Supporting Information
Molecular interactions at the
hexadecane/water interface in the presence
of surfactants studied with second
harmonic generation
Yajun Sang, †,‡ Fangyuan Yang,†,‡ Shunli Chen,† Hongbo Xu,†,‡ Si Zhang,†,‡ Qunhui
Yuan,*,† and Wei Gan*,†
†
Laboratory of Environmental Science and Technology, The Xinjiang Technical Institute
of Physics & Chemistry; Key Laboratory of Functional Materials and Devices for Special
Environments, Chinese Academy of Sciences, Urumqi 830011, China
‡
University of Chinese Academy of Sciences, Beijing 100049, China
Email: yuanqh@ms.xjb.ac.cn, ganwei@ms.xjb.ac.cn
A: The stability of the emulsions and the selection of experimental conditions
B: Detailed analysis of the second harmonic intensities
A: The stability of the emulsions and the selection of experimental conditions
The as-prepared emulsion was with droplet diameter ranging from 200 to 300 nm. With the
hexadecane purified by alumina columns, the prepared emulsion was with a relatively wide size
distribution as shown in Figure S1 and with a polydispersity index as approximately 0.4. The
stability of the emulsion during the SHG experiments was monitored by measuring the diameter of
the oil drops. The addition of MG and the surfactants were controlled in a range within which the
emulsion tends to holds its stability and only measurements from emulsion monitored to be stable
during the experiments (with less than 15% change in droplet diameter, which was comparable to
the fluctuation of the DLS measurement) was used to analyze the adsorption process. The
adsorption free energy and saturated MG density values presented were averaged values based on
at least 3 repetitions.
(a) Stability of the emulsion
Figure S1 shows the time dependent size distribution of the oil droplets in a freshly prepared
emulsion. It could be noticed that the size of the oil droplets kept almost unchanged within 18
hours. In the SHG experiments, the emulsion was used within 5 hours after the preparation to
ensure the stability of the sample.
Figure S1: The time dependent size distribution of the oil droplets in a
freshly prepared hexadecane/water emulsion.
(b) Stability of the emulsion after the addition of MG
Because the adsorption of the positively charged MG ions neutralized the negative charges at
hexadecane/water interface,1-4 the addition of MG was kept below 2 micromolar to ensure the
stability of the emulsion during the SHG measurement. Figure S2 shows the diameter change of
the oil droplet in the emulsion with the addition of MG at various concentrations.
Figure S2: The measured diameter of the oil droplet in hexadecane/water
emulsion after the addition of MG at various concentrations.
(c) Stability of the emulsion after the addition of SDS and MG
The addition of SDS stabilized the hexadecane/water emulsion. In the presence of SDS at
concentration beyond 5 μM, the emulsion after the addition of MG with concentration up to 33
μM was found to be stable. In the presence of SDS with concentration at 0.1 μM, 0.5 μM, and 1
μM, the emulsion kept stable after the addition of MG with concentration of 2 μM, 2 μM, and 17
μM, respectively. Above concentration range was selected for the titration experiments. Typical
data from the DLS measurements are shown in Figure S3.
Figure S3: The left panel is the measured average diameter of the oil droplets
in the hexadecane/water emulsions after the addition of SDS at various
concentrations. The red and blue curves in the right panel are the DLS
measurements of the oil droplets in the presence of 1 M SDS, before and
after the addition of 33 M MG, respectively.
(d) Stability of the emulsion after the addition of Tween80 and MG
The addition of Tween80 also stabilized the oil/water emulsion. In the presence of Tween80 at
concentration beyond 2 μM, the emulsion kept stable after the addition of MG with concentration
up to 17 μM. This concentration range was selected for the titration experiments. In the presence
of Tween80 at lower concentration, the addition of MG was still kept below 2 μM. Typical data
from the DLS measurements are shown in Figure S4.
Figure S4: The left panel is the measured average diameter of the oil droplets
in the hexadecane/water emulsions after the addition of Tween80 at various
concentrations. The red and blue curves in the right panel are the DLS
measurements of the oil droplets in the presence of 2 M Tween80, before
and after the addition of 17 M MG, respectively.
(e) Stability of the emulsion after the addition of CTAB and MG
The emulsion was found to be stable in the presence of CTAB at concentration beyond 0.5 μM.
Once the emulsion was stabilized with CTAB, it kept stable with the addition of MG with
concentration up to 33 μM. This concentration range was selected for the titration experiments.
Typical data from the DLS measurements are shown in Figure S5.
Figure S5: The left panel is the measured average diameter of the oil droplets
in the hexadecane/water emulsions after the addition of CTAB at various
concentrations. The red and blue curves in the right panel are the DLS
measurements of the oil droplets in the presence of 1 M CTAB, before and
after the addition of 33 M MG, respectively.
