Abstract:
‘Surfactants’ or ‘surface active agents’ are chemical agents capable of reducing the surface
tension of a liquid in which it is dissolved. Nonylphenol ethoxylates (NPEs) are a group of
non-ionic surfactants commonly known as Tergitol NP surfactants. Nonylphenol
ethoxylates are classified according to the number of ethoxylate units in the hydrophilic
chain. The properties of the nonylphenol ethoxylates differ with the number of ethoxylate
units in the molecule. The hydrophilic-lipophilic-balance (HLB value) is the key parameter
that surfactant formulators are focused on when studying the properties of non-ionic
surfactants. Even though the HLB values of two surfactants are equal or close to each
other, the expected properties cannot be obtained by replacing one surfactant with another.
This issue leads to the necessity to carry out many trial and error tests to identify the
equivalent surfactants in industrial applications.
To address this issue, a comprehensive investigation of the nonylphenol ethoxylate
molecules was carried out computationally at B3LYP level of theory using 6-311G basis
set in Linux version of Gaussian 09 computer software package. The aqueous phase
investigation focused on deducing the interactions between nonylphenol ethoxylates and
water with extending chain length of the ethoxylate part, branching of the nonyl
hydrocarbon chain and varying the substituent position (ortho-, meta- and para-) of
nonylphenol ethoxylate molecules. To interpret the interactions of nonylphenol ethoxylate
molecules with water, molecular properties such as Gibbs free energy changes of solvation
(ΔGsolv) in aqueous medium, molecular dipole moments and hydrogen bond lengths
between surfactant molecules and water molecules were investigated. The more negative
values (-46.62 to -116.88 kJ/mol) of the Solvation Gibbs free energy (ΔGsolv) indicate that
the stability of the NPEs in aqueous medium increases with the extension of the length of
ethoxylate chain (4 to 16 ethoxylate units). The substituent position and the branching of
nonyl-hydrocarbon chain have not caused the Gibbs free energy of solvation to change
significantly. Although electrical dipole moment of NPEs in aqueous medium varied with
the position of the substituents, it was not affected by branching patterns of the nonyl
hydrocarbon group. Computationally simulated UV-Visible spectra of NPEs revealed that
the absorption maximum of NPEs (λmax) was independent of ethoxylate chain length. UVVisible
spectra generated from selected molecules of NPEs confirmed that absorption
maximum (λmax) of the surfactant species was significantly affected by the change in
position of substituents.
