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Home , Phase transition

Second-order transitions

Second-order transition G

T S

ΔS=0 The coexistence curve ends at a point called the critical point (TC, PC). As one moves along the coexistence curve toward the T ∂S critical point, the distinction between the C T⎛ ⎞ phase on one side and the phase on the P = ⎜ ⎟ → ∞ ⎝ ∂T ⎠P,N other gradually decreases and finally disappears at (T , P ). The T-driven that C C C occurs exactly at the critical point is called a P second-order phase transition. At and/ or beyond the critical point there is no physical distinction between liquid and gas. The density of the gas is so great and the thermal motion of the liquid is so great that gas T and liquid are the same. Unlike the 1st-order transitions, the 2nd-order transition does not require any latent (L=0). In the higher-order transitions (order-disorder transitions or ) the is continuous across the transition. The specific heat CP =T(∂S/∂T)P diverges at the transition (a cusp-like “λ” singularity).

These include the ferromagnetic phase transition in materials such as iron, where the , which is the first derivative of the free energy with respect to the applied magnetic field strength, increases continuously from zero as the is lowered below the . The , the second derivative of the free energy with the field, changes discontinuously. The Clausius-Clapeyron Relation

Along the phase boundary: P G1(P,T)= G2 (P,T) phase G1(P,T)= G2 (P,T) ΔP boundary

− S1dT +V1dP = −S2dT +V2dP ΔT T

dP S1(P,T )− S2 (P,T ) For the slope of the boundary we have: = dT V1(P,T )−V2 (P,T ) dP L(T ) Clausius-Clapeyron Relation : = dT T ΔV (T )

For the liquid-gas phase transition, we can make the following reasonable assumptions:

• molar volume of the liquid is much smaller than that of the gas (neglect Vliquid) • molar volume of gas is given by the law V = RT/P • is almost T-independent, L ≠ L(T) ⎛ dP ⎞ L LP ⎜ ⎟ ≈ = 2 ⎝ dT ⎠vap TVgas RT 4He (mbar) (mbar) P dP L dT ⎛ L ⎞ P = Pvap ∝ exp⎜− ⎟ P R T 2 ⎝ RT ⎠

T (K)

The : Gliq (Ptr ,Ttr ) = Ggas (Ptr ,Ttr ) = Gsol (Ptr ,Ttr )

Lvap = Ttr (SG − SL ) Lmelt = Ttr (SL − SS ) Lsub = Ttr (SG − SS )

Lmelt (Ttr )+ Lvap (Ttr ) = Lsub (Ttr ) of

The critical point and the orange line in the -one phase space refer to the low-density (LDA) and high- density (HDA) forms of amorphous water (ice)

Supercritical water is created in submarine volcanoes. Many lie at such great depths that the tremendous from the weight of the water above them prevents the explosive release of steam and . This causes the water to be heated to over 375 degrees C, turning the water in the hottest parts of the vents into a since the pressure at this depth of over 3 km is over 300 atmospheres, well above the 218 atmospheres required.

Nature 396, 329 (1998) Phase diagram of quark-gluon van der Waals model Phases exist because the particles interact with each other, in contrast to an ideal gas. The interactions are complicated (quantum mechanics), so we create simplified, effective models of the interparticle interactions in order to figure out what properties of the interactions lead to the observed phase diagrams.

The is an for a fluid composed of particles that have a non-zero volume and a pairwise attractive inter-particle force (such as the van der Waals force). It was derived in 1873 by Johannes Diderik van der Waals, who received the Nobel prize in 1910 for "his on the equation of state for gases and ". The equation is based on a modification of the and approximates the behavior of real fluids, taking into account the nonzero size of molecules and the attraction between them. Johannes van der Waals (1837 – 1923)

The main reason for the transformation of gas into liquid at decreasing T and (or) increasing P is the short-distance interaction between the molecules. repulsion 4 Lennard-Jones U(r) 3 12 6 2 "σ % "σ % 1 U r ∝$ ' −$ '

Energy 0 ( ) # r & # r & -1

-2 -3 NNa2 1.5 2.0 2.5 3.0 3.5 4.0 ΔUN=−⋅ ⋅ a=− distancer VV r = σ Na22 Na long-distance UU=− ⇒ PP=+ attraction vdW IG VVeff 2 the constant a is a measure Veff = V − Nb of the long-range attraction the constant b (~ 4πσ3/3) is a measure of the short-range repulsion, the “excluded volume” per particle The vdW equation of state

! N 2a $ NkT N 2a #P + 2 &(V − Nb) = NkT P = − 2 " V % (V − Nb) V b – roughly the volume of a molecule, (3.5·10-29 – 1.7 ·10-28) m3 ~(few Å)3 a – varies a lot [~ (8·10-51 – 3 ·10-48) J · m3] depending on the intermolecular interactions (strongest – between polar molecules, weakest – for inert gases).

