Mussel Matan, Basser Peter J, Horkay Ferenc
Section on Quantitative Imaging and Tissue Sciences, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, USA.
Soft Matter. 2019 May 22;15(20):4153-4161. doi: 10.1039/c9sm00464e.
Measurements are reported on the effect of monovalent and divalent salts on the swelling behavior and supramolecular structure of sodium polyacrylate gels (NaPA) made by osmotic swelling pressure and small angle neutron scattering measurements. The swelling response of the gels in solutions of ten different monovalent salts is found to be practically identical indicating that the principal effect of monovalent ions is screening the electrostatic repulsion among the charged groups on the polyelectrolyte chains; i.e., chemical differences between the monovalent ions do not play a significant role. Introducing Ca2+ ions into the equilibrium NaCl solution results in a sharp volume transition of the gels. The threshold Ca2+ ion concentration at which the transition occurs increases with increasing NaCl concentration in the surrounding bath. It is demonstrated that the swelling behavior of NaPA gels exhibits universal properties in solutions containing both NaCl and CaCl2. Osmotic swelling pressure measurements reveal that both the second and third virial coefficients decrease with increasing CaCl2 concentration until the volume transition is reached. The macroscopic measurements are complemented by small angle neutron scattering that reveals the variation of the thermodynamic length scales as the volume transition is approached. The thermodynamic correlation length L increases with increasing CaCl2 concentration.
通过渗透压和小角中子散射测量，报告了单价盐和二价盐对聚丙烯酸钠凝胶（NaPA）的溶胀行为和超分子结构的影响。发现凝胶在十种不同单价盐溶液中的溶胀响应几乎相同，这表明单价离子的主要作用是屏蔽聚电解质链上带电基团之间的静电排斥；即单价离子之间的化学差异不起重要作用。将Ca2+离子引入平衡NaCl溶液会导致凝胶发生急剧的体积转变。转变发生时的阈值Ca2+离子浓度随周围浴中NaCl浓度的增加而增加。结果表明，NaPA凝胶在含有NaCl和CaCl2的溶液中的溶胀行为具有普遍特性。渗透压测量表明，第二和第三维里系数均随CaCl2浓度的增加而降低，直至达到体积转变。宏观测量由小角中子散射补充，小角中子散射揭示了接近体积转变时热力学长度尺度的变化。热力学相关长度L随CaCl2浓度的增加而增加。
