Nonionic Surfactants with Linear and Branched Hydrocarbon Tails:  Compositional Analysis, Phase Behavior, and Film Properties in Bicontinuous Microemulsions

Abstract

Nonionic alcohol ethoxylates are widely used as surfactants in many different applications. They are available in a large number of structural varieties as technical grade products. This variety is mainly based on the use of different alcohols, which can be linear or branched and contain primary, secondary, or tertiary OH groups. Technical grade products are poorly defined as they are composed of alcohol mixtures being different in chain length and structure. On the other hand, monodisperse alcohol ethoxylates are commercially available; however, these surfactants exist only with primary and linear alcohols. In the field of microemulsion research the monodisperse alcohol ethoxylates are widely used. The phase behavior and film properties of these surfactants were studied intensively with respect to the size of the hydrophilic and hydrophobic moieties. Due to the lack of appropriate model surfactants until now, there is little information on how the structure of the hydrocarbon tail influences the microemulsion behavior. To examine structural influences, we synthesized a series of surfactants with the composition C10E5 and having different linear and branched hydrocarbon tails. The surfactants were monodisperse with respect to the hydrocarbon tail but polydisperse with respect to the ethoxylation degree. However, a detailed characterization showed that they were similar concerning the average ethoxylation degree and EO chain length distribution. The phase behavior was investigated for bicontinuous microemulsions, and the film properties were analyzed by small-angle neutron scattering (SANS). Our results show that the structure of the hydrocarbon tail strongly influences the microemulsion behavior. The most efficient surfactant is obtained if the hydrocarbon tail is linear and the hydrophilic group is attached in the C-1 position. Surfactants having the hydrophilic group bound to the C-2 or C-4 position or which contain a branched hydrocarbon tail are less efficient and exhibit in most cases visibly lower phase inversion temperatures. Both the efficiency and temperature behavior mainly can be explained on the basis of increased bulkiness of the branched structures compared to the fully linear version. The phase behavior results are largely confirmed by the SANS investigations. Those results show that the fully linear surfactant exhibits the most rigid interfacial film. In additional experiments this surfactant was compared with its monodisperse analogue. According to the phase diagrams, the surfactant having the polydisperse hydrophilic moiety is drastically more efficient although the film stiffnesses are almost identical.

Adsorption Properties and Composition of Binary Kolliphor Mixtures at the Water–Air Interface at Different Temperatures

“…This indicates that some changes in the hydration number of surfactants head can take place and/or the changes in the surfactant molecules packing as a result of their configuration variation. As follows from the literature [ 26 , 27 ] the possibility of forming different configuration by surfactants having large and branched molecules is greater than for the classical surfactants whose molecules are linear and not very long. It is possible that the oxyethylene and/or hydrocarbon chains are coiling.…”

Surfactant Monolayer Bending Elasticity in Lipase Containing Bicontinuous Microemulsions

“…The statistical errors from the fits were ≈ 1 Å for d and ξ , and 0.01 kT for the bending rigidities determined from SANS measurements. The Porod contribution at high-Q is included into the model (Frank et al, 2007):…”

Multi-stage freezing of HEUR polymer networks with magnetite nanoparticles

“…the distance between the hydrophobic domains of the HEUR polymer network, having a width proportional to 1/x, where x is the correlation length of the density fluctuations of the dense polymer network. 66 The SANS data were fitted with a model where a simple power law describing the long range fluctuations and the Teubner-Strey (TS) theory 67 were combined. The latter theory is a thermodynamic approach with a simple order parameter indicating essentially two major phases, i.e.…”