Substance a’ b’ Pc Tc (J. m3/mol2) (x10-5 m3/mol) (MPa) (K)

Air .1358 3.64 3.77 133 K

Carbon Dioxide (CO2) .3643 4.27 7.39 304.2 K

Nitrogen (N2) .1361 3.85 3.39 126.2 K

Hydrogen (H2) .0247 2.65 1.30 33.2 K

Water (H2O) .5507 3.04 22.09 647.3 K

Ammonia (NH3) .4233 3.73 11.28 406 K (He) .00341 2.34 0.23 5.2 K

Freon (CCl2F2) 1.078 9.98 4.12 385 K

! N 2a $ #P + 2 &(V − Nb) = NkT PV = NkT " V %

Nb << V - low densities # Na & PV ≈ N %kT − ( $ V ' Na kT - high temperatures V (kinetic energy >> interaction energy) Entropy of monatomic van der Waals gas

N 3/2N Multiplicity of ideal gas: 12VmU⎛⎞π Ω≈IG (NVU,,) 2 Δ p NN!3 /2!⎜⎟ h2 ( ) ⎝⎠ 3/2N N Na2 1 VNb− ⎛⎞2πmU+ V Multiplicity of van der ( ) ( ) Ω≈vdW (NVU,,) ⎜⎟2 2 Δ p Waals gas: NN!3/2!⎜⎟ h ( ) ⎝⎠

3/2 ⎧⎫⎡⎤Na2 ⎪⎪VNbm− ⎛⎞45π U + V SNVUNkvdW (,,)=+B ⎨⎬ ln⎢⎥2 NhN⎜⎟32 ⎩⎭⎪⎪⎣⎦⎢⎥⎝⎠ 1 ⎛⎞∂S 2 = 3 N a ⎜⎟ U vdW = NkT − TU⎝⎠∂ VN, 2 V ⎛⎞∂S NkT N 2a PT=⋅ P = − ⎜⎟V 2 ⎝⎠∂ UN, (V − Nb) V S 3/2 ⎛⎞∂ ⎡⎤VNb− ⎛⎞2π mkB T 2 NaNb µ =−T ⋅ µ =−kTB ln ⎢⎥ −+ ⎜⎟ Nh⎜⎟2 VVNb ⎝⎠∂N UV, ⎣⎦⎢⎥⎝⎠ −

van der Waals Isotherms A real isotherm cannot have a negative slope dP/dn (or a positive slope dP/dV), since this corresponds to a negative compressibility. The

black isotherm at T=TC corresponds to the critical behavior. Below TC, the system becomes unstable against the phase separation (gas ↔ liquid) within a certain range V(P,T). The horizontal straight line shows the true isotherm in the liquid-gas coexistence region. Phase Separation in the vdW Model

T = const (< TC) P The phase transformation in the vdW model is easier to analyze by minimizing F(V) rather than G(P) (dramatic changes in the term PV makes the dependence G(P) very complicated, see below).

At T< TC, there is a region on the F(V) curve in which F makes a concave protrusion (∂2F/ V1 V2 V ∂V 2<0) – unlike its ideal gas counterpart. Due to this protrusion, it is possible to draw a common tangent line so that it touches the F bottom of the left dip at V = V1 and the right V dip at V = V2. Since the common tangent line lies below the free energy curve, molecules F 1 can minimize their free energy by refusing to (liquid) be in a single homogeneous phase in the region between V1 and V2, and by preferring F2 to be in two coexisting phases, gas and liquid: (gas)

N gas V −V1 Nliquid V2 −V N = N gas + Nliquid = = N V2 −V1 N V2 −V1

F1 F2 V −V2 V1 −V F = Nliquid + N gas = F1 + F2 N N V1 −V2 V1 −V2

V < V1 V1 < V < V2 V2 < V Since the tangent line F(V) maintains the same P T < T 7 C slope between V1 and V2, the pressure remains constant between V1 and V2: 3 6 2 ⎛ ∂F ⎞ Pvap 4 ⎜ ⎟ = −P ⎝ ∂V ⎠T ,N 1 5

V In other words, the line connecting F points on the PV plot is horizontal and the two coexisting phases are in V a mechanical equilibrium. For each Fliq temperature below TC, the phase transformation occurs at a well- defined pressure Pvap, the so-called pressure. Fgas

P P critical point

Pvap(T1)

triple point

Vliq Vgas V T1 T

Two stable branches 1-2-3 and 5-6-7 correspond to different phases. Along branch 1-2-3 V is large, P is small, the density is also small – gas. Along branch 5-6-7 V is small, P is large, the density is large – liquid. Between the branches – the gas-liquid phase transformation, which starts even before we reach 3 moving along branch 1-2-3. µ µ

P / P n / nC C

For T

The kink on the G(V) curve is a signature of the 1st order transition. When we move along the gas-liquid coexistence curve towards the critical point, the transition becomes less and less abrupt, and at the critical point, the abruptness disappears.

G 3 4 7 5 2,6

1

the lowest branch P represents the stable phase, the other branches are unstable