Fig. 1(a). Dynamic contact angle values expressed a function of
time for 0.10% 14-6-14 GS system; (b) Sessile drop profiles of
surfactant fluid at different time intervals for sandstone rock at 303
K.
Surfactant adsorption behavior
Surfactant adsorption is a major problem encountered during chemical EOR
owing to material losses observed within reservoir pore-throats.
Adsorption profile, obtained from UV spectroscopic analysis as a
function of 14-6-14 GS concentration for sandstone, is presented in Fig.
2. At low 14-6-14 GS concentrations (> CMC), surfactant
molecules begin to form aggregates/micelles and are subsequently
attracted to dispersed rock particle charge as electrical double layer
[48,49]. This ultimately leads to a sharp increase in adsorbed
surfactant. With further addition, 14-6-14 GS dimer molecules start to
occupy active “adsorption” sites; and repel previously adsorbed
micelles or aggregates [48]. Consequently, rock adsorption for
14-6-14 GS increased gradually with increasing concentration. At this
stage, solid surface shows a very slow increasing trend for 14-6-14 GS
adsorption density profiles owing to nearly complete saturation of
rock-liquid interfaces with surfactant dimer molecules. The main
influencing factor responsible for 14-6-14 GS adsorption is the
formation of an ionic or electrostatic pairing bond between cationic
head-groups of surfactant molecules and negatively charged rock surface
[49]. However, attractive forces such as electrostatic pairing
between surfactant head and charged rock surface, dispersive forces,
hydrophobic forces among adsorbed and free surfactant molecules,
hydrogen bonding and covalent interactions also aid in enhancing the
adsorption behavior of surfactant species [48,49].
Fig. 2. Adsorption density onto sand surface for 14-6-14 GS at
303 K.
Experimental data for rock adsorption was investigated by Langmuir and
Freundlich isotherms. The model parameters obtained by fitting
experimental data with adsorption theories are depicted in Table 2.
Though both isotherms have their respective behavioural traits and
applications, a careful evaluation of adsorption density versus
surfactant concentration plots with these theories may help in
understanding the molecular arrangement of adsorbed 14-6-14 GSs onto
sandstone rock. Langmuir model assumes that surfactant molecules form a
single layer onto adsorbent rock surfaces, wherein Freundlich model
suggests a multiple layer stacking arrangement of adsorbed surfactant
species [50]. Langmuir model exhibited better results as compared to
Freundlich adsorption model, which was evident from larger values of
coefficient of determination (R2). Furthermore,
smaller values of chi-square statistic (χ2) were
obtained during Langmuir isotherm fitting, which further corroborated
our inclination to the Langmuir isotherm model to predict surfactant
adsorption data. Therefore, monomolecular layer of adsorbed 14-6-14 GS
molecules is proposed with no stacking behavior during rock adsorption
studies.
Table 2. Langmuir and Freundlich model parameters obtained by
fitting experimental adsorption data