Download Terahertz absorption by dilute aqueous solutions

Terahertz absorption by dilute aqueous
solutions
Dmitry Matyushov
Center for Biological Physics
ASU
Questions asked
•Absorp(on of THz radia(on by solu(ons (large solutes in water)
•Dielectric (broad-­‐band) spectroscopy of solu(ons
•Polariza(on of the solute-­‐liquid interface 2
Maxwell: Polarization of the interface
Polarization of the solute interface
2
Ms = M − Ω0 P − (�s − 1)Mint
0
3
Polariza(on excluded
liq
by the solute
_
_
+
Maxwell scenario:
Mint
0
_
+
_
+
_
+
3Ω0 P
=−
2�s + 1
Polariza(on of the solvent by interface dipole
Polarization of a liquid
Liquid
Electrode
The structure of a polar liquid (water!) tends to diminish fluctua(ons of the surface dipoles in the direc(on normal to the dividing surface.
What to expect for a structured interface?
Ms = M
liq
2
− Ω0 P − (�s − 1)Mint
0
3
DVM, PRE 81 (2010) 021914
6
Access to alpha from simulations
Kihara solute (“cavity”):
In-­‐plane orienta(on of the surface dipoles (broken Maxwell’s boundary condi(on)
Lee, MaCammon, Rossky, J. Chem. Phys.’86
angle between water dipole and surface normal
7
Field inside Kihara solute
�s + 2
2(�s − 1)2
χc =
−α
3�s
3�s (2�s + 1)
int
M0z
=0
Kihara solute yields alpha=0, i.e., no interface dipole!
Mar(n, Friesen, DVM, JCP 135 (2011) 084514
Transverse vs Longitudinal
x
_
_
+
_
+
_
+
_
+
M
M0x
3�s
= −Ω0 P
2�s + 1
M
M0z
3
= −Ω0 P
2�s + 1
M
M
M0x
/M0z
= �s
z
Absorp(on of of transverse electromagne(c waves is more sensi(ve to details of the solute-­‐solvent interface than dielectric measurements
9
Deviations from Maxwell’s scenario (absorption)
M
α = x̂ · Mint
/M
0
0x
Parameter quan(fying the devia(on
from the Maxwell scenario
Maxwell interface dipole projected
on x-­‐axis of the external field
Characterizing interface in terms of solu(on absorp(on:
Absorp(on coefficient of the solu(on of spherical voids in water
4πω
χ�� (ω)
�
αabs (ω) =
c
1 + 4πχ� (ω)
�
∆χ(ω)
�s (ω) − 1
= −η0 1 + α(ω)
χs (ω)
2�s (ω) + 1
�
Volume frac(on of
solutes in solu(on
10
THz absorption of sugars and amino acids (aq)
Rota(ons of a large solute are dynamically frozen on the THz (me-­‐scale,
solutes are approximated by dielectric voids
Heyden et al, JACS 130 (2008) 5773
Niehues et al. Farad. Disc. Chem. Soc. 150 (2011) 193
11
What does THz absorption tell us?
Maxwell: alpha = 1
Sugars: alpha = -­‐0.2 -­‐ 0, no interface dipole!
Amino acids: alpha = (-­‐5) -­‐ (-­‐0.1), opposite to the field!
M
12
Alternative access to alpha
Cavity field:
Ec
χc =
E0
M
χc
3
=
2�s + 1
Maxwell scenario
2(�s − 1)2
3�s χc = �s + 2 − α
2�s + 1
DVM, JCP 136 (2012) 085102
13
Deviation from Maxwell’s scenario (longitudinal)
�s (ω)
= 1 − 3η0 + 3η0 �s (ω)χc (ω) (1 − y0 (ω))
�mix (ω)
Experimental input
Volume frac(on
Response of the lysozyme dipole,
taken from MD
Maxwell scenario
DVM, JPCM 24 (2012) 325105
Camec et al., JPCB 115 (2011) 7144
Vinh et al, JACS 133 (2011) 8942
14
What does dielectric spectroscopy tell us?
•Hydra(on shell over-­‐screen the external field, the low-­‐
frequency (< 1 GHz) response of lysozyme is dia-­‐electric (~223 D dipole moment repels from a higher field). Why?
�δM0 · δMw �
χc = 1 +
= 1 + χ0s /χ00
2
�(δM0 ) �
χ0s ∝ �δM0 · Ms � < 0
χ00 ∝ �(δM0 )2 � > 0
15
It works where it is supposed to!
Maxwell limit
16
Summary
•Hydrophobic/weakly hydrophilic amino
solutes: no polariza(on footprint in solu(on.
proteins
•Hydrophilic solutes (and proteins): increased polarity rela(ve to bulk water
acids
M
sugars
•Dependence on frequency: die-­‐
electric effect at low frequencies. Dan
Mar(n
$$ NSF (CHE)