Innovative systems and applications - Physical Gels from Biological and Synthetic Polymers - page 343

solubility of such hydrophobically modi ed alginates is strongly reduced as ionic strength

increases. Particles can then be formed by dropping a solution containing the polysaccharide
into a Na + solution or into a solution containing Na + and Ca 2+ . Appropriate conditions to
induce gel particle formation, and avoid polymer precipitation, were determined by adjusting
concentrations of NaCl, CaCl 2 and amphiphilic alginates. Amphiphilic derivatives of Na +
alginate were prepared by esterifying long alkyl chains (12 or 18 carbon atoms) on to the
hydrophilic backbone. Highly viscous solutions and strong gels were obtained in water,
within a narrow range of substitution degree, i.e. 3

- 5% and 13 - 16% for C 18 -andC 12 -

modi ed alginates, respectively, since at higher degree of substitution the polymers were no

longer water-soluble. The microstructure of Ca 2+ modi

ed alginate gels was more hetero-

geneous than that of Ca 2+ unmodi

ed alginate gels, because the dense hydrophobic domains
restrict egg-box formation ( Chapter 5 ).
Another method of preparing microparticles from hydrophobically modi

ed alginate

derivatives uses an procedure (Leonard et al., 2004 ); see Figure 11.11 . The

main problem arising from the use of amphiphilic alginates is the high pre-gel viscosity
of the solution, so dispersion in NaCl solution was dif

' all-aqueous '

cult and particle sizes much higher

than required.

Figure 11.11 Schematic representation of the process used for preparation of microparticles of amphiphilic
alginate. Adapted from Leonard et al.( 2004 ) with permission from John Wiley & Sons.