Renewable Materials and Nanotechnology Research Group, Department of Chemical Engineering, KU Leuven, Campus Kulak Kortrijk, Etienne Sabbelaan 53 box 7659, 8500 Kortrijk, Belgium.
Phys Chem Chem Phys. 2018 Jul 4;20(26):17637-17647. doi: 10.1039/c8cp01532e.
Negatively charged ions readily interact with the surface of positively charged pyridinium-grafted cellulose nanocrystals. In this work we investigated the thermodynamics of these interactions using isothermal titration calorimetry. We investigated the effect of the ionic charge, using carboxylate salts with different valence (1-4), and compared it with sodium sulfate as another delocalized ionic charge. Experiments performed using cellulose nanocrystals with three different degrees of substitution of pyridinium grafts showed that the number of ions adsorbed onto the surface of cellulose nanocrystals is directly linked to the number of grafted cationic functionalities. The adsorption of anions onto pyridinium-grafted nanocrystals was further found to be endothermic and driven by an increase in entropy upon adsorption of the anions due to the release of surface bound water. The association constant increased with the net charge of the anions from a low value for monovalent sodium acetate. Both entropy and enthalpy increased linearly with the net charge of the anions, demonstrating that a stronger depolarization occurred when anions of higher net charge were adsorbed. The values of the stoichiometric number determined were lower than the values relative to processes involving ions in solution, suggesting a more complex mode of interaction, involving hydrogen-bonding and bridge formation. However, the number of adsorbed ions was directly correlated with the amount of cationic surface grafts. This makes it possible to control the amount of surface interactions directly with the degree of substitution on the nanocrystal surfaces. Comparison with zeta potential measurements showed that zeta-potential measurements can be used as a direct method to determine the stoichiometry of binding between the positive surface grafts and the carboxylate salts.
带负电荷的离子容易与带正电荷的吡啶接枝纤维素纳米晶体的表面相互作用。在这项工作中，我们使用等温滴定微量热法研究了这些相互作用的热力学。我们研究了离子电荷的影响，使用了具有不同价态（1-4）的羧酸盐，并将其与硫酸钠进行了比较，硫酸钠是另一种离域离子电荷。使用三种不同取代度的吡啶接枝纤维素纳米晶进行的实验表明，吸附在纤维素纳米晶表面上的离子数量与接枝的阳离子官能团的数量直接相关。还发现阴离子吸附到吡啶接枝纳米晶上是吸热的，并且由于表面结合水的释放，阴离子吸附导致熵增加。吸附常数随着阴离子的净电荷的增加而增加，从单价的乙酸钠的低值开始。熵和焓都随阴离子的净电荷线性增加，表明当吸附更高净电荷的阴离子时，发生了更强的去极化。确定的化学计量数的值低于与溶液中离子有关的过程的值，表明涉及氢键和桥形成的更复杂的相互作用模式。然而，吸附的离子数量与阳离子表面接枝的数量直接相关。这使得可以直接通过纳米晶表面的取代度来控制表面相互作用的数量。与ζ电位测量的比较表明，ζ电位测量可以用作确定正表面接枝与羧酸盐之间结合的化学计量比的直接方法。
