Alvarez Margarita Angel, Black Nathan, Blanco Saylor Estelle, Reid Katelyn Ruth, Billiot Eugene J, Billiot Fereshteh H, Morris Kevin F
Department of Physical and Environmental Sciences, Texas A&M University-Corpus Christi, 6300 Ocean Drive, Corpus Christi, TX 78412, USA.
Department of Chemistry, Carthage College, 2001 Alford Park Drive, Kenosha, WI 53140, USA.
Molecules. 2024 Sep 18;29(18):4436. doi: 10.3390/molecules29184436.
Electrical conductimetry and dynamic light scattering (DLS) were used to investigate the aggregation behaviors of four amino acid-based surfactants (AABSs; undecanoyl-glycine, undecanoyl--alanine, undecanoyl--valine, undecanoyl--leucine) in the presence of five linear diamine counterions (1,2-diaminoethane, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane). Electrical conductimetry was used to measure the CMCs for each system, which ranged from 5.1 to 22.5 mM. With respect to counterions, the obtained CMCs decreased with increases in the interamine spacer length; this was attributed to the improved torsional binding flexibility in longer counterions. Strong linear correlations (mean R = 0.9443) were observed between the CMCs and predicted surfactant partition coefficients (logP; water/octanol), suggesting that micellization is primarily driven by the AABS's hydrophobicity for these systems. However, significant deviations in this linear relationship were observed for systems containing 1,2-diaminoethane, 1,4-diaminobutane, and 1,6-diaminohexane ( = 0.0774), suggesting altered binding dynamics for these counterions. pH measurements during the CMC determination experiments indicated the full deprotonation of the AABSs but did not give clear insights into the counterion protonation states, thus yielding an inconclusive evaluation of their charge stabilization effects during binding. However, DLS measurements revealed that the micellar size remained largely independent of the counterion length for counterions longer than 1,2-diaminoethane, with hydrodynamic diameters ranging from 2.2 to 2.8 nm. This was explained by the formation of charge-stabilized noncovalent dimers, with each counterion bearing a full +2 charge. Conductimetry-based estimates of the degrees of counterion binding (β) and free energies of micellization (ΔG°) revealed that bulky AABSs exhibit preferential binding to counterions with an even number of methylene groups. It is proposed that when these counterions form noncovalent dimers, perturbations in their natural geometries result in the formation of a binding pocket that accommodates the AABS steric bulk. While the direct application of these systems remains to be seen, this study provides valuable insights into the structure-property relationships that govern AABS aggregation.
采用电导法和动态光散射（DLS）研究了四种氨基酸基表面活性剂（AABSs；十一烷酰甘氨酸、十一烷酰丙氨酸、十一烷酰缬氨酸、十一烷酰亮氨酸）在五种直链二胺抗衡离子（1,2 - 二氨基乙烷、1,3 - 二氨基丙烷、1,4 - 二氨基丁烷、1,5 - 二氨基戊烷、1,6 - 二氨基己烷）存在下的聚集行为。用电导法测量了每个体系的临界胶束浓度（CMC），其范围为5.1至22.5 mM。对于抗衡离子，随着胺间间隔长度的增加，所获得的CMC降低；这归因于较长抗衡离子中扭转结合灵活性的提高。在CMC和预测的表面活性剂分配系数（logP；水/辛醇）之间观察到强线性相关性（平均R = 0.9443），这表明对于这些体系，胶束化主要由AABS的疏水性驱动。然而，对于含有1,2 - 二氨基乙烷、1,4 - 二氨基丁烷和1,6 - 二氨基己烷的体系，在这种线性关系中观察到显著偏差（R² = 0.0774），表明这些抗衡离子的结合动力学发生了改变。在CMC测定实验期间的pH测量表明AABS完全去质子化，但没有对抗衡离子的质子化状态给出清晰的见解，因此对它们在结合过程中的电荷稳定作用的评估尚无定论。然而，DLS测量表明，对于长度大于1,2 - 二氨基乙烷的抗衡离子，胶束尺寸在很大程度上与抗衡离子长度无关，流体动力学直径范围为2.2至2.8 nm。这可以通过形成电荷稳定的非共价二聚体来解释，每个抗衡离子带有完整的 +2电荷。基于电导法对抗衡离子结合度（β）和胶束化自由能（ΔG°）的估计表明，体积较大的AABS对具有偶数个亚甲基的抗衡离子表现出优先结合。有人提出，当这些抗衡离子形成非共价二聚体时，它们自然几何形状的扰动会导致形成一个容纳AABS空间体积的结合口袋。虽然这些体系的直接应用还有待观察，但这项研究为控制AABS聚集的结构 - 性质关系提供了有价值的见解。
