Influence of Temperature and Concentration on the Self-Assembly of Nonionic C_iE_j Surfactants: A Light Scattering Study

Abstract

Nonionic poly(ethylene oxide) alkyl ether (C i E j ) surfactants self-assemble into aggregates of various sizes and shapes above their critical micelle concentration (CMC). Knowledge on solution attributes such as CMC as well as aggregate characteristics is crucial to choose the appropriate surfactant for a given application, e.g., as a micellar solvent system. In this work, we used static and dynamic light scattering to measure the CMC, aggregation number ( N agg ), and hydrodynamic radius ( R h ) of four different C i E j surfactants (C 8 E 5 , C 8 E 6 , C 10 E 6 , and C 10 E 8 ). We examined the influence of temperature, concentration, and molecular structure on the self-assembly in the vicinity of the CMC. A minimum in the CMC vs temperature curve was identified for all surfactants investigated. Further, extending the hydrophilic and hydrophobic chain lengths leads to an increase and decrease of the CMC, respectively. The size of the aggregates strongly depends on temperature. N agg and R h increase with increasing temperature for all surfactants investigated. Additionally, N agg and R h both increase with increasing surfactant concentration. The data obtained in this work further improve the understanding of the influence of temperature and molecular structure on the self-assembly of C i E j surfactants and will further foster their use in micellar solvent systems.

Solubilization of Aldehydes and Amines in Aqueous CiEj Surfactant Aggregates: Solubilization Capacity and Aggregate Properties

“…The SC was then determined by plotting the measured maximum amounts of solute m ̃ solute max vs the amount of surfactant incorporated in aggregates according to eq Here, m ̃ surfactant tot and m ̃ surfactant aq are the total molality of the surfactant and the molality of the surfactant in the aqueous bulk phase (that is the molality of surfactant molecules not incorporated in aggregates), respectively. We used the CMC in water for m ̃ surf aq determined in a previous work . The molality of solute in the aqueous bulk phase (solute molecules not incorporated into aggregates) is given by m ̃ solute aq .…”

“…Mass fractions were calculated according to eq . Note that in eq , the amount of water in the aggregates is neglected The refractive index increments of the surfactants were taken from a previous work, while the refractive index increments of the solute were measured using the RI detector. Knowing the refractive index increment of the mixed aggregates, eq can be used to determine the M W of the aggregates.…”

“…14,27,28 Increasing the hydrophilic ethylene oxide chain at constant hydrophobic alkyl chain length C i leads to a decrease of N agg of the aggregates and thus also decreases the hydrophobic core volume (as in the previous case). 23,29 Thus, an increase of the ethylene oxide chain length from E 6 to E 8 at fixed C 10 leads to a slight decrease of the SC toward decane from 0.81 to 0.78 molecules solubilized per surfactant molecule within the aggregate. This was also found for the solubilization of decane in C 12 E j surfactants with increasing ethylene oxide chain 20 as well as for other hydrophobic solutes.…”

“…We used the CMC in water for m̃s urf aq determined in a previous work. 23 The molality of solute in the aqueous bulk phase (solute molecules not incorporated into aggregates) is given by m̃s olute aq . Hence, the SC can be determined from the slope and the solubility of the solute in the aqueous phase from the intercept according to eq 1.…”

“…For C i E j surfactant aggregates, it was found that the degree of hydration of the ethylene oxide units is decreasing gradually from the ethylene oxide unit closest to the aggregate/water interface to the ethylene oxide unit closest to the core of the aggregates. 23,29 Therefore, the polarity gradient of the corona region is directly coupled/related to the gradient in hydration of the ethylene oxide units. However, long-chain aldehydes and n-amines are expected to arrange in a C i E j surfactant aggregate in such a way that their polar functional group is orientated toward the corona region, while the hydrophobic alkyl chain is orientated toward the hydrophobic core of the aggregates.…”