B: Detailed analysis of the second harmonic intensities
The obtained signal at 405 nm wavelength in the titration experiment was subjected to
attenuation because of the absorption caused by the emulsion and the MG molecules in the
solution at 405 nm wavelength. So it was corrected with the measured absorbance of the emulsion
and the MG solution with concentration used in the experiments. At 405 nm wavelength, the
emulsions absorb ~44% of the signal, while the MG molecules in the solution absorb 0-29% of the
signal, depending on the concentration (The values presented here is calculated based on the
UV-vis measurements as shown in Figure S6, considering a 0.65 cm pass length for the signal in
the cuvett). After the correction with these two factors considered, the actual value of the signal at
405 nm wavelength generated from the focus point of the laser was obtained. Also, the emulsions
absorb ~14% of the fundamental laser at 810 nm wavelength, thus cause a decrease of ~26% for
the obtained signal. This factor was also corrected in the treatment. The obtained value was further
corrected by subtracting the Hyper-Rayleigh scattering and the relatively weak tail contribution
from the two photon fluorescence of the MG molecules in the emulsions, which was measured
with the same experimental setup as used in the SHG experiment and also corrected considering
its self-absorption. Then the Hyper-Rayleigh scattering of water and the SH radiation from
surfaces of oil droplets, that were significantly smaller than the real signal (after correction
considering the absorption of the emulsion, the sum of the two was less than 10% of the real
signal) was also subtracted. The obtained value was then plot with the concentration of MG added
in the solution for further analysis.
Figure S6: The black curve is the UV-vis spectra of the emulsion used in the
experiments. The red curve is the spectra after adding 33 μM MG in the
emulsion.
The corrected SH intensity is in proportional to the square of MG coverage at surfaces of oil
droplets in the emulsion.5 The surface coverage (𝛳) of MG molecules could be related with MG
concentration in the emulsion with the modified Langmuir model described by Wang et al. 6
𝛳=𝑁
𝑁
𝑚𝑎𝑥
=
𝐶+𝑁𝑚𝑎𝑥 +55.5⁄𝐾−√(𝐶+𝑁𝑚𝑎𝑥 +55.5⁄𝐾)2 −4𝐶𝑁𝑚𝑎𝑥
2𝑁𝑚𝑎𝑥
(S1)
with N as the number of MG adsorbed at the surface of oil droplet, Nmax as the saturated number of
MG adsorbed at the surface of oil droplet, C as the total concentration of MG in the emulsion, K
as the adsorption equilibrium constant, which could be used to calculate the adsorption free energy
by ∆𝐺 = −𝑅𝑇𝑙𝑛𝐾.
Figures S7, S8 and S9 show the SH intensities in the titration experiments after the addition of
MG in the presence of SDS, Tween80 and CTAB at various concentrations. The fitting with
Equation S1 was also plotted as the solid lines. Please be noted that the fitting to data in Figure
S8-d and Figure S9 always resulted in negative Nmax values, which are with no physical meaning.
At the same time, acceptable fitting to the SHG data could be consistently achieved with relatively
small positive Nmax values. This implies small saturated MG densities at the oil/water interface in
these emulsions. Also, the adsorption free energy values obtained from all acceptable fittings with
physically allowed Nmax values only slightly changed within the uncertainty presented in the main
manuscript. In these cases, since the exact Nmax values were not able to be determined with the
fitting, we estimated them based on comparison of the SH intensities, with the SH intensities from
emulsions that are with similar chemical environment and with Nmax values accurately determined
by the model fittings.
It is known that the concentration of MG+ changed with the acidity of the solution because of

 MGOH+H + with a pKa value of 6.9.7-10 The concentration of
the equilibrium MG + + H 2O 
k
k1
-1
the MG+ in the solution at pH 6 was estimated to be 89% of the total MG molecules used to
prepare the solution. This factor was considered in the calculation of the MG density at the
interfaces.
(a) Titration curves with the emulsion containing SDS of various concentrations
Figure S7: The SH intensity curves in the titration from the emulsion
containing SDS with concentration of 0.1 μM (a), 0.5 μM (b), 1.0 μM (c), 5.0
μM (d), 10 μM (e), 50 μM (f), 100 μM (g), 250 μM (h), 500 μM (i), 750 μM
(j), 1 mM (k), 2 mM (l), and 4 mM (m), respectively.
(b) Titration curves with the emulsion containing Tween80 of various concentrations
Figure S8: The SH intensity curves in the titration from the emulsion
containing Tween80 with concentration of 0.5 μM (I), 1.0 μM (II), 2.0 μM
(III), 3.0 μM (IV), 4.0 μM (V), and 5.0 μM (VI), respectively.
(c) Titration curves with the emulsion containing CTAB of various concentrations
Figure S9: The SH intensity curves in the titration from the emulsion
containing CTAB with concentration of 0.5 μM (I), 1.0 μM (II), and 2.5 μM
(III), respectively.
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