Amount Adsorbed on cement In solution Molecular weight • Grinding agent in cement production • Drilling mud additive - removes the cuttings - lubricates and cools the bit • Dispersant for dyes • Dispersant for carbon black • Dispersant for pesticides • Dispersant for ceramics A dispersing agent must… • anchor to the solid particle • repel the particle from other dispersant-covered particles There are two mechanisms for stabilization of dispersions 1. Electrostatic stabilization 2. Steric stabilization A lignosulfonate can be regarded as a poly- electrolyte, providing electrostatic stabilization 100.00 50 vol% alumina, 0.50% CP 55 vol% alumina, 0.35% LS 10.00 55 vol% alumina, 0.25% PAA Viscosity (Pa s) Uncharged comb polymer 1.00 Lignosulfonate Poly(acrylic acid) 0.10 0.01 0.1 1.0 10.0 100.0 1000.0 Shear rate (s-1) QCM measurements showed that while the comb copolymer gave a thick layer (6.4 nm), the lignosulfonate and the poly(acrylic acid) formed thin layers at the surface (0.6 and 0.5 nm, respectively).